The Key to Relay Protection Success: Cooperation Part II
In the first installment of this series, we examined the dependency of relay scheme on other piece of substation equipment, such as current transformers and circuit breakers. In Part II our series we look at how batteries, potential transformers/CCVTs and communication links contribute to the correct operation of the relay protection systems.
The battery system plays a vital role in completing the sequence of tripping circuit breakers when called upon by the protective relaying scheme. The trip coil that is responsible for tripping the circuit breakers is energized when DC voltage is provided to it and the protective relays provide the permissive. If relays provide the permissive but no DC power is available, the trip coil will not energize, and the breaker will not trip. Batteries are a crucial link in the chain of events and must be tested at regular intervals to ensure integrity.
To ensure the battery system is operational and will comply within specified parameters, the following tests are recommended:
- Intercell resistance
- Voltage (Battery float and cell float)
- Specific gravity
- Discharge testing
- Infrared scanning
- Capacity testing
The impact of potential transformers both inductive and capacitive on relay performance is important. However, PTs/CCVTs do not undergo the type of duty that CTs undergo and the performance of CTs has a much greater impact than those of PTs/CCVTs. During short circuit faults, the high current imposes extreme pressure on CTs. On the other hand, the voltage during short circuit faults is depressed and hence the PTs do not experience any stress and continue performing as usual.
The tests recommended are:
- Voltage ratio
- Ratio and phase angle error
- Insulation resistance
- Winding resistance
- Burden check
- Power factor Test
These tests to be conducted and related maintenance interval are listed in the maintenance procedures. In general, the maintenance interval for PTs/CCVTs are much longer than those for CTs. The Vanguard CVT-765 and Doble M4100 deliver the testing capabilities for testing PTs and CCVTs.
Communication is a critical link in several protection schemes. In high-speed communication-assisted protection schemes there are communications between the two substations connected by a transmission line. The relaying at each end depends on information received from the other end to operate correctly. The medium used for communication can be fiber optic, power line carrier, microwave, etc. with transmitters and receivers installed at each end. To test such relay schemes utilities conduct end-to-end testing to ensure the scheme works appropriately and this testing includes both the transmitters and receivers.
As was mentioned in the “Relay” section of Part I of this blog, in a digital substation designed per the IEC 61850 standard, the IEDs communicate with each other via GOOSE (Generic Object-Oriented Substation Event) and Sampled Value (SV) protocols. GOOSE/SV is a Layer 2 protocol, i.e. messages are transported over Ethernet. In a conventional station, communication between IEDs is accomplished via point-to-point wiring. Hence, communication assumes a much bigger role in a digital substation and the relay protection scheme relies heavily on proper communication. Testing such protection schemes involves extensive involvement of debugging the communication network.
The Doble F6150 Power System Simulator, Protection Suite/IEC Testing software will provide all the necessary tools to conduct an effective end-to-end testing and testing of protective schemes in a digital substation.
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