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

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

Introduction: Differential protection - the ultimate defense against internal faults in generators

In the protection system of generator sets and large electric motors, differential protection is known as the "ultimate defense line" against internal short-circuit faults. When there is a turn to turn short circuit, phase to phase short circuit, or grounding fault in the winding, the fault current can cause serious thermal damage and mechanical stress in a very short period of time, and even lead to equipment scrapping. Traditional overcurrent protection often requires a long delay due to its coordination with the system, and cannot provide sufficiently fast protection. Differential protection is based on Kirchhoff's current law - comparing the vector difference of current flowing into and out of the protected equipment, it can quickly act within half a cycle of the fault occurrence, minimizing damage.

The DEIF MDR-2 differential current relay is a specialized protection device designed specifically for differential protection of generators and motors. As a member of the Multi line 2 product family, MDR-2 integrates precise current measurement, customizable differential action characteristics, flexible relay output, and intuitive local display and programming interface, widely used in ship power systems, land power stations, and industrial drive fields. This article will provide a systematic engineering interpretation of the technical principles, hardware architecture, wiring specifications, parameter tuning methods, and on-site debugging points of MDR-2 based on the official technical manual, providing practical technical references for protection engineers.

Product positioning and technical specifications: Differential protection for harsh industrial environments

MDR-2 is a differential current monitoring and protection device based on microprocessor technology. Its design core is to quickly and reliably detect small differential currents inside generators or motors, and issue warnings or trip commands before they develop into serious faults.

1. Core protection principle

MDR-2 measures the three-phase current on both sides (neutral point side and outgoing line side) of the protected equipment and calculates the differential current (Id) phase by phase. During normal operation or external faults, the currents on both sides are basically equal, and the differential current is close to zero; When an internal fault occurs, the current on both sides is no longer balanced and the differential current increases. The device determines whether to trigger a warning or trip based on the preset action characteristic curve and delay.

2. Key technical specifications

Accuracy: Within the range of 0.1 × IN to IN, the measurement accuracy is 1% IN; when it is greater than IN, the accuracy is 1% of the measured value (IN is 1A or 5A, secondary side value). This accuracy ensures the reliability of the setting value.

Response time: The action response time of differential protection is less than 50ms (calculated from the end of the measured current cycle), achieving rapid fault removal.

Wide working range: The auxiliary power supply supports 8-36V DC (nominal 12/24V) and can withstand a voltage drop of 10ms (from 24V to 0V). The working temperature range is -25 ° C to+70 ° C, meeting the environmental requirements of the cabin and outdoor cabinets.

Measurement input: Suitable for 1A or 5A current transformers (CT), with a maximum power consumption of 0.3VA per phase and low requirements for CT load.

Electrical isolation: The isolation voltage between AC current input and other I/O can reach up to 3250V AC (50Hz, 1 minute), ensuring personal safety and equipment anti-interference ability.

Electromagnetic compatibility: Compliant with EN-61000 series and IEC 255-3 standards, suitable for strong electromagnetic interference environments in ships and industrial sites.

Protection level: shell IP40, terminal IP20, front panel optional IP54 (with sealing gasket installation), meeting the installation requirements inside the cabinet.

3. Certification and Materials

All plastic components meet the UL94 V1 self extinguishing rating to ensure fire safety. The system complies with HSE climate rating (DIN 40040) and can withstand high humidity and condensation environments.

Hardware architecture and wiring specifications: Building reliable physical connections

MDR-2 adopts a modular rack design, with a clear hardware structure that is easy to install and maintain.

1. Slot allocation and functional modules

The MDR-2 chassis is divided into 8 slots, of which the following slots have been defined:

Slot # 1: Power Supply and Digital I/O (Standard Configuration). Includes DC power input, status relay (normally closed for processor monitoring), 5 configurable relay outputs (R1~R5), and 5 digital inputs (DI1~DI5). Digital input is used for functions such as differential trip suppression and alarm confirmation.

Slot # 7: AC current measurement (standard configuration). Contains 6 sets of CT input terminals, corresponding to the primary side currents (I1L1, I1L2, I1L3) and secondary side currents (I2L1, I2L2, I2L3) of the three phases. Each set of terminals is labeled with S1 (k) and S2 (l), used to connect the secondary winding of CT.

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:

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