GB2111222A - Series-resonant test-voltage source - Google Patents

Abstract

A test voltage source of the series resonant type having a high- voltage transformer 32 whose primary is fed from an A.C. mains supply via a voltage regulator (34) and whose secondary is connected in series with the test load (Cx) via a series reactance (Ls) is made to behave in the same manner as a shunt-resonant circuit fed with constant voltage by disposing a constant voltage/constant current converter (30) either (a) between the series reactance (Ls) and the secondary winding of the high- voltage transformer (T1), (b) between the primary of the high-voltage transformer (T1) and the regulator (34) Figure 6, or (c) between the regulator and the A.C. mains supply. <IMAGE>

Description

SPECIFICATION Series-resonant test-voltage source The present invention is concerned with testvoltage sources of the series-resonant type, for use in A.C. dielectric test equipment for testing loads which are mainly capacitive, such as, for example, the insulation of electrical equipment.

When the high-voltage insulation of electrical equipment has to be subjected to a specified withstand voltage, then a source is needed which will supply both the specified voltage and the outof-phase current taken by the insulation under test. A similar source is needed when highvoltage insulation has to be subjected to losstangent tests or partial-discharge tests.

The conventional source for tests of this kind has been a high-voltage transformer fed from the supply (usually 50/60 Hz a.c. mains) via a voltage regulator. In the case of partial discharge tests, an isolating transformer and a HF filter are also needed. It is usual practice to incorporate one or more shunt reactors in either the primary or the secondary circuit of the HV transformer, in order to compensate for some or all of the reactive power taken by the sample under test. This makes it possible to reduce the ratings of the regulator, the isolating transformer and the filter. It also reduces the power taken from the supply, so reducing the cost of the supply circuit.

Furthermore, if the reactors are connected in the secondary circuit of the HV transformer (i.e.

directly across the sample), then the size and cost of the HV transformer will also be reduced.

If the value of the shunt reactance is adjusted to achieve resonance with the capacitance of the sample, then the supply circuit has to provide only the losses in the transformers and reactors, and the ratings can be correspondingly reduced.

However, it is inconvenient in practice to make reactors that are continuously adjustable in reactance. Hence it is usual practice to employ fixed reactors having reactance values in the series 1, 2, 4 and to switch these in and out as required. When any given combination of reactors is in circuit, the range of capacitance accommodated is Cr+AC, where the current taken by Cr is provided by resonance with the shunt reactance and the current taken by AC is provided directly from the supply. Since the value of AC can be made considerably smaller than the maximum sample capacitance for which the source is rated, substantial economies are possible. Typical circuits are shown in the accompanying Figures 1 and 2 referred to again hereinafter.

An alternative known way of using a reactor to reduce the rating of the supply circuit is to connect it in series with the sample under test, as shown in accompanying Figure 3. This method also confers three further advantages, namely: (i) The secondary voltage of the HV transformer is greatly reduced, thus making it much easier to construct and less expensive. In the case of a source to be used for paritaldischarge testing, the source usually has to be discharge-free. This is greatly facilitated if the transformer secondary voltage is reduced.

(ii) The series reactor may take the form of a number of series-connected elements. The voltage rating and the reactive power rating of each element are then both reduced. This makes the reactor easier to manufacture, easier to render discharge-free, and less expensive.

(iii) A separate filter is no longer needed, because the series-resonant circuit itself acts as a filter.

However, the known series-resonant circuit shown in Figure 3 has the following practical disadvantages: (i) If the magnification factor of the reactors is high (for example Q=50 would be typical), a much greater range of adjustment is needed for the voltage regulator. For most applications a range of output voltage of 10:1 would be considered adequate. However, in order to achieve this range with the circuit of Figure 3, it might be necessary for the voltage regulator to have a range of 60:1.

(ii) For reactors with a high magnification factor, the tuning of the circuit is critical. Small changes in either frequency or reactance can then produce large changes in output voltage.

Incorrect setting of the voltage regulator can also produce an excessive output voltage. It is thus easy to accidentally apply too high a voltage to the sample, with consequent risk of breakdown.

(iii) Excessive voltage can also be applied accidentally to the sample due to ferroresonance.

Due to non-linearity of the iron-cored reactors, the circuit can go from off-resonance to resonance as the voltage is raised. The effect is then that, as the voltage is slowly raised by the regulator, it can suddenly jump to a higher value. Under some conditions the voltage jump can amount to about 2:1. This characteristic is clearly unsatisfactory.

Ferroresonance can be avoided by designing reactors to operate at a low value of maximum flux density (preferably below 1T), but this has the effect of increasing the size and cost by a substantial factor.

It is an object of the present invention to eliminate, or at least to reduce considerably, the aforegoing problems of the known circuits.

In accordance with the present invention this is achieved by the introduction into the seriesresonant circuit of a constant voltage/constant current converter at a position either (i) between the series reactance and the high-voltage transformer, (ii) between the high-voltage transformer and the regulator, or (iii) between the regulator and the basic A.C. supply.

The action of the converter is to convert a constant-voltage source into a constant-current source. The effect on the series-resonant circuit is to make it behave in the same manner as a shuntresonant circuit fed with constant voltage. The current fed to the series circuit (Ls in series with Cx) is independent of its impedance, and depends only on the voltage fed to the CV/CC converter.

Hence the voltage across the sample depends only on the capacitance in the circuit and the setting of the regulator. This means that the voltage across the sample is not affected by the tuning of the resonant circuit, and hence is not critical.

By using such a CV/CC converter, the disadvantages referred to above for seriesresonant sources are eliminated.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a known testvoltage source with a high-voltage transformer and having reactive compensation in the primary circuit; Fig. 2 is a circuit diagram of a known testvoltage source with a high-voltage transformer and having reactive compensation in the secondary circuit; Fig. 3 is a circuit diagram of a basic known series-resonant test-voltage source; Figs. 4a, 4b and 4c are basic circuit diagrams of three embodiments of series-resonant testvoltage source in accordance with the present invention; Figs. 5a to 5j are circuit diagrams of different forms of constant voltage/constant current converters capable of being used in sources embodying the invention;; Fig. 6 is a more detailed circuit diagram of an embodiment in accordance with the present invention; and Fig. 7 is a circuit diagram of another embodiment in accordance with the invention.

The known test-voltage source of Fig. 1 comprises a high-voltage transformer 10 whose secondary winding provides an output voltage Vx across a test load of capacitance Cx. The primary of the transformer 10 is connected via a filter F to the secondary of an isolating transformer 12 whose primary is connected to an A.C. main supply via a regulator 14. Connected across the primary of the high-voltage transformer 10 are a plurality of shunt reactors L1, L2 . . . selected ones of which can be introduced in parallel with the primary of transformer 10 by means of series switches 16.

The known source of Fig. 2 is identical to that of Fig. 1 except that the shunt reactors LX, L2 etc.

are connected in the secondary circuit of the highvoltage transformer 1 0.

The known source of Fig. 3 is an example of a basic series-resonant arrangement where a reactor L is disposed in series with the secondary winding of a transformer 20 and a capacitive load Cx, the primary winding of the transfomer 20 being coupled to the A.C. main supply via a regulator 22.

Fig. 4 illustrates the basic manner by which the known arrangement of Fig. 3 is modified in accordance with the present invention. In Fig. 4a, a constant voltage/constant current converter is disposed between the series reactor Lx and the high-voltage transformer 20. In Fig. 4b, the constant voltage/constant current converter is disposed between the high voltage transformer 20 and the regulator 22. In Fig. 4c, the constant voltage/constant current converter is disposed between the A.C. mains supply and the regulator 22.

Fig. 5 shows a number of possible configurations for the constant voltage/constant current converter. The efficiency of any particular design may be expressed in terms of the ratio of the aggregate kVAr rating of the constituent inductors and capacitors to the power rating of the load. Of the illustrated examples, circuits (i) and (j) are considerably more efficient on the latter basis. Circuit (j) is the most efficient when the load is purely resistive. However, for a load consisting of a series resonant circuit, it can be shown that circuit (i) is the most efficient. Hence, circuit (i) is the preferred embodiment for the present purposes.

Fig. 6 shows a practical embodiment utilising the circuit of Fig. 5(i). In this case, the constant voltage/constant current converter 30 is disposed between the high-voltage transformer 32 (T1) and the regulator 34, which is preferably of the conventional motorised brush type, including zero volt interlocks and overcurrent trip facilities (not shown), and which comprises a variable reactor Tr connected via a circuit breaker 36 to the A.C.

mains. The secondary circuit of the transformer 32 includes the series reactor Lx and the load Cx.

The converter 30 comprises a double wound, 1:1 transformer To and two equal value capacitors CO connected so as to provide a constant voltage/constant current source to feed the high voltage transformer. By this means, the series resonant circuit is fed with a constant current and then behaves in the same way as a shunt resonant circuit fed with constant voltage. The test voltage is then controlled by the setting of the regulator but is nearly independent of any change into or out of resonance. Hence no special reactor tuning is required. The system behaves as a simple high voltage source with the advantages gained by a high Q Series Resonant circuit.

Ferroresonance is completely eliminated.

It will be noted that the output can be shortcircuited without damage to the HV source or to the test sample. However, if the output voltage is allowed to rise above the rated voltage, then both the source and the sample may be damaged. For this reason, there is provided across the output a spark gap 38 (for example operating at 1 7 kV). It is also usually necessary to prevent the voltage on transformer T, from rising above the rated voltage. This is accomplished by means of a plurality of further spark gaps S,, S2, S3 connected in series across the secondary of T1, and which act to trip the earth-leakage circuit breaker 36 which in turn interrupts the mains supply to the regulator. One or more shunt capacitors can be inserted across the output in order to increase the range of the sample capacitance.

Fig. 7 shows another embodiment in accordance with the invention.

The use of a constant voltage/constant current source in the aforegoing manner has the following additional advantages: (i) The range of adjustment required for the regulator is moderate. For example, a range of output voltage of 10:1 could be obtained with a regulator range of 12.5:1; (ii) The adjustment of the output voltage is not critical, even when high-Q reactors are used; (iii) Voltage jumps due to ferroresonance are entirely eliminated.

Claims (Filed on 1/10/82) 1. A test-voltage source of the series resonant type for use in the testing of electrical loads which are mainly capacitive and comprising a highvoltage transformer whose primary is fed from an A.C. supply via a voltage regulator and whose secondary is connected in series with the test load via a series reactance, wherein a constant voltage/constant current converter is disposed either (i) between the series reactance and the secondary of the high-voltage transformer, (ii) between the primary of the high-voltage transformer and the regulator, or (iii) between the regulator and the A.C. supply.

2. A test-voltage source as claimed in claim 1, wherein the constant voltage/constant current converter comprises a transformer having primary and secondary windings, one end of the primary winding of the transformer being connected to one end of the secondary by way of a first capacitor and the other end of the primary winding being connected to the other end of the secondary winding by way of a second capacitor, the input to the converter being applied across the junction of the primary winding and the first capacitor and the junction of the secondary winding and the second capacitor and the output of the converter being taken across the junction of the secondary winding and the first capacitor and the junction of the primary winding and the second capacitor.

3. A test-voltage source as claimed in claim 1 or 2, including a spark gap disposed across the source output for limiting the output voltage of the source.

4. A test-voltage source as claimed in claim 1, 2 or 3, including one or more spark gaps disposed in series across the secondary winding of the high-voltage transformer for limiting the voltage at the latter secondary winding.

5. A test-voltage source substantially as hereinbefore described with reference to and as illustrated in Fig. 6 or Fig. 7 of the accompanying drawings.

6. A test-voltage source as claimed in claim 1, wherein the constant voltage/constant current converter is constructed as illustrated in any of Figs. 5(a) to (j) of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. The use of a constant voltage/constant current source in the aforegoing manner has the following additional advantages: (i) The range of adjustment required for the regulator is moderate. For example, a range of output voltage of 10:1 could be obtained with a regulator range of 12.5:1; (ii) The adjustment of the output voltage is not critical, even when high-Q reactors are used; (iii) Voltage jumps due to ferroresonance are entirely eliminated. Claims (Filed on 1/10/82)

1. A test-voltage source of the series resonant type for use in the testing of electrical loads which are mainly capacitive and comprising a highvoltage transformer whose primary is fed from an A.C. supply via a voltage regulator and whose secondary is connected in series with the test load via a series reactance, wherein a constant voltage/constant current converter is disposed either (i) between the series reactance and the secondary of the high-voltage transformer, (ii) between the primary of the high-voltage transformer and the regulator, or (iii) between the regulator and the A.C. supply.

2. A test-voltage source as claimed in claim 1, wherein the constant voltage/constant current converter comprises a transformer having primary and secondary windings, one end of the primary winding of the transformer being connected to one end of the secondary by way of a first capacitor and the other end of the primary winding being connected to the other end of the secondary winding by way of a second capacitor, the input to the converter being applied across the junction of the primary winding and the first capacitor and the junction of the secondary winding and the second capacitor and the output of the converter being taken across the junction of the secondary winding and the first capacitor and the junction of the primary winding and the second capacitor.

3. A test-voltage source as claimed in claim 1 or 2, including a spark gap disposed across the source output for limiting the output voltage of the source.

4. A test-voltage source as claimed in claim 1, 2 or 3, including one or more spark gaps disposed in series across the secondary winding of the high-voltage transformer for limiting the voltage at the latter secondary winding.

5. A test-voltage source substantially as hereinbefore described with reference to and as illustrated in Fig. 6 or Fig. 7 of the accompanying drawings.

6. A test-voltage source as claimed in claim 1, wherein the constant voltage/constant current converter is constructed as illustrated in any of Figs. 5(a) to (j) of the accompanying drawings.