Solutions   for   Protecting   andManaging   the   Electrical   Network

NPDT 620 / 630

2 or 3 Windings Transformer Differential Protection

The digital differential protection relays NPDT620 and NPDT630 are designed to provide fast and selective protection of 2 and 3-winding three-phase transformers.
The NP600 relays can detect various types of fault in the zone to be protected, such as phase-phase faults, inter-winding faults and earth-fault.
Restricted earth-fault protection, on primary and secondary sides, is available as an optional function for 3-winding transformers.

In addition to the protection functions, NPDT relays also perform monitoring, measurement and recording of the electrical quantities of the network.
Parameters can be set locally, using either the integrated display/keypad or the RS232 port, or remotely using the RS485 port.
Electrical values calculation is achieved by Fast Fourier Transforms.
Setting, reading, measurement and recording functions are available in local mode as well as in remote mode.


  • Differential protection [87]
  • Breaker Failure [50BF]
  • Restricted earth-fault protection [64REF] (optional on NPDT 630 model)


  • Configuration and parameter setting by local keypad or by off-line or on-line PC
  • Measurement of electrical data:
    – transformer primary and secondary currents (NPDT 620 and 630) and tertiary currents (NPDT 630)
    – phase differential currents (R, S, T)
    – phase through currents (R, S, T)
    – display of CT measurements expressed in primary current
  • Vector group compensation function
  • Detection of transformer inrush (2nd harmonic) and magnetising currents (5th harmonic)
  • Stability during CT saturation, using 2nd and 3rd harmonic measurement
  • Stability for out-of-zone faults
  • Stability to DC currents
  • Self-diagnosis: RAM, ROM, EEPROM, output relays, A/D converters, auxiliary supply, cycles of software run, hardware anomaly
  • 8 output units / 5 programmable digital inputs
  • Time stamping of internal events with 1 ms resolution
  • Disturbance recording according to Comtrade format: eight 52-period recordings
  • Modbus® Communication through RS 485 (option)


  Device Doc. Type Lang. Ref. Index
NPDT 620 / 630 Leaflet EN A586 A
NPDT 620 Connection/Definition Diagram -- S38247 B
NPDT 630 Connection/Definition Diagram -- S38248 C
NPDT 620 / 630 Database Modbus EN A431 B