AQ-C255D Capacitor bank protection device


The AQ-C255D capacitor bank protection device has been specifically designed for the protection of capacitor banks. It includes capacitor bank current unbalance, capacitor bank neutral unbalance, and overload protections in addition to the standard overcurrent, earth fault and voltage protections. Additionally, it includes the power factor controller that functions as the control unit of an automatic capacitor bank system and can control up to four (4) capacitor banks.

AQ-C255D offers a modular protection and control solution for applications that require a large I/O capacity. You can add up to eleven (11) I/O or communication modules into the device for extensive monitoring and control applications. The AQ-C255D capacitor bank protection device communicates using various protocols, including the IEC 61850 substation communication standard.


  • A large I/O capacity.
  • Capacitor bank module with capacitor bank-specific protection functions.
  • Current- and voltage-based protections.
  • Power factor controller for up to four (4) capacitor banks.
  • A controllable color mimic with active feedback.
  • Power and energy measurement accuracy of up to 0.2 %.

See all AQ-C255 variants


Technical data

  • Capacitor bank module (CBP)
  • Capacitor bank current unbalance (Iuc>; 46C)
  • Capacitor bank neutral unbalance protection (Cnu>; 50UB)
  • Capacitor bank overload protection (Icol>; 49OL)
  • Non-directional overcurrent (I>; 50/51) - 4 stages (INST, DT or IDMT)
  • Non-directional earth fault (I0>; 50N/51N) - 4 stages (INST, DT or IDMT)
  • Directional overcurrent (Idir>; 67) - 4 stages (INST, DT or IDMT)
  • Directional earth fault (I0dir>; 67N/32N) - 4 stages (INST, DT or IDMT)
  • Intermittent earth fault (I0int>; 67NT)
  • Negative sequence overcurrent/ Phase current reversal/ Current unbalance (I2>; 46/46R/46L) - 4 stages (INST, DT or IDMT)
  • Harmonic overcurrent (Ih>; 50H/51H/68H) - 4 stages (INST, DT or IDMT)
  • Non-directional undercurrent (I<; 37)
  • Circuit breaker failure protection (CBFP; 50BF/52BF)
  • High-impedance or low-impedance restricted earth fault/ Cable end differential (I0d>; 87N)
  • Overvoltage (U>; 59) - 4 stages (INST, DT or IDMT)
  • Undervoltage (U<; 27) - 4 stages (INST, DT or IDMT)
  • Neutral overvoltage (U0>; 59N) - 4 stages (INST, DT or IDMT)
  • Sequence voltage (U1/U2>/<; 47/27P/59PN) - 4 stages (INST, DT or IDMT)
  • Overfrequency and underfrequency (f>/<; 81O/81U) - 8 stages (INST or DT)
  • Rate-of-change of frequency (df/dt>/<; 81R) - 1 stage (DT)
  • Power protection (P, Q, S>/<; 32) - 4 stages (DT)
  • Resistance temperature detectors (RTD)
  • Line thermal overload (TF>; 49F)
  • Voltage memory
  • Programmable stage (PGx>/<; 99)
  • Arc protection (IArc>/I0Arc>; 50Arc/50NArc) (optional)

  • Number of objects to control and monitor: 10
  • Number of indicators to monitor: 10
  • Number of setting groups: 8
  • Power factor controller (PFC) - up to 4 banks
  • Cold load pick-up
  • Switch-on-to-fault

  • Phase, sequence and residual currents (IL1, IL2, IL3, I01, I02)
  • Phase, sequence and residual voltages (UL1, UL2, UL3, U12, U23, U31, U0)
  • Frequency (f)
  • Power (P, Q, S, pf) and Energy (E+, E-, Eq+, Eq-)
  • Power and energy class 0.5
  • Power and energy measurement accuracy of up to 0.2 % (optional)
  • Current transformer supervision
  • Voltage transformer supervision (60)
  • Circuit breaker wear monitoring
  • Total harmonic distortion (current)
  • Total harmonic distortion (voltage)
  • Measurement recorder
  • Measurement value recorder
  • Event recorder (max. 15 000 permanent event records)
  • Disturbance recorder (max. 100 records á 5 seconds at 3.2 kHz sampling)

  • Current inputs: 5
  • Voltage inputs: 4
  • Digital inputs (fixed): 3
  • Digital outputs (fixed): 5
  • Number of empty slots: 11
  • Digital inputs: +8/16/24/32/40/48/56/64/72/80/88 (optional)
  • Digital outputs: +5/10/15/20/25/30 (optional)
  • Milliampere I/O module (4 mA outputs + 1 mA input)
  • Arc protection module (4 sensors + 2 HSO + 1 BI)
  • Communication media (see "Communication" below)
  • External I/O modules (see "Accessories" below)

  • Communication inputs
  • RJ-45 100 Mbps Ethernet (front panel, fixed)
  • RJ-45 100 Mbps Ethernet and RS-485 (rear panel, fixed)
  • 2 x RJ-45 100 Mbps Ethernet with an IRIG-B input (optional)
  • 2 x ST 100 Mbps Ethernet with an IRIG-B input (optional)
  • 2 x LC 100 Mbps Ethernet (PRP/HSR) (optional)
  • RS-232 serial fiber (PP/PG/GP/GG) (optional)
  • Communication protocols
  • IEC 61850 (edition 1)
  • IEC 60870-5-101/104
  • IEC 60870-5-103
  • Modbus/RTU and Modbus/TCP
  • DNP3
  • SPA

  • AX007 External 6-channel 2-/3-wire RTD input module (pre-configured)
  • AX008 External 8-channel thermocouple and mA input module (pre-configured)
  • AX013 Raising frame (120 mm)
  • AX014 Raising frame (40 mm)
  • AX015 Wall mounting bracket

Additional material

Application Drawing

An application image for AQ-C255D capacitor bank protection device with full protection functionality as well as a power factor controller for up to four banks, with one version drawn according to IEC standard symbols and the other according to IEEE/ANSI standard symbols.

Device Rear Image

Device rear image_AQ-255-AAAAAAAAA

Device and Cut-out Image

AQ-250 dimensions and cut-out

Order Code


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