Protection Management [Industrial Electrical Power Systems]

Management of protection

A protective system is considered 100% perfect if the number of circuit breakers opened under a fault are as per the design configuration. However, there are occasions when a few protective relays incorrectly operate or fail to operate. There could be many reasons but the principal reasons could be:

1. • The internal faults in the relays
2. • Defects in the wiring to the relays
3. • Wrong and poorly coordinated settings
4. • Unforeseen faults at the design stage
5. • Mechanical failures

Protection systems must be kept 100% operational at all times as one never knows when or where faults are likely to occur. The systems must therefore be maintained and managed properly to ensure safe and efficient operation of the power network.

Although the relays are tested prior to commissioning a system, it’s most likely that the relays may not be operating due to the soundness of the system. However, it cannot be assumed that the relay did not operate because of the system healthiness. Hence, it’s very vital that the relays should be periodically checked and tested at fixed intervals. It’s also important that the records must be kept about the tests being conducted and the details of results for future reference and records.

The functions required for good maintenance are listed on the following schedule A and it’s important that good records are kept of the system parameters, wiring schematics, relay settings and calculations, CT magnetization curves and so on. Some suggested formats of test sheets are attached to give some idea of the sort of information that should be kept on file.

Schedule A

Schedule A is nothing, but the basic functions that are considered essential to ensure that the relays are kept in good form during their life. The tests will give the idea about any internal parts that are to be corrected or replaced. The records will also give an idea on the frequency of failures expected in typical relays and the replacements that are needed at regular intervals. Such frequent replacement parts can be kept as spares so that the relays can be put back in perfect conditions immediately on noticing the defects.

+--+ generally outlines this schedule.

Functions of Maintenance-- Routine inspection and testing

Annual trip testing (random)

Full scheme test every 4th year

Investigations

Defects

Incorrect operations

Spares and repairs

Performance assessment

Modifications

Refurbishment

Replacements/up-grading

+--+ Schedule A Protection management also involves addressing some of the following issues listed in schedule B. Issues -- Technology

Organization

Privatization

Skilled technical staff

Environment

Access for work

+--+ Schedule B

Schedule B

The technology has been changing at a rapid rate in recent times and it’s important that staffs are trained to be skilled in the area and are kept up to date. Good forward planning is essential to get access to plant for maintenance.

If the budget cannot carry permanent staff, then bring in specialist private companies to do the annual checks. Above all, make sure that the relays are:

• Applied correctly for job
• Commissioned properly
• Set correctly
• Maintained in good condition and working order

Test sheets

A typical test sheet standard format can be as seen below and the format can be redesigned based on the relay type and the tests needed.

Test Certificate Station: Circuit: Customer: Circuit: Relay: Test Details: Injection Current/Voltage: Fault Simulated: Results Obtained:

Date: Tested By: Engineer: Date:

If the budget cannot carry permanent staff, then bring in specialist private companies to do the annual checks. Above all, make sure that the relays are:

• Applied correctly for job
• Commissioned properly
• Set correctly
• Maintained in good condition and working order.

Test sheets

A typical test sheet standard format can be as seen below and the format can be redesigned based on the relay type and the tests needed.

Test Certificate Station: Circuit: Customer: Circuit: Relay: Test Details: Injection Current/Voltage: Fault Simulated: Results Obtained:

Date: Tested By: Engineer: Date:

===

Motor Protection Station: Circuit: Date: Panel No.: CT Ratio: Class: Relay Type: Serial No.: Rated: A Min Tap Set: % Load to Trip: % Aux Volts: V R Stab: Inst O/CL A E/F: A Thermal Checks: (100% Tap) Three Current Elements in Series Check center pointer read '0' Yes No Ad Check outer elements are central Yes No Adj

Inject Ir A Load Reads % Adjusted

1. Inject XIr = A Elements move together

Adjusted

2. Time (from cold): Rated S Actual S Error Allowed

Check Running Load Values: 80% % Check 90% Tap @ 90% Ir Load Reads: % Check 80% Tap @ 80% Ir Load Reads % Instantaneous Checks Overcurrent: Phase Set A Operate A

Phase Set A Operate A Ground Fault: Set A Operate A

Set A Operate V Remarks:

Date: Tested By: Engineer: Date:

===

When a system is put into service, it’s necessary that proper records should be available for the various tests to be conducted. Above all, a checklist is mandatory to ensure that all basic tests are carried out before putting the system into use. Following table is a typical commissioning checklist, which should be planned, well in advance before taking up the commissioning of any electrical system, whether simple or complex.

Commissioning Check List

Station: Circuit:

1. Current transformer tests 5. Functional tests (DC)

Magnetization curve --Tripping circuits

Polarity --Closing circuits

Ratio --Supervisory circuits

Megger --Fuse ratings

SEC resistance 6. Phasing tests

2. Voltage transformer tests Primary circuits

Polarity VT secondaries

Ratio Auxiliary supplies

3. Primary injection tests 7. On load checks

Protection CTs 8. TRFR Buch alarm/trip

Metering CTs TRFR Wdg temp alarm/trip

Bus Zone CTs TRFR pressure alarm/trip

4. Secondary injection tests TRFR oil temp alarm/trip

Relays 9. NEC Buch alarm/trip

Metering NEC temp alarm/trip Comments:

Engineer: Date:

Primary and secondary injection tests are the most common tests applicable for voltage and current sensing relays, whose functions depend on the correct sensing characteristics.

Following table shows typical test sheet for such a purpose.

A secondary injection test serves the purpose when there is no possibility to apply the primary voltage or pass the primary current to the voltage and current transformers, connected to a relay. For e.g. a 110 V supply and a 5 A current source would be able to complete most of the functional tests of typical relays.

Primary Injection Test Station: Circuit: Ratio: Function: Phases Injected Primary Current Secondary Red Secondary Yellow Secondary Blue Secondary Neutral

R/R R/Y R/B