GB2489262A

GB2489262A - Testing apparatus for testing a switching valve in an HVDC power converter

Info

Publication number: GB2489262A
Application number: GB1104863.4A
Authority: GB
Inventors: Donald Alasdair Picken
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2011-03-23
Filing date: 2011-03-23
Publication date: 2012-09-26
Anticipated expiration: 2031-03-23
Also published as: GB201104863D0; KR101899031B1; KR20120109379A; GB2489262B

Abstract

A testing apparatus for a switching valve in a power converter of a high voltage direct current network comprises primary and secondary terminals 40,42 with a capacitor 44 connected between them. A DC supply 50 and the switching valve 52 under test are respectively connected to the primary and secondary terminals. First 46 and second 48 electronic blocks interconnect the capacitor and the terminals. The first electronic block includes first and second modules 58,60 and a first inductor; the second electronic block includes a third module 62 and a second inductor. Each module includes a switching element, the switching element of the first module connecting the first inductor in series with the capacitor and supply to form a first resonant circuit, the switching element of the second module connecting the first inductor in series with the capacitor to form a second resonant circuit, and the switching element of the third module connecting the second inductor in series with the capacitor and the valve to form a third resonant circuit. Each of the resonant circuits is formed in sequence to selectively charge or discharge the capacitor 44 and transfer power from the supply 50 to the valve 52.

Description

TESTING APPARATUS

This invention relates to a testing apparatus and a method of operating a testing apparatus.

In the field of high voltage direct current (HVDC) power transmission, high capacity power electronic converters typically undergo a series of tests to determine whether they meet the relevant international standards. This is to ensure the compatibility of the converter equipment with the intended power application. As such, there is a need for a test circuit which is capable of carrying tests related to the relevant international standards.

One form of known test circuit is shown in Figure 1. in the test circuit, a capacitor 20 is connected in parallel with a series arrangement of a DC power supply 22, a first inductor 24 and a first switch 26 in the form of a mercury arc valve, and with a series arrangement of a test object 28, a second inductor 30 and a second switch 32 in the form of a pair of parallel-connected mercury arc valves. The test object 28 is a semiconductor valve, which is required to be subjected to a test sequence including a high voltage prior to turn-on, followed by a controlled rate of rise of current during turn-on, a period of constant high current, a high reverse voltage and a controlled rate of fall of current during turn-off. A reverse blocking voltage can be applied during recovery of the test object 28 to the off-state.

In order to perform the test sequence, the first and second switches 26,32 are operated to respectively form a first series resonant circuit, which results in the charging of the capacitor 20 from the DC power supply 22, and a second series resonant circuit, which results in the discharging of the capacitor 20 into the test object 28. This therefore leads to the application of a test waveform across the test object 28 which includes a changing current that is dependent on the inductance value of the second inductor 30. An auxiliary valve 34 is connected in parallel with the test object 28 to simulate the generation of voltage waveforms across the test object 28 when the test object 28 is in a non-conducting state while a current source 36 can be connected in series with the test object 28 to apply the required constant current.

The voltage applied to the test object is controlled by manually modifying the voltage level of the DC power supply 22 and by switching resistors in and out of circuit to initiate changes in voltage levels. These methods of voltage control however are relatively slow to change and affect both positive and negative voltages by the same degree.

Additionally some tests require the application of equal magnitudes of positive and negative voltage to the test object. The test circuit of Figure 1 is limited in that the magnitude of the maximum applied positive voltage is always larger than the magnitude of the maximum applied negative voltage, which means that any attempt to obtain a desired level of reverse voltage can potentially lead to excessive voltage stress of the test object in the positive direction.

According to a first aspect of the invention, there is provided a testing apparatus comprising primary and secondary terminals; a capacitance element connected between the primary and secondary terminals; a first electronic block interconnecting the capacitance element and the primary terminals; and a second electronic block is interconnecting the capacitance element and the secondary terminals, the capacitance element including at least one capacitor, the primary and secondary terminals being operably connected in use to first and second electrical networks respectively, the first electronic block including first and second modules and at least one first inductor, the second electronic block including a third module and at least one second inductor, each module including at least one primary switching element, the or each primary switching element of the first module being controllable in use to connect at least one first inductor in series with the capacitance element and the first electrical network to form a first series resonant circuit, the or each primary switching element of the second module being controllable to connect at least one first inductor in series with the capacitance element to form a second series resonant circuit, the or each primary switching element of the third module being controllable in use to connect at least one second inductor in series with the capacitance element and the second electrical network to form a third series resonant circuit, wherein, in use, each of the series resonant circuits is formed in a predetermined sequence in order to selectively charge or discharge the capacitance element and thereby facilitate the transfer of power from the first electrical network to the second electrical network.

The provision of the second module allows rapid and accurate control over the voltage level across the capacitance element when charging the capacitance element from the first electrical network. Conventionally the capacitance element is charged from the first electrical network and discharged into the second electrical network to define one cycle of operation, which can be repeated depending on the requirements of the testing application. The formation of the second series resonant circuit between each cycle of operation allows the discharged capacitance element to be charged up to a specific voltage with the desired polarity before the formation of the first series resonant circuit.

Subsequently, during the formation of the first series resonant circuit, the level of the specific voltage affects the amount of energy injected into the capacitance element from the first electrical network and thereby the voltage across the capacitance element prior to the formation of the second series circuit. The level of the specific voltage depends on the duration between the formation of the second series resonant circuit and the formation of the first series resonant circuit and thereby relies on the switching speed of the or each primary switching element of the first module. This therefore results in faster and more accurate control over the voltage across the capacitance element when compared to the direct modification of the first electrical network, which is relatively slow to change.

is Additionally the provision of the second module allows equal magnitudes of positive and negative voltages to be applied across the secondary terminals during a test sequence by varying the specific voltage across the capacitance element prior to different cycles of operation in the test sequence without having to modify the characteristics of the first electrical network.

The or each primary switching element of the first module may be controllable in use to form the first series resonant circuit subsequent to the formation of the second series resonant circuit. In such embodiments, the primary switching elements of the first and second modules may be controllable in use to provide a time delay between the formation of the second series resonant circuit and the formation of the first series resonant circuit.

The formation of the series resonant circuits based on a predetermined sequence allows energy to be injected from the first electrical network into the capacitance element.

Preferably the or each primary switching element of the third module is controllable in use to connect the capacitance element in series with the secondary terminals when the second electrical network is an open circuit.

As such, the voltage across the charged capacitance element is applied across the secondary terminals, which ensures that the correct voltage is applied across the second electrical network when the second electrical network is switched into circuit. a

Each of the capacitance element and the second module may be connected in parallel with a first series arrangement of the first module and the primary terminals and with a second series arrangement of the third module and the secondary terminals; the third module includes at least one second inductor connected in series with the or each primary switching element of the third module; and each module of the first electronic block includes at least one first inductor connected in series with the or each respective primary switching element.

io Alternatively each of the capacitance element and the second module may be connected in parallel with a first series arrangement of the first module and the primary terminals and with a second series arrangement of the third module and the secondary terminals; the third module includes at least one second inductor connected in series with the or each switching element of the third module; and at least one first inductor is connected in series between a parallel arrangement of the second module and the first series arrangement and between a parallel arrangement of the or each capacitance element and the second series arrangement.

In further embodiments, at least one of the modules of the first electronic block may include at least one first inductor connected in series with the or each respective primary switching element.

In embodiments of the invention, the or each primary switching element of the third module may be a bidirectional switching element controllable in use to conduct current in either direction.

The bidirectional switching element allows current to flow through the third series resonant circuit in either direction and thereby allows a reverse voltage to be applied to the second electrical network.

In such embodiments, the or each primary switching element of the third module is controllable in use to form the third series resonant circuit so as to apply a reverse voltage to the second electrical network.

The application of the reverse voltage to the second electrical network leads to the turn-off of the second electrical network.

In other embodiments, the testing apparatus may include first and second primary terminals, one of the first and second primary terminals being operably connected in use to one of positive and negative poles of the first electrical network, the other of the first and second primary terminals being operably connected in use to the other of positive and negative poles of the first electrical network, and wherein the first electronic block further includes a fourth module, the or each primary switching element of the first module is controllable in use to connect at least one of the plurality of inductors in series with the capacitance element and the first electrical network via the first primary terminal to form the first series resonant circuit, and the or each primary switching element of the fourth module is controllable in use to connect at least one first inductor in series with the capacitance element and the first electrical network via the second primary terminal to form a fourth series resonant circuit.

The provision of the fourth module allows the capacitance element to be charged to different polarities so as to generate positive and negative waveforms to the secondary terminals to accommodate the testing of a second electrical network that can conduct current in both directions.

In such embodiments, the primary switching elements of the second and fourth modules may be controllable in use to provide a time delay between the formation of the second series resonant circuit and the formation of the fourth series resonant circuit.

The testing apparatus may further include a tertiary terminal, wherein the fourth module is connected in series between the second primary terminal and the junction interconnecting the first module and a first end of the second module, and a second end of the second module is operably connected in use via the tertiary terminal to ground. In such embodiments, the first electronic block may further include first and second switching links, the first switching link being operably connected between the second end of the second module and the second primary terminal, the second switching link being operably connected between the second end of the second module and the tertiary terminal, the first and second switching links being controllable in use to selectively switch the second end of the second module into circuit with either the second primary terminal or the tertiary terminal.

The above arrangement results in an integrated test circuit, which is compatible with the testing of different second electrical networks that can conduct current either in one direction or both directions. This therefore leads to savings in cost and space because it would otherwise be necessary to employ the use of separate test circuits to test the different second electrical networks.

In further embodiments having first and second primary terminals, the or each primary switching element of the second module may be a bidirectional switching element controllable in use to conduct current in either direction.

The bidirectional switching element allows current to flow through the second module in either direction and thereby allows the capacitance element to be charged to either polarity.

In embodiments of the invention, the first electronic block may further include a fifth module connected between the capacitance element and the first module and between the capacitance element and the second module, the or each primary switching element of the fifth module being controllable in use to switch the capacitance element into circuit to form the first and second series resonant circuits.

In such embodiments having first and second primary terminals, the fifth module may be connected between the fourth module and the capacitance element, and the or each primary switching element of the fifth module may be controllable in use to switch the capacitance element into circuit to form the fourth series resonant circuit.

The fifth module may be connected in series between the parallel arrangement of the second module and the first series arrangement and between the parallel arrangement of the or each capacitance element and the second series arrangement.

The fifth module forms an isolation switch between the capacitance element and the first electrical network via the respective module of the first electronic block. This allows the or each primary switching element of each module of the first electronic block to have a lower voltage rating. It would otherwise be necessary to rate the or each primary switching element of each module of the first electronic block to be compatible with the high voltage levels across the capacitance element, which leads to an increase in component size and cost.

In other embodiments employing the use of a fifth module, the or each primary switching element of the fifth module may be a bidirectional switching element controllable in use to conduct current in either direction.

The bidirectional switching element allows current to flow through the fifth module in either direction and thereby allows the capacitance element to be charged to either polarity from a bi-pole first electrical network.

In embodiments of the invention, the second electronic block may further include a sixth module connected in parallel with the secondary terminals, the or each primary switching element of the sixth module being controllable in use to connect the sixth module in series with the capacitance element and at least one second inductor to form a fifth series resonant circuit.

The provision of the sixth module allows the energy in the capacitance element to be injected into the sixth module during the build-up of a specific voltage across the capacitance element instead of injecting the energy into the second electrical network.

is In addition, the provision of the sixth module enables the simulation of a changing voltage across the second electrical network when the second electrical network is in a non-conducting state.

In other embodiments, the testing apparatus may further include auxiliary terminals; and one or more auxiliary switching elements connected in series with the auxiliary terminals, the series arrangement of the or each auxiliary switching element and the auxiliary terminals being connected in parallel with the secondary terminals, wherein the auxiliary terminals are connected in use to a current source; and the or each auxiliary switching element is controllable in use to switch the current source into and out of circuit with the secondary terminals.

This allows the second electrical network to be subjected to a constant current condition.

For example, the operation of the or each auxiliary switching element allows the second electrical network to be subjected to a constant current after a controlled rise of current or a controlled rise of current after the application of the constant current.

In other embodiments having auxiliary terminals, the or each auxiliary switching element may be a bidirectional switching element controllable in use to conduct current in either direction.

The bidirectional switching element allows the second electrical network to be subjected to a constant current flowing in either direction.

Preferably each switching element has reverse blocking capability.

The reverse blocking capability of the switching element results in the automatic turn-off of the respective switching element when the flow of current through the switching element is reversed. This results in more accurate control over the respective switching process when compared to the manual turn-off of the respective switching element.

The or each switching element preferably includes at least one semiconductor device, at least one mercury arc valve or at least one mechanical switch. The or each semiconductor device may be a thyristor, an insulated gate bipolar transistor, a gate turn-off thyristor, a field effect transistor, a gate commutated thyristor or an integrated gate commutated thyristor.

The use of semiconductor devices is advantageous because such devices are small in size and weight and have relatively low power dissipation, which minimizes the need for cooling equipment. It therefore leads to significant reductions in power converter cost, size and weight.

The fast switching characteristics of semiconductor devices allows the testing apparatus to respond quickly to changes in voltage and thereby minimise the risk of any fluctuations in the voltage characteristics of the capacitance element.

At least one inductor is preferably a variable inductor.

This allows the testing apparatus to subject the second electrical network to a wide range of current pulses by varying the inductance of one or more inductors.

In further embodiments, the testing apparatus may further include a closed loop so controller to monitor and regulate the voltage across the capacitance element. The or each primary switching element of the first module may therefore be controllable in use to vary the length of the time delay in response to control signals from the closed loop controller, and/or the or each primary switching element of the fourth module may therefore be controllable in use to vary the length of the time delay in response to control signals from the closed loop controller.

The provision of the closed loop controller improves fast and accurate operation of the testing apparatus by ensuring that the voltage across the capacitance element stays within set values so as to achieve desired test conditions for the second electrical network.

In embodiments of the invention, the second electronic block may further include a snubber circuit connected in parallel with the or each primary switching element of the third module. For example, the snubber circuit may include at least one resistor connected in series with at least one capacitor.

The provision of the snubber circuit allows control of the reverse voltage imposed across the secondary terminals.

In other embodiments, the testing apparatus may further including a low pass filter operably connected to the primary terminals so as to interconnect in use the primary terminals and the first electrical network. For example, the low pass filter may include one or more capacitors connected in series between the primary terminals; and one or more inductors connected in series with the respective primary terminal so as to interconnect in use the respective primary terminal and the first electrical network.

The provision of the low pass filter not only minimises the presence of voltage ripple in the testing apparatus, but also results in a low impedance voltage source for the testing apparatus.

In further embodiments, in use, the predetermined sequence is repeated at a frequency equal to the operating frequency of the second electrical network.

This allows the second electrical network to be repeatedly subjected to a test sequence in order to simulate actual operating conditions.

Preferred embodiments of the invention will now be described by way of non-limiting examples with reference to the accompanying drawings in which: Figure 1 shows, in schematic form, a prior art test circuit; Figure 2 shows, in schematic form, a testing apparatus according to a first embodiment of the invention; Figure 3 shows a simplified representation of the testing apparatus when it is operated in conjunction with a unidirectional second electrical network; Figure 4 shows the formation of the first series resonant circuit and the charging of the capacitance element from the first electrical network during a positive half-cycle; Figure 5 shows the formation of the third series resonant circuit and the discharging of the capacitance element into the second electrical network during the positive half-cycle; Figure 6 shows the formation of the second series resonant circuit during the positive half-cycle; io Figure 7 shows the respective change in current of the thyristor of the first module and the bidirectional switching element of the second module and the change in voltage of the capacitance element during the first and second charging phases; Figure 8 shows the change in current in the second electrical network during the formation of the third series resonant circuit following the second charging phase; is Figure 9 shows a simplified representation of the testing apparatus when it is operated in conjunction with a bidirectional second electrical network; Figure 10 shows the formation of the third series resonant circuit and the discharging of the capacitance element into the second electrical network during a negative half-cycle; Figure 11 shows the change in current in the second electrical network during the formation of the third series resonant circuit during the negative half-cycle; Figure 12 shows the formation of the second series resonant circuit during the negative half-cycle; Figure 13 shows the formation of the fourth series resonant circuit and the charging of the capacitance element from the first electrical network during the negative half-cycle; Figure 14 shows the changes in current of the bidirectional switching element of the second module and the thyristor of the fourth module and the change in voltage of the capacitance element during the negative half-cycle; Figure 15 shows a typical test sequence consisting of positive and negative half-cycles; and Figures 16 and 17 show typical changes in voltage and current in the second electrical network during the operation of the testing apparatus sequence to generate an unidirectional or bidirectional current pulse at the secondary terminals of the testing apparatus.

A testing apparatus according to an embodiment of the invention is shown in Figure 2.

The testing apparatus comprises primary and secondary terminals 40,42; a capacitance element 44; and first and second electronic blocks 46,48.

In use, the primary terminals 40 of the testing apparatus are connected to a first electrical network 50 in the form of a DC power supply, while the secondary terminals 42 of the testing apparatus are connected to a second electrical network 52 in the form of a switching valve, which may, for example, form part of an external power electronic converter.

The primary terminals 40 include first and second primary terminals 54a,54b. In use, the first primary terminal 54a is operably connected to the positive pole of the DC power supply 50, while the second primary terminal 54b is operably connected to the negative pole of the DC power supply 50.

The testing apparatus further includes a low pass filter 56, which includes two capacitors connected in series between the first and second primary terminals 54a,54b; and two inductors, each inductor being connected in series with the respective primary terminal 54a,54b so as to interconnect in use the respective primary terminal 54a,54b and the respective pole of the DC power supply 50.

The provision of the low pass filter 56 not only minimises the presence of voltage ripple in the testing apparatus, but also results in a low impedance voltage source for the testing apparatus.

The capacitance element 44 is in the form of a single capacitor. In other embodiments, the capacitance element 44 may include a plurality of capacitors.

The first electronic block 46 includes first and second modules 58,60 while the second electronic block 48 includes a third module 62. Each of the capacitance element 44 and the second module 60 is connected in parallel with a first series arrangement of the first module 58 and the primary terminals 40 and with a second series arrangement of the third module 62 and the secondary terminals 42.

The first electronic block 46 further including a fourth module 64 connected in series between the second primary terminal 54b and the junction interconnecting the first module 58 and a first end 66 of the second module 60; and a tertiary terminal 68, a second end 70 of the second module 60 being connected in use via the tertiary terminal 68 to a ground terminal of the DC power supply 50.

Each of the first and fourth modules 58,64 includes a thyristor connected in series with a first inductor while each of the second and third modules 60,62 respectively include a first inductor connected in series with a bidirectional switching element and a second inductor connected in series with a bidirectional switching element. Each bidirectional switching element is in the form of a pair of thyristors connected in parallel and controllable in use to permit current flow in either direction.

It is envisaged that in other embodiments, each module may include a series connection of a thyristor or bidirectional switching element with a plurality of inductors or a series connection of a plurality of thyristors or bidirectional switching elements with an inductor.

In further embodiments, it is envisaged that each thyristor may be replaced by a plurality is of thyristors connected in series or parallel.

The reverse blocking capability of each thyristor results in the automatic turn-off of the respective thyristor when the flow of current through each thyristor is reversed. This results in more accurate control over the switching of each thyristor when compared to other manually controlled switches.

In other embodiments, the thyristor may be replaced by a mercury arc valve or a mechanical switch. In further embodiments, the thyristor may be replaced by an insulated gate bipolar transistor, a gate turn-off thyristor, a field effect transistor, a gate commutated thyristor or an integrated gate commutated thyristor.

The fast switching characteristics of semiconductor devices allows the testing apparatus to respond quickly to changes in voltage and thereby minimise the risk of any fluctuations in the voltage characteristics of the capacitance element 44.

The second electronic block 48 further includes a snubber circuit 72 connected in parallel with the bidirectional switching element of the third module 62. The snubber circuit 72 includes a resistor connected in series with a capacitor.

The first electronic block 46 further includes first and second switching links 74,76, the first switching link 74 being operably connected between the second end 70 of the second module 60 and the second primary terminal 54b, the second switching link 76 being operably connected between the second end 70 of the second module 60 and the tertiary terminal 68. In use, the first and second switching links 74,76 are controllable to selectively switch the second end 70 of the second module 60 into circuit with either the second primary terminal 54b or the tertiary terminal 68. This enables the testing apparatus to generate a unidirectional or bidirectional current pulse at the secondary terminals 42 depending on the current characteristics of the second electrical network 52.

The first electronic block 46 also includes a fifth module 78 connected in series between the parallel arrangement of the second module 60 and the first series arrangement and between the parallel arrangement of the capacitance element 44 and the second series arrangement. The fifth module 78 includes a first inductor connected in series with a bidirectional switching element.

The fifth module 78 forms an isolation switch between the high voltage capacitance element 44 and each of the modules 58,60,64 of the first electronic block 46. This allows the thyristor of each module 58,60,64 of the first electronic block 46 to have a lower voltage rating. It would otherwise be necessary to rate the thyristor of each module 58,60,64 of the first electronic block 46 to be compatible with the high voltage levels across the capacitance element 44, which leads to an increase in component size and cost.

In Figure 3, the second electronic block includes a sixth module 80 in the form of a switching valve connected in parallel with the secondary terminals 42.

Additionally the testing apparatus includes auxiliary terminals and an auxiliary switching element in the form of a bidirectional switching element. The auxiliary switching element is connected in series with the auxiliary terminals, while the series arrangement of the auxiliary switching element and the auxiliary terminals is connected in parallel with the secondary terminals 42.

In use, the auxiliary terminals are connected to a current source 82, which can be either a DC or AC current source depending on the requirements of the testing apparatus.

Switching the current source 82 into circuit with the secondary terminals 42 therefore allows the second electrical network 52 to be subjected to a constant or variable current.

The testing apparatus also includes a closed loop controller 84, which serves to monitor and regulate the voltage across the capacitance element 44, as shown in Figure 2. The closed loop controller 84 is configured to send control signals to the first and fourth io modules 58,64 so as to control the timing of the firing of their respective thyristor.

The operation of the testing apparatus can be divided into two main sequences, a start-up sequence and a test sequence.

When the second electrical network is capable of conducting current in only one direction, the first switching link is closed and the second switching link is open so that the DC power supply of the testing apparatus defines a single power supply connected in series with the first module. A simplified representation of the testing apparatus configured in this manner is shown in Figure 3.

During the start-up of the testing apparatus, the voltage across the capacitance element is built up over multiple cycles until equilibrium has been reached. This is carried out by forming first and second resonant circuits to define one cycle of the start-up sequence.

The first resonant circuit is initially formed by firing the thyristor of the first module 58 and the bidirectional switching element of the fifth module 78 to switch the first and fifth modules 58,78 in series with the capacitance element 44 and the first electrical network to form the first series resonant circuit, as shown in Figure 4. The structure of the first series resonant circuit causes a sinusoidal current to flow within the first series resonant circuit. After each of the thyristor of the first module 58 and the bidirectional switching element of the fifth module 78 conducts a half-sine current pulse, the subsequent reversal of current causes the thyristor of the first module 58 and the bidirectional switching element of the fifth module 78 to turn off, which results in the capacitance element 44 being charged to a positive voltage with a magnitude that is nearly twice the voltage of the DC power supply 50.

The voltage on the capacitor is given by Equation 1.

V1 =VDc+kI(VDcJ'enhgial) (1) Where V1 is the voltage across the capacitance element after being charged from the DC power supply following the formation of the first series resonant circuit; VDC is the voltage across the DC power supply; k1 is the efficiency of the first series resonant circuit, which lies between 0 and 1; Vinitiai is the voltage across the capacitance element before the formation of the first series resonant circuit.

This is followed by firing the bidirectional switching element of the third module 62 to switch the third module 62 into circuit with the capacitance element 44 and the second electrical network 52 to form the third series resonant circuit, as shown in Figure 5. This causes a sinusoidal current to flow in the third series resonant circuit. After the bidirectional switching element of the third module 62 conducts a half-sine current pulse, the subsequent reversal of current causes the bidirectional switching element of the third module 62 to turn off, which results in the capacitance element 44 being charged to a negative voltage with a magnitude that is slightly less than the preceding positive voltage across the capacitance element 44. The value of this negative voltage is calculated using Equation 2. (2)

Where 42 is the voltage across the capacitance element after being discharged following the formation of the second series resonant circuit; k2 is the efficiency of the second series resonant circuit, which lies between 0 and I. In the next cycle of the start-up sequence, the formation of the first series resonant circuit again leads to the charging of the capacitance element to a positive voltage with a magnitude that is nearly four times the voltage of the DC power supply. The voltage across the capacitance element is given by Equation 3.

V3 =VDC--kJ.[VDC+k2(V(1+k1))j (3) Where V3 is the voltage across the capacitance element after being charged from the DC power supply following the formation of the first series resonant circuit in the second cycle of operation of the start-up sequence.

Voltage amplification continues for each cycle of the start-up sequence until equilibrium is reached between the losses in the first series resonant circuit and the energy supplied by the DC power supply. The voltage amplification capability of the first series resonant circuit is advantageous in that it enables the use of a low voltage DC power supply to provide high voltage test conditions for the second electrical network.

For practical values of k1 and k2, which lie between 0 and 1, equilibrium is reached when successive peak positive and negative voltages across the capacitance element in consecutive cycles of the start-up sequence are equal. The peak positive and negative voltages across the capacitance element are calculated using Equations 4 and 5.

v (4) P -(i-k, .Jç2) V VDCk2(1+kl) 5 -(i-k, .k2) Where V is the peak positive voltage across the capacitance element; V is the peak negative voltage across the capacitance element Alternatively each cycle of the start-up sequence is defined by the formation of the first series resonant circuit followed by a fifth series resonant circuit instead of the second series resonant circuit. The fifth series resonant circuit is formed by turning on the thyristors of the third and sixth modules to switch the sixth module in series with the capacitance element. This allows the energy in the capacitance element to be injected into the sixth module during the build-up of a specific voltage across the capacitance element instead of injecting the energy into the second electrical network and thereby minimises the risk of damage to the second electrical network before the start of the test sequence.

The test sequence is initiated after equilibrium in the first series resonant circuit is achieved.

During the test sequence, the charging of the capacitance element subsequent to the formation of the third series resonant circuit is divided into first and second charging phases. The first charging phase includes the firing of the bidirectional switching elements of the second and fifth modules 60,78 to switch the second and fifth modules 60,78 into circuit with the capacitance element to form the second series resonant circuit, as shown in Figure 6. This results in the flow of a sinusoidal current in the second series resonant circuit, which charges the capacitance element 44 to a positive voltage with a magnitude that is slightly less than the magnitude of the preceding negative voltage. The difference between these positive and negative voltage levels is attributed to circuit losses.

The second charging phase includes the further injection of energy from the DC power supply to replace the lost energy so as to charge the capacitance element up to a desired positive voltage. This is carried out by firing the thyristor of the first module to switch the first electrical network into circuit with the capacitance element, which leads to the reversal of the current flowing through the second module and thereby causes the bidirectional switching element of the second module to turn off. This results in the formation of the first series resonant circuit as seen in Figure 3. After each of the thyristor of the first module and the bidirectional switching element of the fifth module conducts a half-sine current pulse, the subsequent reversal of current leads to the turn-off of both the thyristor of the first module and the bidirectional switching element of the fifth module. This leaves the capacitance element with the desired positive voltage, which can then be discharged into the second electrical network by forming the third series resonant circuit.

Figure 7 shows the respective change in current 86,88 of the thyristor of the first module and the bidirectional switching element of the second module and the change in voltage of the capacitance element during the first and second charging phases. As outlined above, the thyristor of the first module is fired after a controlled delay 92 subsequent to the firing of the bidirectional switching element of the second module.

The second charging phase is followed by the formation of the third series resonant circuit to discharge the capacitance element into the second electrical network. The change in current 94 in the second electrical network is shown in Figure 8.

The amount of energy injected into the capacitance element from the DC power supply and therefore the voltage across the capacitance element at the end of the second charging phase is dependent on the voltage across the capacitance element at the start of the second charging phase, as seen in Equation 3. The voltage across the capacitance element at the start of the second charging phase is dependent on the extent to which the capacitance element is charged during the first charging phase and therefore is dependent on the time spent during the first charging phase, which is equal to the time delay between the formation of the second series resonant circuit and the formation of the first series resonant circuit. Consequently the voltage across the capacitance element can be controlled by modifying the length of the time delay between the initiation of the first and second phases. An increase in the time delay leads to an increase in the amount of injected energy while a decrease in the length of the time delay leads to a decrease in the amount of injected energy.

This results in faster and more accurate control over the voltage across the capacitance element when compared to the conventional methods of directly modifying the voltage is characteristics of the first electrical network, which is relatively slow to change, and switching resistors in and out of circuit to initiate changes in voltage levels The thyristor of the first module is associated with the closed loop controller that monitors the voltage across the capacitance element and sends control signals to the thyristor of the first module so as to control the timing of firing of that thyristor and thereby control the length of the time delay.

To carry out Periodic Firing and Extinction testing, according to the requirements of IEC Standard 60700-1 for high voltage direct current valve testing, the testing apparatus is operated in the following sequence: Initially the testing apparatus undergoes the start-up sequence to charge the capacitance element to the desired voltage level.

so Once the desired voltage across the capacitance element is achieved, the third series resonant circuit is formed to simulate the turn-on of the second electrical network at a set positive voltage, followed by a controlled rise of current flowing through the second electrical network. The bidirectional switching element of the third module is fired in advance of the turn-on of the second electrical network in order to apply the full positive voltage of the capacitance element across the second electrical network. The rate of rise of current is determined by the inductance value of the second inductor of the third module.

At this stage, a current source is switched into circuit with the auxiliary terminals so as to provide a near-constant flow of current through the second electrical network.

Meanwhile, the capacitance element is recharged from the DC power supply up to the desired voltage by initiating the first and second charging phases as outlined earlier.

To simulate the turn-off of the second electrical network, the third series resonant circuit is formed and the current source is switched out of circuit, which results in a controlled fall in current in the second electrical network. After the bidirectional switching element conducts a half-sine current pulse, the subsequent reversal of current causes both the bidirectional switching element of the third module and the second electrical network to turn off, which leaves a negative voltage across the capacitance element. The bidirectional switching element of the third module is again fired to permit the flow of current in the reverse direction and thereby apply the negative voltage of the capacitance element as to apply a reverse recovery voltage across the second electrical network.

Once the capacitance element is recharged from the DC power supply, each of the thyristor of the sixth module and the bidirectional switching element of the third module is turned on to switch the capacitance element in series with the sixth module to define a fifth series resonant circuit. This allows the testing apparatus to simulate the switching of other valves in the power electronic converter which leads to voltage notches on the voltage across the non-conducting second electrical network. The simulation is carried out by discharging the capacitance element into the sixth module instead of the second electrical network, which is kept in a non-conducting state. After each of the bidirectional switching element of the third module and the thyristor of the sixth module conducts a half-sine current pulse, the subsequent reversal of current leads to the turn-off of both the bidirectional switching element of the third module and the thyristor of the sixth module.

The above sequence may be repeated over multiple cycles at the operational frequency of the second electrical network Various specific test conditions can be created using the testing apparatus of Figure 1.

A prospective reverse voltage develops across the second electrical network when the second electrical network is in a non-conducting state. This prospeclive reverse voltage develops from the negative voltage across the capacitance element reduced by the voltage divider comprising the resistor and capacitor of the snubber circuit and any damping networks across the second electrical network and other networks connected in parallel with the second electrical network. In order to apply the full reverse voltage of the capacitance element to the second electrical network, it is necessary to fire the bidirectional switching element of the third module.

The peak forward voltage across the second electrical network can be controlled in the following manner. When the bidirectional switching element of the third module is fired soon after the formation of the first series resonant circuit to charge the capacitance element, the voltage across the second electrical network will be equal to the existing voltage across the capacitance element. However, if the bidirectional switching element of the third module remains turned off, a forward voltage develops across the second electrical network from the positive voltage across the capacitance element reduced by the voltage divider comprising the resistor and capacitor of the snubber circuit and any damping networks across the second electrical network and other networks connected in parallel with the second electrical network. At this stage, during the rise in forward voltage as the capacitance element charges from the DC power supply, the second electrical network can be turned on so as to be in a conducting state and thereby limit the peak forward voltage of the capacitance element. The bidirectional switching element of the third module is fired soon after so as to form the third series resonant circuit and thereby allow discharging of the capacitance element into the second electrical network.

The rate of change of current flowing in the second series resonant circuit is dependent on the inductance value of the second inductor of the third module. In order to accommodate a range of values for the rate of change of current in the second series resonant circuit, the second inductor is controllable in use to vary its inductance value. In other embodiments, it is envisaged that each inductor in the testing apparatus is controllable in use to vary its respective inductance value.

As outlined earlier, the voltage across the capacitance element during the test sequence is controlled by the time delay between the formation of the second series resonant circuit and the formation of the first series resonant circuit. For example, during a cycle of the test sequence, the formation of the third series circuit results in the application of the positive voltage of the capacitance element to the second electrical network. The subsequent magnitude of the negative voltage on the capacitance element after the turn-off of the bidirectional switching element of the third module will however be lower than the preceding positive voltage due to circuit losses. When recharging the capacitance element from the first electrical network, the length of the time delay between the first and second charging phases is controlled so that the magnitude of the negative voltage in the next cycle of the test sequence is equal to the positive voltage in the previous cycle of the test sequence. The second electrical network is therefore subjected to equal magnitudes of positive and negative voltages during the same test sequence.

When the second electrical network 52 is capable of conducting current in both directions, the first switching link is open and the second switching link 76 is closed so that the DC power supply 50 of the testing apparatus defines positive and negative power supplies with respect to its ground terminal. A simplified representation of the testing apparatus configured in this manner is shown in Figure 9.

In order to generate a bidirectional current pulse at the secondary terminals, the operation of the testing apparatus is divided into positive and negative half-cycles to define a cycle of the test sequence.

During the positive half-cycle, the testing apparatus is operated in a similar manner to the above-described sequence for testing of a unidirectional second electrical network so as to charge the capacitance element from the DC power supply and to discharge the capacitance element into the second electrical network. At the end of the positive half-cycle, the voltage across the capacitance element is negative as a result of the capacitance element being discharged into the second electrical network.

The negative half-cycle is initiated by firing the bidirectional switching element of the third module 62 to permit the flow of current in the reverse direction and thereby apply the negative voltage of the capacitance element 44 to the second electrical network 52 as shown in Figure 10. After the bidirectional switching element of the third module 62 conducts a half-sine current pulse, the subsequent reversal of current leads to the turn-off of the bidirectional switching element of the third module 62, which leaves a positive ao voltage across the capacitance element 44. The bidirectional switching element of the third module 62 may be fired to apply this positive voltage so as to apply a reverse recovery voltage across the second electrical network 52. The described change in voltage 90 of the capacitance element and change in current 94 of the second electrical network is shown in Figure 11.

This is followed by the firing of the bidirectional switching elements of the second and fifth modules 60,78 to form the second series resonant circuit, which leads to the flow of ¶ ¶ a sinusoidal current in the second series resonant circuit and thereby charges the capacitance element 44 to a negative voltage with a magnitude that is slightly less than the magnitude of the preceding positive voltage, as shown in Figure 12.

This in turn is followed by the further injection of energy from the DC power supply 50 to replace the lost energy so as to charge the capacitance element 44 up to a desired negative voltage. This is carried out by firing the thyristor of the fourth module 64 to switch the DC power supply 50 into circuit with the capacitance element 44, which leads to the reversal of the current flowing through the second module and thereby causes the bidirectional switching element of the second module to turn off. This leads to the formation of a fourth series resonant circuit as shown in Figure 13. After each of the thyristor of the fourth module 64 and the bidirectional switching element of the fifth module 78 conducts a half-sine current pulse, the subsequent reversal of current leads to the turn-off of the thyristor of the fourth module 64 and the bidirectional switching element of the fifth module 78. This leaves the capacitance element 44 with the desired negative voltage, which can then be discharged into the second electrical network by forming the third series resonant circuit. The described changes in current 88,96 of the bidirectional switching element of the second module and the thyristor of the fourth module and the change in voltage 90 of the capacitance element can be seen in Figure 14. As outlined above, the thyristor of the fourth module is fired after a controlled delay 92 subsequent to the firing of the bidirectional switching element of the second module.

The charging of the capacitance element to a negative voltage is followed by the formation of the third series resonant circuit to discharge the negatively charged capacitance element into the second electrical network.

The negative voltage across the capacitance element can be controlled by modifying the length of the time delay between the formation of the second series resonant circuit and the formation of the fourth series resonant circuit. The thyristor of the fourth module is associated with the closed loop controller that monitors the voltage across the capacitance element and sends control signals to the thyristor of the first module so as to control the timing of firing of that thyristor and thereby control the length of the time delay.

The structure of the testing apparatus therefore allows the testing apparatus to provide a bidirectional current pulse to the second electrical network in the same test sequence. A typical test sequence consisting of positive and negative half-cycles can be seen in Figure 15, which also shows the change in voltage 90 of the capacitance element and the respective change in current 86,96 in the respective thyristor of the first and fourth modules Figures 16 and 17 show typical changes in voltage 100 and current 94 in the second electrical network during the operation of the testing apparatus sequence to generate an bidirectional current pulse at the secondary terminals of the testing apparatus.

To carry out Periodic Firing and Extinction testing, according to the requirements of lEG Standard No. 61954 for static Var compensator valve testing, the testing apparatus is operated in the following sequence: Initially the testing apparatus undergoes the start-up sequence to charge the capacitance element to the desired positive voltage level.

The testing apparatus is operated according to the positive half-cycle of the test sequence to simulate the turn-on of the second electrical network followed by a controlled rise of current, constant current through the second electrical network, a controlled decrease in current followed by the turn-off of the second electrical network as outlined above and finally the application of a negative reverse recovery voltage to the second electrical network.

This is followed by the operation of the testing apparatus according to the negative half-cycle of the test sequence to charge the capacitance element to a negative voltage.

Once the desired negative voltage across the capacitance element is achieved, the third series resonant circuit is formed to simulate the turn-on of the second electrical network at a set negative voltage, followed by a controlled rise of current flowing through the second electrical network. The bidirectional switching element of the third module is fired in advance of the turn-on of the second electrical network in order to apply the full negative voltage of the capacitance element across the second electrical network. The rate of rise of current is determined by the inductance value of the second inductor of the third module.

Meanwhile, the capacitance element is recharged from the DC power supply up to the desired voltage by forming the second series resonant circuit followed by the fourth series resonant circuit.

To simulate the turn-off of the second electrical network, the third series resonant circuit is formed and the current source is switched out of circuit, which results in a controlled fall in current in the second electrical network. After the bidirectional switching element conducts a half-sine current pulse, the subsequent reversal of current causes both the bidirectional switching of the third module and the second electrical network to turn off, which leaves a positive voltage across the capacitance element. The bidirectional switching element of the third module is again fired to permit the flow of current in the reverse direction and thereby apply the positive voltage of the capacitance element as to apply a reverse recovery voltage across the second electrical network.

The above sequence may be repeated over multiple cycles at the operational frequency of the second electrical network.

It is envisaged that in other embodiments of the invention, the series resonant circuits may be formed in different sequences in order to generate different test conditions for the second electrical network.

Claims

ICLAiMS 1. A testing apparatus comprising primary and secondary terminals; a capacitance element connected between the primary and secondary terminals; a first electronic block interconnecting the capacitance element and the primary terminals; and a second electronic block interconnecting the capacitance element and the secondary terminals, the capacitance element including at least one capacitor, the primary and secondary terminals being operably connected in use to first and second electrical networks respectively, the first electronic block including first and second modules and at least one first inductor, the second electronic block including a third module and at least one second inductor, each module including at least one primary switching element, the or each primary switching element of the first module being controllable in use to connect at least one first inductor in series with the capacitance element and the first electrical network to form a first series resonant circuit, the or each primary switching element of the second module being controllable to connect at least one first inductor in series with the capacitance element to form a second series resonant circuit, the or each primary switching element of the third module being controllable in use to connect at least one second inductor in series with the capacitance element and the second electrical network to form a third series resonant circuit, wherein, in use, each of the series resonant circuits is formed in a predetermined sequence in order to selectively charge or discharge the capacitance element and thereby facilitate the transfer of power from the first electrical network to the second electrical network.
2. A testing apparatus according to Claim I wherein the primary switching elements of the first and second modules are controllable in use to provide a time delay between the formation of the second series resonant circuit and the formation of the first series resonant circuit.
3. A testing apparatus according to Claim I or Claim 2 wherein the or each primary switching element of the third module is controllable in use to connect the capacitance element in series with the secondary terminals when the second electrical network is an open circuit.
4. A testing apparatus according to any preceding claim wherein: each of the capacitance element and the second module is connected in parallel with a first series arrangement of the first module and the primary terminals and with a second series arrangement of the third module and the secondary terminals; the third module includes at least one second inductor connected in series with the or each primary switching element of the third module; and each module of the first electronic block includes at least one first inductor connected in series with the or each respective primary switching element.
5. A testing apparatus according to any of Claims 1 to 4 wherein: each of the capacitance element and the second module is connected in parallel with a first series arrangement of the first module and the primary terminals and with a second series arrangement of the third module and the secondary terminals; the third module includes at least one second inductor connected in series with the or each switching element of the third module; and at least one first inductor is connected in series between a parallel arrangement of the second module and the first series arrangement and between a parallel arrangement of the or each capacitance element and the second series arrangement.
6. A testing apparatus according to any preceding claim wherein at least one of the modules of the first electronic block includes at least one first inductor connected in series with the or each respective primary switching element.
7. A testing apparatus according to any preceding claim wherein the or each primary switching element of the third module is a bidirectional switching element controllable in use to conduct current in either direction.
8. A testing apparatus according to Claim 7 wherein the or each primary switching element of the third module is controllable in use to form the third series resonant circuit so as to apply a reverse voltage to the second electrical network.
9. A testing apparatus according to any preceding claim including first and second primary terminals, one of the first and second primary terminals being operably connected in use to one of positive and negative poles of the first electrical network, the other of the first and second primary terminals being operably connected in use to the other of positive and negative poles of the first electrical network, and wherein the first electronic block further includes a fourth module, the or each primary switching element of the first module is controllable in use to connect at least one of the plurality of inductors in series with the capacitance element and the first electrical network via the first primary terminal to form the first series resonant circuit, and the or each primary switching element of the fourth module is controllable in use to connect at least one first 1' inductor in series with the capacitance element and the first electrical network via the second primary terminal to form a fourth series resonant circuit.
10. A testing apparatus according to Claim 9 wherein the primary switching elements of the second and fourth modules are controllable in use to provide a time delay between the formation of the second series resonant circuit and the formation of the fourth series resonant circuit.
11. A testing apparatus according to: Claim 4 or 5; and Claim 9 or Claim 10, the testing apparatus further including a tertiary terminal, wherein the fourth module is connected in series between the second primary terminal and the junction interconnecting the first module and a first end of the second module, and a second end of the second module is operably connected in use via the tertiary terminal to ground.
12. A testing apparatus according to Claim 11 wherein the first electronic block further includes first and second switching links, the first switching link being operably connected between the second end of the second module and the second primary terminal, the second switching link being operably connected between the second end of the second module and the tertiary terminal, the first and second switching links being controllable in use to selectively switch the second end of the second module into circuit with either the second primary terminal or the tertiary terminal.
13. A testing apparatus according to any preceding claim wherein the or each primary switching element of the second module is a bidirectional switching element controllable in use to conduct current in either direction.
14. A testing apparatus according to any preceding claim wherein the first electronic block further includes a fifth module connected between the capacitance element and the first module and between the capacitance element and the second module, the or each primary switching element of the fifth module is controllable in use to switch the capacitance element into circuit to form the first and second series resonant circuits.
15. A testing apparatus according to Claims 9 and 14 wherein the fifth module is connected between the fourth module and the capacitance element, and the or each primary switching element of the fifth module is controllable in use to switch the capacitance element into circuit to form the fourth series resonant circuit. C"
16. A testing apparatus according to: Claim 4 or 5; and Claim 14 or Claim 15 wherein the fifth module is connected in series between the parallel arrangement of the second module and the first series arrangement and between the parallel arrangement of the or each capacitance element and the second series arrangement.
17. A testing apparatus according to any of Claims 14 to 16 wherein the or each primary switching element of the fifth module is a bidirectional switching element controllable in use to conduct current in either direction.
18. A testing apparatus according to any preceding claim wherein the second electronic block further includes a sixth module connected in parallel with the secondary terminals, the or each primary switching element of the sixth module being controllable in use to connect the sixth module in series with the capacitance element and at least one second inductor to form a fifth series resonant circuit.
19. A testing apparatus according to any preceding claim further including auxiliary terminals; and one or more auxiliary switching elements connected in series with the auxiliary terminals, the series arrangement of the or each auxiliary switching element and the auxiliary terminals being connected in parallel with the secondary terminals, wherein the auxiliary terminals are connected in use to a current source; and the or each auxiliary switching element is controllable in use to switch the current source into and out of circuit with the secondary terminals.
20. A testing apparatus according to Claim 19 wherein the or each auxiliary switching element is a bidirectional switching element controllable in use to conduct current in either direction.
21. A testing apparatus according to any preceding claim wherein each switching element has reverse blocking capability.
22. A testing apparatus according to any preceding claim wherein the or each switching element includes at least one semiconductor device, at least one mercury arc valve or at least one mechanical switch.
23. A testing apparatus according to Claim 22 wherein the or each semiconductor device is a thyristor, an insulated gate bipolar transistor, a gate turn-off thyristor, a field effect transistor, a gate commutated thyristor or an integrated gate commutated thyristor.
24. A testing apparatus according to any preceding claim wherein at least one inductor is a variable inductor.
25. A testing apparatus according to any preceding claim further including a closed loop controller to monitor and regulate the voltage across the capacitance element.
26. A testing apparatus according to Claims 2 and 25 wherein the or each primary switching element of the first module is controllable in use to vary the length of the time delay in response to control signals from the closed loop controller.
27. A testing apparatus according to: Claim 10; and Claim 25 or Claim 26 wherein the or each primary switching element of the fourth module is controllable in use to vary the length of the time delay in response to control signals from the closed loop controller.
28. A testing apparatus according to any preceding claim, wherein the second electronic block further includes a snubber circuit connected in parallel with the or each primary switching element of the third module.
29. A testing apparatus according to any preceding claim further including a low pass filter operably connected to the primary terminals so as to interconnect in use the primary terminals and the first electrical network.
30. A testing apparatus according to any preceding claim wherein, in use, the predetermined sequence is repeated at a frequency equal to the operating frequency of the second electrical network.