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25 - Synchronism-Check Relay

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25 - Synchronism-Check Relay


Description

This block models a generic 25 synchronism-check or sync-check relay. The primary application of this relay is in situations that require verification that synchronism exists prior to closing a circuit breaker. The model also includes the Voltage Check function which can be used for closing the circuit breaker on de-energized networks. The model works for 50 Hz or 60 Hz system.

Table of Contents

The figure below shows the tripping logic of the relay.


Details on the Synchronism-Check Model Functionality

Sync-check relay is used for checking the conditions at circuit breaker closing. The purpose of the sync-check function is to find the instant when the voltages on both sides of the circuit breaker are in synchronism. The schematic for the relay operation is shown below:

The relay verifies that Bus frequency, voltage magnitude and phase angle match the Line frequency, voltage magnitude and phase angle before allowing the circuit breaker to be closed. The relay uses the a three-phase Phase Lock Loop (PLL) to calculate the bus and line frequencies. Also, the model uses fundamental Phase A to Ground voltage as the operating quantity. The relay has a freezing time to avoid misoperation when the simulation starts. A breaker operation time can also be defined to take into account the breaker operation delay.

The relay works in both energized network and de-energized network, which will be discussed individually in the following sections.

Live Bus and Live Line

The function will compare the values of the bus and line voltage with the set values for VMax which is a supervision to check whether both the bus and the line are live. 

Further, the Bus and Line voltages can be static or slipping with respect to each other:

  1. When the voltages are static:

    If the magnitude of the frequency difference/slip frequency (fBus – fLine) is less than or equal to 0.01 Hz—the relay presumes that voltages Bus and Line are “static” (not ‘slipping’ with respect to each other).

    In this case, if the voltage magnitude difference and angle difference are within the threshold bounds for a certain time i.e. Ton delay (internally pre-set as 1 power cycle),                                                                                                         

    then the network voltages are considered to be synchronized and a close command to the CB is delivered. (Output CloseCB will become high).                                                                                                                                             

    Note: If the voltages are static, breaker operate time will not be considered while issuing the CloseCB command.                                                                                                                                                                                                                          

  2. When the voltages are slipping: 

    If the slip frequency magnitude is more than 0.01 Hz—the relay presumes that the voltages are not static and are slipping with respect to each other.

    In this case, if the condition that slip frequency is within the threshold limit fails, the CloseCB command will not be asserted.