Thytronic DMC926
Residual overcurrent relay for the medium voltage neutral in networks with a Petersen coil.
The Thytronic DMC926 is a residual overcurrent relay that monitors the current in the Petersen coil of medium voltage networks in primary substations and signals earth faults. This Modbus RTU template reads the residual current, the start and trip counters, the event log and the self-test status, and lets you set the threshold and operate time and send store, reset and test commands from ModbusCloud.
- Protocol
- Modbus RTU
- Connection
- RS485, 9600 baud
- Registers
- 30, read and write
- Brand
- Thytronic
- Key readings
- Residual current · Start counter · Trip counter · Event log · Self-test status
- Available commands
- Io> threshold · Operate time · Relay pulse times · Store and reset commands · Protection test
- Compatible models
- DMC926
- Last updated
Registers (30)
24 read, 12 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| Residual Current Io | Read | ||||
| Start Count Io> | Read | ||||
| Trip Count Io> | Read | ||||
| Self-Test Level | Read | ||||
| Settings Command | Write | ||||
| Reset Command | Write | ||||
| Test Command | Write | ||||
| Recorded Data Default Command | Write | ||||
| Io> Pickup (x Ion) | Read and write | ||||
| Io> Operate Time | s | Read and write | |||
| Relay K1 Minimum Pulse | s | Read and write | |||
| Relay K2 Minimum Pulse | s | Read and write | |||
| Measurement Reading Mode | Read and write | ||||
| Rated Residual Current Ion | Read and write | ||||
| Residual CT Primary Current | Read | ||||
| Serial Baud Rate | Read | ||||
| Login User | Read | ||||
| Login Password | Read | ||||
| Login Command | Write | ||||
| Logout Command | Write | ||||
| Set Clock Year | Read | ||||
| Events Stored | Read | ||||
| Event 1 Reading Mode | Read | ||||
| Event 2 Reading Mode | Read | ||||
| Event 3 Reading Mode | Read | ||||
| Event 4 Reading Mode | Read | ||||
| Event 5 Reading Mode | Read | ||||
| Event 6 Reading Mode | Read | ||||
| Clock Year | Read | ||||
| Device Identification | Read |
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Use cases
- Show the residual current of the Petersen coil on a dashboard and keep a year of history.
- Send an e-mail or webhook alert when the trip counter changes.
- Follow the start and trip counters to see how often earth faults occur on the network.
- Watch the self-test status to spot a relay fault early.
- Check the Io> threshold and operate time remotely and adjust them during commissioning.
Installation
Connect the front RS232 port of the DMC926 to an RS232 to RS485 converter, and wire the converter to the RS485 port of the ModbusCloud Gateway with 120 Ω termination.
Check the serial settings in the Communication menu of the relay (default 9600 baud, no parity, 1 stop bit).
In ModbusCloud, add a Modbus device on the RS485 port of the Gateway with address 1, 9600 baud, 8 data bits, no parity and 1 stop bit.
Import the Thytronic DMC926 template and check that the residual current and the counters come in.
Read Device Identification: the template fits a relay that reports DMC926_B.
Things to watch for
The Gateway gets no answer from the DMC926. What should I check?
The DMC926 only has a front RS232 port, so it needs an RS232 to RS485 converter between relay and Gateway. Check the converter wiring and that baud rate, parity and stop bits match the device in ModbusCloud (default 9600 baud, no parity, 1 stop bit). The relay menu has no address setting; the template uses address 1.
Which DMC926 does this template fit?
It fits the DMC926 that reports DMC926_B (34) as Device Identification and keeps six event records. The original DMC926, which reports DMC 926 (21), keeps eight event records, so its residual current and counters sit at other addresses and read wrong with this template.
The residual current does not match the relay. Why?
With the default Relative reading the relay reports the current as a multiple of the rated current Ion; with Direct reading it reports primary amps, calculated from the residual CT primary current. Check the reading mode and the Ion setting, which must match the dip switch on the rear.
I wrote a new threshold, but it is gone after a restart. Why?
The DMC926 keeps changed settings in temporary memory until it receives the store command. After writing, write Store to the Settings Command register, or Clear to discard the changes. Read a setting first before you write it, so you know the current value.
Can a test command operate the output relays?
Yes. Test Io> with output relays switches the start and trip contacts and can make the protection of the plant operate, so only use it during commissioning or maintenance with the network in a safe state. Test Io> LEDs only leaves the contacts alone. A test ends with Test off, or by itself after five minutes.
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