Primary and per-unit values - Arc protection - Auto synchronization - Bay control and measurement - Merging unit - Transformer protection - 2 winding - Feeder protection - Voltage regulation - Capacitor bank protection - Petersen Coil control - Grid automation - Busbar differential protection (high impedance) - Back-up protection - Motor protection - Busbar protection (voltage and frequency) - Transformer protection - 3 winding - Interconnection protection - Generator protection - Power management/Load shedding - Modbus Communication Manual - REX640 Protection and control - PCL4 - IEC - ANSI - 16.02.2023

REX640 Modbus Communication Protocol Manual

Measurands originating from CT or VT measurements can be obtained from the protection relay in two ways. They can be viewed either as primary values or as per-unit values.

The primary values are represented internally as decimal numbers. The primary units are [A] for current and [kV] for voltage. The internal representation of the per-unit values is always 1.0 at nominal current or voltage. A typical range for a per-unit value is 0.00...40.00, that is 0 to 40 times nominal. With Communication Management in PCM600, the user can select how these values are presented in the Modbus register. It may be necessary to upscale or downscale the primary values to fit the register's 16-bit integer value. The register's scale factor and offset parameters can be used for this purpose. By default, this protection relay shows per-unit values multiplied by the scaleFactor 1000.

Note: If the primary value representation is selected but no CT or VT ratio parameters are configured in the protection relay, the Modbus values remain as per-unit values. Check the protection relay configuration to find out the CT or VT ratio being used.
Note: If scaling of primary values is used, the protection relay must be rebooted if the CT/VT ratio settings are changed. Otherwise, Modbus continues using the old CT/VT settings in its internal scaling algorithms.