The protection relay is internally modeled according to the IEC 61850 standard. The Modbus Master protocol is implemented on top of this model.
The Modbus protocol standard defines one-bit digital data and 16-bit register data as RTU application data alternatives. The protocol does not define how this protocol application data should be used by a protection relay application. The usage depends on system specific application needs.
The relay provides a set of IEC 61850 data objects or function designs to which both measurements and one-bit data polled from Modbus Slaves can be mapped to. These data objects can then be read from a SCADA system over the IEC 61850 or other protocols available in the relay.
The IEC 61850 objects available for mapping polled data are listed in Table 1.
| Name | Description | Number of instances | Type | CDC | Min/Max input values Note |
|---|---|---|---|---|---|
| MMVGAPC(x) | Received Modbus binary value information | 2 | 8-input/output binary type | SPS – Single Point Status n | 0 – False / 1 - True |
| MMVI4GAPC(x) | Received Modbus 32-bit integer value information | 5 | 4 input/output 32-bit integer type | INS – measurement 32-bit Integer value | -2000000.0 / 2000000.0 * |
| MMVF4GAPC(x) | Received Modbus measured value information | 10 | 4-channel 32-bit float type | MV – measurement 32-bit float value | -2147483648 / 2147483647 * |
* Values outside these ranges saturate the input to min or max values and affects the quality of the mapped signal.
| Name | Type | Values (Range) | Unit | Description |
|---|---|---|---|---|
| Reset counters | BOOLEAN |
0=False 1=True |
Reset counters | |
| Received frames | INT32 | -1...2147483647 | Number of received frames | |
| Transmitted frames | INT32 | -1...2147483647 | Number of transmitted frames | |
| Checksum errors | INT32 | --1...2147483647 | Number of frames that are discarded because of incorrect checksum | |
| Received exc A | INT32 | -1...2147483647 | Number of exceptions with code 01 or 02 | |
| Received exc B | INT32 | -1...2147483647 | Number of exceptions with code 03 | |
| Connection errors | INT32 | -1...2147483647 | Number of unexpected communication disconnections |
Dataset and Report Control in IEC 61850
Data update rate is dependent of data set and report control engineering in IEC 61850 tool. In most cases default values are suitable, as typically essential
measurement and indication data are by default mapped to IEC61860 dataset and report control block. However, it may be necessary to understand all dependencies, if some customization or modification to update rates is needed:
- Data set content
- Trigger options for report control
- Signals selected in communication management (instantaneous or deadband supervised value)
Measurement data
Polled Modbus measurement data is included in a IEC61850 dataset and can be cyclically updated. The update rate is determined by the integrity period IntgPd (see IEC61850 Engineering Manual) of the report control block. There is no deadband setting for these measurements.
Indication data
Commonly used indication data, such as protection start/trip and other important values are by default included in a IEC61850 dataset. If the protocol has CMT mappings of indication data that does not belong to a IEC61850 dataset, the protocol adds this data automatically to an internal update mechanism that enables still a fast update for the time-critical events.
Control operations
This protection relay does not support control operations to slave units.
Application data compatibility
This protection relay is designed to operate with a wide range of Modbus slaves spanning from battery chargers to energy meters. The application solutions have been chosen to achieve the highest possible level of compatibility with the systems.
- Application data is polled from different Modbus memory areas. Digital data is readable as packed bits in registers.
- Primarily 16-bit register sizes are used for measurands; however, 32 bits are used only in some cases.
- The measurands can be freely rescaled by the user in the communication management tool.
- The addressing of the application data in the documentation and tools follows the so-called Modbus-PLC addressing principle, where the base address 1 is used. The assignable application data addressing for slaves in this relay spans between 1 and 65535.
- The mapped data from slaves is assigned to the respective IEC61850 object signals. At the same time, these signals can be mapped to another protocol e.g. Modbus Slave, DNP3.0 and a SCADA system can then scan or poll these.
Quality update for measurement and indication data
Modbus Master objects or function designs’ signals quality is updated in the following cases:
- Good Quality
- The mapped slave unit(s) replies with the requests within the set timing constraints and its data value doesn’t saturate the input limits in the function design.
-
Bad/Old Quality
- The quality status of the Modbus Master objects’ signals will change to BAD when
- The mapped slave is unable to reply to the master within the set delay constraints, e.g. the slave is physically disconnected from the bus or shutdown.
- The value received saturates the minimum or maximum value conditions of and object’s signal
- The quality status of the Modbus Master objects’ signals will change to BAD when