Requirements of Line Protection Relay Commissioning

Bilateral (trilateral) protection scheme between any generation station or switching station and the utility station(s) on the other side of the utility transmission line(s) is called tele-protection and is achieved through identical line protection relays at the stations.

In order to ensure compatibility between the stations, it is important that not only the make and model of the relays but also the firmware revision number and its boot revision number is the same, since the boot revision number might be different from the firmware revision number.

The single line diagram, the DC schematics of the line protection relays, as well as a copy of uploaded files to the relays are to be sent to the utility company for approval. At the end of commissioning the as left files shall also be sent to the utility for their approval and records.

Prior to direct transfer trip (DTT) testing, the communications equipment and devices are to be installed and commissioned.  BERT tests (Bit Error Rate Test) are to be conducted to make sure the communication channels are working properly. Then an end to end testing would wrap up the communication tests. 

It is also important to verify that the communication medium and all communication devices such as multiplexers are:

• Capable of working at the data transfer (bit rate) required by the protection relays.
• Have the same fiber optic wavelength.
• All are single mode or multi-mode.

A BERT typically consists of a test pattern generator and a receiver that can be set to the same pattern. They can be used in pairs, with one at either end of a communication chanell, or singularly at one end with a loopback at the remote end.

A loopback test is a test used for debugging physical connection problems in communication channel components in which a signal in sent from a communications device and returned (looped back) to it as a way to identify a failing node in a network. A comparison of the returned signal with the transmitted signal conveys the integrity of the transmission path. One type of loopback test is performed using a special plug, called a Wrap Plug, that is inserted in a port on a communications device.

A wrap plug, also known as a loopback plug, is a special plug that can be inserted into a port on a communications device to perform a diagnostic test called a loopback test. The effect of a wrap plug is to cause transmitted (output) data to be returned as received (input) data, simulating a complete communications circuit using a single computer. There are numerous possible configurations, depending on the hardware and the nature of the test to be performed.

The purpose of the communications channel is to transmit information about the system conditions from one end of the protected line to the other, including requests to initiate or prevent tripping of the remote circuit breaker. The former arrangement is generally known as a 'transfer tripping scheme' while the latter is generally known as a 'blocking scheme'. 

DTT Tests are generally done in two phases:

• Dead Zone Transfer Trip, DZTT
• Live Zone Transfer Trip, LZTT

DZTT is the set of tests done before the energization of the station. During DZTT the trip situations are simulated and signals are sent from either side to verify the intended operation is initiated on the other side. This would verify that the transfer trip equipment are effectively talking to each other and can operate in live conditions.

LZTT is the set of tests implemented after the energization of the station. The signals sent during LZTT actually trip the circuit breaker on each side of the line and de-energize the station and the transmission line.

References:

•          Communication Networks NSD570 – Teleprotection Equipment, ABB
•          SEL-2595 Teleprotection Terminal
•          https://en.wikipedia.org/wiki/Loopback
•          http://searchnetworking.techtarget.com/
•          https://www.techopedia.com/
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