HIGH VOLTAGE TESTING

GENERATION AND MEASUREMENTS

1. INTRODUCTION

why high voltage test ?

2. HIGH VOLTAGE GENERATION

a) Generation of direct high voltages

b) Generation of alternating high voltages

c) Generation of impulse voltages

3. HIGH VOLTAGE MEASUREMENTS

a) Measurement of direct voltages

b) Measurement of alternating voltages

c) Measurement of impuls voltages

4. GENERATION OF HIGH IMPULSE CURRENTS

5. MEASUREMENT OF IMPULS CURRENTS

a) Rogo wski coil method

b) Current shunt method

Difinition : high voltage = voltage > 1 kV

Aims of HV test

i) Test HV equipment used in power systems

Non-destructive test (NTD) consist of

• Type tests • Routine tests

• Monitoring tests

Used to measure

• Loss angle • Partial disharge level

• Withstand level

ii) Study of overvoltage effects on equipment

External overvoltages (lightNing)

System overvoltages (switching, temporary)

1. INTRODUCION

HIGH VOLTAGE TESTS RELATED

STANDARDS AND ORGANISATION

IEC : International Electrotechnical Committee

BSI : British Standard Institution

i) IEC 60 (Part I and Part II) = BS 923 : high voltage testing techniques

ii) IEC 71 (Part I and Part II) = BS 5622 : insulation Co-ordination

iii) IEC 52 = BS 358 : Method for the measurement of voltage with sphere

gaps.

iv) Other bodies

CIGRE : Conférence International des Grands Réseaux Electriques

ISH : International Symposium on High Voltages Engineering

2. HIGH VOLTAGE GENERATION

a) Generation Of High Direct Voltage

Existing methods

• Rectifier circuits

• Multiplier rectifier circuits

• Cascaded transformer/rectifier circuits

• Special circuits; Engetron, Deltatron

• Electrostatic generators

V

iL (t)

RL (load)

(a)

C

c

DV~(t)

i (t)

a

b

h.t

transformer

i(t)

V(t)

V min

V max

t

V~ (t)

(b)

T = 1/f

I) Rectifier Circuits : Background definition

i) Ripple δδδδV :

which gives

To reduce ripple

1. Increase size of smoothing capacitor

2. Increase frequency

3. Increase number of phases (if possible)

ii) Amlitude V

iii) Ripple Factor

iv) Ripple Factor

Ripple factor ≤≤≤≤ 3 %

Regulation

Change / variation of voltage

1 minute tests →→→→ ≤≤≤≤ 1%

More than 1 minute tests →→→→ ≤≤≤≤ ±±±± 1%

Voltage doubler circuit

Greinacher doubler circuit = (Villard doubler + rectifier/smoothing) circuit

Loaded multipliers

If unloaded the diodes do not conduct and the ripple is

Because of charge transfer, the capacitors Ci are charged to a voltage

equal (Vci - ∆Vi)

The total voltage drop is then,

2 V

2 V

2 V1

1

1

1

2

2

2

8 V

4-stage cascade rectifier

Voltages at nodes ‘1’ are oscillating

Voltages at nodes ‘1’ are constant 9dc)

HV out is 2* 4* Vmax

2nVmax

(no load)

Vo (t) with load

Vo max

+ Vmax

t2t1

T = 1/f

V (t)t

0

∆Vo

2 δ V

Loaded cascade circuit, definitions of voltage drop ∆ Vo and ripple δV

GENERATION OF ALTERNATING

VOLTAGE

i) Test transformer ii) Casaded transformer iii) Resonant circuit

I. Test Transformer

Simple design ; usually, one side is earthed

IEC Standard Specifikations

� f = 45 – 65 Hz

� Shape :

�V (1 minute test) ≤ 1%

� Transformer ratings

* dry tests: current rating of ≥ 100mA

* wet tests: current rating of ≥ 500mA

* pollution tests: current rating ≅ 15 A

3

3 V

3

3

3P 2

2P 2P

22V

2

1

1

1

P

P

PP

I = P/V

V

Transf. I

Transf. II

Transf. III

Cascaded transformers

- Power is shared by the three units

- Voltage stress is reduced (shared along the cascade)

Basic circuit of cascaded transformers. (1) Primary widings. (2) Seconddary

h.t. widings. (3) Tertiary exciting windings.

Series Resonant Circuits

For testing large capacitive loads (cable and GIS) and inductive loads

(HV reactors)

~

R L

V1 V

2

For an RLC series circuit the resonant frequency is

The output voltage acros the test object is

At resonance, Zc = Zl

In practice Zc >> R

Which gives V2 >> V1 (20 to 50 times)

Advantages

• Better waveshape (fundamental times 50)

• Better

rkVA output

• Voltage callapse after flashover (no damage to equipment)

• Easily cascaded, lighter than transformers.

Resonant Circuits

a) Previous designs (no reactors)

~ CVi f

~ CVi f

b) Modern designs

GENERATION OF IMPULSE VOLTAGE

Why impulse voltage

To study the effect of transient overvoltages generated by lightning

or swiching operations on the system

I. Standard Definition of Lightning Surges

T1 front time 1.2 µs

T2 time to half-value 50 µs

Surge defined T1/T2 = 1.2 /50 µs

Tolerances : (IEC60, BS923)

peak value ± 3%

front time ± 30%

time to half-value ± 20%

II. Standard Definition of Switching Surges

Tp time to peak (also known as T1) 250 µs

T2 time to half-value 2500 µs

TD time above 90% is sometimes specified

Surge defined as T1 /T2 = 250/2500 µs

Tolerances : (IEC60, BS923)

peak value ± 3%

front time ± 20%

Time to half-value ±60%

C2

R2

C1

G R1

Vo

Vout(t)

Analysis of impulse generators

With β>>α

Parameters of V out (t)

-Time to peak

- Time to half peak value