ABB Relion 620 Series Protocol Manual

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ABB Relion 620 Series Protocol Manual Manual pdf 34 page image

3.4.5.2 Primary and per-unit valuesMeasurands originating from CT or VT measurements can be obtained from the IEDin 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 unitsare [A] for current and [kV] for voltage. The internal representation of the per-unitvalues is always 1.0 at nominal current or voltage. A typical range for a per-unit valueis 0.00...40.00, that is 0 to 40 times nominal. With CMT the user can select how thesevalues are presented in the Modbus register. It may be necessary to upscale ordownscale the primary values to fit the register's 16 bit integer value. The register'sscaleFactor and offset parameters can be used for this purpose. As a default, this IEDshows per-unit values multiplied with the scaleFactor 1000.If the primary value representation is selected but no CT or VT ratioparameters are configured in the IED, the Modbus values remain asper-unit values. Check the IED configuration to find out the CT or VTratio being used.If scaling of primary values is used, the IED must be rebooted if theCT/VT ratio settings are changed. Otherwise, Modbus continuesusing the old CT/VT settings in its internal scaling algorithms.3.4.5.3 Register sizesIn most cases the measurands or counters are located in single 16 bit registers. Themeasurands are either unsigned or signed two's complement values while the countersare always unsigned values.In some cases the measurands or counter values can be located in two consecutiveregisters, thus forming a single 32 bit integer value. The 32 bit value is always codedso that the high word part, that is, the higher 16 bits, is located first in this registeraddress. The low word part, that is, the lower 16 bits, is then always in the next registeraddress.Register sizes and types are clearly stated in the Modbus memory map list.3.4.5.4 Register saturationAfter a re-scaling operation the Modbus value may exceed the limit of the Modbusregister representation. The Modbus value then saturates to the closest max or minvalue of the register size in question.Section 3 1MRS757645 BVendor-specific implementation28 620 seriesCommunication Protocol Manual