Beckwith M-3520 Interconnection Protection Relay

Function Analysis and Application Guide of Beckwith M-3520 Interconnected Protection Relay

With the large-scale integration of distributed energy (DER) into the distribution network, the protection requirements for interconnected lines are becoming increasingly complex. The distributed power grid interface not only needs to have conventional overcurrent and grounding protection, but also must be able to detect special working conditions such as islanding operation, abnormal power flow, frequency fluctuations, etc., and automatically reconnect to the grid when conditions are met. M-3520 Integrated Protection System launched by Beckwith Electric ® The digital interconnection protection relay designed specifically for this requirement integrates 21 protection functions, synchronization check, reclosing enable, and waveform capture capabilities, providing a complete integrated solution for DER interconnection. Based on the technical specifications, this article systematically analyzes the protection function system, key parameter tuning, and engineering application points of M-3520, providing reference for engineers engaged in distributed energy grid connection and distribution protection.

Product positioning and design philosophy

M-3520 is an interconnection protection relay based on microprocessor and digital signal processing technology, specifically designed for the point of interconnection between distributed generation (DER) and the power grid. Its protection functions cover three dimensions: network loss protection (detecting islanding operation), abnormal power flow protection (reverse power/over power direction discrimination), and fault backup protection (phase to phase and ground faults). Simultaneously integrating synchronous inspection, reclosing, and frequency change rate monitoring to meet the requirements of interconnection protection in grid standards such as IEEE 1547 and UL 1741.

The core design features include:

Comprehensive features: Standard configuration includes 18 protection functions, with optional options of 21 (phase to phase distance), 78 (out of step), and 81R (frequency change rate) to expand to 21 types.

Flexible configuration: Supports two configurations: standard protection system or basic system, and users can choose functional modules according to their needs.

High precision measurement: Voltage accuracy ± 0.5% or ± 0.5V, current accuracy ± 3% or ± 0.1A (5A CT), frequency accuracy ± 0.02Hz.

Powerful data management: 170 cycle oscilloscope recording, 32 target storage, event sequence recording, supports COMTRADE format waveform export.

Open communication: Supports MODBUS and BECO 2200 protocols, equipped with two RS-232 ports and one RS-485 port.

Detailed explanation of standard protection functions

2.1 Synchronization check and dead bus/dead line closing (25)

M-3520 is equipped with high-precision synchronous inspection function, which is used to monitor the closing conditions of circuit breakers. Its core parameters include:

Phase angle limit: 0 °~90 °, accuracy ± 1 °

Voltage upper and lower limits: upper limit 60~140V, lower limit 40~120V

Δ V limit: 1.0~50.0V, accuracy ± 0.5V

Δ F limit: 0.001~0.500Hz, accuracy ± 0.0007Hz or 5%

Synchronization check delay: 1~8160 cycles

Dead bus/dead line voltage limit: 0~60V, dead zone delay 1~8160 cycles

Synchronization check can operate as an independent function or be supervised by the 79 (reclosing) function. Support multiple "hot/dead" bus/line combination closing schemes, suitable for various scenarios such as conventional grid connection, black start, and system recovery. The extremely high Δ F resolution (0.001Hz) ensures reliable discrimination even under extremely low slip, avoiding false closing.

2.2 Voltage Protection

27- Phase Undervoltage

Two independent settings, each adjustable between 5-180V, with a delay of 1-8160 cycles. Used to detect system voltage drops, lock protection or initiate tripping under undervoltage conditions. When the VT wiring is line to place and needs to be converted to line to line voltage, the accuracy is ± 0.8V or ± 0.75%.

59- Phase Overvoltage

Two independent settings, adjustable from 5 to 180V. Used to detect system overvoltage and prevent distributed power sources from injecting excessive voltage into the grid, which may cause equipment damage or power quality exceeding standards.

59G/27G - Neutral point overvoltage/undervoltage

Detecting neutral point voltage offset for ground fault detection and system grounding status monitoring. The setting range is 5-180V, with a delay of 1-8160 cycles.

59I - Peak Overvoltage

The setting range is 1.05~1.50PU, specifically designed for ferromagnetic resonance detection. This function is based on instantaneous voltage peak response and can quickly identify overvoltage caused by ferromagnetic resonance, with a delay of 1~8160 cycles. For signals with severe harmonic distortion, the accuracy will decrease as the harmonic order increases (applicable to voltages below 180V).

2.3 Direction power protection (32)

Provide active power direction detection with dual setting values, with a setting range of -3.000~+3.000PU and an accuracy of ± 0.002PU or 2%. Can be configured as:

Positive Overpower (Positive Setting): Detect forward power exceeding the limit.

Reverse power (negative value setting): detects reverse power (suitable for anti islanding and anti backflow).

Underpower: The detected power is lower than the set value.

Single phase detection mode is optional (applicable to line ground VT wiring). In the application of distributed power grid connection, reverse power detection is one of the important means of anti islanding protection.

2.4 Negative sequence protection

46- Negative sequence overcurrent

Provide two modes of time limit and inverse time limit:

Time limit: Set to 0.10~20.00A (5A CT) or 0.02~4.00A (1A CT), with a delay of 1~8160 cycles.

Inverse Time: Supports Definite Time, Inverse, Very Inverse, Extremely Inverse, and IEC standard curves, with a time dial of 0.5-11.0 (or 0.05-11.0 IEC).

Negative sequence overcurrent is used to detect asymmetric faults and system imbalances, and is particularly important for rotor protection of rotating motors.

47- Negative sequence overvoltage

Two independent settings, adjustable from 5 to 180V, with a delay of 1 to 8160 cycles. Used to monitor system voltage imbalance and assist in distinguishing PT disconnection or system asymmetric operation.

2.5 Overcurrent protection

50- Instantaneous phase overcurrent

Set 1.0~240.0A (5A CT) or 0.2~48.0A (1A CT) with a fixed delay of 2 cycles. Used for quickly cutting off phase to phase short circuit faults.

50G - Instantaneous Neutral Overcurrent

Set 0.5~2400A (5A CT) or 0.1~48.0A (1A CT). Can be supervised by 67N (directional grounding) components to achieve directional instantaneous grounding protection.

51V - Voltage control/voltage suppression inverse time overcurrent

Inverse time overcurrent protection with voltage control (VC) or voltage suppression (VR) function. Set to 0.50~12.00A (5A CT), supporting multiple sets of inverse time curves. In voltage control mode, the lock is only released when the voltage is lower than the set value; In voltage suppression mode, the action value decreases linearly with the decrease of voltage. Suitable for load feeder protection in distribution networks, improving protection sensitivity when system voltage drops.

51G - Inverse time neutral point overcurrent

Set to 0.25~12.00A (5A CT) or 0.05~2.40A (1A CT), supporting Define Time, Inverse, Very Inverse, Extremely Inverse, and IEC curves. Can be supervised by 67N directional components.

2.6 Direction overcurrent protection (67/67N)

67- Directional phase overcurrent

Provide two modes: fixed time limit and inverse time limit. The directional component adopts positive sequence voltage polarization, with a maximum sensitivity angle (MSA) adjustable from 0 ° to 359 °, and has an 8-cycle fault memory function. The sensitivity at MSA is 0.5VA. Direction sensing can be independently disabled and is suitable for situations where direction discrimination is not required. When operating at high speed, the delay is set to 1 cycle and the response time is less than 1.5 cycles.

67N - Directional neutral overcurrent

Supports multiple polarization modes:

Zero sequence voltage polarization

Negative sequence voltage polarization

Current polarization (requires external current source)

Dual polarization (zero sequence+negative sequence combination)

Polarization methods 1, 3, and 5 are not applicable to line to line and line to ground to line VT configurations. When using polarization modes 4 or 5, MSA is not applicable and should be set to zero. Direction sensing can also be disabled.

2.7 Frequency Protection (81)

Four independently set over/under frequency protection, each segment can be independently selected as over frequency or under frequency. Setting range:

60Hz model: 50.00~67.00Hz

50Hz model: 40.00~57.00Hz

Accuracy: ± 0.02Hz within the range of 57~63Hz (60Hz model)/47~53Hz (50Hz model), ± 0.1Hz beyond the range

Delay: 2~65500 cycles

Distributed power sources must be disconnected within the specified time limit when the system frequency is abnormal, and the 81 function is a key protective component to achieve this requirement.

2.8 VT fuse breakage detection (60FL)

Detecting VT secondary circuit disconnection or fuse melting by comparing the positive and negative sequence components of voltage and current. After detecting VT disconnection, programmable locking of relevant voltage protection functions (such as 27, 59, 21, etc.) can be implemented to prevent misoperation. Wire breakage detection can come from internal logic or external input contacts, with a delay of 1~8160 cycles.

2.9 Re closing Enable (79)

Provide programmable reclosing delay function. When all contacts designated as trip outputs have dropped out, the reclosing timer starts and can issue a reclosing permission signal after a delay of 2-65500 cycles. This function works in conjunction with 25 (synchronous check) to achieve "checking synchronous reclosing" or "checking no pressure reclosing".

Optional protection function

For more complex application scenarios, M-3520 offers three optional protection functions:

21- Dual zone interphase distance protection

Suitable for backup protection of interconnected lines, providing two segments of M-O characteristic impedance protection. Each segment can independently set the circle diameter (0.1~100 Ω), offset (-100~+100 Ω), and impedance angle (0 °~90 °), with a delay of 1~8160 cycles.

78- Out of Step Protection

Based on the M-O characteristic, detect system oscillation or out of step. The key parameters include: circle diameter, offset, impedance angle, blind spot (0.1~50.0 Ω), delay, whether to trip when the M-O circle exits, sliding pole count (1~20 times), and sliding pole reset delay (1~8160 cycles). Suitable for safe disconnection in the event of power oscillation in interconnected systems.

81R - Rate of Change in Frequency (ROCOF)

Two independent settings, with a setting range of 0.10~20.00Hz/Sec and a delay of 1~8160 cycles. Support negative sequence voltage suppression (0-99%). ROCOF protection is an important means of active islanding detection, which acts quickly when the system frequency changes rapidly.

Data collection and communication capabilities

4.1 Oscillographic recording

The oscilloscope has a storage capacity of 170 cycles and can be configured with 1-4 partitions. The sampling rate is 16 times the rated frequency (800Hz for 50Hz system, 960Hz for 60Hz system). The trigger source can come from status input, trip output, or serial communication. Supports exporting COMTRADE format files for easy processing by third-party analysis software.

4.2 Target Storage

The 32 target records include: the type of action function, the activated functional component, the input/output contact status, timestamp, timer status, phase current at the time of tripping, and neutral point current. Each target is marked with a millisecond time scale.

4.3 Real time measurement

Provide real-time measurement values of voltage (phasor, neutral point, sequence component), current (phasor, neutral point, sequence component), active power, reactive power, power factor, and impedance, which can be used for SCADA system monitoring.

4.4 Communication Interface

Front panel RS-232: Local debugging and parameter settings

Rear RS-232: Can connect to RTU or modem

Rear RS-485: 2-wire system (optional 4-wire), supports multi-point bus network

IRIG-B: The BNC interface supports modulation or demodulation type time synchronization signals

Supports MODBUS and BECO 2200 protocols. IPScom software (M-3822) provides complete set value management, real-time monitoring, waveform data download, and target analysis functions.

Key points of setting calculation

5.1 Synchronization check and setting

The Δ F limit should be calculated based on the maximum slip allowed by the system and the closing time of the circuit breaker. It is generally recommended to set it at 0.1~0.3Hz (lower should be set for slow speed circuit breakers). The dead bus/dead line closing function is only enabled during system recovery or black start, and there should be external locking logic during normal operation.

5.2 Direction power protection setting

The reverse power protection setting of distributed power grid connection points should consider factors such as inverter absorption of reactive power and system load fluctuations. Usually, the reverse power setting is set to -2% to -5% of the rated power, with a delay of 0.5 to 2 seconds. The rated power is determined based on the inverter capacity and the thermal stability limit of the circuit.

5.3 Frequency change rate setting

The tuning of 81R needs to be based on the maximum frequency change rate that may occur during normal system operation. Typical value is 0.5~2.0Hz/Sec. The negative sequence voltage suppression function can prevent misoperation in case of asymmetric faults in the system, and it is recommended to set it to 5%~10%.

5.4 Direction overcurrent protection

The key to setting the directional components of 67/67N is the determination of the maximum sensitivity angle (MSA), which should be based on the phase difference between the voltage and current during the fault. For three-phase short circuits, MSA is usually the line impedance angle (60 °~85 °); For grounding faults, the zero sequence network characteristics need to be considered. In the scenario of distributed power grid connection, special attention should be paid to the different fault current characteristics of the inverter compared to traditional synchronous generators (amplitude limited, phase angle affected by control strategies).

Hardware selection and installation

6.1 Selection Points

CT rating: 1A or 5A can be selected, and it needs to be matched with the on-site CT.

Frequency: 50Hz or 60Hz model.

Power supply: Standard single power supply (85~265Vac or 80~288Vdc), optional redundant power supply.

Display and operation: Optional M-3931 HMI module (2 × 24 character LCD) and M-3915 target module (24 target LED+8 output LED).

Protection configuration: Standard protection system or basic system, optional 21/78/81R expansion.

6.2 External Wiring

PT connection: 5 voltage inputs, configurable as line ground or line line mode, supporting line ground to line conversion (applicable to 69V rated PT).

CT connection: 4 current inputs (three-phase+neutral point), pay attention to CT polarity and secondary grounding.

Circuit breaker status: INPUT1 should be connected to the 52 contact for circuit breaker position indication and logic judgment.

6.3 Environmental specifications

Mechanical dimensions: 19 "× 5.21" × 10.2 "(3U height)

Temperature range: -20 ℃~+70 ℃ (IEC test conditions)

Protection level: cULus Listed, suitable for Type 1 enclosed environments

Weight: Approximately 17 pounds (25 pounds for transportation)