Power System Protective Relaying-Part Four

Part Four – Protection Part Four – Protection Fundamentals and Basic Design Fundamentals and Basic Design PrinciplePrinciple

Wei-Jen Lee, Ph.D., PE

Professor of Electrical Engineering Dept.The Univ. of Texas at Arlington

IntroductionIntroduction

• The best protection technique now and for more than 50 years is that known as differential protection.

• Differential protection is universally applicable to all parts of the power system.

• Other protections, such as overcurrent, over/under frequency, and over/under voltage protections, are also use among utility industry.

The Differential PrincipleThe Differential Principle

• For normal operation and all external faults, the Iop of the relay is very small.

• During external faults, the transient of the CTs can produce rather large currents. Therefore, it is difficult and impractical to apply an instantaneous relay. Time delay relay can be used with care.

• For internal faults, the differential relay operating current is the sum of the input currents feeding the faults.

• To provide high sensitivity to light internal faults with high security for external faults, most the relays are of the percentage differential type. The secondary of the CT is connected to restraint coil.

• For the circuit with 50% characteristics, an external or through current of 10A would require a difference or operating current of 5 A or more for the relay to operate.

• The through current characteristics apply only for external fault.

• Differential relays are very sensitive to the internal faults.

• Typical pick-up currents for differential relays are on the order of 0.14 – 3.0 A

Overcurrent and Distance Overcurrent and Distance ProtectionProtection• Where differential is not utilized, overcurrent and

distance relays are the major protection schemes.• The minimum operating criteria for overcurrent

relays is shown below:

Overcurrent and Distance Overcurrent and Distance ProtectionProtection• Relay coordination

Basic Design PrinciplesBasic Design Principles

• Electromechanical• Solid State• Hybrid• Numerical (Microprocessor)

• Time-Overcurrent Relays

• Time-Overcurrent Relays• Definite minimum time (CO-6)

• Inverse (CO7 or CO-8)

• Very inverse (CO-9)

• Extremely inverse (CO-11)

• Tap setting and time dial setting

• Example: Time-overcurrent relays for radial system protection

System diagram

• Example: Time-overcurrent relays for radial system protection• Arrangement of breaker, CT, and relay

BREAKER BREAKER OPERATING TIME

CT RATIO

RELAY TYPE

B1 5 Cycles 400:5 CO-8

• Example: Time-overcurrent relays for radial system protection• System loading and fault current

Bus Load (MVA) PF (Lagging)

1 11.0 0.95

2 4.0 0.95

3 6.0 0.95

• Example: Time-overcurrent relays for radial system protection• I-T curve of Co-8 relay

• Example: Time-overcurrent relays for radial system protection• System loading and fault current

Bus Max. fault current Min. fault current

1 3000 2200

2 2000 1500

3 1000 700

• Example: Time-overcurrent relays for radial system protection• Calculation and coordination

• Select Tap setting that the relay will not operate for maximum load current.

• For CT at B3, the maximum CT current for maximum load L3 is:

• For this example, 3-A TS was selected for B3 relay.

51.2)5/200(

4.10010*5.34*3

• For CT at B2, the maximum CT secondary current for maximum load is:

18.4)5/200(

35.167

35.16710*5.34*3

10*)46(3

• For CT at B1, the maximum CT secondary current for maximum load is:

39.4)5/400(

43.351

43.35110*5.34*3

10*)1146(3

• The largest fault current through B3 is 2000A. (Fault at right of B3)

• Ignore CT saturation, the fault-to-pickup current ratio at B3 is 2000/(40*3)=16.67

• Since the speed of operation is the main concern, 0.5 Time-Dial setting (TDS) is selected.

• Under this fault current, the operating time of B3 is 0.05 second

• Adding the breaker operating time (0.083 second), primary protection needs 0.113 second (0.05 + 0.083) to clear this fault.

• For the same fault, the fault-to-pickup current ratio at B2 is 10.0. (2000/40*5)

• Adding the B3 relay operating time, breaker operating time, and 0.3 second coordination time interval, the B2 relay’s operating time should be 0.43 seconds.

• Select TDS=2 for the B2 relay.

• The largest fault current through B2 is 3000A. The fault-to-pickup ratio for B2 is 15.

• The operating time of B2 relay under this fault current is 0.38 seconds.

• For the same fault, the fault-to-pickup ratio of B1 relay is 7.5.• Adding the B2 relay operating time, breaker operating time,

and 0.3 second coordination time interval, the B1 relay’s operating time should be 0.76 seconds (0.38+0.3+0.083).

• Select TDS=3 for B1 relay. [Confirm this results with Min. fault current]

• Instantaneous current-voltage relays

• Directional sensing power relay

• Polar unit

• Phase distance relay• Balance beam type

• Phase distance relay• Distance relay characteristics on the R-X diagram

Impedance mho Offset mho

• Phase distance relay• Distance relay characteristics on the R-X diagram

lens Simple blinder reactance

• Phase distance relay• Figures (b) through (e) can operate on a fault current

less than the load current.

• A load of 5 A (secondary CT) and 120 V line to line appear to the relay as

86.135*3

120loadZ

• Phase distance relay• The equation of the mho circle through the origin is

is the offset from the origin

is the radius from the offset point

• When the mho circle is tilted, is the angle of R of the offset

(Typo in the book)

• Phase distance relay• Various operating point of the mho circle characteristic

are determined by the following equation:

ZX is the impedance from the origin to any point on the circle at angle X.

)cos( XRRX ZZ

• Phase distance relay• For example, determine the reach of a mho relay unit

along a 75o angle line if the maximum load into the line is 5 A secondary at 30o lagging. From previous calculation, the load impedance is 13.86 secondary. 13.86 = ZR*cos(75o-30o)=19.6.(secondary)

• This can be translated into primary line ohms by considering the CT and VT ratio.

• Zone protection of phase distance relay

• Single phase MHO unit• Three MHO units are required for a protective zone.

All three units operate for three-phase fault, but for phase-to-phase and double-phase-to-ground faults, only one unit operates. Thus,

• The A unit, energized by Iab, and Vab, operates for ab and ab-gnd faults.

• The B unit, energized by Ibc, and Vbc, operates for bc and bc-gnd faults.

• The C unit, energized by Ica, and Vca, operates for ca and ca-gnd faults.

• Single phase MHO unit• Single phase MHO unit (shown for the A unit)

• An air-gap transformer provides a secondary voltage IabZc for the A unit. Leading the primary current for lee than 90o.

• The combined output voltage is equal to IabZc – Vab. This voltage with a polarizing voltage is compared to provide the mho circle.

• Poly-phase MHO unit• This type of MHO unit has two units for a zone

protection:• An mho circle through the origin for three-phase faults.

• A phase-to-phase unit, with a large operating circle

protection:• Three-phase faults unit

• The cylinder unit is like a two phase motor operating negative sequence xzy is applied and retrains on positive sequence xyz.

• When a solid comparator is used, Vxy = Vab – (Ia –Ib)Zc and Vzy = -jkVab are compared.

)3(5.1 0

protection:• Phase-to-phase fault unit

• For a phase-to-phase faults at the balance or decision point, VxVyVz provides a zero-area triangle (Vzy and Vxy in phase for solid state comparator)

• Any fault inside the triple-zone negative sequence xzy, or when Vzy lags Vxy cause operation.

cbccnz

cbaanx

• Poly-phase MHO unit• This type of MHO unit has two units for a zone protection:

• Phase-to-phase fault unit

• Ground distance relays• Consider a phase-a-to-ground fault on a line with Z1L and

Z0L as the positive and zero sequence line impedance and n is the location of the fault from the relay. The fault currents through the relay are I0, I1, and I2. Then for a fault at nZ1L with a single-phase unit:

• For voltage compensation, subtract out nZ1L(I1+I2) and use I0 (not Ia), then:

00211 )(

InZIInZ

(typo)

IInZVZ 0

211 )(

• Ground distance relays• For current compensation, let nZ0L=pnZ1L (p=Z0L/Z1L). Then

ZZmwhere

pIIInZ

IpnZIInZ

01211' ))1(()()(

• Ground distance relays• Considering fault resistance and mutual coupling from an

adjacent parallel line, the complete formula for current compensated single-phase ground distance relay is:

where I0E is the zero sequence current in the parallel line and Z0M is the mutual coupling impedance between two lines.

relayfaultL

(typo)

Microprocessor Based System Microprocessor Based System ProtectionProtection

Introduction

In the Utility Industry, Regardless Their Operation Mechanisms, the Protective Relays are Categorized and Evaluated by Their Functions.

Same Test Procedures are Applied to a Group of Relays With the Same Protective Function.

The Performance of a Microcomputer Based Relay Depends on the Hardware Design, the Accuracy of the Input Signals, and the Algorithms Embedded Inside the Unit

Introduction

Each Vendor Has Its Own Hardware Selection and Software Design Preference. Therefore, the Physical Capability and Constraints of a Microcomputer Based Relay Are Different from Vendor to Vendor Even with Similar Protective Functions

General Structure of a Microcomputer Based Relay

SUBSTATION & SWITCH YARDCurrent & Voltage Contacts I/P Contacts O/P

SurgeSupressor

SignalConditioning

Sample/HoldA/D Conversion & Multiplexer

CentralProcessor

SurgeSupressor

CommunicationModule

SurgeSupressor

Memory Sub-SystemRAM, EPROM, EEPROM

SignalConditioning

OutputDriver

Hardware Design Consideration

Central Processing Unit (CPU) Word Length Clock Speed Floating Point Calculation Single or Multiple Processor Technical Support

Memory Subsystem Memory Types

– Read Only Memory (ROM): It is designed to permanently store a fixed program which is not alterable. It can only be programmed once and requires special equipment to program the chips (Most of them are pre-programmed by manufacturers). The information is preserved even without power.

– Random Access Memory (RAM): It is designed so that information can be written into or read from any unique location. There are two types of RAM: Static RAM and Dynamic RAM. RAM does not retain its contents if power is lost.

– Programmable Read Only Memory (PROM): The characteristics of PROM is similar to ROM except that it is user programmable.

Memory SubsystemMemory Types

– Erasable Programmable Read Only Memory (EPROM): The EPROM is a specially designed PROM that can be reprogrammed after being entirely erased with the use of ultra-violet (UV) light source. The EPROM can be considered as a semi-permanent data storage device.

Memory SubsystemMemory Types

– Non-Volatile Random Access Memory (NOVRAM): Combination of RAM and EEPROM on a single chip. This chip can protect data against power failure.

Usage of Memory Real-Time Data and Pre/Post-Fault Recorder:

RAM Program Memory: ROM, PROM, EPROM, or

Combination of RAM and EPROM Settings: Flash Memory or EEPROM

Signal Conditioning The Outputs of the PT and CT Require Auxiliary

Transformers or Divider Circuits to Convert Current and Voltage Signals Into Low Level Voltage Signals Before They Can be Processed

Note: Some relay manufacturers are pushing the possibility of low

level input signals

Signal Conditioning (Conti.) Since the Signals Are Highly Distorted During a

Disturbance and the Relay Operation is Mainly Based on the Magnitude of Some Specific Signals, an Analog Filter Circuit is Generally Used to Perform First Stage Signal Screening. Later, it Relies on the Digital Filter to Perform Further Clean Up

Signal Conditioning (Conti..) Anti-Alaising Filter Design

– Band-Pass Filter

– Low-Pass Filter

Frequency Response of a Band-Pass Filter

Frequency (rad/sec)

Gain dB

Q=1Q=5

Phase Shift of a Band-Pass Filter

Phase deg

Frequency (rad/sec)o

Q=10Q=1

Frequency Response of a Low-Pass Filter

0Gain dB

Frequency (rad/sec)

1st Order

2nd Order

3rd Order

Phase Shift of a Low-Pass Filter

0Phase deg

Frequency (rad/sec)c

1st Order

2nd Order

3rd Order

Data Sampling Circuit Designs

BUFFER

(a) (b)

(c) (d)

A/D Conversion Techniques– Dual Slope Tracking A/D Converter

A/D Conversion Techniques– Dual Slope Successive-Approximation A/D

Converter

A/D Conversion Techniques– Flash or Parallel Conversion

Sampling Algorithms of A/D Converter– Sampling speed and available calculation time

– Constant time interval sampling or constant samples per cycle

– Frequency calculation technique

Errors Associated with A/D Conversion– Word Length

– Quantization Error of a N-bit A/D Converter is Equal to 2-N

– Gain, and Offset Errors

Errors Associated with A/D Conversion

OffsetError

QuantizationError

Output

10% 50% 90%

Desired Output

ActualOutput

Errors Associated with A/D Conversion– Sampling Speed and Alaising Errors

Errors Associated with A/D Conversion– Reconstruction Errors (Original Signal)

Errors Associated with A/D Conversion– Reconstruction Errors (Sampled Data)

Errors Associated with A/D Conversion– Reconstruction Errors (Reconstructed Waveform)

Errors Reduction Techniques for A/D Conversion– Word Length Selection of A/D Converter

– Full Scale Value Selection

– Coordination Between Anti-Alaising Filter and Sampling Speed

– Compensation of Gain and Offset Errors

– Programmable Gain Control

Data Communication Subsystem Isolation of Communication Network

– Isolation of Power Supply

– Isolation of Signal Inputs (RJ-11 and RS232)

– Isolation of Grounding

– Survivability of Protection Function After Communication Failure

Data Communication Subsystem External or Internal Modem

– Baud Rate– Future Upgrade– Compatibility– Power Supply– Automatic Answer

Data Communication Subsystem More Than One Units Share One Phone Line

– Mini Switch Board

– Local Area Network

– Communication Software Compatibility of Different Relays From Different Venders

Data Communication Subsystem Surge Withstand Capability and Walkie Talkie Test

– All the Open Connectors Should Comply With Surge Withstand Capability and Radiated Electromagnetic Interference Specified in IEEE Std. C-37.90.1 and C-37.90.2.

Software Implementation Considerations

Programming Languages Execution Speed Portability Upgrade Graphical User Interface (GUI)

Computation Algorithms Digital Filtering

– Non-Recursive Filter

Y X X X

m m m m

3, 9, …

DC, 6, 12, …

5, 7, ...

Computation Algorithms Digital Filtering

– Recursive Filter• Kalman Filter

• Fast Fourier Transform

• Curve Fitting

• Walsh Filter

Computation Algorithms Convergence of a Recursive Filter

Computation Algorithms Magnitude Calculation

| | ( cos ) / sin

V v v v v T T

m m m m

Computation Algorithms Phase Calculation

| |*| |*cos * *

| |*| |*sin * *

V I v i v i

V I v i v im m m m

m m m m

Moving Windows for Magnitude Estimation

Moving Data Window

Computation Algorithms

Calculation Algorithms

– Fault Detection Algorithm

– Fault Classification Algorithms

– Calculate Between Samples or Calculate After Several Samples

Decision Making Process

– Trip After One Violation

– Trip After Several Consecutive Violations

– Trip After Majority Violation of N Consecutive Samples

– Similar Conditions Also Apply to Reset Procedure

Auto Execution, Watch Dog Timer, and Self Diagnostic – Auto Execution After Power Failure

– Watch Dog Timer to Protect Against Software Failure

Auto Execution, Watch Dog Timer, and Self Diagnostic – Self Diagnostic to Protect Against Hardware Failure.

• Frequency of Self Diagnostic Function

• Completeness of the Diagnostic Function

• Failure Isolation and Indication

Data Communication Algorithm

Communication Protocol

– Compatibility and Security

– EPRI’s UCA

Error Checking– General properties of error detection and correction

• If the distance between any two code words of a code C is >dmin, the code is said to have minimum distance of dmin

• In general, a code provides t error correction plus detection of s additional errors if and only if the following inequality is satisfied:

2t + s + 1 < dmin

Error Checking– Definition of the distance

• The distance between I and J, d(I, J), is equal to the number of bit position in which I and J differ.

• For example: I = 01101100 and J = 11000100

I = 0 1 1 0 1 1 0 0

J = 1 1 0 0 0 1 0 0

d(I, J) = 3

Error Checking– Distance and error detection/correction

valid code word

dmin=2 dmin=4

Error Detection Algorithms– Parity Check

• Even parity

• Odd parity

– Check Sum• CRC

• LRC

• CX-ORC

Error Correction Algorithm– Hamming code

i3 i2 i1 i0 c2c1c0

1 0 0 0 1 1 1 1 1 1 0 1 0 0

G= 0 1 0 0 1 1 0 H= 1 1 0 1 0 1 0

0 0 1 0 1 0 1 1 0 1 1 0 0 1

0 0 0 1 0 1 1

Error Correction Algorithm– Hamming code

• c2: even parity check of i3, i2, i1

• Transmit code word, c = iG• HcT = 0• If the receiving code, d, with error

d = c + e• Syndrome, s = HdT = HeT

Error Correction Algorithm– Syndrome table

Syndrome Meaning0 0 0 No error0 0 1 Error in c0

0 1 0 Error in c1

1 0 0 Error in c2

0 1 1 Error in i0

1 0 1 Error in 11

1 1 0 Error in i2

1 1 1 Error in i3

Error Correction Algorithm– H matrix for (15, 11) hamming code

1 1 1 1 0 1 1 1 0 0 0 1 0 0 0

H = 1 1 1 0 1 1 0 0 1 1 0 0 1 0 0

1 1 0 1 1 0 1 0 0 1 1 0 0 1 0

1 0 1 1 1 0 0 1 1 0 1 0 0 0 1

• Code length: n = 2m - l -1

• Number of information bits: k = 2 m - m - l -1

• Number of check bits m = n - k

Example: Design a Hamming code for encoding five (k =5) information bits

• Four check bit, m = 4, is required

• Delete six (6) columns from previous H matrix 1 1 1 1 0 1 0 0 0 1 0 0 0 0 1 1 1 1

H = 1 1 1 0 1 0 1 0 0 0 1 0 0 0 1 1 1 0

1 1 0 1 1 0 0 1 0 G = 0 0 1 0 0 1 1 0 1

1 0 1 1 1 0 0 0 1 0 0 0 1 0 1 0 1 1

0 0 0 0 1 0 1 1 1

Unauthorized Access Prevention and Security Operation

Levels of Password Master Password Operation Reconfirmation Background Operation Settings Update Algorithm

Future Development of Microcomputer Based Relay Systems

Artificial Intelligence Multiple Function Relays Common Hardware Configuration Software (Firmware) Driven Protective

Function(s) Stronger Communication Capability Serve as Pre-/Post-Fault Recorder

Power System Protective Relaying-Part Four

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