Generation of high AC voltages-Testing transformer – single unit testing transformer, cascaded transformer – equivalent circuit of cascaded transformer – generation of high frequency AC voltages- series resonance circuit – resonant transformer – voltage regulation.

1. MODULE 2
Generation of high AC voltages-Testing transformer – single unit testing
transformer, cascaded transformer – equivalent circuit of cascaded
transformer – generation of high frequency AC voltages- series
resonance circuit – resonant transformer – voltage regulation

2. Alternating Current
• ac predominates dc in our transmission and distribution system-
Most common form for testing-ac voltages
• ‘rated power-frequency short duration withstand voltage’ V is
different for different apparatus used in the transmission system
t

3. Testing Transformers
• The power frequency single-phase transformer is the most common form of
HVAC testing apparatus.
• High Voltage power frequency test transformers are required to produce
single phase very high voltages.
• Their continuous current ratings are very low, usually ≈ 1A.
• Even 1A is a very high current rating.
- This is because a HV test transformer has to supply only the capacitive
charging current to the capacitance formed by the dielectric of the test object.
• However, since the voltage rating requirements are high, the test
transformers are required to be produced with very high insulation level.
This increases the size of the test sets tremendously.
• Hence, single units of test transformers produced maximum upto 700 kV.

4. A 600 kV, 3.33A Testing Transformer for
continuous operation [Courtesy TUR Dresden,
Germany]

5. Single unit testing transformer
• Primary-designed for voltages <1kV
• Iron core is fixed at earth potential

6. Single unit testing transformer with mid-point
potential at core: . (1) Iron core. (2) Primary winding.
(3a & b) High-voltage windings. (4a & b) compensating windings.
(5) Exciting winding

7. • Primary winding may be switched in series or parallel to increase the
regulation capabilities
• Iron core is fixed at earth potential.
• Core and the winding is held in the metal tank unit
• HV bushing-to bring out voltage out of the tank
• Co-axial cable for connecting testing transformer and testing objects.
• Inorder to reduce the height the active part is placed in the isolating
cylinder avoiding the bushing
• Vessels will be filled with high quality transformer oil-as most of the
windings are oil paper insulated

8. • Two additional windings-rated for low voltage-compensatory winding
are included and placed closer to core-
• Reduces leakage reactance between high voltage winding and the
transformer core
• Exciting winding for cascading of transformer

9. drawback
• For high voltage requirement –single unit construction becomes
difficult
• Excitation and transportation becomes difficult
• Solution
• Cascading:high voltage terminals comes in series
• Series connection

10. CASCADE TRANSFORMERS
• When test voltage requirements are < 300 kV to 500kV-cascading –
transport and errection –simpler
• DOMINATING HVAC TESTING UNIT IN HIGH VOLTAGE
LABORATORIES

11. CASCADE TRANSFORMER
3VI

12. It consists of:
HV and LV windings
Meter winding - to measure the output voltage
• Circuit configuration:
– 1st transformer, T1 is at ground potential
– 2nd transformer, T2 is kept on insulators and maintained at a potential of V2 (i.e. the output voltage
of the first unit above the ground).
– The HV winding of the T1 is connected to the tank of T2.
– The LV winding of T2 is supplied from the excitation winding of T1.
– Excitation winding of T1 is in series with the HV winding of T1 at its high voltage end.
– The rating of the excitation winding is almost identical to that of the primary or the low voltage
winding.
– The HV connection from T1 and the excitation winding terminal are taken to T2 through
a bushing.
– In a similar manner, the 3rd transformer T3 is kept on insulators above the ground at a potential of
2V2 and is supplied likewise from T2.

13. • The numbers of stages -very often, three stages are adopted to facilitate a three-
phase operation so that (3V2) can be obtained between the lines.
•

14. Equivalent circuit of cascaded transformer

15. Zps=Zp+Zs …………..(leakage impedance measured on primary side with
secondary short circuited and teritiary open)….(1)
Zpt=Zp+Zt…………..(leakage impedance measured on primary side
with teritiary short circuited and secondary open)….(2)
Zst=Zs+Zt…………..(leakage impedance measured on secondary side
with teritiary short circuited and primary open)….(3)
Impedance associated with each winding
Zp=0.5 (Zps+Zpt- Zst)…..(4)….impedance associated with each winding
Zs=0.5 (Zps+Zst- Zpt)……(5)
Zt=0.5 (Zpt+Zst- Zps)……(6)

17. EQUIVALENT CIRCUIT OF 3STAGE
TRANSFORMER
2P
P3P
P
P
P
P-POWER

18. • Assuming negligible magnetizing current,sum of ampere turns of
all windings must be zero
• Assuming lossless transformer we have

19. Short circuit reactance
Xres= 𝒊=𝟏
𝒏
[ 𝒏 − 𝒊 + 𝟏 𝟐 𝑿 𝒑𝒊 + 𝑿 𝒔𝒊 + (𝒊 − 𝟏) 𝟐 𝒙 𝒕𝒊]
For n stages
𝑿 𝒑𝒊 short circuit reactance of primary winding
𝑿 𝒔𝒊 short circuit reactance of secondary winding
𝒙 𝒕𝒊 short circuit reactance of teritiarywinding

21. • Impedance of two stage transformer is 3-4 times impedance of 1unit
• And three stage impedance is 8-9 times that of 1stage transformer.
• Inorder to have low impedance ,impedance of individual units should
be as small as possible

22. Cascade TX unit with isolation TX for
excitation
• In Fig, a second scheme for providing the excitation to the second
and the third stages is shown

23. Disadvantages of this scheme:
- Expensive and requires more space.
• The advantages of this scheme is that:
-Natural cooling is sufficient
-The transformers are light and compact.
-Transportation and assembly is easy.
- The construction is identical for isolating transformers and the HV
cascade units.
Cascade transformer

24. RESONANT TRANSFORMER
• A high-voltage transformer in which the secondary circuit is tuned to
the frequency of the power supply.
It consists of:
• Leakage reactances of the windings
• Winding resistances
• – Magnetizing reactance
• – Shunt capacitance across the output terminal due to the bushing of
the HV terminal and test object.

25. L-EXCITING
INDUCTANCE OF
THE TRANSFORMER
>>L1

26. • The advantages of this principle are:
1. – Gives an output of pure sine wave
2– Power requirements are less (5 to 10% of total kVA required)
3– No high-power arcing and heavy current surges occur if the test object fails (since
resonance ceases at the failure of the test object) .[POWER ARC CAN RESULT IN
EXPLOSION OF CABLE TERMINATION]
4– Cascading is possible for very high voltages
5– Simple and compact test arrangement
6– No repeated flashovers occur in case of partial failures of the test object and
insulation recovery
disadvantages:
7– Require additional variable chokes capable of withstanding the full test voltage &
the full current rating.

27. SERIES RESONANT CIRCUIT

28. PARALLEL RESONANT CIRCUIT

29. • Parallel resonant a.c. test system
• – A voltage regulator is connected to the supply mains
• – The secondary winding of the exciter transformer is connected across the
high voltage reactor L and the capacitive load C.
• – L is varied by varying its air gap and operating range is set in the
• ratio 10:1
• – C comprises of the capacitance of the test object, capacitance of the
measuring voltage divider, capacitance of the high voltage bushing etc.
Advantages of parallel resonant circuit:
• – More stable output voltage
• – High rate of rise of test voltage (independent of the degree of tuning and the
Q-factor)

30. SERIES RESONANT CIRCUIT

31. (a)Series resonance circuit with variable
h.t.reactors(b)Equivalentcircuitof(a)

32. • At resonance
Q-quality factor Varies between 40 and 80

33. Tesla coil
• Design a basic air cored resonant transformer known as a tesla coil,
rated at over 300kV output.

34. problem
• . A 100 kVA 250 V/200 kV feed transformer has resistance and
reactance of 1% and 5% respectively. This transformer is used to test a
cable at 400 kV at 50 Hz. The cable takes a charging current of 0.5 A
at 400 kV. Determine the series inductance required. Assume 1%
resistance of the inductor. Also determine input voltage to the
transformer. Neglect dielectric loss of the cable.

35. • The resistance and reactance of the transformer are

38. Advantages of series resonant circuit
• The power requirements in KW of the feed circuit are (kVA)/Q where
kVA is the reactive power requirements of the load and Q is the quality
factor of variable reactor usually greaterthan 40. Hence, the
requirement is very small.
• The series resonance circuit suppresses harmonics and interference to
a large extent. The near sinusoidal wave helps accurate partial
discharge of measurements and is also desirable for measuring loss
angle and capacitance of insulating materials using Schering Bridge.

39. Advantages continued
• In case of a flashover or breakdown of a test specimen during testing
on high voltage side,the resonant circuit is detuned and the test voltage
collapses immediately. The short circuit current is limited by the
reactance of the variable reactor. It has proved to be of great value as
the weak part of the isolation of the specimen does not get destroyed.
In fact, since the arc flash over has very small energy, it is easier to
observe where exactly the flashover is occurring by delaying the
tripping of supply and allowing the recurrence of flashover.
• No separate compensating reactors (just as we have in case of test
transformers) are required.This results in a lower overall weight.

40. • When testing SF6 high voltage switchgear, multiple breakdowns do
not result in high transients. Hence,no special protection against
transients is required.

41. questions
• Explain with neat diagram the principle of operation of (i) series (ii)
parallel resonant circuits for generating high a.c. voltages. Compare
their performance.
• Draw equivalent circuit of a 3-stage cascaded transformer and
determine the expression for short circuit impedance of the
transformer. Hence deduce an expression for the short-circuit
impedance of an n-stage cascaded transformer.