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3.5.2 DIRECTIONAL OVERCURRENT LOGIC

POC1 I>1 TBlk

POC1 I>1 Start A

POC1 I>1 Trip

POC1 IH2 Any St

I> Blocking

POC1 IA2H Start

I> Blocking

V00689

2H Blocks I>1

2H 1PH Block

Timer Settings

I> Threshold

IDMT/DT

VTS Slow Block

I> Blocking

VTS Blocks I>1

Directional

check

I>1 Direction

Non-Directional

Directional FWD

Directional REV

VAB

Notes: Shown for phase A only . Same principle applies for phases B and C .

This diagram does not show all stages . Other stages follow similar principles.

Figure 95: Directional overcurrent logic diagram

The dir

ectional 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 Operate Current Polarising Voltage

A Phase IA VBC

B Phase IB VCA

C Phase IC VAB

When the element is selected as directional, blocking of the Voltage Transformer Supervision (VTS Block) is

ailable (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

inv

erse 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 Chapter 9 - Current Protection Functions

P64x-TM-EN-1.3 207

Brand	GE
Model	P642
Category	Relays
Language	English

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