X - Fachgebiet Hochspannungstechnik

Generating High Alternating Voltages 

Tests at alternating voltage are performed as 

• Design Test (which is not defined acc. to IEC) 

• Type test 

• Routine Test 

• Acceptance Test 

• Commissioning Test, On-Site-Test 

Fachgebiet 

Hochspannungstechnik 

High-Voltage Technology / Chapter 2 - 1 -


Required test voltage levels up to 2 MV (extreme value: 3 MV) 

Example 1: 

Standard short-duration power-frequency withstand voltage up to 3·U m 

• for type, routine and acceptance tests: factor of 1.1 required 

• for design tests at least factor of 1.4 recommended 

(U m = 245 kV ⇒ 

Standard short-duration power-frequency withstand voltage 

max. 460 kV factor of 1.4 644 kV) 

Example 2: 

Line-to-earth voltage at U m = 800 kV: U LE = 462 kV 

factor of 1.1: 508 kV 

Line-to-earth voltage at U m = 1200 kV: U LE = 693 kV 

factor of 1.1: 762 kV 





Root mean square value of a voltage: 

T 

1 2 

Urms 

u() 

t dt 

= ∫ 

T 

0 

(root mean square value) 

• Usually the voltage shape is not ideally sinusoidal. 

• In high-voltage technology in most cases only the peak value is of interest. 

⇒ Specification of a test voltage: 

û 

U 

rms 

= (IEC 60060-1, 1989-11) 

2 





Requirements (IEC 60060-1): 

Deviation from sinusoidal shape: 

û 

2 5% 

U = ± 

(which is more or less the same as a requirement for a 

harmonics content < 5%) 

rms 






Differences in peak values of two 

subsequent periods



Voltage dips (even if transient) 



Usually, the requirements are fulfilled automatically if the transformer 

short-circuit current is: 

For material investigations 

0.1 A for tests under dry conditions 

on just small test samples of solid or liquid insulation or 

a combination of both 

For tests on high-voltage equipment 

• at least 0.1 A for tests on self-restoring, external insulation 

under dry conditions 

• 0.5 A to 1 A under rain 

For tests under artificial pollution 

15 A or more (!!) 






Further possible means to fulfill the requirements: 

Connection of a supporting capacitor 

C >= 1 nF to the high-voltage terminals 

Damping resistance R = 1 kΩ ... 100 kΩ 

(or value such that τ = 10 µs) 

(avoidance of resonant oscillations, 

limiting of du/dt) 





Absolutely forbidden! 

Sudden connection of the transformer to the test sample 

at more than 50% of rated output voltage 

(else: resonant oscillations) 

⇒ Increase voltage slowly from zero! 




Differences Power Transformator – Test Transformator 

Power transformers: 

• mostly of three phase design (but also single phase) 

• tank and core always earthed 

• voltage regulation stepwise by tap changer 

• optimized for high power, high continuous and high short-circuit currents 

⇒ mechanical stress! (dynamic short-circuit current forces) 

⇒ thermal stress! (cooling ducts between winding layers; 

cooling by forced circulation; Buchholz relay) 

• insulated for high overvoltages 

Bilder Leistungstransformatoren 




Differences Power Transformator – Test Transformator 

Test transformers: 

• only of single phase design 

• tank either earthed or on potential 

• tank often made from insulating material 

•coreeither earthed or on potential 

• very precise (nearly stepless) voltage regulation on the lv side 

by help of a regulation transformer; no regulation on the hv side 

• optimized for short-time duty 

• very high rated voltages, but not insulated against 

high overvoltages 

• mainly capacitive loading 




Capacitive Loading 

Kind of equipment 

Station posts, line insulators 

Bushings 

Inductive instrument transformers 

Power transformers < 1 MVA 

Power transformers > 1 MVA 

Cables 

Gas-insulated lines (GIL) 

Gas-insulated switchgear (GIS) 

Capacitance (estimated) 

20 pF 

100 pF ... 500 pF 

200 pF ... 500 pF 

1 nF ... 3 nF 

1 nF ...25 nF 

200 pF/m ... 700 pF/m (300 pF/m) 

60 pF/m 

1 nF ... 10 nF 





• Test voltage 1 MV 

• Capacitive load 3,2 nF 

Current 

I = U⋅ω⋅ C = ⋅ ⋅ ⋅ ⋅ ⋅ = 

6 −9 

110 V 2π 

50Hz 3,210 F 1A 

Power (leading reactive power) 

S U C 

( ) 2 

2 6 −9 

= ⋅ω⋅ = ⋅ ⋅ π ⋅ ⋅ ⋅ = 

110 V 2 50Hz 3,210 F 1MVA 





Additional capacitive loading by 

• shielding electrodes 

• high voltage connections 

• voltage measurement 

(e. g. capacitive divider) 

 

Up to 100 kV test voltage ⇒ 

100 pF 

Up to 1000 kV test voltage ⇒ 1000 pF 




Capacitive Loading by Shielding Electrodes 

Sphere electrode for 1 MV alternating voltage 

Goal: to chose the radius 

such that there are no 

partial discharges. 

Maximum electrical field stress 

on the electrode surface: 

E 

max 

U 

= 

r 

K 

K 





Maximum allowed electrical field 

stress in air to achieve freedom 

from partial discharges: 

E max = 25 kV/cm 

Required sphere radius: 

r 

K 

U 

1,4 MV 

K 

= = = 

Emax 

25 kV/cm 

56 cm 





Capacitance of a sphere to ground 

(ground in infinite distance): 

C K = 4 πε 0 ε r r k 

ε 0 = 8,86 pF/m 

C K / pF = 1,11 r K / cm = 62 pF 

⇒ capacitive current: 20 mA 

⇒ reactive power: 20 kVA 




Basic Design of Test Transformers 

Grading electrodes for field stress reduction 

High-voltage winding 

Energizing winding 

Core 

Optimal capacitive voltage distribution by close winding 

and approximately trapezoidal shape of high-voltage winding 




Basic Design of Test Transformers 

How to get even capacitances between the individual layers? 

Length l 

Radius r 

Distance d 

Capacitance between two layers: 

A 2π 

r⋅l 

C ≈εε 

0 r⋅ = εε 

0 r⋅ (for d

Transformer Equivalent Electric Circuits 

Φ h 

I 1 I 2 

U 1 

U 2 

Φ 1σ 

Φ 2σ 

Electrical circuit 

U 

I 

R 

G = 1/R 

Magnetical circuit 

N·I 

Φ 

l m 

/µA 

Λ = µA/l m 

Φ nσ ... Stray flux of winding n 

Φ h ... Common working flux 

N·I ... magnetic potential, ampere-turns 

Φ ... magnetic flux 

µ= µ 0·µ r 

µ 0 ... Permeability of the vacuum 

4π·10 -7 = 1,256·10 -6 Vs/Am 

µ r ... relative Permeability 





I Φ h 

1 

I 2 

X 2σ 

R 1 X 1σ 

R 2 

U 1 

U 2 

X σ = ωL σ (with: L σ = N 2 Λ σ ) 

X σ ..."Stray reactance" 





I' 1 

I 2 

X 2σ 

R' 1 X' 1σ 

R 2 

U' 1 

U 2 

I µ 

X h 

R Fe 

I Fe 

X h ..."Magnetizing reactance" 

X σ ..."Stray reactance" 

Conversion of dashed values: 

U 

N 

N 

' 2 

1 = ⋅U1 

1 

I 

N 

N 

' 1 

1 = ⋅ I 1 

2 

R 

2 

⎛N 

⎞ 

= ⋅ R 

⎝ ⎠ 

' 2 

1 ⎜ 

1 

N 

⎟ 

1 

X 

' 2 

1σ 

⎜ 

1σ 

N 

⎟ 

1 

2 

⎛N 

⎞ 

= ⋅ X 

⎝ ⎠ 

Magnitudes: 

• R 

≈ 

R 

' 

1 2 

X 

≈ 

X 

' 

1σ 

2σ 

• for big transformers: 

3 4 

• X 

h, RFe 

≈10 ...10 ⋅X σ 

X 

≈ 1,2 

20...30 ⋅ 

σ 

R1,2 





Simplified transformer equivalent electric circuit 

I 

R k 

X k 

U' 1 U 2 

reduced to short - circuit impedance Z = R + X = R + jωL 

k k k k k 




Performance at Capacitive Loading 

I 

I 

R k X k R k X k 

U' 1 C T U 2 

C a U' 1 U 2 

C 

R k·I 

jωL k·I 

C = C T + C a 

U 2 

U' 1 

X k = ωL k 

I 

U' 1 = R k·I + jωL k·I + U 2 





The mainly capacitive loading of test transformers 

(as virtually always the case) 

results in a 

Magnification of the secondary voltage! 

⇒ The secondary voltage can only roughly be estimated from the 

transformer ratio. 

⇒ The measurement of the high-voltage has to be performed 

directly on the hv side of the transformer. 





Calculation of capacitive voltage magnification 

at loading with rated voltage and current 

1 st step: further simplification of the equivalent electrical circuit by 

neglecting the ohmic losses .... 

I 

X k 

U' 1 U 2 

C 

10 

8 

6 

y = 1/(1-0,1x) 

y 

U 

U′ 

2 

1 

= 

1 

2 

1−ω 

LC 

k 

4 

2 

0 

0 2 4 6 8 10 

x 







2 nd step: Calculation of relative short-circuit voltage 

for the particular case .... 

U k = I n ·Z k = I n · ωL k 

Short-circuit voltage = voltage that must be applied to the primary side 

in order to drive nominal current in case of a shorted secondary side 

u 

k 

U In 

⋅ωL 

U U 

k 

= = 

2, n 2, n 

k 

I = U ⋅ωC 

n 

2, n 

(Assumption that the current through C 

at rated voltage just equals rated current.) 

u 

k 

2 

= ω L C 

k 







3 rd step: insert the relative short-circuit voltage .... 

U 

U 

U 

U 

2 

′ 

1 

2 

′ 

1 

= 1 

2 

1 − ω LC 

1 

= 

1 − u 

k 

k 

u 

k 

2 

= ω L C 

k 

Relative short-circuit voltage u k = 20% 

⇒ Voltage magnification at capacitive loading 

with rated current: 25%! 




Design Variants of Test Transformers 

2 different circuit variants: 

x 

HW 

U isol 

One pole insulated 

K 

EW 

x 

x 

Fully insulated 

K 

HW 

U isol 

x 

Center tap earthed 

⇒ balanced-to-earth voltage 

EW 

One of the external terminals earthed 

⇒ non balanced-to-earth voltage 





Design variants 

Epoxy resin insulated 

Oil insulated 

SF 6 -insulated 

Tank type 

Insulated enclosure type 

Single stage 

or 

Multi-stage (cascading) 





Test transformer with epoxy resin insulation 

(very simplified depiction) 

1 High-voltage winding 

2 Energizing winding 

3 Iron core 

4 Base 

5 High-voltage terminal 

6 Epoxy resin insulation 

Up to rated voltages of about 100 kV 





Test transformer with epoxy resin insulation 

(very simplified depiction) 

MWB test transformer 2 x 50 kV 

"piglet" 





Oil insulated tank type test transformer 



3 Iron core 

4 Base 


6 Bushing 

7 Metal tank 






Rated voltages up to about 400 kV (800 kV) 

• good cooling (metal tank!) 

• high power 

• comparatively small oil volume 

• expensive bushing 

• expensive shielding electrode 






Aktive parts 550 kV (MWB) 

• aufwendige Durchführung 

• aufwendige Schirmelektrode 






TuR, 600 kV, 3.33 A 

• aufwendige Durchführung 

• aufwendige Schirmelektrode 





Oil insulated enclosure 

type test transformer 



3 Iron core 

4 Base 


8 Insulated enclosure 







• no bushing required 

• simple shielding electrode 

• large oil volume 

• bad cooling 







HighVolt, 100 kV 

MWB, 100 kV 

HAEFELY, 100 kV 







MWB, 550 kV 





9 

3 2 

1 

5 

SF 6 -insulated test transformer 



3 Iron core 


9 Pressure resistant vessel (Steel, Aluminum) 





9 

3 2 

1 

5 


• compact, lightweight 

• direct connection to GIS/GIL 






Direct coupling of an SF 6 -insulated 

test transformer to the GIS in order to 

avoid mounting of an open air test bushing 

alternatively… 









Transformer Cascades 

Cascading of transformers usually above about 300 kV (latest above 800 kV) 

Energizing winding 

Coupling winding 

High-voltage winding 

⇒ Three-winding transformer 

max. 4 stages! 





With the exception of the bottom stage 

all stages must be installed insulated. 

U 

2/3 U 

1/3 U 





First 1-MV test transformer 

cascade worldwide 

(year of manufacturing 1923) 

(2·500 kV in phase opposition) 





Two stage transformer cascade (2·600 kV, 2 A continuous duty) 





2-stage cascade 

1.8 MV 

HVTS 





TU Darmstadt 

1.2 MV (2·2·300 kV) 





Top transformer 

Intermediate transformer 

U 

Siemens Berlin 

1.8 MV (3·600 kV) 

TuR 

Base transformer 





Power requirements 

Energizing winding base stage: 300% 

Coupling winding base stage: 200% 

Energizing winding top stage: 100% 

Energizing winding intermed. stage: 200 % 

Coupling winding intermed. stage: 100% 





Compensation of capacitive reactive power by reactors 





... also as insulated enclosure type 

3·100 kV 

2·100 kV 






Siemens Berlin 

2·400 kV 

TuR 






EDF, Les Renardières 

MWB, 1.65 MV 

Lab closed! 





Cascade arrangement with 2 stages on one core 

Core at 50% potential 

Lower requirements on hv 

winding insulation against 

core 

Requires two bushings 

One energizing winding 

used as energizing winding 

The other energizing 

winding used as coupling 

winding for the next stage 

Leakage suppressing 

windings to balance the 

magnetic flux 

⇒ reduced stray reactance 






... epoxy resin insolated 

MWB 

2 x 50 kV 






... tank type 

TuR 






... tank type 

2-stage 

1.2 MV, 1.25 A 






... tank type 

2-stage 

TuR 






... tank type 

3-stage 

2.25 MV (3·2·375 kV), 1 A 

TuR 






... tank type 

TU Munich 

1.2 MV (3·2·200 kV) 

Siemens 





Largest test tranformer cascade worldwide (3 MV, 12.6 MVA, Moscow 1991) 

Modell 

TuR 





Largest test tranformer cascade worldwide (3 MV, 12.6 MVA, Moscow 1991) 

Siemens 

(TuR) 






... insulated enclosure type 

2·400 kV CESI (Milano) 

2·400 kV 

2·400 kV 




Short-Circuit Reactance of Transformer Cascades 

Electrical equivalent circuit diagram 

of a 3-stage cascade 

If losses are neglected and under the assumption 

of even winding ratios: 

N 

N 

Eν 

Hν 

= 

N 

N 

Kν 

Hν 

for ν = 1...3 





Equivalence of power for both equivalent circuit diagrams: 

3 

2 '2 ' '2 ' 2 

H⋅ res 

= ∑ Eν ⋅ 

Eν + 

Kν ⋅ 

Kν + 

Hν ⋅ 

Hν 

ν = 1 

( ) 

I X I X I X I X 





3 

∑( ν ν ν ν ν ν) 

I ⋅ X = I ⋅ X + I ⋅ X + I ⋅X 

2 '2 ' '2 ' 2 

H res E E K K H H 

ν = 1 

not present! 

I ⋅X I ⋅X I ⋅X I ⋅X I ⋅X I ⋅X 

X X X X 

'2 ' '2 ' '2 ' '2 ' '2 ' '2 ' 

E1 E1 K1 K1 E2 E2 K2 K2 E3 E3 K3 K3 

res 

= + + 

2 2 H1 + + + 

2 2 H2 

+ + + 

2 2 

H3 

IH IH IH IH IH IH 





I = I = I = I ⇒ I = I = I 

' ' '2 '2 2 

E3 K2 H E3 K2 

H 





I = I = I 

'2 '2 2 

E3 K2 H 

3 

∑( ν ν ν ν ν ν) 

I ⋅ X = I ⋅ X + I ⋅ X + I ⋅X 

not present! 

2 '2 ' '2 ' 2 


ν = 1 

res 

'2 ' '2 ' '2 ' '2 ' '2 ' '2 ' 

IE1 ⋅XE1 IK1 ⋅XK1 IE2 ⋅XE2 IK2 ⋅XK2 IE3 ⋅XE3 IK3 ⋅XK3 

= + + 

2 2 H1 + + + 

2 2 H2 

+ + + 

2 2 


H3 

X X X X 

I ⋅X I ⋅X I ⋅X 

X X X X X X 

'2 ' '2 ' '2 ' 

E1 E1 K1 K1 E2 E2 ' 

' 

res 

= + + 

H1+ + 

2 2 2 

K2 

+ 

H2 

+ 

E3+ 

H3 

IH IH IH 





I = I = 2⋅ I = 2⋅I ⇒ I = I = 4⋅I 

' ' '2 '2 2 

E 2 K1 H E2 K1 

H 






X X X X X X 

'2 ' '2 ' '2 ' 

E1 E1 K1 K1 E2 E2 ' 

' 

res 

= + + 

H1+ + 

2 2 2 

K2 

+ 

H2 

+ 

E3+ 

H3 

IH IH IH 

I = I = 4⋅I 

'2 '2 2 

E2 K1 H 

3 

∑( ν ν ν ν ν ν) 

I ⋅ X = I ⋅ X + I ⋅ X + I ⋅X 

I ⋅ X 

X X X X X X X X 

'2 ' 

E1 E1 ' ' ' ' 

res 

= + 4⋅ K1+ 2 

H1+ 4⋅ E2 

+ 

K2 

+ 

H2 

+ 

E3+ 

H3 

IH 

not present! 

2 '2 ' '2 ' 2 


ν = 1 

res 

'2 ' '2 ' '2 ' '2 ' '2 ' '2 ' 

IE1 ⋅XE1 IK1 ⋅XK1 IE2 ⋅XE2 IK2 ⋅XK2 IE3 ⋅XE3 IK3 ⋅XK3 

= + + 

2 2 H1 + + + 

2 2 H2 

+ + + 

2 2 


H3 

X X X X 

I = I = I 

'2 '2 2 

E3 K2 H 





I = 3⋅ I = 3⋅I ⇒ I = 9⋅I 

' '2 2 

E1 H E1 

H 





I 

= 9⋅I 

'2 2 

E1 H 

3 

∑( ν ν ν ν ν ν) 

I ⋅ X = I ⋅ X + I ⋅ X + I ⋅X 

X = 9⋅ X + 4⋅ X + X + 4⋅ X + X + X + X + X 

' ' ' ' ' 

res E1 K1 H1 E2 K2 H2 E3 H3 

( ) 

X = X + X + X + X + X + 4⋅ X + X + 9⋅X 

' ' ' ' ' 

res H1 H2 H3 K2 E3 K1 E2 E1 

not present! 

2 '2 ' '2 ' 2 


ν = 1 

res 

'2 ' '2 ' '2 ' '2 ' '2 ' '2 ' 

IE1 ⋅XE1 IK1 ⋅XK1 IE2 ⋅XE2 IK 2 

⋅XK 2 

IE3 ⋅XE3 IK3 ⋅XK3 

= + + 

2 2 H1 + + + 

2 2 H2 

+ + + 

2 2 


H3 

X X X X 

I = I = I 

'2 '2 2 

E3 K2 H 


X X X X X X 

'2 ' '2 ' '2 ' 

E1 E1 K1 K1 E2 E2 ' 

' 

res 

= + + 

H1+ + 

2 2 2 

K2 

+ 

H2 

+ 

E3+ 

H3 

IH IH IH 

I = I = 4⋅I 

'2 '2 2 

E2 K1 H 

I ⋅ X 

X X X X X X X X 

'2 ' 

E1 E1 ' ' ' ' 

res 

= + 4⋅ K1+ 2 

H1+ 4⋅ E2 

+ 

K2 

+ 

H2 

+ 

E3+ 

H3 

IH 





( ) 

X = X + X + X + X + X + 4⋅ X + X + 9⋅X 

' ' ' ' ' 

res H1 H2 H3 K2 E3 K1 E2 E1 

The resulting short-circuit reactance of a multi-stage cascade 

is higher than the linear sum of the individual stage reactances! 




Series Resonant Circuits 

Resonant frequency 

f 

0 

= 

1 

2π 

LC 

jωL·I 

U 2 

U' 2 

Voltage magnification 

Q-factor 

Characteristic impedance 

U 

U 

Q 

Z 

2 

' 

2 

K 

= 

= Q 

ZK 

= 

R 

L 

C 

R T·I 

jωL T·I 

U' 1 

I 





Drive 

core movement 

Lead screw (the core is actually in 100% position) 

Fixed part of the core 

Active part of a high-voltage reactor with variable core 





Advantages: 

• Extremely low content of harmonics in high-voltage 

• Very low short-circuit power 

• Only low power requirements on transformer (just compensation of losses) 

• Weight of high-voltage reactor only one third of that of a comparable 

high-voltage transformer 





1 

3 

4 

5 

Adjustable 

high-voltage reactor 

Q ≈ 50 

C max /C min ≈ 20 (because Lmax/Lmin ≈ 20) 

Specific weight 3...8 kg/kVA 

4 

2 

3 

Variable frequency 

5 

Q ≈ 100...200 

C max /C min = 225 (because f = 20...300 Hz) 

Specific weight 0,8...1,5 kg/kVA 





Mobile series resonant test setup of variable frequency; 150 kV/90 A 

HighVolt 

Energizing transformer 

Reactor (tank type) 

Coupling capacitor for partial 

discharge measurement 





On-site testing with a two stage series resonant test setup 700 kV 

HVTS 

Transformer! 





Three stage series resonant test setup 1200 kV/4 A 

with adjustable reactors 

HighVolt 





On-site testing with a three stage series resonant test setup 

GIS 765 kV, ESKOM, South Africa 













Research project at Fachgebiet 

Hochspannungstechnik of TUD 




Power Supply of Test Transformers 

Most common: 

Regulation transformer fed from the line (low or medium voltage) 

But also: 

Motor-Generator set 

• Synchronous converter 

• Ward-Leonard converter 




Protection Circuit 

Damping resistance R = 1 kΩ ... 100 kΩ 

(or value such that τ = 10 µs) 

(avoidance of resonant oscillations, 

limiting of du/dt) 




Protection Circuit 

Current breaker and short-circuiter with power electronic devices 

Regulation - 

transformer 

Thyristor - 

short-circuiter 

Thyristor breaker 

Test - 

transformer 




Basic Insulation Levels acc. to IEC 60071-1 

Range I: 

U m = 1 kV up to and including U m = 245 kV 

The standard voltage values are 

all the same for 

• Phase-to-earth-, 

• Phase-to-phase-, 

• Longitudinal 

insulation!

X - Fachgebiet Hochspannungstechnik

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