Primary And Per-Unit Values; Register Sizes; Register Saturation - ABB Relion 620 Series Protocol Manual

Section 3
Vendor-specific implementation
3.4.5.2
3.4.5.3
3.4.5.4
28

Primary and per-unit values

Measurands originating from CT or VT measurements can be obtained from the IED
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 CMT 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
scaleFactor and offset parameters can be used for this purpose. As a default, this IED
shows per-unit values multiplied with the scaleFactor 1000.
If the primary value representation is selected but no CT or VT ratio
parameters are configured in the IED, the Modbus values remain as
per-unit values. Check the IED configuration to find out the CT or VT
ratio being used.
If scaling of primary values is used, the IED 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.

Register sizes

In most cases the measurands or counters are located in single 16 bit registers. The
measurands are either unsigned or signed two's complement values while the counters
are always unsigned values.
In some cases the measurands or counter values can be located in two consecutive
registers, thus forming a single 32 bit integer value. The 32 bit value is always coded
so that the high word part, that is, the higher 16 bits, is located first in this register
address. The low word part, that is, the lower 16 bits, is then always in the next register
address.
Register sizes and types are clearly stated in the Modbus memory map list.

Register saturation

After a re-scaling operation the Modbus value may exceed the limit of the Modbus
register representation. The Modbus value then saturates to the closest max or min
value of the register size in question.
1MRS757645 B
620 series
Communication Protocol Manual