This blog is created as part of University Technology Malaysia (UTM) M.Engg course assignment for the High Voltage Insulation & Co-ordination subject. UTM provides multiple opportunities to the students to enhance their knowledge and skills on the latest technologies. As part of university’s ambitious plan our Professor Dr. Hussein Bin Ahmad assigned this assignment to the students to develop this blog for boosting the student’s confidence at e-skills and the knowledge on latest technological developments with respect to the subject. I sincerely thank my beloved Prof. Dr. Hussein and UTM for providing such a great opportunity to me and my classmates.

The blog content is based on internet resources and most of the details collected from Indian Institute of Technology (IIT) Kanpur. I would like to thank IIT as well in this opportunity for providing such valuable information on line for the global students benefit.

This blog contents the details of high voltage testing is one of the most important chapter at high voltage 7 insulation co-ordination.

High Voltage Testing

Introduction

Reliability and safe operation of all electrical equipment depend on the integrity of its insulation. A high potential test is used to examine this integrity and to determine the ability of the insulation to meet its design specification.  Basically high voltage tests often called as withstand voltage test, flash over test, break down test or dielectric strength test. This high voltage test performed to a equipment with a voltage higher than its normal working voltage, for a specific period of time, to discover if the insulation withstands or breakdown under that voltage.

By definition, a breakdown test is a destructive technique to measure the dielectric strength of insulation and usually made on a sample piece of material. However because of the equipment high cost usually performed this test by a non destructive technique designed to ensure that level of insulation is adequate for service condition.

The purpose of high voltage tests from customer’s view point is to demonstrate that the apparatus has met the guaranteed specifications of dielectric strength. From manufactures standpoint, the tests ensures the insulation design of the equipment, determination of insulation strength of the materials used in the equipment and a guide to the development of better equipment.

To perform this high voltage testing, generation of desired or required high test voltage is indeed a greater challenge. This blog presents the details of generation of high voltage.

Generation of High Test Voltages

This includes the following methods of generation of high voltages.

-          Power frequency high test voltage generation

·         Transformers in cascade using

Ø  Single Testing Transformer

Ø  Transformers in cascade

·         Resonance Transformers

-          Generation of high DC voltage

Ø  Basic Half wave rectifier circuit

Ø  Multistage voltage doubler circuit in cascade

-          Impulse Voltage Generator

Ø  Single stage impulse generator

Ø  Multistage impulse generator

Power Frequency high test voltage generation                              

          This section captures

                                    Single testing transformers and

                                    Transformers in cascade

                                    Disadvantage of cascading

Single Testing Transformers

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 are produced maximum upto 700 kV.

Fig .1  A 600 kV, 3.33A Testing Transformer for continuous operation [Courtesy TUR Dresden, Germany]

For the production of higher voltages, a number of identical units are put in cascade to add up their voltages as shown in Fig 2.

Fig .2   A.C. Testing Cascade, 1.2 MV, 1.25 A, short-time operation 4 hours (at an ambient temperature of 35⁰C). Transformer tanks made of sheet aluminum [TUR Dresden]

Cascaded transformers

Cascading a number of single identical units makes transportation, production and erection simpler.

The cascading principle is illustrated with the basic scheme shown in Fig. 3 below in which it can be seen that output of a stage transformer becomes input for the next stage.

Fig 3  Three Transformers in cascade
(1) Primary windings,
(2) Secondary, HV, windings,
(3) Tertiary/ excitation windings (4) Core

·         The HV supply is connected to the primary winding "1" of transformer I, designed for a HV output of V. The other two transformers too are connected in the same fashion.

·         The excitation winding "3" of Transformer I supplies the primary voltage for the second transformer unit II; both windings are dimensioned for the same low voltage, and the potential gain is fixed to the same value V.

·         The HV or secondary windings "2" of both units are connected in series, so that a voltage of 2 V is produced at the output of 2nd unit.  The unit III is added in the same way.

·         The tanks or vessels containing the active parts (core and windings) are indicated by dashed lines.

·         For a metal tank construction and the HV windings shown in this basic scheme, the core and the tank of each unit would acquire the HV level of the previous unit as indicated. Only the tank of transformer I is earthed.

·         The tanks of transformers II and III are at high potentials, namely V and 2 V above earth, and must therefore be suitably insulated, hence raised above the ground on solid post insulators.

·         Through HV bushings the leads from the excitation windings "3", as well as the tapings of the HV windings "2", are brought   to the next transformer.

For voltages higher than about 600 kV, the cascade of such transformers is a big advantage. The weight and the size of the testing set are sub-divided into single units of smaller size and lower weight. The transportation and erection of the test set in cascade becomes simpler. However, there is a disadvantage that the primary windings of the lower stages are more heavily loaded with higher current in such sets.

There are several methods of designing the cascade test sets. In Fig. 4 schematic diagram of another power frequency test set cascade of 3 x 750 = 2250 kV rating is shown. This circuit has a third winding, known as "Balancing Winding''. These windings are designed to acquire the intermediate potentials between two stages. In this circuit, the transformers of the upper stages have their excitation windings arranged over the HV windings of the transformers of the lower potential. Fig. 5 shows a photograph of this test set installed in open air.

Fig. 4    Schematic of an ac test set circuit in cascade

Disadvantage of cascading transformers

The disadvantage of transformer cascading is the heavy loading of primary windings for the lower stages.

In above fig.3 this is indicated by the figure P, the product of current and voltage for each of the coils.

For this three-stage cascade the output kV A rating would be 3P, and therefore each of the H.V. windings "2" would carry a current of I = P/V.

Also, only the primary winding of transformer III is loaded with P, but this power is drawn from the exciting winding of transformer II.

Therefore, the primary of this second stage is loaded with 2P. Finally, the full power 3P must be provided by the primary of transformer I.

Thus an adequate dimensioning of the primary and exciting coils is necessary.

Resonance Transformers

Tuned Resonant Circuits for HV ac power frequency test equipment.

Testing of HV equipment having high capacitance, for example, long length of HV power cables, power capacitors, GIS etc. may draw excessive capacitive charging current. Necessity for "Tuned series resonant HV power frequency test equipment" arose in particular by the cable manufacturing industry when they required testing long lengths of HV cables drawing large capacitive current on the HV side.

 Fig 6 shows such a circuit. The capacitance Ct represents the capacitance of the test object. A variable reactor is connected on the LV (primary) winding of the test transformer. If the inductance of this reactor is tuned to match the impedance of the capacitive load, the capacitive power can be completely compensated.

The equivalent circuit diagram for this is a low damped series resonant circuit. The high output voltage can be controlled by a variable ac supply, i.e. a voltage regulator transformer (Feed Transformer) if the circuit was tuned before. The Feed Transformer is rated for the nominal current of the inductor and its voltage rating could be very low.

Fig 6 Series resonant circuit for transformer/reactor.
(a) Single transformer/reactor.
(b)  Two or more units in series.

For simple high capacitive loads the series resonant circuit, shown in Fig. 6  and also in Fig 7 are suitable. However, for high capacitance and ohmic loads (loads with high real power losses), the parallel resonant circuit shown in Fig 7.b is more suitable. Both these series and parallel circuits can be made at the same system by changing the connections of the variable reactor 'L' . Fig 8 shows a HV variable reactor which is tuned automatically to the desired value of the capacitive load.

Fig. 7 Series and Parallel resonant transformer circuit.

Fig. 8  A variable reactor

Advantages of the Series Resonant Circuit

Dimensions and weight of such test sets are much smaller.

100% compensation of capacitive reactive power is possible. Under this condition, the only power drawn from the mains is the active power required.

The magnitudes of the short circuit currents, in case of insulation failure, are minimized.

The voltage wave shape is improved by attenuation of harmonic components already in the power supply. A practical figure for the amplification of the fundamental voltage amplitude at resonance is between 20 and 50 times. Higher harmonic voltages are divided in the series circuit to a decreasing proportion across the capacitive load. Good wave shape helps accurate HV measurement and it is very desirable for Schering Bridge measurements.

The power required from the supply is lower than the kVA in the main test circuit. It represents only about 5% of the main kVA with a unity power factor.

If a failure of the test object occurs, no heavy power arc will develop, as only the load capacitance will be discharged under this condition. As the voltage collapses, immediately the load capacitance is short-circuited. This has been of great value to the cable industry where a power arc can sometimes lead to the dangerous explosion of the cable termination. It has also proved invaluable for the development work as the weak part of the test object is not completely destroyed. Additionally, as the arc is self-extinguishing due to this voltage collapse, it is possible to delay the tripping of the supply circuit. This results in a recurring flashover condition with low energy level, making it simpler to observe the path of   the flashover in air over the surface of the test object.

By detuning the resonant circuit, breakdown or disruptive discharge can be achieved

Generation of high dc voltage, Voltage Multiplier Circuits and Ripple Minimization

This section includes

-          High Direct Voltage generation

-          Basic Halfwave rectifier circuit and Ripple in output voltage

-          Multistage voltage doubler circuit in cascade

HIGH DIRECT VOLTAGES

In HV technology direct voltages are mainly used for pure scientific research work and for testing equipment used in HVDC transmission systems. HVDC test sets are also suitable as mobile test units for testing the equipment at site after installation since these are very light weight.

High dc voltages are even more extensively used in physics (accelerators, electron microscopy, etc.), electro medical equipment (x-rays), industrial applications (precipitation and filtering of exhaust gases in thermal power stations and cement industry; electrostatic painting and powder coating, etc.), or communications electronics (TV; broadcasting stations). Very high static voltages, produced by electrostatic generators, are used in nuclear physics.

Therefore, the requirements of voltage shape, voltage level, current rating, short - or long-term stability for every HVDC generating system may differ strongly from each other. With the knowledge of fundamental generating principles, it is possible, however, to select proper circuits for any special application.

The high dc voltages are generally obtained by means of rectifying circuits applied to ac voltage. Voltage doubler circuits in desired number are then used in cascade for the multiplication of the dc voltage. These are described in the following:

Half Wave rectifier circuit

 

Shown in the  Fig 9 is a basic half wave (single phase)rectifier circuit.

Fig 9  Half wave rectifier circuit

where:  D - Diode, C - smoothing capacitor, RL - the resistive load

In Fig 10 the voltage output of this circuit is shown

Fig 10  Voltages and ripple in the output

As it can be seen, the dc output voltage is not of constant magnitude. The periodic oscillation/deviation from its mean value due to load is known as ripple.

According to IEC 60/1962, the definition of ripple is the periodic deviation from the arithmetic mean value of the dc output voltage of the test set. The magnitude of the ripple is defined as half the difference between the maximum and minimum values of the dc output voltage during one cycle from the generator. The ripple factor is the ratio of the ripple magnitude to the arithmetic value of the dc output voltage.

 is the arithmetic mean value of the dc output, it is given as (defined by IEC and IEEE) following:

Where T represents the periodic time of oscillation or deviation from the mean value and its periodic oscillation frequency is given by:

Let the amplitude of the ripple be δU , then

δU = 0.5 (Umax-Umin)

And the ripple factor is given by  

, which should be less than 5% for the dc test voltages.

In the Fig 10 above, the time of conduction of the diode tc is equal to αT . Hence the charge Q transferred to the load RL is given by:

Considering the load current iL (t) I

but Q = 2δUC , therefore

Thus the ripple in the output voltage can be reduced by increasing the frequency of the ac voltage and also by increasing the capacitance of the smoothing capacitor.

, means failure of the insulation. This condition must be taken in to consideration for the rectifier current while designing the test equipment and smart tripping device should be provided at the transformer input to protect the equipment.

Voltage Doubler Circuit in Cascade

 

HV dc test sets are produced up to 2.0 MV using half wave rectifier circuit and voltage multiplier (doubler) circuit in cascade. In 1920 Greinacher proposed such a circuit for the first time which was improved upon by Cockcroft & Walton in 1932. In the Fig. 11(a) a n - stage, single phase voltage doubler circuit cascade is shown.

Fig 11 (a) Cascade circuit according to Cockcroft-Walton or Greinacher.
          (b) Waveform of potentials at the nodes of cascade circuit, no load.

Fig 11(a) shows a Multi-stage halfwave rectifier voltage doubler circuit in cascade by Cockcroft-Walton.

Considering the load current value to be zero, the steady state potentials at all the nodes of the circuit are shown in Fig 11(b). It can be seen that,

The potentials at the nodes 1', 2' ....n' oscillate because of the applied ac voltage U(t).

The potentials at the nodes 1, 2, .... n remain constant with reference to the ground potential.

The voltages across all the capacitors are dc, the magnitude of which are 2Umax except the capacitor C'n   which is stressed with Umax only.

All the rectifier  D1 , D1' ..... Dn , Dn'   are stressed with 2Umax.

The HV dc output is of the magnitude 2nUmax

For this multistage cascade, the peak to peak ripple is given by,

All the rectifier  D1 , D1' ..... Dn , Dn'   are stressed with 2Umax.

The HV dc output is of the magnitude 2nUmax

For this multistage cascade, the peak to peak ripple is given by,

Since all the diodes  D'1......D'n  transfer the same charge on the capacitors in the smoothing column the total ripple reduces to

Usually equal capacitance values are provided in each stage as smoothing column hence  C = C1 = C2 = ......Cn.   The above equation can be written as,

A photograph of a HVDC Test set of  2000 kV, at 100 mA  continuous current rating is shown in Fig 12

Fig 12   2000 kV HVDC Test Set  [ TUR Dresden ]

The dc test set shown in this figure has a ripple factor less than 1% at full load. Such equipment are capable of allowing higher continuous currents at lower voltage output than rated. For instance, the dc generator shown in Fig 12 can be used for continuous current operation of 150 mA at 1900 kV or 200 mA at 1800 kV. These test sets can be operated without external damping resistor as they have high internal impedance.