hv2_22_12_11

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Cascade Connection of

Testing Transformers• Size of Testing Transformer

• Transportation and Erection• Advantageous for voltages >500 kV

• Whole weight is subdivided• Transportation becomes easier

•Prerequisite• Tertiary winding within each transformer

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Schematic of Cascaded TT

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Cascading of TT

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• All the three units have identical ratingshaving same voltage ratio is V 1:V2.

•Units at Stage-I and Stage-II are provided withTertiary winding.

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Primary Voltage of Testing

Transformer • Power Frequency AC voltage of 300 V to 500 V(continuously variable).

• Output voltage of a testing transformer must have a peakfactor of 2 5%, contains low harmonics as possible.

• Wave generators are used for supply sinusoidal inputvoltage (Pitch factor and distribution factors are suitably

designed to eliminate harmonics)• Buck-boost transformer is also used to supply the input

voltage

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Operation of Cascaded Units

The output of the Stage – I is V 2 kV with respect toground and tertiary develops V 1 to feed to Stage – II

• Point A is floating

• The lowermost terminalsof primary and secondary

of stage – I unit are to beearthed

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• Primary of Stage – II is feed from tertiary of stage – I and

output of Stage – II is V2.

• Stray capacitance is avoided by connecting tank and lowermost terminal of the winding together.

• The output of Stage-II wrt earth is 2V 2.

• Output of Stage – III is 3V2 wrt earth.

• V2 is in the range of 200 kV to 300 kV.

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KVA Grading of CascadedTesting Transformer Units

• Output current of the Transformer is I A and voltageis 3V 2 kV.

•kVA Supplied to the load = 3V 2 I• kVA rating of the secondary of stage – III alone

P = V 2 I• kVA rating of the primary of stage – III = P• kVA rating of the tertiary of stage – II = P • kVA rating of secondary of stage – II = V2 I = P • kVA rating of primary of stage – II = P + P = 2P

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Continued……. • kVA rating of tertiary of stage – I = 2P

• kVA rating of secondary of stage – I = V2 I = P • kVA rating of primary of stage – I = P + 2P = 3P • total installed capacity for the three units

• Summation of power rating of primary of all the threeStages

= 3P + 2P + P = 6P kVA• From cascaded TT output to load is 3P kVA• Utilization of installed capacity

• Output to Load / Installed Capacity

= 3P/6P *100% = 50%

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3 Cascaded Testing Transformer UnitsOutput = 3P kVA, 3V 2 kV

StageNo.

Insulation levelof tank wrt

earth

Primarywinding rating

Tertiary windingrating

Secondarywinding rating

I 0 V1,3P kVA

V1,2P kVA

V2 kVP kVA

II V2

kV V1,

2P kVA V

1,

P kVA V

2kV

P kVA

III 2V 2 kV V1,P kVA

-- V2 kVP kVA

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Restriction to more number of

stages• Inefficient utilization of installed capacity.

• Overloading of the lower stagetransformers.

• Relatively high internal impedances of the

whole cascaded circuit that causes poorvoltage regulation.

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Series Resonance Circuit

• High voltage equipments - insulation is to betested - offers capacitive load to the testcircuit.

• Test circuit (Transformer) has resistance andinductance

• This forms series R-L-C circuit.• Resonance may take place (Z = R) and voltage

increases to very high value.

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Let, R = 0.1 , XC = 1000 (C 3 F) and v = 1000V

Current at resonance = 1000/0.1 = 10 4

At resonance condition V c=VL andVC = VL =Impedance x Current = 10 4 x 10 3 = 10 7 V

Output VA ( across the test Object)= VL X I = 10 7 x 10 4 = 10 11 VA

Input VA = v X I = 10 3 x 10 4 = 10 7 VA

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Continued……..

• In series resonance circuit, large voltage andlarge output kVA is obtained with a small amountof input voltage and input kVA .

• Capacitance of the test object is normally low and hence, L should be higher .

• Capacitance is dependent on the test object andhence, cannot be controlled . So, L should beadjusted to achieve resonance .

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Series Resonance Circuit withTransformer/ Reactor

•To achieve resonance the inductance (L) has to be adjusted.

• While changing L, step variation is not allowed.

• High Voltage spikes may occur due to L(di/dt).

• It may affect the insulation of test object.

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SRC Continued…

• Feed transformer - inject the current

• Initially, small current is injected

• Achieve resonance

• One the circuit is in resonance more current isinjected into the circuit till the required voltage levelis achieved

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Multiple Transformer/Reactor

units in Series

Why?kVA rating of the reactor-transformer becomesvery high for higher test voltages

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Continued….

•Total voltage across thetest object is supportedequally by the differentunits.

•Reactors in the different

units are normally placedon the same shaft.

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Cable Testing by Testing Transformer

R 1 & L 1 = Equivalent resistance and leakage inductance of testingtransformer primary (LV) side w.r.t. secondary (HV)side.

R 2 & L 2 = Resistance and leakage inductance of testing transformersecondary (HV) side.

L m = Equivalent magnetizing inductance of testing transformerw.r.t. secondary (HV) side

In practice, Lm (L1 + L2)

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Continued…. Approximate Equivalent Circuit

Thus it becomes a series R-L-C circuit, where resonance may occur

•As harmonics are present due to transformer iron-core,

resonance may occur due to harmonics.•Even for fundamental frequency resonance may occuroccasionally depending upon the cable length and cabletype and rating.

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Consequences of Resonance whileTesting Cable with Testing

Transformer• The circuit becomes resistive and a large current flow through

the circuit.• Voltage across the cable capacitance becomes very large of

the order of 20 – 50 times the rated test voltage. • Puncture of the cable insulation causing short-circuit across

the cable capacitance.

• The testing transformer continues to feed this short circuitcurrent at a very high voltage.

• This causes severe explosion of the cable.• Protective relay connected to primary side of the testing

transformer cannot protect the cable.

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Advantages of Series ResonanceCircuit, if used to test cable

• Circuit explosion of the cable is avoided, if the cable insulationbreaks down, then the capacitance is shorted and resonance is lost.

•Impedance increases to Z=R+jX L from Z=R

• The test voltage waveshape is improved compared to input voltagedue to attenuation of harmonics present in the input supply.

•Any number of units may be put in series without the problem ofhigh impedance and hence poor regulation associated with acascaded testing transformer group.

• The power required from the supply is lower than the kVA in themain circuit.

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