
Manual MRI3 GB
Woodward
DOK-TD-MRI3 Rev.A
9
Figure 3.5: Voltage transformer in V-connection for the directional detection at overcurrent and short-circuit protection.
The V-connection cannot be applied at earth fault directional feature.
3.1.1 Analog input circuits
The protection unit receives the analog input signals of the phase currents IL1 (B3-B4), IL2 (B5-
B6), IL3 B7-B8) and the current IE (B1-B2), phase voltages U1 (A3), U2 (A5), U3 (A7) with A2 as
star point, each via separate input transformers.
The constantly detected current measuring values are galvanically decoupled, filtered and finally
fed to the analog/digital converter.
For the unit type with earth fault directional features (ER/XR-relay type) the residual voltage UE in
the secondary circuit of the voltage transformers is internally formed.
In case no directional feature for the phase current path is necessary the residual voltage from the
open delta winding can directly be connected to A3 and A2.
See Chapter 4.5 for voltage transformer connections on isolated/compensated systems.
3.1.2 Output
relays
The MRI3 is equipped with 5 output relays. Apart from the relay for self-supervision, all protective
functions can be optionally assigned:
•
Relay 1: C1, D1, E1 and C2, D2, E2
•
Relay 2: C3, D3, E3 and C4, D4, E4
•
Relay 3: C5, D5, E5
•
Relay 4: C6, D6, E6
•
Relay 5: Self-supervision C7, D7, E7
All trip and alarm relays are working current relays, the relay for self supervision is an idle current
relay.