Votage Sags and Interruptions

•A voltage sag is a shortduration (typically 0.5 to 30 cycles) reduction in rms voltage caused by faults on the power system and the starting of large loads, such as motors. •Momentary interruptions (typically no more than 2 to 5 s) cause a complete loss of voltage and are a common result of the actions taken by utilities to clear transient faults on their systems. •Sustained interruptions of longer than 1 min are generally due to permanent faults.

Chapter 02- Votage Sags and Interruptions

GV: Nguyễn Hữu Phúc

Chapter 02- Votage Sags and Interruptions

•A voltage sag is a short-duration (typically 0.5 to 30 cycles) reduction in rms

voltage caused by faults on the power system and the starting of large loads, such

as motors

•Momentary interruptions (typically no more than 2 to 5 s) cause a complete loss of voltage and are a common result of the actions taken by utilities to clear transient faults on their systems

•Sustained interruptions of longer than 1 min are generally due to permanent faults

Utilities have been faced with rising numbers of

complaints about the quality of power due to

sags and interruptions

There are a number of reasons for this, with the

most important being that customers in all

sectors (residential, commercial, and industrial)

have more sensitive loads

Voltage sag due to a single line-to-ground fault

•If there is a fault on the same feeder, the customer will experience a

voltage sag during the fault followed by an interruption when the breaker opens to clear the fault

•If the fault is temporary in nature, a reclosing operation on the breaker should be successful and the interruption will only be temporary

•It will usually require about 5 or 6 cycles for the breaker to operate,

during which time a voltage sag occurs

• The breaker will remain open for typically a minimum of 12 cycles up to

5 s depending on utility reclosing practices

•Sensitive equipment will almost surely trip during this interruption

3.1 Sources of Sags and Interruptions:

Voltage sags and interruptions are generally caused by faults (short

circuits) on the utility system

•Note that to clear the fault shown on the transmission system, both breakers A and B must operate

•Transmission breakers will typically clear a fault in 5 or 6 cycles In this case there are two lines supplying the distribution substation and only one has a fault

Example of fault locations that caused misoperation of sensitive production equipment

at an industrial facility (the example system had multiple overhead distribution feeders and

an extensive overhead transmission system supplying the substation)

Voltage sag due to a short-circuit fault on

a parallel utility feeder

•Figure clearly shows the

voltage sag prior to fault

clearing and the

subsequent two fast

recloser operations

•The reclose time (the time

the recloser was open) was

a little more than 2 s, a very

common time for a utility

line recloser

•Apparently, the fault—

perhaps, a tree branch—

was not cleared completely

by the first operation,

forcing a second

•The system was restored

after the second operation

Utility short-circuit fault event with two fast trip operations of utility line recloser

3.2 Estimating Voltage Sag Performance

The following is a general procedure for working with industrial customers to

assure compatibility between the supply system characteristics and the facility

operation:

1 Determine the number and characteristics of voltage sags that result from

transmission system faults

2 Determine the number and characteristics of voltage sags that result from

distribution system faults (for facilities that are supplied from distribution systems)

3 Determine the equipment sensitivity to voltage sags This will determine the

actual performance of the production process based on voltage sag performance calculated in steps 1 and 2

4 Evaluate the economics of different solutions that could improve the

performance, either on the supply system (fewer voltage sags) or within the

customer facility (better immunity)

3.2.1 Area of vulnerability

The concept of an area of vulnerability has been developed to help evaluate

the likelihood of sensitive equipment being subjected to voltage lower than its

minimum voltage sag ride-through capability

The latter term is defined as the minimum voltage magnitude a piece of equipment

can withstand or tolerate without misoperation or failure

This is also known as the equipment voltage sag immunity or susceptibility limit

An area of vulnerability is determined by the total circuit miles of exposure to faults that can cause voltage magnitudes at an end-user facility to drop below the

equipment minimum voltage sag ride-through capability

Figure shows an example of an area of vulnerability diagram for motor

contactor and adjustable-speed-drive loads at an end-user facility served from

the distribution system

The loads will be subject to faults on both the transmission system and the

distribution system

Illustration of an area of vulnerability

3.2.2 Equipment

sensitivity to voltage sags

Equipment sensitivity to voltage

sags can be divided into three

categories:

■ Equipment sensitive to only the

magnitude of a voltage sag

■ Equipment sensitive to both the

magnitude and duration of a voltage

sag This group includes virtually all

equipment that uses electronic

power supplies

■ Equipment sensitive to

characteristics other than =>

magnitude and duration

Some devices are affected by other sag characteristics such as the phase

unbalance during the sag event, the in-the wave at which the sag is initiated, or any transient oscillations occurring

point-during the disturbance

Typical equipment voltage sag through capability curves

ride-•The voltage sag performance for

a given customer facility will

depend on whether the customer

is supplied from the transmission

system or from the distribution

system

•For a customer supplied from the

transmission system, the voltage

sag performance will depend on

only the transmission system fault

performance

3.2.3 Transmission system

sag performance

evaluation

•On the other hand, for a customer

supplied from the distribution system, the

voltage sag performance will depend on

•the fault performance on both the

transmission and distribution systems

•Most utilities have detailed short-circuit models of the

interconnected transmission system available for programs such

as ASPEN* One Liner (Fig.)

•These programs can calculate the voltage throughout the system resulting from faults around the system

•Many of them can also apply faults at locations along the

transmission lines to help calculate the area of vulnerability at a specific location

•The area of vulnerability describes all the fault locations that can cause equipment to misoperate

•The type of fault must also be considered in this analysis =>

Single-line-to-ground faults will not result in the same voltage sag

at the customer equipment as a three-phase fault

Example of modeling the transmission system in

a short-circuit program for calculation of the area

of vulnerability

Voltage sag types at end-use equipment that result from different types of faults and

transformer connections

The relationships in Table 3.1 illustrate the fact that a single-line

to-ground fault on the primary of a delta-wye to-grounded transformer does not result in zero voltage on any of the phase-to-ground or phase-to-

phase voltages on the secondary of the transformer

The magnitude of the lowest secondary voltage depends on how the

3.2.4 Utility distribution system sag

performance evaluation

•Customers that are supplied at distribution voltage levels are impacted by faults on both the transmission system and the distribution system

•The analysis at the distribution level must also include

momentary interruptions caused by the operation of protective devices to clear the faults

•These interruptions will most likely trip out sensitive

Figure 3.10 shows a typical distribution system with multiple feeders and fused branches, and protective devices The utility protection scheme plays an

important role in the voltage sag and momentary interruption performance

The critical information needed to compute voltage sag performance can be summarized as follows:

■ Number of feeders supplied from the

substation

■ Average feeder length

■ Average feeder reactance

■ Short-circuit equivalent reactance at the

substation

■ Feeder reactors, if any

■ Average feeder fault performance which

includes three-phase-line to- ground (3LG)

faults and single-line-to-ground (SLG) faults in

faults per mile per month The feeder

performance data may be available from

protection logs

•However, data for faults that are cleared by

downline fuses or downline protective devices

may be difficult to obtain and this information

may have to be estimated.

Typical distribution system illustrating protection devices

3.3 Fundamental Principles of Protection

Several things can be done by the utility, end user, and equipment

manufacturer to reduce the number and severity of voltage sags and to

reduce the sensitivity of equipment to voltage sags

Approaches for voltage sag ride-through

1 Equipment manufacturers should have

voltage sag ride-through capability

curves available to their customers so that

an initial evaluation of the equipment can

be performed

2 The company procuring new equipment

should establish a procedure that rates the

importance of the equipment If the

equipment is critical in nature, the

company must make sure that adequate

ride-through capability is included when the

equipment is purchased

3 Equipment should at least be able to

ride through voltage sags with a minimum

voltage of 70 percent (ITI curve)

3.4 Solutions at the End-User Level

1 Protection for small loads [e.g., less than 5 kilovoltamperes (kVA)]

2 Protection for individual equipment or groups of equipment up to about 300 kVA

3 Protection for large groups of loads or whole facilities at the low-voltage level

4 Protection at the medium-voltage level or on the supply system

3.4.1 Ferroresonant transformers (constant-voltage xfmr)

Schematic of ferroresonant constant-voltage

transformer

Ferroresonant transformers (CVT) are basically 1:1 transformers which are excited high on their saturation curves, thereby providing an output voltage which is not

significantly affected by input voltage variations

Voltage sag improvement with ferroresonant

transformer

VOLTAGE SAG MITIGATION TECHNIQUES

INTRODUCTION

• Voltage sags are most costly of all power quality disturbances

• Lead to disruption of manufacturing processes due to equipment being

• unable to operate correctly at the reduced voltage levels

• Industrial equipment such as variable speed drives and some control

• systems are particularly sensitive to voltage sags.

• In many manufacturing processes, loss of only a few vital pieces of equipment may lead to a full shut down of production leading to

significant financial losses

• For some processes which are thermally sensitive a significant loss of material as well as the time taken to clean up and restart the process must also be considered

1 Ferroresonant transformers

• FERRORESONANT transformers are designed to

achieve regulation with non-linear operation They provide line regulation, reduce harmonics, and are current limiting

• Also known as Constant Voltage Transformers(CVT)

• Operates in the saturation region of the transformer B-H curve

REGION OF OPERATION

• A ferroresonant transformer consists of a core, a primary winding, two secondary

windings (one for the load and one for the capacitor) and a magnetic shunt that separates the primary and secondary windings

• The magnetic shunt provides a path for the

imbalanced flux of the primary and secondary by

allowing a portion of the primary flux to return to the primary winding without coupling the secondary

• At the same time, it allows the secondary flux to return

to the secondary winding without coupling the primary

• OPERATION:

• When a voltage is applied to the primary winding the

secondary voltage increases as the primary voltage

increases

• As the primary voltage increases the secondary voltage

continues to increase up to a point of discontinuity, or

secondary resonance, where an abrupt increase, about 20 %,

in secondary voltage occurs

• The resonance effect immediately increases the secondary flux density and causes saturation of that portion of the core

• This partial core saturation is the key to the magnetic design

of the ferroresonant transformer

• The voltage induced in the capacitor winding by the primary flux causes a capacitive current to flow

• The flux due this current is in phase with the primary flux This flux addition occurs in the secondary portion of the

core

• The increased flux saturates the portion of the core on the secondary winding only

• The primary portion of the core is operating below

saturation or below the “knee” of the magnetization curve

• FERRORESONANT TRANSFORMERS are

inherently self-protected against short circuits, and are able to supply large surge currents if required because of the large amount of energy stored in the secondary circuit

• Ferroresonant transformers are simple and relatively maintenance free devices which can be very

effective for small loads

• Ferroresonant transformers are available in sizes up to

around 25 KVA

• Voltage sags down to 30 % retained voltage can be

mitigated through the use of ferroresonant

transformers

• Typically ferroresonant transformer regulators can

maintain secondary voltage to within ±0.5% for

changes in the primary voltages of ±20%

• The disadvantages of a ferroresonant transformer

STATIC TRANSFER SWITCH

• For facilities with a dual supply, one possible method

of voltage sag mitigation is through the use of a

automatic static transfer switch

• Upon detection of a voltage sag, these devices can transfer the load from the normal supply feeder to the alternative supply feeder within half a cycle

• Conventional transfer switches will switch from the primary supply to a backup supply in seconds

• Fast transfer switches that use vacuum breaker

technology are available that can transfer in about 2

electrical cycles This can be fast enough to protect many sensitive loads

• Static switches use power electronic switches to

accomplish the transfer within about a quarter of an

electrical cycle

VOLTAGE REGULATOR

VOLTAGE REGULATOR

• Voltage regulators are devices that can maintain a constant

voltage (within tolerance) for voltage changes of predetermined limits above and below the nominal value

• A switching voltage regulator maintains constant output voltage

by switching the taps of an autotransformer in response to

changes in the system voltage

• The electronic switch responds to a signal from the

voltage-sensing circuitry and switches to the tap connection necessary to maintain the output voltage constant

• The switching is typically accomplished within half of a cycle,

which is within the ride-through capability of most sensitive

devices

UNINTERRUPTIBLE POWER SUPPLIES

• Ups systems have the advantage that they can mitigate all

voltage sags including outages for significant periods of time

(depending on the size of the ups)

• ONLINE UPS

• OFFLINE/STANDBY UPS

• HYBRID UPS

ONLINE UPS

•The load is always fed through

the UPS The incoming ac

power is rectified into dc power,

which charges a bank of

batteries This dc power is then

inverted back into ac power, to

feed the load

•If the incoming ac power fails,

the inverter is fed from the

batteries and continues to

supply the load

•However, the on-line operation

increases the losses and may

be unnecessary for protection

of many loads

OFFLINE/STANDBY UPS

•A standby power supply is

sometimes termed off-line UPS

since the normal line power is

used to power the equipment until

a disturbance is detected and a

switch transfers the load to the

battery backed inverter

•The transfer time from the normal

source to the battery-backed

inverter is important

•8 ms is the lower limit on interruption through for power-conscious

manufacturers Therefore a transfer time of 4 ms would ensure continuity

of operation for the critical load

•A standby power supply does not typically provide any transient

protection or voltage regulation as does an on-line ups

•This is the most common configuration for commodity UPS units

available at retail stores for protection of small computer loads

HYBRID UPS

•Similar in design to the standby UPS, the hybrid UPS

utilizes a voltage regulator on the UPS output to provide

regulation to the load and momentary ride-through when the transfer from normal to UPS supply is made

• UPS specifications include kilo-voltampere capacity, dynamic and static voltage regulation, harmonic distortion of the input current and output voltage, surge protection, and noise attenuation

• The specifications should indicate, or the supplier should furnish, the test conditions under which the specifications are valid