WO2010069401A1

WO2010069401A1 - Current source element

Info

Publication number: WO2010069401A1
Application number: PCT/EP2008/068058
Authority: WO
Inventors: Colin Donald Murray Oates
Original assignee: Areva T&D Uk Ltd
Priority date: 2008-12-19
Filing date: 2008-12-19
Publication date: 2010-06-24
Also published as: EP2368316A1; ES2856002T3; EP2368316B1

Abstract

A current source element (10) for a current source converter comprises first and second voltage terminals (V₁, V₂). The current source element (10) also comprises a chain of modules (M₁, M₂, M₃....M_N) and an inductor (L) connected in series between the voltage terminals (V₁, V₂). Each module (M₁, M₂, M₃....M_N) includes at least one pair of semiconductor switches (12, 14) connected in parallel with a capacitor (20). The semiconductor switches (12, 14) are controllable in use to provide a continuously variable voltage source to directly control the rate of change of current in the inductor (L).

Description

CURRENT SOURCE ELEMENT

The invention relates to a current source element for use in a current source converter, and a current source converter incorporating one or more current source elements.

The nature of power electronics is to employ switches based on semiconductor technology that direct current . When combined with passive components that provide temporary energy storage, the ability of these switches to operate rapidly permits electrical power in practically any form to be converted to a different form with minimal losses. The emphasis on making such power conversion efficient and compact has focused development on increasing the frequency of operation so that the size of the passive components can be reduced.

For power conversion equipment involving high voltages, high frequency switching is not desirable since it creates high stressing on insulation, and series semiconductor switches must be configured to operate within the operating parameters of the poorest of the switches. The converter losses therefore become exaggerated.

The path of the circulating current passing through each switch must also have a large area to maintain the insulation, thereby creating a large self- inductance that will inherently limit the converter performance. A number of multilevel topologies exist that allow the semiconductor switches to operate independently. GB 2 294 821 A discloses a static compensator (STATCOM) circuit, which is a chainlink converter consisting of a series of modules that each include charged capacitance that can be switched in and out of circuit using semiconductor switches to yield a continuously variable voltage source. The circulating current path for the semiconductor switching is contained within the respective modules, which results in minimum self-inductance and thereby improves the efficiency of the switching operation.

Figure 1 shows a 50Hz power waveform being synthesized by staggering the switching of the modules such that, although many switching operations are taking place, the switching operations are contained within individual modules and the number of switching operations per module may be as low as the frequency of the power voltage. The concept underlying the chainlink topology disclosed in GB 2 294 821 A originates from work that was carried out at the Massachusetts Institute of Technology (MIT) in the 1970s to produce a high voltage converter from linked circuits, each circuit containing a moderate voltage source that could be switched in or out of circuit.

This work was embodied in US 3,909,685, US 3,866,643 and US 3,971,976, which disclose circuits in which the voltage source is initially a battery, and later the transformer isolated output of a high frequency converter, and a switch is included so that the batteries can be connected in parallel for charging.

These patents disclose the basic concept of the multilevel converter principle, demonstrate how a sinusoidal voltage can be synthesized from a stepped waveform and formed the inspiration for the STATCOM circuit of GB 2 294 821 A.

In the circuit of GB 2 294 821 A the voltage source is replaced by a charged capacitor within a 4- quadrant module, which is shown in Figure 2.

The term "4-quadrant" refers to the ability of the module to conduct current in both directions and develop both positive and negative voltage. This is exemplified in the graph of voltage against current shown in Figure 3 in which the numbers shown indicate the respective quadrants.

The ability to develop positive and negative voltage and to conduct current in both directions allows a string of such modules to be operated against an AC power network to provide only reactive power.

In the circuit of GB 2 294 821 A there is no net power flow between the power network and the

STATCOM circuit except to counter losses. This means that once the capacitors are charged they remain charged.

DE 101 03 031 Al recognises that power can be transferred from an AC circuit to a DC circuit using the 2-quadrant capacitor module shown in Figure 4.

The significance of the "2-quadrant" operation is that the module can only develop zero or positive voltage but can conduct current in both directions. This is exemplified in the graph of voltage against current shown in Figure 5 in which the numbers shown indicate the respective quadrants.

The principle of the circuit disclosed in DE 101 03 031 Al has been embodied within topologies disclosed in WO 2007/028349 Al and WO 2007/0328350 Al for voltage source converter (VSC) type high voltage direct current (HVDC) schemes.

GB 2 294 821 A, DE 101 03 031 Al, WO 2007/028349 Al and WO 2007/0328350 Al however consider the chainlink converter as a series of power electronics modules that can be controlled to give a continuously variable voltage source. An aim of the invention is to extend these principles to one that can be applied to a range of topologies and that will give rise to converters suitable for power conversion at high voltages.

According to an aspect of the invention there is provided a current source element for a current source converter, the current source element comprising first and second voltage terminals and a chain of modules and an inductor connected in series between the voltage terminals, wherein each module includes at least one pair of semiconductor switches connected in parallel with a capacitor, the semiconductor switches being controllable in use to provide a continuously variable voltage source to directly control the rate of change of current in the inductor . The inclusion of the inductor connected in series with the modules allows the variable voltage to be used to control the current in the inductor to the extent that the current through the element is independent of the voltage across it so that it can be considered as an ideal current source. The current source element preferably includes a control circuit connected across the first and second voltage terminals to measure the potential difference across the terminals and the current through the inductor and a control element operably associated with the semiconductor switches to control operation of the semiconductor switches in response to the measured voltage and current values in order to regulate current in accordance with a received control signal.

Preferably each semiconductor switch comprises an insulated-gate bipolar transistor. In other embodiments it is envisaged that other semiconductor switches may be used.

The semiconductor switches of each module may be connected with the respective capacitor in a half-bridge arrangement to define a 2-quadrant unipolar module that can develop zero or positive voltage and can conduct current in both directions.

Alternatively the semiconductor switches of each module may be connected with the respective capacitor in a full-bridge arrangement to define a 4- quadrant bipolar module that can develop positive or negative voltage and can conduct current in both directions .

The choice of whether the modules of the current source elements are 2-quadrant or 4-quadrant depends on whether the application requires a voltage or current reversal.

In use at least one current source element is connected between an input circuit and an output circuit to form a converter and facilitate a net power transfer between the input and output circuits.

The semiconductor switches of each module of the or each current source element are preferably controllable to switch modules into and out of the chain of modules so as to maintain the sum of the voltages over the chain of modules and the inductor equal to the potential difference across the first and second voltage terminals whilst varying the voltages over the modules so as to vary the voltage over the inductor and thereby control the rate of change of current in the inductor.

The converter preferably includes a controller that calculates the current profile required from the or each current source element in the converter and sends a control signal to the control element of the or each current source element so that the control element can control the semiconductor switches to produce the required current profile.

The or each current source element is preferably connected between the input circuit and the output circuit so that input and output currents can circulate only within the input and output circuits respectively, and the or each current source element defines a circulating path for current within the converter that permits an exchange of power between the input and output circuits. Preferably the power transfer is effected by means of a difference current through the or each current source module.

In particularly preferred embodiments, a converter includes a plurality of the current source elements connected in a matrix array to interconnect the input and output circuits.

In one such embodiment the converter includes nine current source elements are connected in a 3x3 matrix array to interconnect in use two three- phase AC networks .

In another such embodiment the converter includes six current source elements connected in a 3x2 matrix array to interconnect in use a three phase AC network and a DC network.

In another embodiment the converter includes a pair of current source elements and a transformer to interconnect in use a DC network connected to a first side of the transformer via the current source elements and an AC network connected to the opposite side of the transformer .

In another embodiment the converter includes a first pair of current source elements connected back to back with a second pair of current source elements via a transformer to interconnect in use two DC networks .

In any of these embodiments, the modules of the current source elements may be 2-quadrant unipolar modules or 4-quadrant bipolar modules depending on whether the polarity of the voltage reverses between the input and output circuits. Embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying figures in which:

Figure 1 shows the synthesis of a 50Hz power waveform by staggering the switching of a series of modules in a chainlink converter;

Figure 2 shows a 4-quadrant module;

Figure 3 shows a graph of voltage against current for the 4-quadrant module of Figure 2; Figure 4 shows a 2-quadrant module;

Figure 5 shows a graph of voltage against current for the 2-quadrant module of Figure 4;

Figure 6 shows a current source element according to an embodiment of the invention; Figure 7 shows a current source element according to another embodiment of the invention;

Figure 8 shows a schematic circuit arrangement of a three-phase to three-phase converter according to another embodiment of the invention; Figure 9 shows a detailed circuit topology of the three-phase to three-phase converter of Figure 8;

Figures 10a and 10b demonstrate the AC voltages and currents when the three-phase to three- phase converter of Figure 8 is used to interconnect a 50Hz network and a 25Hz network;

Figures 11 and 12 show the operation of two of the current source elements of the three-phase to three-phase converter of Figure 8; Figure 13 shows a detailed circuit topology of a three-phase to single-phase converter according to another embodiment of the invention;

Figure 14 shows a detailed circuit topology of a three-phase to symmetrical ±DC converter according to another embodiment of the invention;

Figure 15 shows a schematic circuit arrangement of a DC to symmetrical single phase AC converter according to another embodiment of the invention;

Figure 16 shows a detailed circuit topology of the DC to symmetrical single phase AC converter of Figure 15;

Figure 17 demonstrates the top constant current element voltages and currents of a simulation of the DC to symmetrical single phase AC converter of Figure 15;

Figure 18 demonstrates the capacitor voltages of the simulation of the DC to symmetrical single phase AC converter of Figure 15;

Figure 19 demonstrates the separated DC and AC currents when the AC voltage and the AC current reference of the simulation are both reversed in polarity; Figure 20 shows a schematic circuit arrangement of a galvanically isolated DC to DC converter according to another embodiment; and

Figure 21 shows a detailed circuit topology of the galvanically isolated DC to DC converter of Figure 20. A current source element 10 according to an embodiment of the invention is shown in Figure 6. The current source element 10 includes first and second voltage terminals Vi, V₂ and a chain of modules Mi, M₂, M₃....M_N and an inductor L connected in series between the voltage terminals.

In the embodiment shown in Figure 6, each of the modules Mi, M₂, M₃....M_N includes two pairs of semiconductor switches 12,14,16,18 in the form of insulated gate bipolar transistors connected in a full- bridge arrangement with a capacitor 20 to define a 4- quadrant bipolar module.

In another embodiment, which is shown in Figure 7, each of the modules M_x, M₂, M₃....M_N includes a pair of semiconductor switches 12,14 in the form of insulated gate bipolar transistors connected in a half- bridge arrangement with a capacitor 20 to define a 2- quadrant unipolar module.

The current source element 10 shown in each of Figures 6 and 7 includes a control circuit 22 connected across the first and second voltage terminals Vi, V₂. The control circuit 22 measures the voltage across the first and second voltage terminals Vi, V₂ to determine the potential difference V_i2 across the two terminals Vi, V₂ and measures the current Ii₂ through the inductor L.

The current source element 10 also includes a control element 24 operably associated with the modules Mi, M₂, M₃....M_N to control operation of the semiconductor switches in response to the measured voltage and current values V_i2, I₁₂ received from the control circuit 22 in order to regulate the current through the inductor L in accordance with a received control signal IDEMAND-

In use, to form a converter, one of more current source elements 10 are connected between an input circuit and an output circuit in order to facilitate a net power transfer between the input and output circuits.

In the simplest form of converter, the input circuit is connected to the first voltage terminal Vi of a current source element 10 and an output circuit is connected to the second voltage terminal V₂ of the current source element 10.

The use of 2-quadrant unipolar modules Mi, M₂, M₃....M_N or 4-quadrant bipolar modules M_x, M₂, M₃....M_N is dependent on whether the polarity of the voltages reverses between the input and output circuits.

The current source element 10 is connected between the input and output circuits such that input and output currents can circulate only within the context of the input and output circuits and a circulating path for all the currents is provided permitting interaction for an exchange of power between the input and output circuits. A controller is provided that calculates the current profile required from the current source element 10 according to the nature of the input and output circuits. The controller communicates with the control element 24 of the current source element 10, providing the control signal I_DEMAND required to allow the control element 24 to control the current through the inductor L.

The control element 24 controls the current through the inductor L by controlling switching of the semiconductor switches of the modules Mi, M₂, M₃....M_N in order to switch modules into and out of the chain of modules to maintain the sum of the voltages over the chain of modules Mi, M₂, M₃....M_N and the inductor equal to the difference V_i2 between the voltages at the first and second voltage terminals Vi, V₂ whilst varying the voltages over the individual modules M₁, M₂, M₃....M_N SO as to vary the voltage e over the inductor L and thereby control the rate of change of current in the inductor L in accordance with the following equation:

τ di e = L — dt

where L is the inductance value of the inductor L so that:

where V_CLM is the voltage required from the chain of modules, i_o is the measured current and i_D is the current required at the end of the sample period At.

This allows the current in the input and output circuits to be controlled by regulating the current source element to follow the instantaneous current difference between the current passing through the current source element for the input and output circuit .

The inductance L of the inductor L, the capacitance of the capacitor 20 of each module Mi, M₂, M₃....M_N and the sample period Δt must be chosen to suit the application.

Each of the modules Mi, M₂, M₃....M_N provides its voltage from the capacitor 20 and the switching of the semiconductor switches is controlled to ensure that the charge of the capacitor 20 of each of the modules

Mi, M₂, M₃....M_N is maintained within predetermined limits.

The transient energy storage provided by the capacitor 20 of each of the modules M_x, M₂, M₃....M_N allows for a net power transfer between the input and output circuits whilst maintaining the input and output circuits independent of each other.

The power transfer is effected by means of a difference current through the current source module, which must reverse in polarity sufficiently to reset the charge on each capacitor 20.

In preferred embodiments of the invention, a matrix array of current source elements 10 is constructed and one such embodiment is shown in schematic form in Figure 8 in which nine current source elements 10 are connected in a 3x3 matrix array 30 in order to interconnect two three-phase networks.

This arrangement, a detailed circuit topology of which is shown in Figure 9, requires only one converter when compared with a conventional method of connecting two HVDC systems back to back with a coupling DC link. Consequently there is only one set of losses. There are also fewer components making the converter smaller and cheaper to construct.

Practical uses of the topology shown in Figure 9 include static frequency changing, such as the frequency change from 50Hz to 400Hz that is required in aerospace and shipboard applications.

Referring to Figure 8, each phase connection of the matrix array 30 contributes one third of the current requirement of the phase to which it is connected.

Assuming that the input and output circuits are balanced, any transient difference between the two sets of waveforms is provided by the energy in the capacitors 20 of the modules Mi, M₂, M₃....M_N of the current source elements 10.

If the two three-phase networks are balanced an in phase with their respective voltages so that only power is being transferred and the voltages have peak magnitudes Vi and V₂, then from the power balance:

and the currents Ii, I₂ in the input and output circuits will have the following magnitudes:

9. P V₇ / = =1

3-V₁ V₁

Since each phase connection of the matrix array 30 contributes one third of the current requirement of the phase to which it is connected, and noting that there is a reversal in the polarity of the current to give a power balance, the matrix of the current demand is:

(Kl⁺Kl) UaI⁺ hi) UaI⁺Kl)

3 UbI⁺Kl) UbI⁺Kl) UbI⁺Kl)

where ^lb\ = h

Similarly the voltage across the elements is

(^Val -^Val) (V_fll -V_M) (V_fll -V_c2)

V₁₈ = (^Vbl -^Val) (^Vbl -^Vbl) (V_fcl -V_c2) (^Vd -^Val) (V_cl -V_M) (V_cl -V_c2)

"αl sin(ω • t) where ^vb\ :V, sin(<»-t-2^-/3) sin(ω-? + 2^"/3)

The AC voltages and currents of a MATLAB model based on the circuit topology shown in Figure 9 and representing two three-phase HkV AC systems being operated at 20MW using chains of 16 bipolar modules in which one system is operating at 50Hz and the other is at 25Hz are shown in Figures 10a and 10b with the 50Hz system being shown on the left and the 25Hz system being shown on the right. The operation is open loop so that there is no distortion in the AC currents, including DC offset. However adding in feedback control and optimizing the system operation would reduce or eliminate this. Figures 11 and 12 show the operation of two of the current source elements 10, Figure 11 showing top chain voltage and element voltage for the two elements and Figure 12 showing the capacitor voltages for the two elements. These figures show, in particular, that there is no drift in capacitor voltages, and thereby highlight that the AC power transfer is balanced.

In other embodiments six current source elements 10 are connected in a 3x2 matrix array 30 in order to interconnect a three-phase AC network and a DC network such as might be required in high voltage DC transmission .

The circuit topology for one such embodiment is shown in Figure 13 in which the DC voltage is lower than the peak AC voltage. In this embodiment each of the current source elements 10 includes 4-quadrant bipolar modules Mi, M₂, M₃....M_N to ensure capacitor balancing.

The circuit topology for another such embodiment is shown in Figure 14 in which the DC voltage is higher than the peak AC voltage. In this embodiment each of the current source elements 10 includes 2-quadrant unipolar modules Mi, M₂, M₃....M_N because the capacitor balancing comes from a current reversal rather than a voltage reversal. A schematic arrangement of a converter according to another embodiment is shown in Figure 15 in which a "biphase converter" (active equivalent to the bi-phase rectifier) is formed by connecting a pair of current source elements 10a, 10b to the antiphase terminals of a centre tapped transformer 32, the free ends of the current source elements being commoned to form the DC voltage terminal and the centre tap forming the common OV connection. A converter constructed in accordance with the arrangement shown in Figure 15, a detailed circuit topology of which is shown in Figure 16, is suitable for connecting a DC network to a symmetrical single phase AC network. Each of the current source elements 10 includes 4-quadrant bipolar modules Mi, M₂, M₃....M_N and, unlike the topology shown in Figure 14, the DC voltage can be less than the peak AC voltage in the topology shown in Figure 16. To illustrate the operation of the circuit topology shown in Figure 16, the applicant has simulated an application in which a 25kV, 4MW, 50Hz AC supply, such as that used for traction, to a DC level that might be used to directly control the traction motors .

In this example, the DC divides and passes equally and in opposition through the transformer windings .

In order to minimize this causing DC flux to appear in the transformer core it requires that the windings are bifilar wound so that the DC flux in the two windings cancel.

The voltage and current are thus:

/_AC 4 = 113A A^C 25 £ V

and for balance :

N -V_M - h = \ -V_c I₂ - costø)

where the right hand side of the equation represents the AC power, V_M is the voltage of the individual modules and N represents the number of modules being switched per quarter cycle. The factor N may vary according to the conditions and, in the simulation a nominal value of 8 was used for a 16-module string.

In the simulation if the modules have a voltage of 4.5kV, the DC current will be 400A at 1OkV DC. The AC impedance is 156.6Ω and so a line reactor of

20% of this value is 10OmH.

The capacitance of the modules is chosen to give a maximum peak to peak ripple under maximum current and in the simulation a value of 3mF was used. The top constant current element voltages and currents resulting from the simulation, in which the control is operating with a sample frequency of

2kHz, are shown in Figure 17. From the results shown in Figure 17 it can be seen that the converter current I₀ is able to track the reference current I_REF with reasonable accuracy.

The capacitor voltages are shown in Figure 18 and are stable, thereby demonstrating that power balance is maintained. In particular Figure 18 shows that the peak AC current is less than the DC current so that the current never reverses.

Extending this model to include a duplicated form of the above with the AC voltage and the AC current reference both reversed in polarity allows the full operation of the converter to be demonstrated. Figure 19 shows the resulting AC and DC currents separated to give the terminal conditions. In a yet further embodiment, two of the converters shown in Figure 16 are connected back to back in order to interconnect two DC networks and a schematic arrangement is shown in Figure 20.

It is envisaged that a converter in the form shown in Figure 20, a detailed circuit topology for which is shown in Figure 21, is suitable for use in HVDC transmission, particularly where disparate existing HVDC schemes need to be connected.

In this embodiment each of the current source elements 10 includes 4-quadrant bipolar modules Mi, M₂, M₃....M_N, which allows the polarity of either of the DC networks to be reversed. It is envisaged therefore that the circuit topology shown in Figure 21 can be applied to conventional HVDC systems employing thyristors to reverse the polarity of the voltage.

Claims

l.A current source element for a current source converter, the current source element comprising first and second voltage terminals and a chain of modules and an inductor connected in series between the voltage terminals, wherein each module includes at least one pair of semiconductor switches connected in parallel with a capacitor, the semiconductor switches being controllable in use to provide a continuously variable voltage source to directly control the rate of change of current in the inductor.

2.A current source element according to Claim 1 further including a control circuit connected across the first and second voltage terminals to measure the potential difference across the terminals and the current through the inductor and a control element operably associated with the semiconductor switches to control operation of the semiconductor switches in response to the measured voltage and current values in order to regulate current in accordance with a received control signal.

3.A current source element according to

Claim 1 or Claim 2 wherein each semiconductor switch comprises an insulated-gate bipolar transistor.

4.A current source element according to any preceding claim wherein the semiconductor switches of each module are connected with the respective capacitor in a half-bridge arrangement to define a 2-quadrant unipolar module that can develop zero or positive voltage and can conduct current in both directions.

5.A current source element according to any of Claims 1 to 3 wherein the semiconductor switches of each module are connected with the respective capacitor in a full-bridge arrangement to define a 4-quadrant bipolar module that can develop positive or negative voltage and can conduct current in both directions.

6.A converter comprising at least one current source element according to any preceding claim connected between an input circuit and an output circuit to facilitate a net power transfer between the input and output circuits.

7.A converter according to Claim 6 wherein the semiconductor switches of each module of the or each current source element are controllable to switch modules into and out of the chain of modules so as to maintain the sum of the voltages over the chain of modules equal to the potential difference across the first and second voltage terminals of the current source element whilst varying the voltages over the individual modules so as to vary the voltage over the inductor and thereby control the rate of change of current in the inductor.

8.A converter according to Claim 7 further including a controller that calculates the current profile required from the or each current source element in the converter and sends a control signal to the control element of the or each current source element so that the control element can control the semiconductor switches to produce the required current profile .

9.A converter according to any of Claims 6 to 8 wherein the or each current source element is connected between the input circuit and the output circuit so that input and output currents can circulate only within the input and output circuits respectively, and the or each current source element defines a circulating path for current within the converter that permits an exchange of power between the input and output circuits.

10. A converter according to any of Claims 6 to 9 wherein the power transfer is effected by means of a difference current through the or each current source module .

11. A converter according to any of Claims 6 to 10 including a plurality of current source elements connected in a matrix array to interconnect the input and output circuits.

12. A converter according to Claim 11 including nine current source elements connected in a 3x3 matrix array to interconnect in use two three-phase networks,

13. A converter according to Claim 11 including six current source elements connected in a 3x2 matrix array to interconnect in use a three-phase AC network and a DC network.

14. A converter according to Claim 11 including a pair of current source elements and a transformer to interconnect in use a DC network connected to a first side of the transformer via the current source elements and an AC network connected to the opposite side of the transformer.

15. A converter according to Claim 11 including a first pair of current source elements connected back to back with a second pair of current source elements via a transformer to interconnect in use two DC networks .

16. A converter according to any of Claims 11 to 15 wherein the modules of each current source element are 2-quadrant unipolar modules.

17. A converter according to any of Claims 11 to 15 wherein the modules of each current source element are 4-quadrant bipolar modules.

18. A current source element generally as herein described with reference to and/or as illustrated in the accompanying drawings.

19. A converter generally as herein described with reference to and/or as illustrated in the accompanying drawings .