Protection Relays in Power System
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| Protection Relays |
Protection Relay Functions
Protective devices that are used to protect electrical
systems & equipment usually have one or more functions. Protective
relays detect fault conditions and initiate circuit breaker trips to
de-energize faulted equipment or circuits before serious damage can occur.
Basic function includes:
- Alert operating personnel to abnormal or potentially dangerous conditions or to the fact that a trip circuit has been energized.
- Automatically interrupt current flow to equipment if a potentially dangerous fault occurs.
- Automatically energize stand-by equipment as necessary to maintain system operation.
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| Sample Protection Relay Scheme |
Common Characteristics of Protection Relays
- Reliability - Must operate when they are supposed to.
- Speed - Must be able to respond to a fault and isolate the affected equipment before damage can occur.
- Simplicity - Important for economic reasons as well as for maintenance efficiency.
- Sensitivity - Able to detect a fault as soon as it occursSelectivity - Should isolate only the faulted area.
Coordination
The relay co-ordination refers to the tripping of protecting relay in a proper sequence or order in electrical power system. This is to avoid tripping of un-faulted branch in the system. Relay co-ordination is required to isolate the faulty part with minimized relay & circuit breaker operation.
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| The importance of coordination in Protection Relay Scheme |
Zones of Protection
Protected zones are established to protect certain
components such as:
- Generator
- Transformers
- Buses
- Motors
ANSI Device Numbers
Each relay in a protection scheme has a specific functions and responds to a certain type of fault encountered in the power system.
ANSI device numbers. In the design of electrical power systems, the ANSI standard device numbers (ANSI /IEEE Standard C37. 2 Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations ) identifies the features of a protective device such as a relay or circuit breaker.
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| Sample ANSI Device Designation |
Basic Protection
Characteristic
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Protection
|
ANSI Device
Number
|
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A
|
The differential relay operates
whenever there is a difference between the currents going into and coming out
of any of the three phases.
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Generator Short Circuit Protection
|
87
|
B
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The result of a breakdown of the
insulation in one of the phase windings
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Generator Ground Fault Protection
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64
|
C
|
Can be caused by tripping of a field
breaker or by short circuits in the field (rotor) windings
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Generator Loss of Field Excitation
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40
|
D
|
It can happen when there is not enough
steam flow to the turbine to drive the generator
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Generator Motoring Protection
|
32
|
E
|
The relay trips the feeder breaker if
an overcurrent condition occurs
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Bus Overcurrent Protection
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51
|
F
|
If an undervoltage condition occurs,
the relay trips the circuit breakers to the loads on the bus that could be
damaged by the undervoltage condition.
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Bus Undervoltage Protection
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27
|
G
|
If a ground occurs, the relay senses
it and operates an alarm to alert operating personnel
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Bus Ground Detection and Protection
|
64
|
H
|
If a fault occurs in the transformer,
current flow through the current transformers becomes unbalanced.
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Transformer Differential Protection
Relay
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87
|
Electronic Protection Relays
Nowadays, most of the protection relays used are electronic type due to its greater precision and allow closer system coordination. Also, the accuracy of solid state relay is greater than the electromechanical relays.
One of the great features of solid state relays is it keeps the history of the operation. And when the relays are networked together, it can be synchronized to a master clock and all significant events can be recorded.
Communication
The electronic protection relay can be connected with the use of RS232 or RS 485 connections with a local computer. These can be used for relay configuration, monitoring and troubleshooting. Most especially, the relays now provide Ethernet capability for networking. Communication between relays in the power system makes it possible to exchange input and outputs thru a communication link, thus reducing the amount of hard wiring.
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| Protection Relay Communication |
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