Other event record registers - 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

Sequence number

Every Modbus event record is given a sequence number. The sequence number runs from 1 to 65535 and then rolls over to 1 again. The client can check that the sequence numbers of the recorded data are sequential. During the event buffer overflow the client can notice a jump in the sequence numbers when some event records are lost. The gap between the new and the previous sequence number reveals how many event records have been lost.

Unread records left

This register shows how many unread event records still remain for the client in question at a particular moment.

Time stamp registers

Time stamp is either in local time or UTC time. The time stamp alternative is selected with a Modbus parameter.

Time stamp registers usually hold two data values in the high and low byte of the registers. High byte value = RegisterValue DIV 256, Low byte value = RegisterValue MOD 256. The Milliseconds register is an exception as it contains the milliseconds 0...999 coded as such.

Event type

This register contains information to interpret the event data correctly.

Event time stamp format bit 15

The time stamp format can be selected with a Modbus parameter via the HMI or Parameter Setting.

Event time stamp source bit 14

The time stamp can be generated by the protection relay application (accurate time) or by Modbus. If generated by Modbus, the change values are detected by the Modbus background scan task. Since there is a latency time between the value change and the time when Modbus detects the change, the time stamp is not accurate in this case.

Clock not synchronized bit 13

The quality information bit is set in the protection relay's real-time clock if the protection relay has not been synchronized.

Clock failure bit 12

The quality information bit is set in the protection relay's real-time clock if the clock has a severe failure. Do not rely on this time stamp.

Data object ID bit 8

The coding alternatives of the data object ID registers 1 and 2 are the Modbus address or UID. The coding alternatives cannot occur simultaneously in the protection relay but are selected and configured at the system setup phase. The default setting is "Modbus address".

The UID code is 32 bits wide and occupies both registers 9259 and 9260. The word order is high/low. The UID code refers to the functional design of the protection relay platform in which the object resides. Shortly, it means that the UID code is equal in all the platform protection relays in which the same functional design and the same Modbus object is used.

Data value identification bits 5..0

Coding of the event data value is one bit, two bits or 32 bits. The coding depends on the IEC 61850 common data class which is the origin of the Modbus data in question.

The original SEC cnt attribute is actually defined as a 32 bit counter. The Modbus event shows the least significant 16 bits of that counter, that is 0...65535.