Sign Conventions for Energy Meter




Whether you noticed that CT connection to energy meter in S/S are such that we connect S1 to “OUT” terminal of the meter instead of “IN”. Have you noticed that sign convention for import and export of MW and MVAR being reported to LD from EHV S/S are reverse as compared to that used at EHV consumer end? Have you faced difficulties about polarity while doing connection of CT wire to MFM or ABT meter? Then this explanation may help you.
Consider a SLD of small power system as shown in figure-1 having three busses, a source and an inductive load. This SLD also shows CT and PT connected to Bus-B along with primary and secondary terminal identification for polarity.
Consider +Ve half cycle of the voltage. Corresponding to this cycle conventional direction of current shown through line AB and BC.
Accordingly secondary currents also shown for devices DC. From the diagram we can observe that whenever voltage is entering (?) in device Dc; current is also entering. Thus for device Dc and for pure resistive load voltage and current will be in phase. Obviously when load is inductive current will lag by some angle as shown in vector diagram for device Dc.


From the above figure-1 we can conclude that when active power is exported from bus; angle between voltage at that bus and current is - 900 to + 900  and when active power being imported this angle will be more than + 900 or less than - 900.
Furthermore we know the formula for calculating power as complex number (like a + Jb) is as below

S = V.I*


Where S is complex power V is voltage vector represented as complex number and I* is complex conjugate of current. Using this formula we will calculate complex power for supply voltage of 220kV and current of 400Ampere for different angle and tabulate the result for ready reference.



From above table we can conclude that theoretically for export from bus MW is +Ve and for import to the bus MW is –Ve.
According to this theory if energy meter is connected at consumer end with CONSUMER VIRTUAL BUS as reference (CT P1 is towards consumer); the energy meter will record –Ve for power being consumed. This –Ve recording for power consumption does not seems normal. Hence it is general practice to reverse P1 and P2 of CT at EHV consumer end (P1 towards grid supply and P2 towards consumer) as shown in Figure-2; to have understandable sign convention; (+Ve) for power being consumed.

For EHV S/S as per connection convention shown in Figure-1 and calculations with this convention (Table-1) this export power will be +Ve. Hence to match with consumer end power; EHV S/S end power to be recorded as –Ve and NECESSARY to call it is EXPORT. To happen this we have to REVERSE the CT connections to meter at EHV bus as shown in Figure-2.



Thus considering transmission network bus as reference Export power (active and reactive) becomes –Ve and Import power becomes +Ve. With this convention Table-2 with LD convention is displayed as below.



Energy meter quadrants for MW and MVAR recording AT CONSUMER END are as shown below for reference



From EHV S/S point of view conventions of +Ve and –Ve will reverse i.e. Q1-VI quadrant will be Q3-VI quadrant and Q2-VI quadrant will be Q4-VI quadrant.





1 comment:

  1. This article is very helpful for understanding the concept of sign convention for energy meter. Now I got the basic reason for reversing the IN OUT standard, that is to maintain the harmony with consumer meter.

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