CMC Limited
From the SelectedWorks of Venkateswar P Oruganti FIETE FIE PMP
1991
A method of validating MW MVAR values
Venkateswar P Oruganti, FIETE FIE PMP
Available at: http://works.bepress.com/venkateswar_oruganti/3/
A method for Validating MW/MVAR Values from MW/MVAR values
A method for Validating MW/MVAR Values from MW/MVAR values
The following 2-wattmeter method is used for measuring power and is followed for validating
MW/MVAR values at different Remote Terminal Unit (RTU) locations of a SCADA project in India. ABB
MW/MVAR Transducers are used for Andhra Pradesh State Electricity Board (APSEB) Load Despatch
Center(LDC) project during 1991-1993. This method of checking connections or validating reduces
verification time by about 60% with less number of changes to already made connections thus reducing
any downtime particularly for CT reversals.
3Ø
- Star connected load phasor diagram is given below. V1, V2, V3 represent the different voltages,
where as V13 , V23,V12 etc., represent line voltages. I1, I2, I3 represent the currents in corresponding
phase.
V13
V1
I1
-V3
300
Ø
V23
300
I3
Ø
V3
Venkateswar Panduranga Oruganti
I2
1991
V2
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A method for Validating MW/MVAR Values from MW/MVAR values
3 phase power for an unbalanced star connected load can be represented by the equation
P3Ø = V13I1Cos(30° -Ø) + V23I2Cos(30° +Ø)
………………………………………….(1)
(V3 is common)
For validating the data from transducer both CT and PT connections should be proper.
(i)
CT Connections :- Assuming the PT connections with respect to phase sequence and
one-to-one correspondence in connections between field and transducer terminals,
the problems related to CT connections can be resolved by following the statements
deduced as below.
By referring to equation (1) above,
Case 1
If I1 only is reversed (means CT connections corresponding to I1),
P-I1 = V13I1Cos(180°+30° -Ø) + V23I2Cos(30° +Ø)
= -V13I1Cos(30° -Ø) + V23I2Cos(30° +Ø) -----------------------------------(2)
From Equation (2),
When I1 is in reversed condition,
With I2 shorted,
More –ve value at transducer output (compared to when nothing is shorted)
With I1 shorted,
Less +ve value at transducer output (compared to when nothing is shorted)
The above observational information can be put in an equation form as below,
P-I1 = V23I2Cos(30° +Ø) [ I1 is shorted i.e. I1 = 0 ]
P-I1 = -V13I1Cos(30° -Ø) [ I2 is shorted i.e. I2 = 0 ]
Generally V13, V23, V12 all will be same. Also I1, I2, I3 all equal to the line current.
Case 2
If I2 only is reversed ,
Venkateswar Panduranga Oruganti
1991
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A method for Validating MW/MVAR Values from MW/MVAR values
P-I2 = V13I1Cos(30° -Ø) + V23I2Cos[180° –(30° +Ø)]
= V13I1Cos(30° -Ø) - V23I2Cos(30° +Ø) -----------------------------------(3)
Here from equation (3),
When I2 is in reversed condition,
P-I2 = -V23I2Cos(30° +Ø) [ I1 is shorted i.e. I1 = 0 ]
P-I2 = V13I1Cos(30° -Ø) [ I2 is shorted i.e. I2 = 0 ]
In simple terms, I2 in reversed condition,
We observe
More +ve value at transducer output with I2 shorted.
Less –ve value at transducer output with I1 shorted.
Case 3
If I1 and I2 are reversed ,
P-I1-I2 = - [ V13I1Cos(30° -Ø) + V23I2Cos (30° +Ø)] -----------------------------------(4)
Hence from equation (4), when voltage sequence is proper and only CT connections
are reversed we will get power in entirely different direction [ i.e. Import in stead of
Export, vice versa], though magnitude matches with the actual value (may be from
meter)
(ii)
PT Connections :- These have to be checked for proper phase sequence R,Y,B and
also for one-to-one mapping between the marked terminals on the transducer and
the wires brought from field. Once these are correct, CT connections can be checked
as per the above procedure.
Venkateswar Panduranga Oruganti
1991
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