Directional Overcurrent Logic; Application Notes; Setting Guidelines; Figure 95: Directional Overcurrent Logic Diagram - GE P642 Technical Manual

P64x
3.5.2

DIRECTIONAL OVERCURRENT LOGIC

I> Threshold
POC1 IA 2H Start
I> Blocking
2H Blocks I>1
POC1 IH2 Any St
I> Blocking
2H Blocks I>1
2H 1PH Block
I>1 Direction
Non-Directional
Directional FWD
Directional REV
VTS Slow Block
I> Blocking
VTS Blocks I>1
POC1 I>1 TBlk
V00689

Figure 95: Directional overcurrent logic diagram

The directional overcurrent logic works the same way as non-directional logic except that there is a Directional
Check function, based on the following criteria:
Directional forward: -90° < (angle(I) - angle(V) - RCA) < 90°
●
Directional reverse: -90° > (angle(I) - angle(V) - RCA) > 90°
●
The polarising voltages for each phase are as follows:
Phase of Protection
A Phase
B Phase
C Phase
When the element is selected as directional, blocking of the Voltage Transformer Supervision (VTS Block) is
available (I>Blocking cell). When the relevant bit is set to 1, operation of the VTS will block the stage if
directionalised. When set to 0, the stage will revert to non-directional on operation of the VTS.
3.6

APPLICATION NOTES

3.6.1

SETTING GUIDELINES

The overcurrent inverse time characteristic on the HV side of the transformer must grade with the overcurrent
inverse time characteristic on the LV side which in turn must grade with the LV outgoing circuits. The overcurrent
function provides limited protection for internal transformer faults because sensitive settings and fast operation
times are usually not possible. Sensitive settings are not possible because the pickup should allow overloading of
the transformer when required. Fast operating times are not possible because of the grading required with respect
to downstream overcurrent relays. To allow fast operating times, phase instantaneous overcurrent functions with
low transient overreach are required.
P64x-TM-EN-1.3
IA
&
&
IA
VAB
Directional
check
&
This diagram does not show all stages . Other stages follow similar principles.
IA
IB
IC
& &
Notes: Shown for phase A only . Same principle applies for phases B and C .
Operate Current
Chapter 9 - Current Protection Functions
POC1 I>1 Start A
IDMT/DT
POC1 I>1 Trip
Timer Settings
Polarising Voltage
VBC
VCA
VAB
207