THE ENGINEERING THE HANDBOOK - SECOND EDITION II (end) ppt

Fault Limiters Underground Cable High Reliability Load 3-Phase Pad-Mounted Distribution Transformer Commercial Customer Network Transformer Network Protector Secondary Network Network Tr

Filters (Passive)

111.1 Fundamentals111.2 Applications

Simple RL and RC Filters • Simple RLC Filters • Compound Filters • Constant-k Filters • m- Derived Filters

A filter is a frequency-sensitive two-port circuit that transmits with or without amplification signals in

a band of frequencies and rejects (or attenuates) signals in other bands The electric filter was inventedduring the First World War by two engineers working independently of each other — the Americanengineer G A Campbell and the German engineer K W Wagner O Zobel followed in the 1920s Thesedevices were developed to serve the growing areas of telephone and radio communication Today, filtersare found in all types of electrical and electronic applications from power to communications Filterscan be both active and passive In this section we will confine our discussion to those filters that employ

no active devices for their operation The main advantage of passive filters over active ones is that theyrequire no power (other than the signal) to operate The disadvantage is that they often employ inductorsthat are bulky and expensive

111.1 Fundamentals

The basis for filter analysis involves the determination of a filter circuit’s sinusoidal steady state responsefrom its transfer function T(jw) Some references use H(jw) for the transfer function The filter’s transferfunction T(jw) is a complex function and can be represented through its gain ΩT(jw)Ω and phase–T(jw) characteristics The gain and phase responses show how the filter alters the amplitude and phase

of the input signal to produce the output response These two characteristics describe the frequency response of the circuit since they depend on the frequency of the input sinusoid The signal-processingperformance of devices, circuits, and systems is often specified in terms of their frequency response Thegain and phase functions can be expressed mathematically or graphically as frequency-response plots.Figure 111.1 shows examples of gain and phase responses versus frequency, w

The terminology used to describe the frequency response of circuits and systems is often based on theform of the gain plot For example, at high frequencies the gain in Figure 111.1 falls off so that outputsignals in this frequency range are reduced in amplitude The range of frequencies over which the output

is significantly attenuated is called the stopband. At low frequencies the gain is essentially constant andthere is relatively little attenuation The frequency range over which there is little attenuation is called a

passband. The frequency associated with the boundary between a passband and an adjacent stopband iscalled the cutoff frequency (wC=2pf C ) In general, the transition from the passband to the stopband,called the transition band, is relatively gradual, so the precise location of the cutoff frequency is a matter

of definition The most widely used approach defines the cutoff frequency as the frequency at which thegain has decreased by a factor of 1/ 2=0 707 from its maximum value in the passband

Albert J Rosa

University of Denver

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111-2 The Engineering Handbook, Second Edition

This particular definition is based on the fact that the power delivered to a resistor by a sinusoidalcurrent or voltage waveform is proportional to the square of its amplitude At a cutoff frequency the gain

is reduced by a factor of and the square of the output amplitude, and thusly also its power, isreduced by a factor of one half For this reason the cutoff frequency is also called the half-power frequency.

There are four prototypical filters These are low pass (LP), high pass (HP), band pass (BP), and bandstop

(BS) Figure 111.2 shows how the amplitude of an input signal consisting of three separate amplitude frequencies is altered by each of the four-prototypical filter responses The low-pass filterpasses frequencies below its cutoff frequency wC, called its passband, and attenuates the frequencies abovethe cutoff, called its stopband. The high-pass filter passes frequencies above the cutoff frequency wC andattenuates those below The band-pass filter passes those frequencies that lie between two cutoff frequen-cies, wC1 and wC2, its passband, and attenuates those frequencies that lie outside the passband Finally,the bandstop filter attenuates those frequencies that lie in its reject or stopband, between wC1 and wC2,and passes all others

equal-The bandwidth of a gain characteristic is defined as the frequency range spanned by its passband Forthe band-pass case in Figure 111.2, the bandwidth is the difference in the two cutoff frequencies

of a logarithmic frequency scale involves some special terminology A frequency range whose end pointshave a 2:1 ratio is called an octave and one with 10:1 ratio is called a decade. Straight-line approximations

FIGURE 111.1 Low-pass filter characteristics showing passband, stopband, and the cutoff frequency, wC.

0 Passband Stopband

ω c

ω ω

= 1 — that is, the input and output amplitudes are equal A positive dB gain means the output amplitude

exceeds the input since ΩT(jw)Ω> 1, whereas a negative dB gain means the output amplitude is smaller

than the input since ΩT(jw)Ω< 1 A cutoff frequency usually occurs when the gain is reduced from its

maximum passband value by a factor or 3 dB

Figure 111.3 shows the asymptotic gain characteristics of ideal and real low-pass filters The gain of

the ideal filter is unity (0 dB) throughout the passband and zero (-• dB) in the stopband It also has an

infinitely narrow transition band The asymptotic gain responses of real low-pass filters show that we can

only approximate the ideal response As the order of the filter or number of poles n increases, the

approximation improves since the asymptotic slope or “rolloff ” in the stopband is -20 ¥n dB/decade

On the other hand, adding poles requires additional stages in a cascade realization, so there is a trade-off

between (1) filter complexity and cost and (2) how closely the filter gain approximates the ideal response

Figure 111.4 shows how low-pass filter requirements are often specified To meet the specification, the

gain response must lie within the unshaded region in the figure, as illustrated by the two responses shown

in Figure 111.4 The parameter Tmax is the passband gain. In the passband the gain must be within 3 dB

of Tmax and must equal at the cutoff frequency wC In the stopband the gain must decrease

and remain below a gain of Tmin for all w≥wmin A low-pass filter design requirement is usually defined

by specifying values for these four parameters The parameters TmaxandwC define the passband response,

whereas Tmin and wmin specify how rapidly the stopband response must decrease

FIGURE 111.2 Four prototype filters and their effects on an input signal consisting of three frequencies.

Passband Stopband

LOW PASS GAIN

Passband Stopband

111-4 The Engineering Handbook, Second Edition

111.2 Applications

Simple RL and RC Filters

A first-order LP filter has the following transfer function:

(111.3)

FIGURE 111.3 The effect of increasing the order n of a filter relative to an ideal filter.

FIGURE 111.4 Parameters for specifying low-pass filter requirements.

ω C ω

|T(j ω)| dB

TMAX

TMIN3dB

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Filters (Passive) 111-5

where for a passive filter K £ a and a = w C This transfer function can be realized in several ways including

using either of the two circuits shown in Figure 111.5

For sinusoidal response the respective transfer functions are

(111.4)

For these filters the passband gain is equal to one and the cutoff frequency is determined by R/L for the

RL filter and 1/RC for the RC filter The gain ΩT( jw)Ω and phase –T( jw) plots of these circuits are

shown back in Figure 111.1

A first-order HP filter is given by the following transfer function:

(111.5)

where, for a passive filter, K £ 1 and a is the cutoff frequency This transfer function can also be realized

in several ways including using either of the two circuits shown in Figure 111.6 For sinusoidal response

the respective transfer functions are

(111.6)

For the LP filters the passband gain is one and the cutoff frequency is determined by R/L for the RL filter

and 1 / RC for the RC filter The gain ΩT( jw)Ω and phase –T( jw) plots of these circuits are shown in

Figure 111.7

FIGURE 111.5 Single-pole LP filter realizations: (a) RL, (b) RC.

FIGURE 111.6 Single-pole HP filter realizations: (a) RL, (b) RC.

R

C L

11

s

( ) =+ a

=

111-6 The Engineering Handbook, Second Edition

connected to achieve the transfer functions given in Equation 111.7 The gain ΩT( jw)Ω and phase –T( jw)

plots of these circuits are shown in Figure 111.9 through Figure 111.11

FIGURE 111.7 High-pass filter characteristics showing passband, stopband, and the cutoff frequency, wc.

0

Passband Stopband

22

L

2

Bw = 2zw0

FIGURE 111.8 RLC circuit connections to achieve LP, HP, or BP responses.

FIGURE 111.9 Second-order low-pass gain responses.

FIGURE 111.10 Second-order band-pass gain responses.

|K|

10 ω 0

K dB + 20dB

ω 0

K dB

− 20dB

ω 0 K

dB − 40dB

ω 0

K dB + 0dB

ω 0

ω 0 ω

Bζ=0.5

111-8 The Engineering Handbook, Second Edition

Compound Filters

Compound filters are higher-order filters obtained by cascading lower-order designs Ladder circuits are

an important class of compound filters Two of the more common passive ladder circuits are the

constant-k and the m-derived filters (either of which can be configured using a T-section, p-section, or L-section,

or combinations thereof), the bridge-T network and parallel-T network, and the Butterworth and

Chebyshev realizations Only the first two known as image-parameter filters will be discussed in this

section Figure 111.12(a) shows a standard ladder network consisting of two impedances, Z1 and Z2,organized as an L-section filter Figure 111.12(b) and Figure 111.12(c) show how the circuit can beredrawn to represent a T-section or ’-section filter, respectively

T- and ’-section filters (also referred to as “full sections”) are usually designed to be symmetrical sothat either can have its input and output reversed without changing its behavior The “L-section” (alsoknown as a “half section”) is unsymmetrical, and orientation is important Since cascaded sections “load”

each other, the choice of termination impedance is important The image impedance, Z i, of a symmetricalfilter is the impedance with which the filter must be terminated in order to “see” the same impedance

at its input terminals In general the image impedance is the desired load or source impedance to whichthe filter matches The image impedance of a filter can be found from

(111.8)

where Z 1O is the input impedance of the filter with the output terminals open circuited, and Z 1S is itsinput impedance with the output terminals short-circuited For symmetrical filters the output and inputcan be reversed without any change in its image impedance — that is,

(111.9)

The concept of matching filter sections and terminations to a common image impedance permits thedevelopment of symmetrical filter designs

The image impedances of T- and ’-section filters are given as

FIGURE 111.11 Second-order high-pass gain responses.

0.01 0.1 1.0 10 100 1000 0.001

and

Z iT= Z Z1O 1S = Z12+Z Z

1 2

14

Since Z1 and Z2 vary significantly with frequency, the image impedances of T- and ’-sections will alsochange This condition does not present any particular problem in combining any number of equivalentfilter sections together, since their impedances va4ry equally at all frequencies But this does make itdifficult to terminate these filters exactly, causing a limitation of these types of filters However, there is

a frequency within the filter’s passband where the image impedance becomes purely resistive It is useful

FIGURE 111.12 Ladder networks: (a) standard L-section, (b) T-section, (c) ’-section.

Z12Z22Z2

Z12Z22Z2

Z12Z22Z2 2Z2

1 2

1 4( / )

111-10 The Engineering Handbook, Second Edition

to terminate the filter with this value of resistance since it provides good matching over much of thefilter’s passband

To develop the theory of constant-k and m-derived filters, consider the circuit of Figure 111.13 The current transfer function in the sinusoidal steady state is given by T( jw) = ΩT( jw)Ω–T( jw) = I2/I1:

In modern references an R replaces the k Note that the units of k are ohms The advantage of this type

of filter is that the image impedance in the passband is a pure resistance, whereas in the stopband it ispurely reactive Hence if the termination is a pure resistance and equal to the image impedance, all thepower will be transferred to the load since the filter itself is purely reactive Unfortunately, the value ofthe image impedance varies significantly with frequency, and any termination used will result in amismatch except at one frequency

In LC constant-k filters, Z1 and Z2 have opposite signs, so that Theimage impedances become

(111.15)

Therefore, in the stopband and passband, we have the following relations for standard T- or ’-sections,

where n represents the number of identical sections:

FIGURE 111.13 Circuit for determining the transfer function of a T-section filter.

2

2 1

The ultimate roll-off of constant-k filters is 20 dB per decade per reactive element or 60 dB per decade

for the T- or ’-section, 40 dB per decade per L-section Figure 111.14 shows normalized plots of a and

b versus These curves are generalized and apply to low-pass, high-pass, pass, or reject filters Figure 111.15 shows examples of a typical LP ’-section, an HP T-section, and a BP T-section

band-m -Derived Filters

The need to develop a filter section that could provide high attenuation in the stopband near the cutoff

frequency prompted the development of the m-derived filter O Zobel developed a class of filters that had the same image impedance as the constant-k but had a higher attenuation near the cutoff frequency The impedances in the m-derived filter were related to those in the constant-k as

FIGURE 111.14 Normalized plots of attenuation and phase angle for various numbers of sections n.

FIGURE 111.15 Typical sections: (a) LP ’-section, (b) HP T-section, (c) BP T-section.

111-12 The Engineering Handbook, Second Edition

(111.17)

where m is a positive constant £ 1 If m = 1 then the impedances reduce to those of the constant-k Figure 111.16 shows generalized m-derived T- and ’-sections.

The advantage of the m-derived filter is that it gives rise to infinite attenuation at a selectable frequency,

w˚, just beyond cutoff, wC This singularity gives rise to a more rapid attenuation in the stopband than

can be obtained using constant-k filters Equation 111.18 relates the cutoff frequency to the infinite

(both normalized) and m In most applications, m is chosen to be 0.6, keeping the image impedance

nearly constant over about 80% of the passband

FIGURE 111.16 m-derived filters: (a) T-section, (b) ’-section.

1 / 2 mZ1

(b) (a)

1 / 2 mZ1(1 − m)Z 1 4m

Z2m

(1 − m)Z 1 4m

mZ1

2Z2m

2Z2m

Defining Terms

Bridge-T network — A two-port network that consists of a basic T-section and another element

con-nected so as to “bridge across” the two arms Such networks find applications as band rejectionfilters, calibration bridges, and feedback networks

Butterworth filters — Ladder networks that enjoy a unique passband frequency response characteristic

that remains very constant until near the cutoff, hence the designation “maximally flat.” This

FIGURE 111.17 Attenuation curves for a single-stage filter with m = 0.6 and m = 0.9.

FIGURE 111.18 Z iT /R and Z i’ /R versus normalized frequency for various values of m.

0 0.5 1 1.25 1.5 2 2.29 2.5

111-14 The Engineering Handbook, Second Edition

filter has its critical frequency remain fixed regardless of the number of stages employed Itobtains this characteristic by realizing a transfer function built around a Butterworth polyno-mial

Chebyshev filters — A variant of the Butterworth design that achieves a significantly steeper transition

band about its critical frequency for the same number of poles Although the Chebyshev filteralso maintains the integrity of its critical frequency regarding the number of poles, it trades thesteeper roll-off for a fixed ripple — usually specified as 1 dB or 3 dB — in the passband

Chebyshev filters are also called equal-ripple or stagger-tuned filters They are designed by

realizing a transfer function using a Chebyshev polynomial

Parallel-T networks — A two-port network that consists of two separate T-sections in parallel with only

the ends of the arms and the stem connected Parallel-T networks have applications similar tothose of the bridge-T but can produce narrower attenuation bandwidths

References

Herrero, J L and Willoner, G 1966 Synthesis of Filters, Prentice Hall, Englewood Cliffs, NJ.

Thomas, R E and Rosa, A J 2004 The Analysis and Design of Linear Circuits, John Wiley & Sons,

Hoboken, NJ

Van Valkenburg, M E 1955 Two-terminal-pair reactive networks (filters) In Network Analysis, Prentice

Hall, Englewood Cliffs, NJ

Weinberg, L 1962 Network Analysis and Synthesis, W L Everitt (ed.) McGraw-Hill, New York.

Williams, A B 1981 Electronic Filter Design Handbook, McGraw-Hill, New York.

Zobel, O J 1923 Theory and Design of Uniform and Composite Electric Wave Filters Bell Telephone

Syst Tech J 2:1.

Further Information

Huelsman, L P 1993 Active and Passive Analog Filter Design — An Introduction, McGraw-Hill, New York.

Good current introductory text covering all aspects of active and passive filter design

Sedra, A S and Brackett, P O 1978 Filter Theory and Design: Active and Passive, Matrix, Beaverton, OR.

Modern approach to filter theory and design

Power Distribution

112.1 Equipment112.2 System Divisions and Types112.3 Electrical Analysis, Planning, and Design

Phase Balancing • Fault Analysis • Protection and Coordination • Reliability Analysis

112.4 System Control112.5 Operations

The function of power distribution is to deliver to consumers economic, reliable, and safe electricalenergy in a manner that conforms to regulatory standards Power distribution systems receive electricenergy from high-voltage transmission systems and deliver energy to consumer service-entrance equip-ment Systems typically supply alternating current at voltage levels ranging from 120 V to 46 kV

Figure 112.1 illustrates aspects of a distribution system Energy is delivered to the distribution tion (shown inside the dashed line) by three-phase transmission lines A transformer in the substationsteps the voltage down to the distribution primary system voltage — in this case, 12.47 kV Primarydistribution lines leave the substation carrying energy to consumers The substation contains a breakerthat may be opened to disconnect the substation from the primary distribution lines If the breaker isopened, outside the substation there is normally an open supervisory switch that may be closed in order

substa-to provide an alternate source of power for the cussubsta-tomers normally served by the substation Thesubstation also contains a capacitor bank used for either voltage or power factor control

Four types of customers, along with representative distribution equipment, are shown in Figure 112.1

A set of loads requiring high reliability of service is shown being fed from an underground three-phasesecondary network cable grid A single fault does not result in an interruption to this set of loads Aresidential customer is shown being supplied from a two-wire, one-phase overhead lateral Commercialand industrial customers are shown being supplied from the three-phase, four-wire, overhead primaryfeeder At the industrial site, a capacitor bank is used to control the power factor Except for the industrialcustomer, all customers shown have 240/120 V service The industrial customer has 480Y/277 V service.For typical electric utilities in the U.S., investment in distribution ranges from 35 to 60% of totalcapital investment

112.1 Equipment

Figure 112.1 illustrates a typical arrangement of some of the most common equipment Equipment may

be placed into the general categories of transmission, protection, and control

Arresters protect distribution system equipment from transient over-voltages due to lightning orswitching operations In over-voltage situations the arrester provides a low-resistance path to ground forcurrents to follow

Electrical Distribution Design, Inc.

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112-2 The Engineering Handbook, Second Edition

Capacitor banks are energy storage devices primarily used to control voltage and power factor Systemlosses are reduced by the application of capacitors

Conductors are used to transmit energy and may be either bare or insulated Bare conductors havebetter thermal properties and are generally used in overhead construction where contact is unlikely.Insulated cables are used in underground/conduit construction and in overhead applications whereminimum right-of-way is available Concentric neutral and tape-shielded cables provide both a phaseconductor and a return path conductor in one unit

FIGURE 112.1 Distribution system schematic.

Fault Limiters

Underground Cable

High Reliability Load

3-Phase Pad-Mounted Distribution Transformer

Commercial Customer

Network Transformer Network

Protector

Secondary Network

Network Transformer

Network Protector

Feed from alternate source

Supervisory Switch (Normally Open)

Voltage Regulator Power

Fuse

Capacitor Bank

Single Phase Pole Mounted Distribution Transformer

Residential Customer

Transmission Line

Transmission Line

Power Transformer

Capacitor Bank Disconnect

Disconnect Disconnect

Disconnect

12.47 KV Bus Breaker

Recloser 1586_book.fm Page 2 Monday, May 10, 2004 3:53 PM

Power Distribution 112-3

Distribution lines are made up of conductors and are classified according to primary voltage, thenumber of phases, number of conductors, and return path The three-phase, four-wire, multi-groundedsystem is the most common primary system, where one conductor is installed for each of the three phasesand the fourth conductor is a neutral that provides a return current path Multi-grounded means thatthe neutral is grounded at many points, so that the earth provides a parallel path to the neutral for returncurrent Three-phase, three-wire primary systems, or delta-connected systems, are rarely used becausefaults therein are more difficult to detect A lateral is a branch of the system that is shorter in length,more lightly loaded, or has a smaller conductor size than the primary feeder

Distribution transformers step the voltage down from the primary circuit value to the customerutilization level, thus controlling voltage magnitude Sizes range from 1.5 to 2500 kVA Distributiontransformers are installed on poles, ground-level pads, or in underground vaults A specification of 7200/12,470Y V for the high-voltage winding of a single-phase transformer means the transformer may beconnected in a line-to-neutral “wye” connection for a system with a line-to-line voltage of 12,470 V or

in a line-to-line “delta” connection for a system with a line-to-line voltage of 7200 V A specification of240/120 V for the low-voltage winding means the transformer provides a three-wire connection with

120 V mid-tap voltage and 240 V full-winding voltage A specification of 480Y/277 V for the low voltagewinding means the winding is permanently wye-connected with a fully insulated neutral avilable for athree-phase, four-wire service to deliver 480 V line-to-line and 277 V line-to-neutral

Distribution substations consist of one or more step-down power transformers configured with switchgear, protective devices, and voltage regulation equipment for the purpose of supplying, controlling,switching, and protecting the primary feeder circuits The voltage is stepped down for safety and flexibility

of handling in congested consumer areas Over-current protective devices open and interrupt currentflow in order to protect people and equipment from fault current Switches are used for control tointerrupt or redirect power flow Switches may be operated manually, automatically with PLC control,

or remotely with supervisory control Switches are usually rated to interrupt load current and may beeither pad or pole mounted

Power transformers are used to control and change voltage level Power transformers equipped withtap-changing mechanisms can control secondary voltage over a typical range of plus or minus 10%.Voltage regulators are autotransformers with tap-changing mechanisms that may be used throughoutthe system for voltage control If the voltage at a remote point is to be controlled, then the regulator can

be equipped with a line drop compensator that may be set to regulate the voltage at the remote pointbased upon local voltage and current measurements Modern microprocessor-based controls enableregulators and line capacitors to work together to provide optimal voltage regulation

112.2 System Divisions and Types

Distribution transformers separate the primary system from the secondary Primary circuits transmitenergy from the distribution substation to customer distribution transformers Three-phase distributionlines that originate at the substation are referred to as primary feeders or primary circuits Primary feedersare illustrated in Figure 112.1 Secondary circuits transmit energy from the distribution transformer tothe customer’s service entrance Line-to-line voltages range from 208 to 600 V

Radial distribution systems provide a single path of power flow from the substation to each individualcustomer This is the least costly system to build and operate and thus the most widely used

Primary networks contain at least one loop that generally may receive power from two distinct sources.This design results in better continuity of service A primary network is more expensive than the radialsystem design because more protective devices, switches, and conductors are required

Secondary networks are normally underground cable grids providing multiple paths of power flow toeach customer A secondary network generally covers a number of blocks in a downtown area Power issupplied to the network at a number of points via network units, consisting of a network transformer

in series with a network protector A network protector is a circuit breaker connected between thesecondary winding of the network transformer and the secondary network itself When the network is

112-4 The Engineering Handbook, Second Edition

operating properly, energy flows into the network The network protector opens when reverse energyflow is detected, such as may be caused by a fault in the primary system

112.3 Electrical Analysis, Planning, and Design

The distribution system is planned, designed, constructed, and operated based on the results of electricalanalysis Generally, computer-aided analysis is used

Line impedances are needed by most analysis applications Distribution lines are electrically anced due to loads, unequal distances between phases, dissimilar phase conductors, and single-phase ortwo-phase laterals Currents flow in return paths due to the imbalance in the system Three-phase, four-wire, multigrounded lines have two return paths — the neutral conductor and earth Three-phase,multigrounded concentric neutral cable systems have four return paths The most accurate modeling ofdistribution system impedance is based upon Carson’s equations With this approach a 5 ¥ 5 impedancematrix is derived for a system with two return paths, and a 7 ¥ 7 impedance matrix is derived for asystem with four return paths For analysis, these matrices are reduced to 3 ¥ 3 matrices that relate phasevoltage drops (i.e., , , ) to phase currents (i.e., , , ) as indicated by

unbal-Load analysis forms the foundation of system analysis unbal-Load characteristics are time varying and depend

on many parameters, including connected consumer types and weather conditions The load demandfor a given customer or group of customers is the load averaged over an interval of time, say 15 min.The peak demand is the largest of all demands The peak demand is of particular interest since it representsthe load that the system must be designed to serve Diversity relates to multiple loads having differenttime patterns of energy use Due to diversity, the peak demand of a group of loads is less than the sum

of the peak demands of the individual loads For a group of loads,

Loads may be modeled as either lumped parameter or distributed Lumped parameter load modelsinclude constant power, constant impedance, constant current, voltage-dependent, and combinationsthereof Generally, equivalent lumped parameter load models are used to model distributed loads.Consider the line section of length L shown in Figure 112.2(a), with a uniformly distributed load currentthat varies along the length of the line as given by

The total load current drawn by the line section is thus

V V V

I I I

A B C

A B C

È

Î

ÍÍÍÍ

Î

ÍÍÍÍ

Î

ÍÍÍÍ

Diversity factor =Sum of individual load peaks

Power flow analysis determines system voltages, currents, and power flows Power flow results arechecked to ensure that voltages fall within allowable limits, that equipment overloads do not exist, andthat phase imbalances are within acceptable limits For primary and secondary networks, power flow

FIGURE 112.2 (a) Line section model with distributed load current; (b) lumped parameter equivalent model.

FIGURE 112.3 Representative diversified load curve for a residential customer type.

(a) Line Section Model With Distributed Load Current

(b) Lumped Parameter Equivalent Model

Distributed Load Current

2IL3

IL3

0 2 4 6 8 10 12 14 16 18 20 22 24 300

600 900

1500

1200

Hours Peak Day of the Month Hourly Load Curves

112-6 The Engineering Handbook, Second Edition

methods used in transmission system analysis are applied For radially operated systems, the laddermethod is used The actual implementation of the ladder method may vary with the type of load modelsused All ladder load flow methods assume the substation bus voltage is known An algorithm for theladder method consists of the following five steps:

1 Step 1. Assume a value for all node voltages throughout the circuit Generally, assumed voltagesare set equal to the substation voltage

2 Step 2. At each load in the circuit, calculate the current from the known load value and assumedvoltage

3 Step 3. Starting at the ending nodes, sum load currents to obtain line section current estimates,performing summation until the substation is reached

4 Step 4. Having estimates of all line section currents, start at the substation and calculate linesection voltage drops and new estimates of node voltages

5 Step 5. Compare new node voltages with estimates of previous iteration values The algorithmhas converged if the change in voltage is sufficiently small If the algorithm has not converged,return to Step 2

Dynamic load analysis includes such studies as motor-starting studies Rapid changes in large loadscan result in large currents, with a resultant drop in system voltage If the dip in voltage is too large ortoo frequent, then other loads are adversely affected, such as in an annoying flicker of lights This studygenerally employs a power flow calculation that is run at a number of points along the dynamic charac-teristic of the load

Planning involves using load forecasting and other analysis calculations to evaluate voltage level,substation locations, feeder routes, transformer/conductor sizes, voltage/power factor control, and res-toration operations Decisions are based upon considerations of efficiency, reliability, peak demand, andlife cycle cost

Phase Balancing

Phase balancing is used to balance the current or power flows on the different phases of a line section.This results in improved efficiency and primary voltage level balance The average current in the threephases is defined as

The maximum deviation from Iavg is given by

FIGURE 112.4 Representative diversity factor curve for a residential customer type.

1 1 1.20 1.40 1.60

At a multi-phase grounded node in a linear distribution system, post-fault voltages are related to fault voltages and fault currents as given by

where Vfdenotes phase voltages (voltages between phase A and ground, B and ground, and C and ground)

of the node during the fault, V0 is the array of phase voltages before the fault occurs, I is the array offault currents that will flow out of the phases of the node during the fault, and Zth (a 3 ¥ 3 matrix)represents the phase Thevenin matrix looking into the node

Once Zth and pre-fault voltages at the node are available, Vf can be written in terms of I dependinguponconditions imposed by the fault Then Equation (112.1) can be solved for I The pre-fault systemmodel represents the system behavior before the fault occurs On the pre-fault model, a power flowcalculation may be used to obtain the voltages V0

The post-fault model represents the system behavior during the fault For fault calculations, the circuitmodel used is modified in several ways from the pre-fault model Usually, load currents are neglected inthe post-fault model, and instead superposition is used to add load currents obtained from the pre-faultpower flow analysis to fault currents For the fault calculations, the circuit model is assumed to be linear.Other changes for the post-fault circuit analysis include neglecting slow-acting control devices (such assubstation transformer tap changers) that do not have time to react during the time of the fault; andinserting appropriate Thevenin equivalent source impedances, representing the Thevenin impedance seen

by the distribution substation looking back into the transmission or subtransmission system Using thepost-fault circuit model assumptions, a power flow calculation using constant current injections at thefault point may be used to perform the fault calculations This is the approach described here

The calculation of Zth may be performed by inserting a small test load sequentially at every individualphase of the faulted node Prior to any test load insertion, the phase voltages of the node are obtainedfrom a power flow solution Let these voltages be Vi After inserting a test load, the power flow is usedagain to obtain the current flowing into the test load and the voltages at all phases of the node Ratios

of changes in Vi to the current drawn by the test load constitute the columns of Zth For instance, if thetest load is inserted between phase A and ground at grounded node N, the results calculated are the firstcolumn of Zth

To elaborate, refer to Figure 112.5 where a grounded node N is considered Here, a general node atwhich any phase may exist is assumed The power flow is run on the post-fault system model, and phase-to-neutral voltages at node N are obtained as V an, V bn, and V cn for the phases A, B, and C, respectively

DIdev=maximum of {Iavg-I A,Iavg-I B,Iavg-I C }

Phase imbalance dev

avg

=DI

I

112-8 The Engineering Handbook, Second Edition

[Figure 112.5(a)] The neutral n is regarded to be the same as ground A test load will be inserted between

A and n, B and n, and C and n sequentially [Figure 112.5(b) through Figure 112.5(d)] During each load

insertion, line currents and phase-to-neutral voltages may be obtained from a power flow calculation

The elements z ij of Zth may be determined in the following manner:

, ,

, ,

, ,

Zth represents the relationship between the voltage changes and the current changes at N. Suppose

phase-to-neutral voltages at N before the fault are Assume that a fault occurs at N and causes

currents to flow out of phases A, B, and C, respectively, resulting in phase-to-neutral voltages

.Then voltage changes at N are related to the currents drawn, via Zth as

(112.2)

where

Equation (112.2) denotes a general case Suppose node N is a double-phase location having phases A

and B but no phase C Then, all the elements in the third row and third column of Zth are zero

FIGURE 112.5 Constructing Zth at a grounded node N (a) Voltages before inserting any test load (b) A test load

being inserted between phase A and ground (c) A test load being inserted between phase B and ground (d) A test

load being inserted between phase C and ground.

Post-fault System Model

Post-fault System Model

Phases present

at node N

a + b + c + n

Post-fault System Model n

(2) (2) (2)

Van(1)(1) (1)

(1)

Vbn

Vcn

(3) (3) (3) (3)

-( ) ( )

-( ) ( )

-( ) ( )

bn i

cn i

, , and

DDD

V V V

I I I

an bn cn

a b c

È

Î

ÍÍÍ

Î

ÍÍÍ

= - for k=a,b,c

1586_book.fm Page 8 Monday, May 10, 2004 3:53 PM

Various fault cases at N are shown in Figure 112.6 A general case of a three-phase-to-ground fault at

N is represented in Figure 112.6(a) Here, each phase has its own fault impedance (Z a , Z b , and Z c for

phases A, B, and C, respectively) to the common point p Z f is the impedance between p and n Consider solving for a three-phase-to-ground fault Let V i

kn and V f

kn denote pre-fault and post-fault

phase-to-ground voltages of phase k, respectively Then the boundary conditions are:

sub-Any fault event imposes a set of boundary conditions Initial voltages (pre-fault voltages) can be readilycalculated from the power flow The final voltages are expressed under the boundary conditions in terms

of the fault currents and fault impedances Then, Equation (112.2) is solved for fault currents Using thisapproach, fault currents for cases b through d shown in Figure 112.6 may be evaluated For an ungroundednode, the phase-to-phase voltages instead of phase-to-neutral voltages are employed

FIGURE 112.6 Various faults at a grounded node N (a) Three-phase fault (b) Phase-to-phase fault (c) phase-to-ground fault (d) Single-phase-to-ground fault.

p

p

Post-fault System Model

Post-fault System Model

Post-fault System Model

Post-fault System Model

DV cn V cn i V V I Z I Z

cn f cn i

I I I

V V V

a b c

an i bn i cn i

È

Î

ÍÍÍ

Î

ÍÍÍ

112-10 The Engineering Handbook, Second Edition

Protection and Coordination

Over-current protection is the most common protection applied to the distribution system With current protection, the protective device trips when a large current is detected The time to trip is afunction of the magnitude of the fault current The larger the fault current is, the quicker the operation.Various types of equipment are used A circuit breaker is a switch designed to interrupt fault current,the operation of which is controlled by relays An over-current relay, upon detecting fault current, sends

over-a signover-al to the breover-aker to open A recloser is over-a switch thover-at opens over-and then recloses over-a number of timesbefore finally locking open A fuse is a device with a fusible member, referred to as a fuse link, which inthe presence of an over current melts, thus opening up the circuit

Breakers may be connected to reclosing relays, which may be programmed for a number of openingand reclosing cycles With a recloser or a reclosing breaker, if the fault is momentary, then the powerinterruption is also momentary If the fault is permanent, then after a specified number of attempts atreclosing the device locks open Breakers are generally more expensive than comparable reclosers Breakersare used to provide more sophisticated protection, which is available via choice of relays Fuses aregenerally used in the protection of laterals

Protective equipment sizing and other characteristics are determined from the results of fault analysis.Moving away from the substation in a radial circuit, both load current and available fault current decrease.Protective devices are selected based on this current grading Protective devices are also selected to havedifferent trip-delay times for the same fault current With this time grading, protective devices arecoordinated to work together such that the device closest to a permanent fault clears the fault Thusreclosers can be coordinated to protect load-side fuses from damage due to momentary faults

Reliability Analysis

Reliability analysis involves determining indices that relate to continuity of service to the customer.Reliability is a function of tree conditions, lightning incidence, equipment failure rates, equipment repairtimes, and circuit design The reliability of a circuit generally varies from point to point due to protectionsystem design, placement of switches, and availability of alternative feeds Many indices are used inevaluating system reliability Common ones include system average interruption frequency index (SAIFI),system average interruption duration index (SAIDI), customer average interruption frequency index(CAIFI), and customer average interruption duration index (CAIDI) as defined by

SAIFI = Total number of customer interruptions

Total number of customers served

SAIDI = Sum of customer interruption durations

Total number of customers

CAIFI = Total number of customer interruptions

Total number of customers affected

CAIDI = Sum of customer interruption durations

Total number of customers affected

112.4 System Control

Voltage control is required for proper operation of customer equipment For instance, in the U.S., “voltagerange A” for single-phase residential users specifies that the voltage may vary at the service entrance from114/228 V to 126/252 V Regulators, tap-changing under load transformers, and switched capacitor banksare used in voltage control

Power factor control is used to improve system efficiency Due to the typical load being inductive,power factor control is generally achieved with fixed and/or switched capacitor banks

Power flow control is achieved with switching operations Such switching operations are referred to

as system reconfiguration Reconfiguration may be used to balance a load among interconnected bution substations Such switching operations reduce losses while maintaining proper system voltage.Load control may be achieved with voltage control and also by remotely operated switches thatdisconnect load from the system Generally, load characteristics are such that if the voltage magnitude

distri-is reduced, then the power drawn by the load will decrease for some period of time Load control withremotely operated switches is also referred to as load management

Effective system control is essential to provide adequate power quality Power quality may be defined

as the absence of service interruptions, voltage dips and sags, and voltage spikes and surges Propercontrol of system voltage is more critical now than ever before because many microprocessor-basedcontrols and adjustable speed drives have voltage tolerances less than 10%

112.5 Operations

The operations function includes system maintenance, construction, and service restoration nance, such as trimming trees to prevent contact with overhead lines, is important to ensure a safe andreliable system Interruptions may be classified as momentary or permanent A momentary interruption

Mainte-is one that dMainte-isappears very quickly — for instance, a recloser operation due to a fault from a tree limbbriefly touching an overhead conductor Power restoration operations are required to repair damagecaused by permanent interruptions

While damaged equipment is being repaired, power restoration operations often involve tion in order to restore power to interrupted areas With reconfiguration, power flow calculations may

reconfigura-be required to ensure that equipment overloads are not created from the switching operations

Defining Terms

Current return path — The path that current follows from the load back to the distribution substation.

This path may consist of either a conductor (referred to as the neutral) or earth, or the parallelcombination of a neutral conductor and the earth

Fault — A conductor or equipment failure or unintended contact between conductors or between

con-ductors and grounded objects If not interrupted quickly, fault current can severely damageconductors and equipment

Phase — Relates to the relative angular displacement of the three sinusoidally varying voltages produced

by the three windings of a generator For instance, if phase A voltage is 120– 0˚ V, phase Bvoltage 120– -120˚ V, and phase C voltage 120– 120˚ V, the phase rotation is referred to asABC Sections of the system corresponding to the phase rotation of the voltage carried are

commonly referred to as phase A, B, or C.

Tap-changing mechanism — A control device that varies the voltage transformation ratio between the

primary and secondary sides of a transformer The taps may only be changed by discreteamounts, say 0.625%

112-12 The Engineering Handbook, Second Edition

References

Broadwater, R P., Shaalan, H E., Oka, A., and Lee, R E 1993 Distribution system reliability and

restoration analysis Electric Power Sys Res J 29(2):203–211.

Carson, J R 1926 Wave propagation in overhead wires with ground return Bell System Tech J 5:40–47.

Engel, M V., Greene, E R., and Willis, H L (Eds.) 1992 IEEE Tutorial Course: Power DistributionPlanning Course Text 92 EHO 361-6-PWR IEEE Service Center, Piscataway, NJ

Kersting, W H and Mendive, D L 1976 An Application of Ladder Network Theory to the Solution ofThree-Phase Radial Load Flow Problems IEEE Winter Meeting, New York

Further Information

Redmon, J R 1988 IEEE Tutorial Course on Distribution Automation Course Text 88 EH0 280-8-PWRIEEE Service Center, Piscataway, NJ

Electric Utility Engineers, Westinghouse Electric Corporation 1950 Electrical Transmission and

Distri-bution Reference Book, Westinghouse Electric Corporation, Pittbsurgh, PA.

Kersting, W H 2002 Distribution System Modeling and Analysis, CRC Press, Boca Raton, FL.

Lakervi, E and Holmes, E J 1989 Electricity Distribution Network Design, Peter Peregrinus, London Pansini, A J 1992 Electrical Distribution Engineering, Fairmont Press, Liburn, GA.

What Is Electromagnetic Interference? • Causes of EMI •

Solutions to EMI Problems

113.2 Grounding

Characteristics of Ground Conductors • Ground-Related EMI Coupling • Grounding Configurations • Summary of Grounding Considerations

113.3 Shielding

Radiated Electromagnetic Waves • Shielding Theory •

Reflection Loss • Absorption Loss • Shielding Material Characterization • Conductive Coatings • Aperture Leakages •

Summary of Shielding Considerations

113.4 Using Shielded Isolation Transformers to ImproveEquipment Compatibility

Shielded Isolation Transformers

113.5 Using Filtering Technologies to Improve EquipmentCompatibility

An Overview of the Current Standard • Limitations of

MIL-STD- 220B 2• IEEE P1560: A Step Forward

Many electromagnetic compatibility problems are caused by low immunity to emissions and poor facilityand data wiring and grounding which further affect equipment performance Besides, radiated andconducted emissions generated by a piece of equipment may also affect that equipment’s own perfor-mance To avoid problems, electromagnetic interference (EMI) control measures must be incorporatedinto the initial circuit design Grounding, shielding, and filtering are some of the important factors thatmust be considered during the initial design of electronic circuits

The purpose of this chapter is to give engineers and facility engineers who are unfamiliar with related problems insights and information necessary to improve equipment compatibility The goal here

EMI-is not to duplicate information currently available, but rather to collect information in a single locationand then supplement it to provide adequate information and procedures to applications personnel ineffectively limiting the spurious emissions given off by electronics and ensuring that electronic equipment

is not adversely affected by such emissions

William G Duff(First Edition)

Computer Sciences Corporation

Arindam Maitra(Second Edition)

EPRI PEAC Corporation

Kermit Phipps(Second Edition)

EPRI PEAC Corporation

Anish Gaikwad(Second Edition)

EPRI PEAC Corporation

1586_book.fm Page 1 Monday, May 10, 2004 3:53 PM

113-2 The Engineering Handbook, Second Edition

113.1 Analyzing and Solving Problems Associated with

Electromagnetic Interference

What Is Electromagnetic Interference?

Electromagnetic interference (EMI) is any natural or manmade electrical or electromagnetic energy thatresults in unintentional and undesirable equipment responses Electromagnetic energy travels in the form

of emissions, either conducted or radiated

Conducted emissions are generated inside electrical or electronic equipment and may be transmittedoutward through the equipment’s data input or output lines, its control leads, or its power conductors.Conducted emissions may cause an EMI problem between equipment that generates useful emissionsand other equipment with low immunity to those same emissions

Radiated emissions are radio-frequency electromagnetic energy that travels through the air Radiatedemissions are also generated by electrical or electronic equipment and may be emitted from power anddata cables that are poorly shielded or unshielded, leaky equipment apertures, equipment housings thatare inadequately shielded, or equipment antennae that may or may not be operating normally.Current trends in the electronics industry (such as increases in the quantity of electronic equipment,reliance on electronic devices in critical applications, higher clock frequencies of computing devices,higher power levels, lower sensitivities, increased packaging densities, use of plastics, etc.) will tend tocreate more EMI problems Whether conducted or radiated, emissions include three properties: ampli-tude, frequency, and waveform EMI can occur in equipment with low immunity to emissions when any

or all of these properties vary from normal, for example, emissions that are too high in amplitude, aretoo low or too high in frequency, or whose waveforms are distorted EMI can also occur when theseproperties are within normal operating parameters, usually resulting from equipment’s low immunity

to emissions Examples of intentional and unintentional conducted and radiated emissions are illustrated

in Table 113.1

Causes of EMI

EMI is generally common-mode (CM) noise, which is induced onto a signal with respect to a referenceground The noise is coupled to ground from the power cables through the capacitance between thepower cable and ground Figure 113.1 demonstrates this principle

The capacitance between the cable and ground increases as the length of cable increases Therefore,short lengths of cable have a low risk of common-mode noise As the length of cable increases, the risk

of common-mode noise increases and the need for EMI solutions rises

As shown in Figure 113.2, the common-mode ground current Iao = Cl-g dv/dt This characteristic ofcommon-mode current makes the adjustable-speed drive a prime source of common-mode noise because

of its abrupt voltage transitions on the drive output terminal The conducted noise will be created as theindividual pulses on the drive output couple with the ground conductor Some common symptoms ofEMI-related problems are:

• Unexplained drive trips with no correlation with voltage disturbances

• Malfunctions of barcode/vision systems, ultrasonic sensors, and weighing and temperature sensors

• Intermittent data errors in drive-control interfaces such as encoder feedback, I/O, and 0-10-Vanalog out

• Interference with TV, AM radio, and radio-controlled devices

Radiated emissions from many types of electronic equipment, including ASDs, lighting systems,broadcast communication equipment, and medical equipment, have been shown to cause electromag-netic interference with other types of sensitive electronic equipment Figure 113.1 shows how conductedand radiated emissions propagated through the electromagnetic environment may interfere with sensitiveelectronic medical equipment that is microprocessor based

1586_book.fm Page 2 Monday, May 10, 2004 3:53 PM

113-4 The Engineering Handbook, Second Edition

Ungrounded shields or floating shields can act like an antenna to unwanted radiated emissions In thecase of a shielded cable, a cable connector also helps to connect the cable shield to the equipment enclosureelectromagnetically Connectors are used to mechanically secure a cable to a piece of equipment Thepieces of a connector must fit together properly and securely to ensure the electromagnetic integrity ofthe connector Particular attention must be paid to how the pieces fit together during connector instal-lation Improperly fitted or loose connector joints can cause electromagnetic leaks in the connector,which can allow unwanted radiated electromagnetic energy from the electromagnetic environment topenetrate the cable system

Solutions to EMI Problems

Methods to mitigate EMI in an industrial facility could include proper grounding and shielding of sensitiveequipment, attenuating emissions at the source, capturing and returning emissions to the source, etc

FIGURE 113.1 Capacitive-coupling from phase-to-ground conductor.

FIGURE 113.2 Examples of conducted and radiated emissions interfering with equipment.

Incoming Conducted Emissions

Conducted Emissions Coupled to Sensitive Electronics by Transformer

Clock Signal Optional Filter Capacitor 5V Energy Storage Capacitor

Optional Battery

120 Vac

Conducted Emissions Filter (Capacitance Values Leakage Current)

Conducted Emissions After Filtering

Frequency Coupling

Frequency Coupling

High-Display Power (Not Affected by Conducted Emissions When Not Powered by Line Voltage)

Analog Processing Circuitry

Patient Computer Data Cables

Radiated Emissions Source

Interface Card Input/Output

User Inputs (Buttons & Switches)

Display Drive Standby Oscillator

Microprocessor (Powered During Emissions)

ag

netic

Energy

Unshielded Phase Conductor Stray Capacitive Coupling Phase-Ground

Comon Mode Voltage V12

Ground Potential #1

Ground Potential #2 Comon Mode Noise Current

Grounding, Shielding, and Filtering 113-5

Proper Grounding and Shielding of Sensitive Equipment

The practice of using unshielded phase conductors in a cable tray from an ASD to a motor couldintroduce common-mode noise into the system The use of a shielded-armor power cable from a drive

to a motor will provide a path for the common-mode noise to return to the source Figure 113.3demonstrates this concept

Signal shields reduce external coupling but may introduce EMI if the shield is connected to a noisyground potential The standard practice is to ground the shield at the source side of the cable If thestandard practice does not eliminate the EMI, it becomes necessary to do whatever it takes to fix theproblem, including grounding on both ends, grounding on the other end, or not grounding at all.The path of common-mode emissions can be diverted from sensitive equipment by separating controland signal cables from high-voltage wires It is also best if the power conductors include a ground wireand are placed in a conduit The conduit should be bonded to an ASD cabinet, the motor junction box,and the ground wire should be connected to the cabinet ground bus and motor ground stud The groundwire and conduit setup parasitic capacitive paths within the conduit and couple high dV/dt pulses andreturn them back to the source of emissions

Often it is necessary to isolate the conduit coupling at the point of the motor to prevent the coupledemissions from traveling on the outer surface of the conduit Insulated motor pairs and ground arerecommended to prevent inadvertent grounding of the conduit where new ground loops may beestablished to radiate the noise In this practice, it is critical to carry the safety ground within theconduit and ensure proper bonding at the motor ground stud to ensure NEC compliance This practice

is usually necessary where interference levels may be in the low frequency band, for example, 10 to

100 kHz

Another often-recommended and important practice is to separate control and signal cables frompower cables in cable trays The practice of placing covers on a signal cable tray will further reduce thenoise coupled to the signal cables from the power cables

Attenuating Emissions at the Source

The best way to eliminate system emissions is to attenuate emissions at the source The use of a mode choke (CMC) is one way to achieve this A CMC is an inductor with phases A, B, and C conductorswound in the same direction through a common magnetic core It provides high impedance and highinductance to any line-to-ground capacitive current emissions Unlike a line reactor/inductor, a CMCdoes not affect the power-line circuit This device is available from drive vendors Figure 113.4 shows anexample of a CMC application

common-Capturing and Returning Emissions to the Source

Another method to reduce EMI is to capture emissions and return them to the source This can beaccomplished with an EMI filter Figure 113.5 demonstrates the use of an EMI filter This figure showsthat the CM current Iao will collect in the ground conductors and return to the drive through the EMIfilter The filter contains a large common-mode core inductance and individual phase capacitors thatlimit the high frequency ground return current to low levels in the main AC supply

FIGURE 113.3 Use of a shielded/armor cable to reduce EMI.

ASD

Armor

lao

C1-g PVC

C1-g C1-g

113-6 The Engineering Handbook, Second Edition

conduc-a zero-voltconduc-age reference for conduc-an electricconduc-al power distribution system Fundconduc-amentconduc-al design principles forsystem and equipment grounding and the basic factors that influence the selection of the type of system/equipment grounding are extensively covered in IEEE Color books, including IEEE Std 141-1993, IEEEStd 142-1991, IEEE Std 446-1995, IEEE Std 1100-1999, NFPA 70-1999 National Electric Code (NEC),and Federal Information Processing Standards (FIPS) 1994

Note that the basic objectives for grounding circuits, cables, equipment, and systems are to prevent ashock hazard; to protect circuits and equipments; and to reduce EMI due to electromagnetic field,common ground impedance, or other forms of interference coupling However, grounding is one of theleast understood and most significant factors in many EMI problems Most equipment manufacturerswill provide details on grounding of their equipment and may often violate NEC and other importantstandards The EMI part of the problem is emphasized in the subsequent sections

Characteristics of Ground Conductors

Ideally, a ground conductor should provide a zero-impedance path to all signals for which it serves as areference If this were the situation, signal currents from different circuits would return to their respectivesources without creating unwanted coupling between circuits Many interference problems occur because

FIGURE 113.4 Application of a common-mode choke.

FIGURE 113.5 Application of EMI filter.

ASD

ASD Input

CMC

Motor

Motor Leads

EMI/RFI Filter

ASD

lao Motor

1586_book.fm Page 6 Monday, May 10, 2004 3:53 PM

Grounding, Shielding, and Filtering 113-7

designers treat the ground as ideal and fail to give proper attention to the actual characteristics of theground conductor

A commonly encountered situation is that of a ground conductor running along in the proximity of

a ground plane as illustrated in Figure 113.6 The ground conductor and ground plane may be represented

as a short-circuited transmission line At low frequencies the resistance of the ground conductor willpredominate At higher frequencies the series inductance and the shunt capacitance to ground will becomesignificant, and the ground conductor will exhibit alternating parallel and series resonance as illustrated

in Figure 113.7 To provide a low impedance to ground, it is necessary to keep the length of the groundingconductor short relative to wavelength (i.e., less than 1/20 of the wavelength)

Ground voltage equalization of voltage differences between parts of an automated data processing(ADP) grounding system is accomplished in parts when the equipment-grounding conductors are con-nected to the grounding point of a single power supply However, if the equipment grounding conductorsare long, it is difficult to achieve a constant potential throughout the grounding system, particularly forhigh frequency noise Supplemental conductors, low-inductance ground plates, and grounding and bond-ing of raised floor pedestals may be necessary Detailed discussions and standard practices and proceduresare extensively covered in IEEE Color books, including IEEE Std 142-1991 and IEEE Std 1100-1999

Ground-Related EMI Coupling

Ground-related EMI involves one of two basic coupling mechanisms The first mechanism results fromcircuits sharing the ground with other circuits Figure 113.8 illustrates EMI coupling between culprit and

FIGURE 113.6 Idealized equipment grounding.

FIGURE 113.7 Typical impedance versus frequency behavior of a grounding conductor.

113-8 The Engineering Handbook, Second Edition

victim circuits via the common-ground impedance In this case, the interference current flowingthrough the common-ground impedance (Z g) will produce an interfering signal voltage (V i) in the victimcircuit This effect can be reduced by minimizing or eliminating the common-ground impedance.The second EMI coupling mechanism involving ground is a radiated mechanism whereby the groundloop, as shown in Figure 113.9, acts as a receiving or transmitting antenna Ground loops are probablythe most common cause of interference in network systems and also the most common problem withmulti-port devices in general For this EMI coupling mechanism the induced EMI voltage (for thesusceptibility case) or the emitted EMI field (for the emission case) is a function of the EMI drivingfunction (field strength, voltage, or current), the geometry and dimensions of the ground loop, and thefrequency of the EMI signal

Common wisdom on electromagnetic compatibility recommends that radiated effects can be mized by routing conductors as close as possible to ground and minimizing the ground-loop area Eventhough theory holds up that closely routing conductors next to a ground plane will reduce the effects ofcoupling, however, where building steel and in particular in cases of lightning where large currents mayflow, the coupling will still occur The coupling effect applies to stray common-mode currents also Theprimary factor is that in practical installations, ideal placement of the conductor next to the surface ofthe ground plane is difficult Examples of this may be the upgrade of existing plant operation toincorporate multiple remote PLC systems communicating to a central control room These cable runsare seldom able to achieve the ideal installation

mini-Figure 113.10 uses linear scales to emphasize the very rapid rise of induced voltage near a conductorand the small additional gain after a few meters However, if “close to” were interpreted by an installer

as a few centimeters, all the expected benefits from the “close” installation would be lost — in otherwords, it is not very effective to attempt minimizing induced voltages by casual routing of unshielded

FIGURE 113.8 Common-ground impedance coupling between circuits.

FIGURE 113.9 Common-mode radiations into and from ground loops.

Radiation or Pick Up by Ground Loop

1586_book.fm Page 8 Monday, May 10, 2004 3:53 PM

Grounding, Shielding, and Filtering 113-9

cables “near” the ground planes From Figure 113.10 it can be clearly seen that when a cable is just a fewcentimeters above the ground plane, the cable will still have significant amounts of induced noise

Grounding Configurations

Typical electronic equipment may have a number of different types of functional signals as shown in

Figure 113.11 To mitigate interference due to common-ground impedance coupling, as many separategrounds as possible should be used

The grounding scheme for a collection of circuits within equipment can assume any one of severalconfigurations Each of these configurations tends to be optimum under certain conditions and maycontribute to EMI problems under other conditions In general, the ground configurations are a floatingground, a single-point ground, a multiple-point ground, or some hybrid combination The determi-nation to use single-point grounding or multiple-point grounding typically depends on the frequencyrange of interest Analog circuits with signal frequencies up to 300 kHz may be candidates for single-point grounding Digital circuits with signal frequencies in the MHz range should utilize multiple-point grounding

FIGURE 113.10 Voltage induced by the field into the loop.

FIGURE 113.11 Grounding hierarchy.

0 0

100 80 60 40 20 R

di dt

Induced voltage (%)

Radius in meters

Lightning, EMP Ground (Tens of kA,

dc to a few tens of MHz)

Relays, etc , Signaling Grounds (5 V to 50 V

dc to a few kHz)

Low-Level, Low-Frequency Ground ( µV to mV

dc to a few 100 kHz)

Low-Level, High-Frequency Ground.

Radio Communication ( µV to mV, kHz to GHz)

Digital Levels, High-Frequency Ground (Volts dc to 100 MHz)

DC Power Ground (Returns for Loads > 1A)

AC Power Safety Ground (50 Hz/60 Hz or 400 Hz)

113-10 The Engineering Handbook, Second Edition

A floating ground configuration is illustrated in Figure 113.12 The signal ground is electrically isolatedfrom the equipment ground and other conductive objects Hence, equipment noise currents present inthe equipment and power ground will not be conductively coupled to the signal circuits

The effectiveness of floating ground configurations depends upon their true isolation from othernearby conductors; that is, to be effective, floating ground systems must really float It is often difficult

to achieve and maintain an effective floating system A floating ground configuration is most practical

if only a few circuits are involved and power is supplied from either batteries or DC-to-DC converters

A single-point ground configuration is illustrated in Figure 113.13 An important advantage of thesingle-point configuration is that it helps control conductively coupled interference As illustrated inFigure 113.13, EMI currents or voltages in the equipment ground are not conductively coupled into thesignal circuits via the signal ground Therefore, the single-point signal ground network minimizes theeffects of any EMI currents that may be flowing in the equipment ground

The multiple-point ground illustrated in Figure 113.14 is the third configuration frequently used forsignal grounding With this configuration, circuits have multiple connections to ground Thus, in equip-ment, numerous parallel paths exist between any two points in the multiple-point ground network.Multipoint grounding is more economical and practical for printed circuits and integrated circuits

FIGURE 113.12 Floating signal ground.

FIGURE 113.13 Single-point signal ground.

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Grounding, Shielding, and Filtering 113-11

Interconnection of these components through wafer risers, mother boards, and so forth should use ahybrid grounding approach in which single-point grounding is used to avoid low-frequency ground loopsand/or common-ground impedance coupling; multipoint grounding is used otherwise

Summary of Grounding Considerations

It should be noted that both the conducted and radiated EMI coupling mechanisms identified earlierinvolve a “ground loop.” It is important to recognize that ground loop EMI problems can exist without

a physical connection to ground In particular, at RF frequencies, capacitance-to-ground can create aground loop condition even though circuits or equipments are floated with respect to ground

A properly designed ground configuration is one of the most important engineering elements inprotecting against the effects of EMI The ground configuration should provide effective isolation betweenpower, digital, high-level analog, and low-level analog signals In designing the ground it is essential toconsider the circuit, signal characteristics, equipment, cost, maintenance, and so forth In general, eitherfloating or single-point grounding is optimum for low-frequency situations, and multiple-point ground-ing is optimum for high-frequency situations In many practical applications, a hybrid ground approach

is employed to achieve the single-point configuration for low frequencies and the multiple-point figuration for high frequencies

con-113.3 Shielding

Shielding is one of the most effective methods for controlling radiated EMI effects at the component,circuit, equipment, subsystem, and system levels Good EMC design reduces EMI signals to a level wherethey do not cause problems within the equipment or with any other equipment

Shielding effectiveness is a measure of how well a shield blocks radiated emissions Reducing the level

of radiated emissions incident upon the equipment requires selecting the proper shielding material (see

Figure 113.15), and correct installation and maintenance to ensure the integrity of the shield

Radiated Electromagnetic Waves

The performance of shields is a function of the characteristics of the incident electromagnetic fields Allelectromagnetic waves consist of two oscillating fields that operate at right angles to each other One ofthese fields is the electric (E) field, whose strength is measured in volts per meter Perpendicular to the

E field is the magnetic (H) field, whose strength is measured in amps per meter H fields usually aregenerated by high current, low voltage, low impedance circuits In contrast, E fields are produced by

FIGURE 113.14 Multiple-point ground configuration.

113-12 The Engineering Handbook, Second Edition

devices that have high voltage, but relatively low current and high impedance The ratio of E field to Hfield is called the wave impedance The H and E fields vary in relative magnitude according to the distance

of the wave from the generating source and the nature of the source itself

The material selected for the shield depends upon characteristics of the source of the radiated emissionsand the reflective and absorbent properties of the shielding material (see Figure 113.16) These includethe impedance of the electromagnetic fields of the source creating the emissions, which depends on thedistance from the emissions source to the malfunctioning equipment In most cases, this distance (referred

to as the “far-field”) is such that the emissions are primarily due to electric fields resulting in fieldimpedance that is constant and can be easily approximated If the equipment is relatively close to thesource of emissions, the impedance will be lower where the equipment is referred to as being in the

“near-field,” and where the magnetic field component must be considered Before a shielding materialcan be selected, one must determine whether the affected equipment is in the “near-” or “far-field.”

In the “far-field,” a moderately good conductor may be used to shield against radiated emissions fromelectric fields For emissions above about 1 MHz, the conductivity of the material, which is partiallybased on its thickness, does not have a significant effect on the material’s ability to block electric fields.The primary dependence is on conductivity, which is the material’s ability to carry electrons from thefield and thus, provide a low impedance path to ground for the radiated emissions However, at distancescloser to the source of the emissions, more attention must be placed on selecting the shielding material

FIGURE 113.15 A variety of shields can be used to solve EMI problems.

FIGURE 113.16 Diagram of a shielding material absorbing and reflecting emission.

Incident Radiated Electromagnetic Energy

Reflected Radiated Electromagnetic Energy

Radiated Electromagnetic Energy that Passes Through the Shield

Absorbed within Shielding Material

Shielding Material 1586_book.fm Page 12 Monday, May 10, 2004 3:53 PM

Grounding, Shielding, and Filtering 113-13

Unlike the mitigation of electric fields in the “far-field,” which only requires the shield to be reflective,

equipment in the “near-field,” which may be in an area containing both electric and magnetic-field

components, requires a material that will reflect and absorb electromagnetic energy Therefore, shielding

considerations in the near-field region of an EMI source may be significantly different from shielding

considerations in the far-field region

Shielding Theory

If a metallic barrier is placed in the path of an electromagnetic field as illustrated in Figure 113.17, only

a portion of the electromagnetic field may be transmitted through the barrier Several effects may occur

when the incident wave encounters the barrier First, a portion of the incident wave may be reflected by

the barrier Second, the portion of the incident wave that is not reflected will penetrate the barrier interface

and may experience absorption loss while traversing the barrier Third, additional reflection may occur

at the second barrier interface, where the electromagnetic field exits the barrier Usually, this second

reflection is insignificant relative to the other effects that occur, and it may be neglected

The shielding effectiveness of the barrier may be defined in terms of the ratio of the impinging field

intensity to the exiting field intensity For high-impedance electromagnetic fields or plane waves, the

shielding effectiveness is given by

(113.1)

where E1 is the impinging field intensity in volts per meter and E2 is the exiting field intensity in volts

per meter For low-impedance magnetic fields, the shielding effectiveness is defined in terms of the ratio

of the magnetic field strengths

The total shielding effectiveness of a barrier results from the combined effects of reflection loss and

absorption loss Thus, the shielding effectiveness, S, in dB is given by

where RdB is the reflection loss, AdB is the absorption loss, and BdB is the internal reflection loss

Charac-teristics of the reflection and absorption loss are discussed in the following sections

Reflection Loss

When an electromagnetic wave encounters a barrier, a portion of the wave may be reflected The reflection

occurs as a result of a mismatch between the wave impedance and the barrier impedance The resulting

reflection loss, R, is given by

FIGURE 113.17 Shielding of plane waves.

B A

TRANSMITTING WAVE INSIDE OF ENCLOSURE

REFLECTED WAVE OUTSIDE

WORLD BARRIER OF FINITE THICKNESS

ATTENUATED INCIDENT WAVE INCIDENT WAVE

E

113-14 The Engineering Handbook, Second Edition

(113.3)

where Z w is the wave impedance =E/H, and Z b is the barrier impedance

Absorption Loss

When an electromagnetic wave encounters a barrier, a portion of the wave penetrates the barrier As the

wave traverses the barrier, the wave may be reduced as a result of the absorption loss that occurs in the

barrier This absorption loss, A, is independent of the wave impedance and may be expressed as follows:

(113.4)

where t is the thickness in mm, fMHz is the frequency in MHz, mr is the permeability relative to copper,

and sr is the conductivity relative to copper

Shielding Material Characterization

The ability of a material to absorb radiated emissions depends on the frequency of the source, and the

conductivity, permeability, and thickness of the shielding material An increase in any of these variables

will increase absorption However, the variables of concern when selecting a material to absorb emissions

in the “near-field” are the ones that vary widely among available materials These are permeability, which

is the extent to which a material can be magnetized, and thickness Thus, to mitigate against low-frequency

magnetic fields in the “near-field” such as those originating from power distribution equipment such as

transformers from some medical equipment utilizing 60-hertz power and from electrical appliances, a

material of the proper thickness such as iron, which can be magnetized and provide a path for the

magnetic field emissions, is required A screened or solid shield made of copper or other material to

attenuate electric fields, some type of steel composite or other material to attenuate magnetic fields, or

a combination of both may be installed around equipment or inside the walls of the room where

equipment is used

The total shielding effectiveness resulting from the combined effects of reflection and absorption loss

are plotted in Figure 113.18 for copper and iron materials having thicknesses of 0.025 mm and 0.8 mm,

having electric and magnetic fields and plane-wave sources, and having source-to-barrier distances of

2.54 cm and 1 m

As shown in Figure 113.18, good shielding efficiency for plane waves or electric (high-impedance)

fields is obtained by using materials of high conductivity, such as copper and aluminum However,

low-frequency magnetic fields are more difficult to shield because both the reflection and absorption loss of

nonmagnetic materials, such as aluminum, may be insignificant Consequently, to shield against

low-frequency magnetic fields, it may be necessary to use magnetic materials

Conductive Coatings

Conductive coatings applied to nonconductive materials such as plastics will provide some degree of

EMI shielding The principal techniques for metalizing plastic are the following:

w b w