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
1. INTRODUCION 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 Monitoring tests Used to measure Loss angle Withstand level Routine tests Partial disharge level ii) Study of overvoltage effects on equipment External overvoltages (lightning) System overvoltages (switching, temporary)
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) iii) iv) IEC 71 (Part I and Part II) = BS 5622 : insulation Co-ordination IEC 52 = BS 358 : Method for the measurement of voltage with sphere gaps. 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~(t) D il (t) b c i (t) C V R L (load) h.t transformer a (a)
V max V(t) V min i(t) V~ (t) t T = 1/f (b)
I) Rectifier Circuits : Background definition i) Ripple δv : 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 C i are charged to a voltage equal (V ci - V i ) The total voltage drop is then,
4-stage cascade rectifier Voltages at nodes 1 are oscillating Voltages at nodes 1 are constant 9dc) 1 2 HV out is 2* 4* V max 1 1 2 V 2 1 2 V 8 V 2 2 V
V o 2nV max (no load) 2δV V o max Vo (t) with load + V max t 1 t 2 T = 1/f V (t) t 0 Loaded cascade circuit, definitions of voltage drop V o 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
Cascaded transformers - Power is shared by the three units Transf. III - Voltage stress is reduced (shared along the cascade) 3 I = P/V Transf. II 3 P 1 P Transf. I 1 2 3 2P 2P P 3 V 1 P V 2 2V 3P 2 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 V ~ 1 V 2 For an RLC series circuit the resonant frequency is The output voltage acros the test object is At resonance, Z c = Z l In practice Z c >> R Which gives V 2 >> V 1 (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) V i f ~ C
b) Modern designs V i f ~ C
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 T 1 front time 1.2 µs T 2 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 T p time to peak (also known as T 1 ) 250 µs T 2 time to half-value 2500 µs T D time above 90% is sometimes specified Surge defined as T 1 /T 2 = 250/2500 µs Tolerances : (IEC60, BS923) peak value ± 3% front time ± 20% Time to half-value ±60%
Analysis of impulse generators G R 1 V o C 1 R 2 C 2 V out (t)
With β>>α Parameters of V out (t) -Time to peak - Time to half peak value