Motor protection for low-power current transformer
The SIPROTEC 7SK81 provides 4 low-power current transformer inputs and optionally 3 low-power voltage transformer inputs. With the same low-power current transformer (LPCT) a wide range of primary rated line currents can be covered. Objects with rated currents in the range of 20 A to 2500 A can be protected when using low-power current transformers.
The following low-power current transformer ratios are suitable for the following primary current operating ranges:
300 A /225 mV for a primary operating current range of 60 A to 630 A
600 A /225 mV for a primary operating current range of 120 A to 1250 A
1250 A /225 mV for a primary operating current range of 250 A to 2500 A
100 A /225 mV for a primary operating current range of 20 A to 200 A.
The SIPROTEC 7SK81 is a multi-functional motor protection relay. It is designed for protection of asynchronous motors of all sizes. The relays have all the required functions to be applied as a backup relay to a transformer differential relay.
The relay provides numerous functions to respond flexibly to the system requirements and to deploy the invested capital economically. Examples for this are: exchangeable interfaces, flexible protection functions and the integrated automation level (CFC). Freely assignable LEDs and a six-line display ensure a unique and clear display of the process states. In combination with up to 9 function keys, the operating personnel can react quickly and safely in any situation. This guarantees a high operational reliability.
- 4 current inputs
- 0/3 voltage inputs
- 3/7 binary inputs (thresholds configurable using software)
- 5/8 binary outputs (2 changeover)
- 0/5 RTD inputs
- 1 life contact
- Pluggable voltage terminals.
- Instantaneous and definite time-overcurrent protection (phase/neutral) (50, 50N, 51, 51N)
- Directional time-overcurrent protection, ground (67N)
- Directional overcurrent protection, ground (definite/inverse) (67Ns, 50Ns))
- Displacement voltage, zero-sequence voltage (59N)
- Trip-circuit supervision (38)
- Undercurrent monitoring (37)
- Temperature monitoring (38)
- Thermal overload protection (49)
- Load jam protection (51M)
- Locked rotor protection (14)
- Restart inhibit (66/86)
- Undervoltage/overvoltage protection (27/59)
- Forward-power, reverse-power protection (32)
- Power factor (55)
- Overfrequency/underfrequency protection(81O/U)
- Breaker failure protection (50B/F)
- Phase-balance current protection (negative-sequence protection) (46)
- Unbalance-voltage protection and/or phase-sequence monitor-ing (47)
- Start-time supervision (48)
- Lockout (86)
- Rate-of-frequency-change protection (81R)
The SIPROTEC 7SK81 device is specifically designed to protect induction-type asynchronous motors.
- Inputs for low power VTs and CTs according IEC 61869-6 (formerly IEC 60044-7 and IEC 60044-8)
- Removable terminal blocks
- Binary input thresholds settable using DIGSI (3 stages)
- 9 programmable function keys
- 6-line display
- Buffer battery exchangeable from the front
- USB front port
- 2 additional communication ports
- Integrated switch for low-cost and redundant optical Ethernet rings
- Ethernet redundancy protocol RSTP for highest availability
- Relay-to-relay communication through Ethernet with IEC 61850 GOOSE
- Millisecond-accurate time synchronization through Ethernet with SNTP.