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

The remaining slots (# 2~# 6, # 8) are reserved for expansion options (such as overcurrent, ground fault, etc.), but the local differential protection function of MDR-2 does not need to be expanded.

2. Relay output and digital input

5 relay outputs (R1~R5): Each relay provides NO/NC/COM contacts with a rated capacity of 250V AC/8A or 24V DC/1A. These relays can be associated with different alarm events such as differential warning, differential tripping, and battery undervoltage through parameter configuration, and can be used in combination. For example, R1 can be used to trip circuit breakers, and R2 can be used to trip AVR or shutdown solenoid valves.

Status Relay: Located at terminals 3-4-5, it is a normally closed (NC) type that reflects the status of the processor watchdog. During normal operation, the relay is engaged (NO closed) and released in case of a fault (NC closed), which can be connected to the safety circuit.

Digital input (DI1~DI5): The input voltage range is 6~32V DC, with an impedance of 2.4k Ω and bidirectional optocoupler isolation. DI1 is predefined as "Id trip inhibition" and is used to temporarily lock differential protection under specific operating conditions (such as grid connection instant); DI2 is used for "Alarm acknowledgement", while DI3~DI5 can be customized by users (through software configuration).

3. Wiring method of current transformer

MDR-2 supports two CT coupling modes, Star and Delta, to adapt to the winding connection of generators/motors:

Star coupling: suitable for star windings with a neutral point directly grounded. Each phase CT is connected to the corresponding primary and secondary inputs, and the neutral point can be grounded to either end of S1 or S2.

Triangular coupling: For triangular windings, it is necessary to convert phase currents into line currents. At this time, the rated current setting should be the nameplate current divided by √ 3. When wiring, also pay attention to the consistency of S1/S2 polarity.

Important engineering reminder: The polarity of CT (S1 and S2) must be strictly consistent - all primary and secondary CT terminals with the same polarity must be connected to the corresponding S1 terminal, otherwise it will cause differential current misoperation. After the wiring is completed, the polarity correctness should be verified by checking the current and differential current values of each phase through the display menu under no-load or light load.

Protection characteristics and parameter tuning: customized action curve

The differential protection of MDR-2 adopts a dual slope ratio braking characteristic, which is a classic algorithm for modern differential protection. It effectively prevents misoperation caused by unbalanced current due to CT saturation caused by external short circuit by introducing "steady current (Is)" as the braking quantity.

1. Principle of action characteristics

Horizontal axis (X-axis): Stable current Is (approximately the average of primary side current I1 and secondary side current I2), expressed as a percentage of rated current IN.

Y-axis: Differential current Id, expressed as a percentage of rated current IN.

The characteristic curve consists of two folded segments:

Low braking zone (when X is small): The differential action threshold is low, used to detect internal minor faults.

High braking zone (when X is large): As the external fault current increases, the action threshold rises to prevent misoperation.

The curve is determined by three key set points:

SP-X1: Turning point abscissa, range 50%~300% (relative to IN).

Y1: Action threshold for low braking zone, ranging from 5% to 45%.

Y2: Action threshold for high braking zone, ranging from 25% to 95%.

2. Independent setting of warning and tripping

MDR-2 provides two independent sets of curve parameters:

Warning (Curve 1): Y1W and Y2W can be set, and delay (0.01~2.00 seconds) and associated relays can be independently configured. Warning is usually used to remind operation and maintenance personnel to pay attention and not to trip directly.

Trip (Curve 2): Set Y1T, Y2T, and share the same SP-X1 with the warning. The trip delay can also be independently set.

On site tuning suggestions:

The warning curve is usually set to be more sensitive than the trip curve (with a slightly lower Y value) to provide early warning.

The delay should not be too long, generally recommended to be between 0.1 and 0.5 seconds to ensure rapid fault removal and avoid transient imbalance at the moment of closing.

If the system is particularly sensitive to CT saturation, SP-X1 can be appropriately increased (e.g. 200%~300%) to increase the braking area.

3. Fixed limit tripping

In addition to the ratio braking feature, MDR-2 also provides a fixed limit trip, which means that when the differential current is ≥ 100% IN, it will immediately trip according to the set delay. This serves as a backup protection and can quickly operate under high fault currents. Its delay is independently set in menu 1031.

4. Output relay configuration

Each protection event (warning, trip, fixed limit) can select up to two relay outputs (output A and output B). For example: