problems in overall differential protection of generator and gt

7/27/2019 Problems in Overall Differential Protection of Generator and GT

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PROJECT : CPCL, MANALI, 1 X 20 MW GTG, UNIT 3

PROBLEM : PROBLEMS IN OVERALL DIFFERENTIAL PROTECTIONOF GENERATOR AND GENERATOR TRANSFORMER

While commissioning Generator Transformer ( GT ) of gas turbine,it was observed that the overall differential protection of the

Generator was repeatedly acting inspite of all healthy connections.

The electrical system of CPCL is provided with a Generator of 28.5MVA/ 11 kV/ 1478 A rating and a Generator Transformer of 33 MVA/

11 kV/ 7.07 kV DYn11 group. The Current Transformers (3 nos.) on

the Generator neutral side is rated for 2000/ 1 A and currenttransformer on the 7 kV side of the transformer is 3000/1 (3 no.s).

The relay supplied for overall differential protection is DUOBIAS of

EASUN REYROLLE.

Normally in the older relays, all the CT connections are given as per

book theory (star on delta side of transformer and delta on star side

of transformer) and interposing CTs are used to count for the ratiomismatches with the transformer current ratio. The generator does not

contribute to any change in this configuration. In the later designs ofnumerical relays, the same function of interposing CTs and connections

of the interposing CTs can be specified by simple software variables.This has eased the system of protection by eliminating the use of

interposing CTs, standard CT connections irrespective of star or delta

side of Generator Transformer and use of the same relay for multipleprotections as in the case at CPCL. The same DUOBIAS relay was used

for transformer star side Restricted Earth Fault Protection also. At

CPCL both the Generator side CTs and the transformer bushing CTs


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were connected in star and connected to the relay in a 4 wire

configuration. As per the old theory, the CTs on the star side of the

transformer should be delta and because of the use of the numericalrelay star connection was given, the vector group configuration of the

internal interposing CTs as yd1 ( equivalent of DYn11 when seen fromthe transformer star side) is to be given. The delta side of thetransformer however was having only a star CT configuration and hence

a yy0 interposing CT configuration in the relay was sufficient. Suitable

multipliers were also incorporated (1.08 for HT side CTs and 1.10 for

LT side CTs)

Once the relay configurations were set, wiring configurations were

checked and found to be alright as per the schematic andinterconnection diagram. Again, it was checked for differential stability

and the system stability could not be established. System was

analysed again by raising the current slightly under short circuitconnection. It was observed at a particular point of primary current,

the winding current on the HT side read 0.07 A and the winding

Current on LT side also read 0.07 A. An operating current of 0.12 A

was observed through the relay, whereas the current of 0.14 A wasexpected or 0 A in case of polarity problem. As the measured value of

0.12 A is close to 0.14 A, the polarity was reversed. So CT polarities

at the transformer end were reversed as that of Generator CTspolarity. It was observed that the HT side winding, LT side winding,

relay operating currents all read 0.07 A. It was inferred from this

that it can occur only if the phase is shifted by 120 deg. 120 deg.phase shift gives a resultant of 0.07 A and if polarity is reversed,

the angle becomes 60 degrees and it gives a resultant of 0.12 A.Hence phase CTs of R,Y, B on any one side were suspected to have got

shifted by 120 degrees.


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To confirm the same, the vector group configuration of the interposing

CT was changed to yd4 corresponding to 120 degrees and it was found

that relay current became zero. The Generator side Neutral CTs werechecked and it was found to be in order by physical verification and

continuity. The transformer was inspected for any markings of phasedetails. There was no printed Phase details near the CT termination andalso the CTs were totally immersed in oil and hence oil is to be

drained to verify the CTs physically. Assembly drawing from BHEL-

Jhansi was not available.

The vector group of the transformer is confirmed, as the machine hasbeen synchronized many a times already. Core balance test connection

was made with the transformer windings on the LT side shorted and a

small voltage of 6 V was given with a single phase 230 V variac. Based

on the % impedance already known, this current should be around 6 A.Also it was observed that the same current was only flowing in the

single phase winding of the transformer where the voltage was given.

The other two windings acts like a potential divider and hence shouldshow a drop of 3 V across each winding. So the winding with 6 V will

draw a current of 6 A on the same winding whereas the other two

windings were drawing just 3 A, correspondingly the secondary sideinline winding also drew higher current than the other two. The vector

grouping for a Dyn11 configuration is as shown below:

Applied Voltage = 6 V across RY

The other two voltages VYB = 3 V, VBR= 3 V Hence currents in thesetwo windings should also be lesser than the winding RY, so corresponding

secondary current would also be less than the one corresponding to RY.

Now for vector group Dyn11

Primary RY corresponds to secondary nrPrimary YB corresponds to secondary ny

Primary BR corresponds to secondary nb


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Hence with higher voltage across RY should give a higher current across

RY and hence nr under short circuit condition. Practically it was

observed that the current in one phase of the secondary of thesecondary side CT showed 5 mA whereas the other two showed 2.5 mA,

hence it was concluded that this CT corresponds to r phase of thesecondary of the transformer. Similarly the other two phases were also

found out by giving a small voltage in the other two phases one by one

and the CTs for each phase were identified. It was observed that theCTs did not match with the markings.

The CTs were correctly marked, the machine was again rolled and

checked. The values indicated change in polarity of one of the CTs. Sopolarities were then interchanged one by one and checked for stability.

b phase CT polarity was found reversed and corrected. Operating

currents in both the primary side and secondary windings were found to

be 0.07 A and the relay currents were found to be zero and hencestability of the system was proved.

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problems in overall differential protection of generator and gt

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