Calculations of Protective Relay Setting

ABSTRACT CALCULATIONS OF PROTECTIVE RELAY SETTINGS FOR A UNIT GENERATOR FOLLOWING CATASTROPHIC FAILURE by Jaime Anthony Ybarra December 2011 After a catastrophic failure of a unit genera

ABSTRACT CALCULATIONS OF PROTECTIVE RELAY SETTINGS FOR A UNIT

GENERATOR FOLLOWING CATASTROPHIC FAILURE

by Jaime Anthony Ybarra December 2011 After a catastrophic failure of a unit generator system the major components may need to be replaced Many times exact replacement of the failed or damaged components may not be possible In such a case components with electrical characteristics as close to the original may be used Therefore new protective relay settings must be calculated In this thesis, we will examine a type of generator protection relays, evaluate new settings and develop a one-line diagram for a 25 MVA generator system A methodology for the development of a safe and reliable protections scheme for a unit generator system is also presented

CALCULATIONS OF PROTECTIVE RELAY SETTINGS FOR A UNIT GENERATOR FOLLOWING CATASTROPHIC FAILURE

A THESIS Presented to the Department of Electrical Engineering

California State University, Long Beach

UMI Number: 150766

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TABLE OF CONTENTS

Page LIST OF TABLES v LIST OF FIGURES vi CHAPTER

1 INTRODUCTION 1

2 GENERATOR COMPONENTS AND PROTECTION SCHEME 3

The Transformer 5

Short Circuit 6 Per Unit Quantities 10

One Line Diagram 11

Relay and Control Symbols 14

Elementary Diagrams 15

3 UNIT GENERATOR PROTECTION RELAYS 18

Volt/Hertz relay (24) 18

Synchronizing Check Relay (25) 20

Under Voltage Relay (27) 20

Directional Reverse Power Relay (32) 21

Loss of Excitation (Field) Relay (40) 21

Negative Sequence or Unbalance Relay (46) 23

Stator Temperature Relay (49) 24

Inadvertent Energization Protection Relay (50) 25

Voltage Controlled Over Current Relay (51V) 25

Over Voltage Relay (59) 26

Voltage Balance Relay (60) 26

Sudden Pressure Relay (63) 27

Field Ground Relay (64F) 27

Oil Level Relay (71) 27

Out Of Step Relay (78) 28

CHAPTER Page

Differential Relay (87) 31

4 SETTINGS CALCULATIONS AND EXPERIMENTAL RESULTS 33

Preliminary Calculations 34 Typical Relay Settings Calculations and Verification with Experiment 3 5

5 CONCLUSIONS 52

REFERENCES 54

LIST OF TABLES

Page Sample Generator Parameters 33

Sample Unit Transformer Parameters 34

Relay Volt/Hertz Experimental Test Result 37

Under Voltage Test Result 38

Reverse Power Test Result 39

Zone 2 Test Result 41 Loss of Excitation Zone 1 Reach Test Result 41

Current Unbalance Pickups for A, B and C Phases 42

Voltage Controlled Over Current Test Results 43

Over Voltage Relay Test Result 44

Relay Reverse and Forward Reach Z Test Results 46

Relay Right Blinder Reach Z Test Result 46

Left Blinder Reach Z Test Result 47

Equipment Summary Table 48

Relay Settings Summary Table 49

LIST OF FIGURES

Page Graphical representation of 3 phase power generation 4

Basic structure of a cylindrical rotor 4

Brushless excitation system 5 Wye connected windings 5

3 Phase fault with DC component offset 8

Short circuit waveform showing the three transient periods 9

Typical electrical symbols 12 Waveform output with polarities in phase 13

Waveform output with polarities reversed 14

Unit connected generator protection with typical relays 16

Basic elementary diagram 17 Various volts/hertz limit curves 19

Generator, transformer and relay plot for volts/hertz relay plot 20

2 zone protection diagram 22 Typical negative sequence relay curve 24

Out of step protection zone 29

FIGURE Page

18 Experimental setup 36

19 RMS TIME vs VOLTS of volt/hertz relay (24) operation 37

20 Under voltage (27) relay operation graph 38

21 3 phase vector diagram of reverse power relay (32) 39

22 Zone reach impedance and phase angle relationship 40

23 Loss of excitation zone 2 reach test 40

24 Loss of excitation zone 1 reach test 41

25 Unbalance A, B and C phases 42

26 Voltage control relay (51C) results 43

27 Voltage controlled relay (51C) RMS trip graph 44

28 Over voltage relay (59) result plot 44

29 Loss of Synchronization protection boundaries 45

30 Forward reach results 45

31 Reverse blinder result 46

32 Right blinder result 46

33 Left blinder result 46

34 Sample system one line 50

CHAPTER 1 INTRODUCTION

A generator system is designed to provide electric power to customers reliably Failure of any electric component such as the generator, unit-transformer or auxiliary transformers can lead to catastrophic damage If any of these components are damaged beyond repair then they must be repaired or replaced However, due to age and

customized engineered system components exact replacements may not be available or the time for new components to be manufactured may not be economically viable The generator owner or user may have to purchase readably available equipment with

capabilities as close as possible to original components If this is the case new protective device settings must be calculated to properly protect the generation In the event of the replacement of any of the components the following basic steps are recommended:

1 Calculate the new capabilities of the generation system

2 Calculate protective device settings based on new system

3 Develop or update electric system single line diagrams (one-lines) to describe the basic layout of the electrical system as well as basic information of the major

components

4 Verify that the relays will operate as programed or set with simulation of fault

The engineer in charge must produce a system that will provide reliable, economical power to the customer as well as maintain a safe system for generator operation and maintenance personal

This thesis is focused on recalculation of protective relay settings of a generator protection system with replacement components that do not have the same ratings or capabilities as the original and will require new protective relay settings calculations Chapter 2 will discuss generating system component and electrical fundamentals as well

as the symbols used to describe an electrical system Chapter 3 will describe protective relay types and functions Chapter 4 covers the calculation of the new protective relay settings and fault simulation testing of the protective functions with a 3 phase power simulator

CHAPTER 2 GENERATOR COMPONENTS AND PROTECTION SCHEME

An electric generator is a device, which converts mechanical energy into electrical energy (see Figure 1) The prime mover provides the rotational mechanical power into the

AC generator This mechanical power may be derived from fossil fuels, nuclear or

movement of water The mechanical rotational motion is transferred via a shaft to the rotating portion of the generator, which is referred to as the rotor The rotor will contain conductors of either copper or aluminum that will have a DC voltage applied and provides

a current path that will set up a controlled magnetic flux these conductors are referred to as the field windings The moving magnetic flux will induce voltage in the stationary portion

of the generator referred to as the stator (see Figure 2) where the amount of flux being produced by the rotor is controlled by a device called the "Exciter" which controls the amount of current in the in the field windings The DC current may be derived externally and then transferred to the field windings on the rotor via brushes or the DC may be generated on the rotor itself by the addition a small permanent magnet AC generator and electronic circuits that will rectify the AC into DC for use for the field current

(see Figure 3)

In a 3 phase wye connected generator (see Figure 3) the 3 windings offset by 120

The neutral may be solidly connected to the ground or connected through an impedance to ground that will limit the amount of current during a line to ground fault The voltage developed between windings is referred to as line to line voltage and voltage referenced to the grounded common connection is referred to as line to neutral voltage

Rotating Shaft

3-Phase Output

DC Field Variable Source

FIGURE 1 Graphical representation of 3 phase power generation

Field windings

ROTATING ELEMENTS

FIGURE 3 Brushless excitation system [1]

A phase

Volts line to neutral

Volts line to line

FIGURE 4 Wye connected windings

The Transformer

A transformer allows the conversion of one voltage level to another voltage level

A higher voltage level allows for lower losses due to lower current levels for a given amount of power Lower voltage levels in turn allow for higher currents to loads for the same given amount of power A transformer consists of coils of copper or aluminum

magnetic lines intersect each other within this core Mathematically the relationship is expressed by the following equation

N P V S = N P V S

Where Np is the turns of conductor on the primary side

Ns is the number of turns of conductor on the secondary side

Vs is the voltage of the secondary side

and Vp is the voltage on the primary side

Like a generator the transformer windings can be configured as a delta where there is no intention grounding of the conductors or in wye configuration that is

configured such that each phase windings end point are connected together at a common point (see Figure 4) This common point can be solidly connected to the ground or connected through impedance to ground to limit ground fault current The advantage of a delta connected system is that if there were to be an inadvertent grounding of one of the phases only a small amount of current will flow and allow the system to stay online until

it can be safely de-energized and repaired However, the voltages on the other phases will increase thereby stressing the insulation of the cables and equipment With a wye connected system the common point is referred to as the neutral

Short Circuit

A power system is designed to be free of faults as much as possible through system design, equipment selection, installation and maintenance However, even with these practices faults do occur Some of these causes can be from insulation failure, moisture or inadvertent contact with conductive material Regardless of the cause a

and burning will occur as well as mechanical stress to the equipment The system voltage levels will drop proportionally with the magnitude and distance to the point of the fault The "available" short circuit current is the maximum possible value of current that can occur at the location of the fault The contribution to this maximum current comes from generators, synchronous and induction motors The basic short-circuit equation is shown below

and Zsystem ( or X ) is the equivalent system impedance (or reactance)

The system impedance is taken from the point of the fault back to and including the source or sources of the fault current for the power system During a 3 phase fault the current waveform will be offset by a DC component that shifts the sinusoidal waveform away from the horizontal axis (see Figure 5) The amount of DC offset depends on the X/R ratio which is the impedance divided by the resistance of the system A generator will have 3 short circuit constants inherent by design (see figure 6) that are used to set various protection elements

These constants are derived by experiment or by analytical methods by the

manufacture These constants are defined as follows

Phase C

DC Component

FIGURE 5 3 Phase fault with DC component offset [12]

A"' d Subtransicnt reactance: Is the reactance of a generator at the initiation of a fault and is used in calculations of the initial asymmetrical fault current (see Figure 6) The current continuously decreases lasting approximately 0.05 s after an applied fault [1]

vY"d Transient reactance: Is the reactance of a generator between the subtransienl and synchronous states (see Figure 6.) This reactance is used for the calculation of the fault current during the period between the subtransient and steady state period (see Figure 6) The current decreases continuously during this period but are assumed to be steady at this value for approximately 0.25s [1]

X& Direct axis: The steady-state reactance of a generator during fault conditions

used to calculate the steady state fault current after the Subtransient and Transient

components have decayed away (see Figure 6)

[ Subtransient

f\ ^*~ Penod

FIGURE 6 Short circuit waveform showing the three transient periods [12]

Per Unit Quantities

A power system can be made up of various voltage levels there by making system calculations difficult Therefore, to simplify calculations a common set of base values are selected and the remaining quantities are then scaled to these base values The two common base values chosen are voltage and power The other base values are then calculated from these two base values by the following equations:

r MV A Base_3 ph ase 'Base V3 x kV,

BaseJLL

Base MVA

m v ^Base_3 phase

Where ZBase is the base impedance in ohms,

MVABase_3 phase is the chosen apparent power base

and kVease_LL is the base line to line voltage

For per phase quantities are required use line to neutral kV Once the base values have been established then the per-unit quantity of a value can be calculated with the following equation:

actual value Per Unit value =

base value

Electrical components in a power system may have different per unit values based

on its own ratings that differ from the chosen base If this is the case they can be

converted into the chosen base per unit values The following equation will transform an

old per unit impedance value into a new per unit impedance value:

Per unit impedance new = Per unit impedance oid (— = — )L x —— =—

KvBasejiew ^^Base_old

Whatever the value for the base voltage and MVA are chosen to be they will be

designated as 1 per unit or 1 p u

Since it is obvious that electro mechanical and electronic relays cannot directly

operate at high voltages and current magnitudes they must be reduced to a magnitude that a

relay can safely operate The devices used to reduce the voltage and currents are referred

to as potential transformers (PT) and current transformers (CT) This is accomplished by

taking the primary quantities and scaling it down by a known ratio

One Line Diagrams

A one-line diagram graphically illustrates an electrical power system by

representing a 3 phase system with single symbol components It is assumed unless

indicated otherwise in the drawing that each device will have 3 units if they are single

phase devices or 1 unit having 3 phase capabilities For example there will be one CT

(current transformer) for each phase for a total of 3, but a circuit breaker will have 3 phase

capabilities per each unit (See Table 1) A three-line diagram assists in the actual

construction of power equipment Each component is now displayed as a three phase

device This will enable the builders of the system to interconnect the protection and other

components

breaker

3 phase disconnect switch with fuse

JIQ

Distribution bus Impedance ground with CT and

resistor for ground fault detection

•

CT and PT with polarity marks in phase CT and PT symbols with reverse polarity

Relay symbol where the " # " is replaced with the relay number

Normally open and normally closed contacts

FIGURE 7 Typical electrical symbols

The symbols for current transformer (CT) and potential transformer (PT) also are referred to as Voltage Transformers (VT) Both CT and PT have polarities A polarity mark as indicated by the dots in a one line diagram Physically on a CT or PT the

primary will be indicated by HI and H2 and XI and X2 for the secondary where HI and

XI correspond to the dots in the one-line diagram The polarity of an instrument

transformer indicates the phase relationship between the input and the output On a CT the current flowing into the polarity mark or HI will result in current flowing out of the secondary polarity mark or XI with little or no phase shift Likewise for the PT (see Figure 7) A 180-degree phase shift will occur if the CT or PT secondary's are connected

or installed in reverse If this is by design the dots will be reversed in the one line

diagram (see Figure 8)

If the CT and PT are connected with their polarities reversed they would be

indicated with the dots in the opposite side

1 5 ,

I Pnmay current/voltage

—« — Seconday current/voltage

— — Set >nday c jrrer t/voltage

FIGURE 9 Waveform output with polarities reversed

Instrument transformers like power transformers may be connected in multiple ways depending on the application If ungrounded they may be connected as a delta or wye

or if grounded they may be connected as an open delta or grounded wye Note that open delta PT only 2 PTs are used with the secondary center phase grounded

Relay and Control Symbols

On a one line diagram a relay will be represented as a circle with the IEEE relay type number in the center (see Figure 7) The device that is activated when a relay operates that is, closes its alarm or trip contact a dashed line with arrows is often used to show the device that is activated Placing these symbols on a one line and interconnecting the single line elements allows for the representation of any type of electrical system and is the

standard method for the design of electrical systems

Elementary Diagrams

In addition to a one line a control logic schematic must also be developed This diagram is also referred to as an "Elementary Drawing " The Elementary Drawing shows the actual devices that are being activated That is, a DC bus will provide the power to either close a circuit breaker or trip it open and power the motor that will compress a

spring The contacts from the various relays are also shown on Table 1 A circuit breaker opens and closes using stored energy in a compressed (charged) spring When the circuit breaker is inserted secondary contacts in the switchgear make contact with power and control terminal in the cubicle and a motor in the circuit breaker charges the spring When

a signal to close is given a close coil (solenoid) is energized and closes the circuit breaker the same holds for the trip coil Half the spring energy is used to close the circuit breaker and the other half is used to trip the circuit breaker The motor will recharge the springs after the trip operation These solenoids are referred to as close coils (CC) or trip coils (TC) The motor is designated by the capital letter "M." Indicating lights on the

switchgear panels are used to indicate the status of the circuit breaker A red light is used

to indicate the circuit breaker is closed and a green light indicates that the circuit breaker is open Two parallel lines represent contacts Note that these are contact not capacitors A normally open contact or "a" has empty space between the lines and a normally closed contact or "b" contact has a line through it The state as shown on elementary diagrams

is when the circuit breaker is open When the circuit breaker closes the state of the

contact reverses

M - Spring Charging Motor

LS - Limit Switch, contact

CHAPTER 3 UNIT GENERATOR PROTECTION RELAYS

A wide variety of relays are required to protect a generator system Each type of relay will protect the system from a particular type of abnormality If electro-mechanical relays are used it may require up to 3 relays of a single type to protect each phase of a 3 phase system However, with the advent of microprocessor based relays many fiinctions if not all are incorporated into one unit This many relays are required due to the large capital investment of not only the generator and transformer, but the stability of the system

The following sections will describe the typical types of relays used in generator protection Not all relays are used in every instance, but a thorough review is necessary to allow the protection engineer to decide if the application warrants the inclusion of a certain protective function

Volt/Hertz Relay (24)

An internal magnetic field is required by generators and transformers in order to operate They are designed in such a way as provide the necessary flux for their rated load Over excitation occurs when abnormally high flux saturates the core steal and the excess flux flows into the portions of the stator that were not designed to handle this flux

This flux in the unintended areas will create high circulating eddy currents which will in turn generate large amounts of heat This excess heat will degrade lamination and winding insulation leading to equipment failure An example of when over excitation may occur is when a generator is started and has not come up to full speed and due to a voltage regulator malfunction or human error the field is applied Since the voltage is a function of flux times speed the voltage regulator may attempt to increase the field current an attempt to maintain rated output voltage Generator voltage regulators may also monitor and protect against over excitation and the 24 relay in this case would act as

a backup or alarm relay

To develop 24 relay settings the generator manufacture over excitation limit curves should be obtained A sample of limit curves are shown in Figure 11 below

Figure 12 Various volts/hertz limit curves [1]

A transformer is also susceptible to volts/hertz problems A similar transformer curve can be obtained and the relay set to protect both (see Figure 12)

MFC 1 GENERATOR MFG 2 GENERATOR M F G 3 GENERATOR

H 1 1 1 1 I j 1 1 1 1 I 1 1 1 1 1

0 1 1 0 10 100 1000 0 01 0 1 1 0 10 100 1000 0 01 0 1 1 0 10 100 1000 TIME (MINUTES) TIME (MINUTES) TIME (MINUTES)

f » V l — P R O H I B I T E D REGION

t

120-too

01

FIGURE 13 Generator, transformer and relay plot for volts/hertz relay plot [1]

Synchronizing Check Relay (25)

A 25 sync check relay is used to check whether or not two separate portions of a system are of similar phasor quantities such as phase, frequency and magnitude and are within predetermined thresholds If and when the electrical differences between the two systems satisfy the threshold conditions an action may be taken This action in a generation system is the closing of a circuit breaker thereby bringing the generator into parallel with the electrical system If two electrical systems were brought together and if they are

significantly different, large currents will flow through the systems which can exceed those experienced during sudden short circuits The intense currents and torques produced may cause server damages to the generator stator which may require it to be rewound

Synchronizing limits that the two systems must be with specified limits in order to safely parallel Typical limits are circuit breaker closing angle of ±10°, generator side voltage relative to system 0% to +5% and frequency differences of ±0 067 Hz

Under Voltage Relay (27) The under voltage relay operates when the voltage applied drops below a

RELAY CHARACTERISTIC

TIME (MINUTES)

the system may have time to stabilize before any trips or alarms are initiated or they may have definite level thresholds For paralleling a generator and distribution bus the 27 under voltage functions is incorporated into the 25 relay

Directional Reverse power Relay (32)

If a generator loses its prime mover it will go into a condition called "Motoring" which as the word implies the generator will now be powered by the external system and the generator will act as a synchronous motor This will drive the prime mover and

possibly damaging its shaft, couplings, compressors etcetera The manufacture provides the magnitude of reverse power that the generator system can withstand before damage occurs A 32 relay will also contain an adjustable time delay to allow short duration power variations to stabilize The manufacture will often provide the reverse power threshold in primary watts and the amount of time the generator can "motor" before it is damaged

Loss of Excitation (Field) Relay (40) Loss of excitation on a synchronous generator will cause the rotor to accelerate and operate as an induction generator As a result it will draw reactive power from the system instead of providing it to the system Heavy currents will also be induced into the rotor teeth and wedges which will cause thermal damage to the generator if allowed to operate in this condition Common causes of excitation loss can be operator error, excitation system failure, accidental tripping of the field breakers or flashover of the exciter commutator A type of 40 relay is called an offset MHO relay The following information will be required from the manufacture to set the protection level: the generator direct axis reactance Xd,

and X is the reactance of the system (see Figure 30) The inner circle referred to as Zone 1 will trip the system offline and the outer circle referred to as Zone 2 will alarm before tripping the system offline

FIGURE 14 2 zone protection diagram [1]

The following equations are used to calculate the diameters and offsets Note that values must be secondary ohms for use in the relay Time delays are of 1 seconds for Zone 1 and 5 seconds for Zone 2 are suggested Zone 1 (inner circle) is commonly set to

1 0 pu of impedance Z

The offset of the inner and outer circle X is the negative quantity of the half the transient reactance The diameter of the outer circle is equal to the direct access transient with the offset being equal to that of the Zone 1 offset Note that these values are in primary quantities and must be converted into relay base for used with the protection

Negative Sequence or Unbalance Relay (46) Negative sequence stator currents, caused by unbalanced faults such as phase-to-phase, phase-to-ground, double-line-to-ground or load unbalance, induce double frequency currents into the rotor that may eventually overheat elements not designed to be subjected

to such currents as a result of the 11 losses The most serious series unbalance for example

is an open phase, due to a failed circuit breaker pole Two limits are used for 46 relay protection settings the Continues Negative-Sequence current capability limit and the Short Time Unbalance current limit IEEE standards dictate that all generators meet a negative sequence withstand capabilities [5] For generators continues negative-sequence limits range from 5 and 10% of rated current The short time limit is expressed in terms of K, where K= I2t and varies from 5 to 40 The actual values will be provided by the generator manufacture These limits will be used to set the relay pickup values The relay will have

a time delay function to allow downstream circuit breakers to clear the portion of the system that is causing negative sequence currents Typical curves (see Figure 15) provide the protection engineer relay pickup and delay characteristics to coordinate trip functions of protection down the line