DEIF MDR-2 Differential Relay: Engineering Guide for Generator Differential Protection

Warning output → R4 (for alarm indication)

Trip output A → R1 (trip generator circuit breaker)

Trip output B → R2 (trip AVR or stop diesel engine)

This flexible configuration ensures a perfect match between the execution mechanism and the protection action logic.

Local operation and parameter programming: convenient configuration without the need for a PC

MDR-2 is equipped with a front display unit (optional remote installation), which enables complete parameter viewing, alarm confirmation, and tuning operations through 4 buttons and a 2-line 20 character LCD display screen, without relying on PC software, greatly facilitating on-site debugging and emergency modifications.

1. Display content and button functions

Measurement value viewing: It can alternately display the three-phase primary side current, secondary side current, differential current, and stable current (all in percentage and actual value).

Alarm and event logs: 150 timestamp event records can be viewed.

Key operation: Move the cursor with the up, down, left, and right keys, enter the menu with the SEL key, and switch the display content with the VIEW key.

2. Parameter tuning process

Taking Y1T for modifying the trip curve as an example:

In the default display interface, move the cursor to the "PROT" (protection menu) below and press SEL to enter.

Select '1020 Diff. current trip' to enter the tuning menu.

Move the cursor to "Y1T" and press SEL to enter numerical editing (password required, factory default, please refer to the manual).

Use the up and down keys to adjust the value, then move the cursor to "SAVE" and press SEL to save.

All parameters are protected by a three-level password and can be accessed through the "JUMP" button to enter the password modification menu (4971).

3. Other important settings

Status LED indicator (4301): If set to ON, when the protection is activated but the delay has not yet expired, the LED will flash yellow (indicating that an alarm is about to occur) and turn red (tripping) after the delay. This helps predict faults.

Automatic alarm confirmation (4312): can be set to automatic confirmation to avoid misoperation caused by alarm holding.

Service menu (4980): Enter through the JUMP key to view the remaining time of the alarm timer, real-time status of digital inputs and relay outputs, making it a powerful tool for debugging.

Installation, debugging, and troubleshooting

1. Mechanical installation

The MDR-2 is installed on a base (4 M6 screws) or DIN rail, weighing approximately 1kg. The panel opening size can be found in the manual, and the front panel IP54 (with sealing gasket) can meet splash protection requirements.

2. Pre power on checklist

Confirm that the auxiliary power supply voltage is 12/24V DC and the polarity is correct.

Confirm that there is no open circuit in the CT secondary circuit and that the S1/S2 polarity is consistent across all CTs.

Confirm that all relay outputs are correctly connected, especially the trip circuit should be connected in series with necessary safety contacts.

Confirm the voltage type and level matching of digital inputs (such as trip suppression).

3. Power on and functional verification

Turn on the DC power supply and observe that the "Power" LED lights up and the "Self check OK" LED lights up (indicating internal normal operation).

Read the current values of each phase through the display menu, which should be close to the actual load current. If there is a significant difference, check the CT ratio or wiring.

Check the differential current value, which should be close to 0 under normal load (usually within ± 5% IN). If the difference is significant, check the polarity or CT ratio setting.

Simulate differential fault: An adjustable resistor can be connected in series in one side of the CT secondary circuit to artificially generate unbalanced current, verifying whether the warning and tripping actions are correct.

4. Common faults and countermeasures

Possible causes and solutions for the phenomenon

After power on, the "Self check OK" does not light up. If there is an abnormal power supply or internal fault, check the power supply voltage. If it is normal, it needs to be repaired

The differential current has been consistently high for a long time, and the CT polarity is inconsistent, the transformation ratio is not matched, or the wiring is incorrect. Check the wiring phase by phase using the displayed value and adjust the CT transformation ratio setting

Improper setting of stable current for tripping misoperation (SP-X1 too low), increasing SP-X1 value, and increasing braking zone

Protection misoperation during external short circuit, CT saturation causing imbalance. Check if CT specifications are appropriate and increase Y1T or extend delay appropriately

Alarm cannot confirm digital input wiring or fault persists. Check DI2 input signal or manually confirm through display