Beckwith M-3520 Interconnection Protection Relay

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)