For the measurements to be correct it is important to make sure that the measurement signals are
connected to correct inputs and direction of voltages are connected right and the scaling is set
correctly.
Relay calculates scaling factors based on the set VT primary and secondary voltage values. Relay
measures secondary voltages which in this case mean the voltage outputs from the voltage
transformer that is installed into the primary circuit of the application. Voltage can be measured up to
400V system directly as well. For the relay to “know” primary and per unit values the voltage
transformer rated primary and secondary voltages need to be set. In modern IEDs like the scaling
calculation is done internally after the voltage transformer primary and secondary voltages are given.
Normally the primary line to line voltage rating for voltage transformers between 400V and 600kV while
normally secondary voltage ratings are 100-120V. Non-standard ratings can also be directly connected
since the scaling settings are flexible in large ranges.
VT scaling example
The connection of VTs to the IED measurement inputs and the ratings of the voltage transformers are
as in following gure. In gure below line to neutral voltages are connected among with zero sequence
voltage. Other connection possibilities are presented in this chapter.
Figure. 5.2.2. - 10. Example connection with three line to neutral voltages and zero sequence voltage connected. 3LN+U4 mode
has to be selected. U4 channel has to be set as U0.
Initial data of the connection and the ratings are presented in following table.
Table. 5.2.2. - 22. Initial data of previous example connection.
Phase voltage VT:
VT primary 20000V
VT secondary 100V
Zero sequence voltage VT:
U4 VT primary 20000V
U4 VT secondary 100V
Zero sequence voltage is connected similar way with Line to neutral voltages (+U0).
In case of incorrect wiring all polarities can be switched individually by 180 degrees in IED.
If voltage based protection is used the supervised voltage may be based on line to line –or line to earth
voltages. This selection is completed in each protection stage menu separately.
AQ-T257
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
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