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ESP

I I I I 111111li I Ii i1111
Ay-VG/0370
nIBLIOTECA CENTRAL

POWER SYSTEM
PROTECTION
Videotraining Program

PSP-7
Bus Protection
Instructor's Guide

L &K International Videotraining

P.O. Box 940, Station 'U',
Toronto, Ontario M8Z 5P9

Tel. (416) 252-6407

Fax. (416) 252-8331
Toil Free 1-800-668-6064
 PSP SERIES - Introduction

The "Power System Protection " videotraining program is designed for Relay Techni-
cians, Engineers , and System Operators , to aid in up-grading their knowledge and
understanding of the function and application of protective schemes , which are used in
the protection of generators , buses, transmission and distribution lines , transformers,
large motors, etc.

The practical know -how gained from diese 18 videotapes and associated workbooks
will improve the participant's ability to perform his day-to-day activities, whether they
be related to operation , maintenance, or engineering.

Although each power system is unique in design , successful completion of this generic
program will ensure that operating and technical personnel have a thorough under-
standing of how the power system behaves under fault conditions , and the different
types of protection schemes which are installed to detect and protect against diese fault
conditions . The concepts presented will apply to your own specific system.

The program will help Technicians evaluate the performance of their protection
schemes, and the knowledge gained by Operators will greatly assist them in analyzing
fault problems.

The videotape format is ideally suited to individual or group instruction . Each video-
tape is broken -down into four or five segments . The participant should view each
segment of the tape, then review this material in his workbook . He should then com-
plete the set questions before proceeding to the next segment.

There are advantages if several people view the material together, and then, as a group,
complete the workbook tests in the segment breaks . A period of discussion proves to be
most beneficial in assuring retention of the information. A technically qualified person
should act as instructor to coordinate the discussion periods.

Adequate time should be taken to review your actual power system installations and
compare them with the generic examples shown in the videotapes . It will be useful in
diese discussions to review schematic diagrams of your own system and especially to
make periodic field trips to view specific protection schemes and equipment.

PSP-7 BUS PROTECIION 1

 PSP SERIES - Program Titles

POWER SYSTEM PROTECTION

Videotape 1 Elements of Protection

Videotape 2 Types of Protective Relays
Videotape 3 Monitoring System Conditions
Videotape 4 Fault Characteristics
Videotape 5 Generator Protection
Videotape 6 Transformer Protection
Videotape 7 Bus Protection
Videotape 8 Motor Protection
Videotape 9 Line Protection
Videotape 10 Pilot Protection
Videotape 11 Protection for System Stability
Videotape 12 Testing and Commissioning of Protective Schemes
Videotape 13 Power Line Carrier

Videotape 14 Fault Investigation and Analysis

Videotape 15 Introduction to Static Relaying

Videotape 16 Coordination of Protection Devices
Videotape.17 Power Supply for Protection and Control Systems
Videotape 18 Energy Control Center for Relay Technicians

PSP-7 BUS PROTEC7ION 2

 PSP 7 - Learning Objectives

The objective of chis videotape is to review che different bus layouts that are used in
power systems, and to present che different protection schemes that are installed to
protect against bus faults.

After study of chis tape and che associated workbook, che participant should be able
to understand che following overall concepts and apply them to his day-to-day work
activities. He will also be able to answer related test questions on these subjects:

Features of different bus arrangements.

Single breaker - single bus.
Single buses connected with bus tic breaker.
Main and transfer buses with single breaker.
Single bmaker - double bus.
Double breaker - double bus.
Ring bus.
Bmaker-and-a-half bus.
Special problems of bus protection.
The ground fault bus.
Parcial differential protection with overcurrent relays.
Directional comparison schemes.
Residual current differential schemes.
CT saturation problems.
Multi-restraint relays.
Linear coupler.
High impedance relays.

LESSON PLAN

The subject master in Chis tape is broken down into che following lessons:

SEGMENT A - BUS ARRANGEMENTS

SEGMENT B - BUS DIFFERENTIAL PROTECTION WITH OVERCURRENT
RELAYS
SEGMENT C - HIGH SPEED DIFFERENTIAL PROTECTION
SEGMENT D - HIGH IMPEDANCE DIFFERENTIAL PROTECTION

PSP-7 BUS PROTECTYON

 SEGMENT A - Bus Arrangments

Please refer to Segment A of the videotape and the workbook.

This segment demonstrates the different bus arrangements that are used on power
systems, and discusses the advantages and disadvantages of each. The location of CTs
is indicated and the consequent zones of bus protection.

The instructor should emphasize or (wherefeasible) demonstrate the following points:

The bus provides a junction point for a number of lines or feeders. Power will
feed in and out of the bus.
Disconnects are fitted on either side of each breaker to allow isolation during
maintenance.
The measurement of current flow in and out of the bus provides the basis for
bus protection.
CTs for bus protection are norm mally located outboard of the breakers. Con-
versely, CTs are mounted inboand for line protection.
If a fault occurs on the bus, all breakers must be opened.
One disadvantage of the single bus is that all circuits will remain isolated until
the fault can be repaircd.
Installation of a tie -breaker allows the bus to operate in two separare halves,
each half having its own bus protection schemes.
The provision of a transfer bus allows circuit breakers to be removed for main-
tenance. The common bus tie-breaker and transfer bus become parí of the fine.
When using the transfer bus, the protection settings may need resetting due to
different characteristics of the connected circuir.
In the double bus arrangement, both buses A and B are connected to multiple
circuits. Separate bus protection is provided for each bus. Bus protection on
buses associated with the tie breaker must be disabled when the tie breaker is
operated.
In the double breaker double bus arrangement, each circuit has two breakers
connecting to either bus A or bus B.
In the breaker-and-a-half arrangement, two incoming circuits share the middle
breaker to allow flexibility of switching between bus A and bus B.
In the ring bus arrangement, each section of bus is included in its associated ¡inc
protection. A line fault will cause the tripping of two breakers, one on either
side.
The advantage of the ring bus is that a circuir breaker can be removed for
maintenance.
Demonstrate on-site the various bus arrangements throughout your system.
Particularly, examine the path of each circuit to the bus through disconnects and
circuit breakers . Also, note the location of CTs.

PSP•7 BUS PROTECIION

 SEGMENT B - Bus Differential Protection With Overcurrent Relays

Pisase refer to Segment B of the videotape and the workbook.

In Chis segment we look at the simpler forms of bus protection that would typically be
used on a distribution substation . We also look at some of the problems which are
particular to bus protection, namely the wide divergente of current flows in different
circuits with the resultant problem of CT accuracy.

The instructor should emphasize or (wherefeasible ) demonstrate the following points:

The operaron of partial differential protection using time overcurrent relays.

Indicare where Chis protection scheme is employed in your own power system.
Operation of the ground fault bus . Indicate where Chis scheme is employed in
your own power system.
Operation of the simple current differential protection circuir.
For accuracy it is essential that CT characteristics are identical.
For a heavy through -fault the magnitude of currents in and out will be widely
divergent. Under these circumstances the CTs may not accurately represent the
line curten[, ie. error current may cause relay operation. This problem is exag-
gerated if the Cr reaches saturation , especially during the transient which
occurs over the first 20 cycles of the fault.
In order to ride through Chis transient period a time overcurrent relay is used for
differential protection in many installacions.
An alternative method uses directional overcurrent relays. The operating con-
tacts are wired in series, and will close when current feeds hico the bus.
Demonstrate on-site the installation of the types of protection used on your own
power system.

PSP-7 RUS PRO7EC77ON 5

 SEGMENT C - High Speed Differential Protection

Pisase refer to Segment C of the videotape and the workbook.

Tisis segment points out the need for high speed differential protection as the bus is
such a strategic pan of the power system. Numerical examples are shown to indicate
the problem of Cr saturation and the need for multiple restraint coils on the differential
relays. The linear coupler is also introduced and discussed.

The instructor should emphasize or (wherefeasible) demonstrate thefollowing points:

Important power buses should be protected by high speed differential protection

which is reliable , ie. it will not trip for through-faults.
Ttte numerical examples show the effect of CT saturation with resultant inad-
vertent tripping for through faults. Connection of restraint coils can prevent
tripping due to error current. This is shown by a numerical example.
On bus protection schemes the restraint percentage differential is usually set
from 50 to 60%.
The multi-restraint differential relay uses an instantaneous relay which operates
within 3 to 6 cycles.
Separate restraint coils must be used for each major circuir.
Three-phase CTs are normally connected in wye.
The CTs should all be identical , and ratio taps are not fitted.
The linear coupler has no ¡ron core and, therefore, does not Buffer from satura-
tion.
For bus protection, the linear couplers are connected in series. Out-of-balance
conditions will produce a voltage which is measured by the differential relay.
The relay will trip in about one cycle.
Demonstrate application of these relays on your power system.

PSP-7 BUS PROTEC77ON 6

 SEGMENT D - High Impedance Differential Protection

Pisase refer. to Segment D of the videotape and the workbook.

This segment looks at the operation of high impedance differential relays . Also pre-
sented is a typical "check circuir" which may be permissive or alarm.

The instructor should emphasize or (where feasible) dentonstrate the following poinrs:

The impedance of each CT circuit is about 2 Q.

The impedance of the high impedance relay is about 3,000 £2.
Because of the high impedance of the relay, error current flows through che CT
circuits instead. This prevenís inadvertent operation of the relay for through
faults.
The high impedance relay includes a resonant circuit which filters out any har-
monics and the D.C. component.
A varistor is connected across the relay, to by-pass high currents.
An instantaneous relay is connected in series with the varistor. This relay oper-
ates within about 1 /2 cycle.
The high impedance relay operates within about 1-112 cycles.
While not covered in the videotape and workbook, you could mention that
check relays are sometimes added to prevent tripping of the bus due to mis-
operation or faulty relays. The check relay can be used to alarm only or to
actually prevent tripping.
In any bus differential scheme, the selection of CT ratings is particularly impor-
tant because of the wide range of cumnt flows which must be measured.
Demonstrate bus protection schemes within your own power system. Take time
to explain the relay settings and the CT ratings.

PSP-7 BUS PROTECnON

 OTHER REFERENCES

Addltlonal information on thls subject can be obtalned from:

1. Chapter 10 of the textbook Protective Relaying by J. Lewis Blackburn.

2. L&K Videotraining program : Power System Operation, tapes 5104 and 5116.

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Copyright © 1990 L & K International Videotraining

PSP-7 BUS PROTEC71ON $