阅读说明：本技术 高压电源系统 (High-voltage power supply system ) 是由 伯恩特·瓦尔格伦 于 2018-10-09 设计创作，主要内容包括：披露了一种用于为静电除尘器ESP(10)供电的高压电源系统(1)。该系统具有被配置为生成第一AC电源电压和第二AC电源电压的AC电源电路(2)、以及连接在该AC电源电路与该ESP之间的两个电源电路(5,6)。这两个电源电路中的一个电源电路是被配置为对该第一AC电源电压进行变换并将其转换为用于该ESP的DC基电压的DC电源电路(5),而另一个电源电路是具有脉冲形成电路(12)的脉冲电源电路,该脉冲形成电路被配置为生成高压脉冲并将其转发到该ESP。该AC电源电路被配置为使得这些AC电源电压中的每一个都处于中频范围内,即处于100Hz至5000Hz的范围内。因此,提出了一种成本有效、重量轻且紧凑的高压电源系统。(A high voltage power supply system (1) for powering an electrostatic precipitator, ESP, (10) is disclosed. The system has an AC power supply circuit (2) configured to generate a first AC power supply voltage and a second AC power supply voltage, and two power supply circuits (5, 6) connected between the AC power supply circuit and the ESP. One of the two power supply circuits is a DC power supply circuit (5) configured to transform the first AC supply voltage and convert it to a DC base voltage for the ESP, while the other power supply circuit is a pulse power supply circuit with a pulse forming circuit (12) configured to generate and forward high voltage pulses to the ESP. The AC supply circuit is configured such that each of these AC supply voltages is in the mid-frequency range, i.e. in the range of 100Hz to 5000 Hz. Thus, a cost-effective, lightweight and compact high-voltage power supply system is proposed.)

1. A power supply system (1) for generating high voltage pulses superimposed on a DC base voltage, suitable for powering an electrostatic precipitator (10), the power supply system comprising:

an AC power supply circuit (2) configured to generate a first AC power supply voltage and a second AC power supply voltage;

-a DC power supply circuit (5) connectable between the AC power supply circuit (2) and the electrostatic precipitator (10), the DC power supply circuit comprising a first transformer (7) and a first rectifier circuit (8) for transforming the first AC power supply voltage and converting it into the DC base voltage;

a pulsed power supply circuit (6) connectable between the AC power supply circuit (2) and the electrostatic precipitator (10), the pulsed power supply circuit comprising:

a second transformer (9) and a second rectifier circuit (11) for transforming the second AC supply voltage and converting it into a DC pulsed supply voltage sufficient to generate the high voltage pulses;

a pulse forming circuit (12; 12') connectable between the second rectifier circuit (11) and the electrostatic precipitator (10), the pulse forming circuit being configured to generate the high voltage pulse without additional voltage transformation;

wherein the AC power supply circuit is configured such that the frequency of each of the first AC power supply voltage and the second AC power supply voltage is in the range of 100Hz to 5000 Hz.

2. The high voltage power supply system (1) as claimed in claim 1, wherein the pulse forming circuit (12; 12') comprises at least one thyristor (25) and at least one diode (26) connected anti-parallel to the at least one thyristor.

3. The high-voltage power supply system according to claim 1 or 2, wherein the AC power supply circuit (2) includes:

a first power inverter (3) configured to convert a DC feed voltage into the first AC supply voltage;

a second power inverter (4) configured to convert the DC feed voltage to the second AC supply voltage; and is

Wherein the first and second power inverters are configured to control a frequency of each of the first and second AC supply voltages within a range of 100Hz to 5000 Hz.

4. The high voltage power supply system (1) as claimed in claim 3, wherein the first power inverter (3) is a full bridge single phase inverter or a half bridge single phase inverter comprising semiconductor power switches, such as IGBTs or MOSFETs.

5. The high voltage power supply system (1) as claimed in claim 3 or 4, wherein the second power inverter (4) is a full bridge single phase inverter or a half bridge single phase inverter comprising semiconductor power switches, such as IGBTs or MOSFETs.

6. The high voltage power supply system (1) as claimed in any one of the preceding claims, wherein the DC-based voltage and the high voltage pulses are connected in parallel at an output of the power supply system.

7. The high voltage power supply system (1) according to claim 6, wherein the pulse forming circuit (12) comprises:

a storage capacitor (21) connected between output terminals of the second rectifier circuit (11),

a first series inductance (23) and a coupling capacitor (27) connected in series to an output of the power supply system, an

A high voltage switching circuit (24) connected in series between the storage capacitor (21) and the first series inductance (23).

8. The high voltage power supply system (1) of claim 6, wherein the pulse forming circuit (12') comprises:

a high-voltage switching circuit (24') connected between the output terminals of the second rectifier circuit (11),

a first series inductance (23) and a coupling capacitor (27) connected in series to an output of the power supply system, an

A storage capacitor (21 ') connected in series between the high-voltage switching circuit (24') and the first series inductance (23).

9. The high voltage power supply system (1) as claimed in claim 7 or 8, wherein the high voltage switching circuit comprises at least one thyristor (25) and at least one diode (26) connected anti-parallel to the at least one thyristor.

10. The high voltage power supply system (1) as claimed in one of claims 7 to 9, wherein the pulse forming circuit (12; 12 ') further comprises a protection branch (31) connected in parallel with the high voltage switching circuit (24; 24 ') and the reservoir capacitor (21; 21 '), the protection branch (31) comprising a first resistor (33) and a series diode (32) for limiting a voltage peak across the high voltage switching circuit (24).

11. The high voltage power supply system (1) as claimed in one of claims 5 to 8, wherein the pulse forming circuit (12; 12') further comprises a recovery branch (34) connected in parallel with the high voltage switching circuit (24) and the reservoir capacitor (21), the recovery branch (34) comprising a second resistance (36) and a second series inductance (35) for recovering the charge of the coupling capacitor (27) between pulses.

12. The high voltage power supply system (1) as claimed in claim 9, wherein the inductance value of the second series inductance (35) is in the range of 0.1H to 10H.

13. The high voltage power supply system (1) as claimed in any one of the preceding claims, wherein the frequency of each of the first and second AC supply voltages is in the range of 200Hz to 2000 Hz.

14. The high voltage power supply system (1) as claimed in any one of the preceding claims, further comprising: -a first series capacitor (41) connected between said first power inverter (3) and said first transformer (7) of the DC power supply circuit (5), and-a second series capacitor (42) connected between said second power inverter (4) and said second transformer (9) of the pulsed power supply circuit.

15. The high voltage power supply system (1) as claimed in any one of the preceding claims, wherein the frequency of the first AC supply voltage is higher than the frequency of the second AC supply voltage.

16. The high voltage power supply system (1) as claimed in any one of the preceding claims, connected to an electrostatic precipitator (10).

Technical Field

The present invention relates to the field of electrical power engineering, and more particularly to the field of power supplies suitable for powering electrostatic precipitators (ESP).

Background

In industrial processes, electrostatic precipitators (ESP) are commonly used to collect and remove particulate matter from a gas stream. For example, these devices may be used to filter particles from emissions from coal-fired power plants, cement plants, steel plants, and waste incineration plants. Some of the reasons ESP are one of the most commonly used devices for particulate filtration/collection is that they can handle relatively large gas volumes under a variety of inlet temperature, pressure, dust volume and acid gas conditions. In addition, they can be used to collect various particle sizes, and can be collected in a dry state and a wet state.

As the name implies, ESPs use electrostatic forces to separate dust particles from a gas stream. Conventional ESPs have a set of discharge/transmit electrodes, typically in the form of thin wires, evenly spaced between large plates called collecting (collecting) electrodes, which are charged at high voltage, while the collecting electrodes are typically grounded, but can be charged at voltages of opposite polarity. Typically, a negative high voltage (typically pulsed) Direct Current (DC) is applied to the emitter electrode, thereby generating a negative electric field. In short, the flowing gas is arranged to pass through the negative electric field provided by the emitter electrode, thereby negatively charging the solid particles. The negatively charged particles are then adsorbed onto the collection electrode to which they are adhered. By shaking or beating these collecting plates, a large amount of accumulated "dust" is released and caused to fall under its own weight into a dust collecting container (hopper) arranged below. In more detail, there are other steps in the process, such as avalanche multiplication and secondary emission to ionize the gas molecules, which in turn ionizes these solid particles and leads to the end result that negatively charged particles are repelled by the negative field around the emitter electrode and strongly adsorbed onto the collector electrode.

High voltage pulse generators are commonly used in ESPs to superimpose voltage pulses on the DC voltage and thereby improve the performance of particle separation or filtration. The pulse width is typically about 100 mus and the frequency is in the range of 1 to 400 pulses/sec. The average current may be controlled by varying the pulse repetition frequency of the switching devices in the system while maintaining the voltage level applied to the electrostatic precipitator. In this way, the generation of back-corona and the negative effects associated therewith can be eliminated or at least limited.

Pulse systems are generally divided into two broad categories, one based on switching at high potential/voltage (on the secondary side) and the other based on what is known as a pulse transformer system based on switching at low potential (on the primary side). Examples of the latter (switching occurring on the primary side) can be found in e.g. US 4,052,177, US 4,600,411 and EP 1652586, while EP 1293253 discloses an example of a high voltage switch (i.e. switching occurring on the secondary side).

Document US5,575,836 discloses a dust collector with a pulsed power supply. In this case, the switch 12 is arranged on the secondary side of the transformer 10. It is clear, however, that the switching is not performed at the final voltage level. In contrast, in US5,575,836, a pulse transformer 16 is required to increase the voltage to a final level.

However, even though there are many prior art solutions, there is still a need for further improvements to the prior art, in particular in terms of reduced power losses, reduced size, cost, reduced output voltage ripple, and/or robustness/reliability.

Disclosure of Invention

It is therefore an object of the present invention to provide a high voltage power supply system for powering an electrostatic precipitator that alleviates all or at least some of the disadvantages associated with currently known systems in terms of power consumption, size, cost, reduced output voltage ripple, and/or robustness/reliability.

This object is achieved by a high voltage power supply system as defined in the appended claims.

The term exemplary should be understood to be used as an example, instance, or illustration hereinafter.

According to a first aspect of the present invention there is provided a power supply system adapted to power an electrostatic precipitator to generate high voltage pulses superimposed on a DC base voltage. The high-voltage power supply system includes: an AC power supply circuit configured to generate a first AC power supply voltage and a second AC power supply voltage; a DC power supply circuit connectable (i.e., configured/adapted to be connected) between the AC power supply circuit and the electrostatic precipitator, the DC power supply circuit including a first transformer and a first rectifier circuit for transforming the first AC power supply voltage and converting it to a DC base voltage; and a pulse power supply circuit connectable between the AC power supply circuit and the electrostatic precipitator. The pulse power supply circuit includes: a second transformer and second rectifier circuit for transforming the second AC supply voltage and converting it into a DC pulsed supply voltage sufficient to generate high voltage pulses; and a pulse forming circuit connectable between the second rectifier circuit and the electrostatic precipitator, wherein the pulse forming circuit is configured to generate (and forward/supply) high voltage pulses without additional voltage transformation. More specifically, the AC power supply circuit is configured such that the frequency of each of the first AC power supply voltage and the second AC power supply voltage is in the range of 100Hz to 5000 Hz.

Therefore, a cost-effective and compact high voltage power supply system (which may also be referred to as a high voltage pulse generating system) is proposed. The system is particularly suited for powering electrostatic precipitators used in gas flow filtration applications. Moreover, the power supply system is lighter and has lower power consumption than other known conventional systems.

The invention is based on the following recognition: by combining the high voltage switching device with two medium frequency power supplies (100Hz to 5000Hz), a relatively low power loss in the pulse unit box (i.e. the DC power supply circuit and the pulse power supply circuit) can be achieved. In more detail, by feeding the transformers of the DC power supply circuit and the pulse power supply circuit with a medium frequency AC voltage, the power losses are reduced due to the smaller core and the fewer winding turns, and therefore less cooling flange area is needed on the tank (where the circuit is located), making the whole system lighter and smaller. Also, high voltage switches have lower power losses than low voltage switches (such as systems utilizing pulse transformers). Furthermore, the output ripple voltage on the rectified output is reduced compared to a low frequency feed (e.g. 50 Hz). Furthermore, for line commutated DC power supplies, the need for a smoothing filter on the high voltage side of the DC power supply circuit is partially or completely alleviated.

Furthermore, the inventors have realized that the control semiconductors of the AC power supply circuit, e.g. Insulated Gate Bipolar Transistors (IGBTs), may be placed in a control cabinet in a protected indoor environment, and that the generated intermediate frequency AC voltage may be supplied via cables to a pulse cell box, which typically has to be placed outdoors, thereby reducing the risk of system failure and/or the manufacturing costs and complexity. For higher frequency systems such as Switched Mode Power Supply (SMPS) systems, it is often necessary to place the IGBTs near the transformer of the pulse forming circuit and thus in an outdoor environment.

Further in accordance with an exemplary embodiment of the invention, the AC supply circuit comprises a first power inverter configured to convert a DC supply voltage into the first AC supply voltage, a second power inverter configured to convert the DC supply voltage into the second AC supply voltage, wherein the first power inverter and the second power inverter are configured to control a frequency of each of the first AC supply voltage and the second AC supply voltage in a range of 100Hz to 5000 Hz. The DC feed voltage may for example be generated by a three-phase rectifier circuit connected to the three-phase mains (e.g. 380V/480V, 50Hz/60 Hz). The rectifier circuit may be uncontrolled or controlled, and may be a half-wave or a full-wave, depending on the specifications and requirements of the intended application. For example, the power inverter may be a full bridge single phase inverter or a half bridge single phase inverter using semiconductor switches, such as Insulated Gate Bipolar Transistors (IGBTs) or Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). Since the load is an inductive load (transformer), the power inverter may further include an anti-parallel diode or feedback rectifier connected across (in parallel with) each semiconductor switch to provide a path for peak inductive load current during switch turn-off. Conventionally, these anti-parallel diodes are integrated in a semiconductor package.

Further, according to another exemplary embodiment of the present invention, the pulse forming circuit includes: a storage capacitor connected across the second rectifier circuit; a first series inductance and a coupling capacitor connected in series with the reservoir capacitor, the first series inductance and the coupling capacitor being connected downstream relative to the reservoir capacitor towards the electrostatic precipitator; and a high voltage switching circuit connected between the storage capacitor and the first series inductance. Further, the high voltage switching circuit includes at least one thyristor and at least one diode connected in anti-parallel with the at least one thyristor.

In use, a micropulse is formed by closing a switch of a high voltage switching circuit, thereby forming an oscillating circuit (or resonant circuit) through the reservoir capacitor, the series inductance, the coupling capacitor and the ESP (which may be approximated as a capacitive load), causing a rapid rise in the voltage across the ESP and a corresponding voltage drop across the reservoir capacitor. Subsequently, the current changes direction and the voltage across the ESP decreases (down to the voltage level supplied by the DC power circuit), and the reservoir capacitor charges again to about the level output by the second rectifier circuit, completing one oscillation cycle. Preferably, the high voltage switching circuit is controlled to generate pulses having a frequency of 2 to 200Hz, such as 50Hz, 100Hz or 150 Hz. The switches may be controlled by a suitable firing circuit (firing circuit) connected to, for example, a thyristor or chain of thyristors used as the switching element(s) in the high voltage switching circuit.

The coupling capacitor is more particularly arranged between the first series inductance and the connection node of the DC power supply circuit (which provides the DC base voltage to the ESP) in order to forward and add the pulsed voltage above the DC base voltage and also to avoid short-circuiting of the DC power supply by the pulsed power supply.

Still further in accordance with another exemplary embodiment of the present invention, the pulse forming circuit comprises an auxiliary circuit connected in parallel with the high voltage switching circuit and the storage capacitor, the auxiliary circuit being connected between the high voltage switching circuit and the first series inductance, the auxiliary circuit comprising a protection branch comprising a first resistor and a series diode for limiting a voltage peak across the high voltage switching circuit. In other words, one terminal of the protection branch is connected to the node/junction between the high voltage switching circuit and the series inductance, and the other terminal is connected to ground. The series diode and resistor serve to limit the voltage peak across the high voltage switching circuit during spark generation in the ESP.

Still further in accordance with yet another embodiment of the present invention, the pulse forming circuit includes an auxiliary circuit connected in parallel with the high voltage switching circuit and the reservoir capacitor, the auxiliary circuit being connected between the high voltage switching circuit and the first series inductance, the auxiliary circuit including a recovery branch including a second resistance and a second series inductance for recovering charge of the coupling capacitor between pulses. In other words, one terminal of the recovery branch is connected to the node/junction between the high voltage switching circuit and the series inductance, and the other terminal is connected to ground. Naturally, the above two exemplary embodiments may be combined, and the pulse forming circuit may comprise an auxiliary circuit with the protection branch and the recovery branch. To improve the ability to restore the voltage across the coupling capacitor to the same value as the DC base voltage, the second series inductance is preferably arranged to have a relatively high inductance value in the range 0.1 henry to 10 henry, and preferably above 1 henry.

According to yet another exemplary embodiment of the present invention, each of the first AC supply voltage and the second AC supply voltage has a frequency in a range of 200Hz to 2000Hz, such as 200Hz to 600 Hz. In the first frequency range (200 to 2000Hz), a good compromise between output voltage ripple and transformer power loss is achieved. However, the latter frequency range (200 to 600Hz) is preferred for most conventional transformer designs.

Still further in accordance with another exemplary embodiment of the present invention, the high voltage power supply system further comprises a first series capacitor connected between said first power inverter and said first transformer of the DC power supply circuit, and a second series capacitor connected between said second power inverter and said second transformer of the pulsed power supply circuit. By adding a capacitor on the primary side of the transformer, a series resonant circuit is formed that allows any semiconductor switch used in the AC circuit (e.g., an IGBT in a power inverter) to turn off at a lower current magnitude, thereby reducing the strain on the IGBT and also reducing the output voltage ripple. In more detail, when only an inductive load (transformer winding) is present in the circuit, the current in the circuit continues to increase until cut off by the semiconductor switch (the current will have a sawtooth waveform). By adding a series capacitor, the circuit will form a series resonant circuit. At full power, the rectified current will then take the shape of a half-wave sinusoid, whereby the semiconductor switch can be switched off at a lower current magnitude. Furthermore, the series capacitor protects the transformer from any undesired DC components (e.g. in case of false control) which may cause problems with high primary saturation currents.

Further, according to yet another exemplary embodiment, the frequency of the first AC supply voltage is higher than the frequency of the second AC supply voltage. For example, the AC supply voltage forwarded to the DC supply circuit (first AC supply voltage) may have a frequency of 400Hz, and the AC supply voltage forwarded to the pulsed supply circuit (second AC supply voltage) may have a frequency of 200 Hz. Since the leakage inductance of the primary winding of the transformer provides sufficient inductance and thus alleviates the need for a primary choke so that the primary choke (a large contributor to noise in the circuit) can be eliminated, undesirable acoustic noise can be reduced by using frequencies in the range of 400 to 700 Hz. Naturally, in other exemplary embodiments of the present invention, the two frequencies may be the same.

These and other features of the invention will be further elucidated with reference to the embodiments described hereinafter.

Drawings

For purposes of illustration, the invention will be described in more detail below with reference to embodiments shown in the drawings, in which:

FIG. 1 illustrates a schematic block diagram of a high voltage power supply system for powering an electrostatic precipitator, in accordance with an embodiment of the present invention;

FIG. 2 illustrates a schematic circuit representation of a high voltage power supply system for powering an electrostatic precipitator, in accordance with an embodiment of the present invention;

FIG. 3A illustrates a schematic waveform representing the voltage across the storage capacitor of the pulse forming circuit during an oscillation period, in accordance with an embodiment of the present invention;

FIG. 3B illustrates a schematic waveform representing the current in the pulsed power supply circuit and into the ESP during an oscillation period, in accordance with an embodiment of the present invention;

fig. 3C illustrates a schematic waveform representing the voltage across an ESP connected to the high voltage power supply system during an oscillation period, in accordance with an embodiment of the present invention.

Fig. 4 is a schematic block diagram of an alternative embodiment of the pulsed power supply circuit of fig. 2.

Detailed Description

In the following detailed description, preferred embodiments of the invention will be described. However, it should be understood that features of different embodiments are interchangeable between embodiments and can be combined in different ways, unless anything else is specifically noted. Although in the following description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures or functions have not been described in detail so as not to obscure the present invention.

Fig. 1 is a schematic block diagram of a high voltage (pulsed) power supply system 1 particularly suitable for powering an electrostatic precipitator (ESP) 10. The system 1 may be divided into two parts, namely a pulse cell box 5, 6 and a control cabinet 2, wherein the control cabinet may be understood as a controllable AC power circuit configured to generate a supply voltage to the pulse cell box and to transform the supply voltage to a suitable level for powering the ESP 10. More specifically, the pulse unit box comprises a high voltage pulse power supply circuit 6 and a high voltage DC power supply circuit 5, whereby the pulse unit box is arranged to supply a higher DC base voltage (e.g. of a magnitude in the range of 20kV to 150 kV) for superimposed high voltage micro pulses (of a magnitude in the range of e.g. 40kV to 120 kV) at a rate of 2 to 200 pulses/sec, preferably 100 pulses/sec. Typically, the voltage applied to the discharge electrode (of the ESP) is of negative polarity, whereby the previous voltage range may be understood as ranging from-20 kV to-150 kV for DC-based voltages and-40 kV to-120 kV for micro-pulses.

The AC power supply circuit 2 is configured to generate a first AC power supply voltage and a second AC power supply voltage for the DC power supply circuit 5 and the pulse power supply circuit 6, respectively. The frequency of the AC mains voltage is in the mid-frequency range, i.e. between 100Hz and 5000Hz, preferably in the range of 200Hz to 2000 Hz. By using this arrangement (intermediate frequency power supply together with high voltage switching circuit), advantages of low loss, reduced tank size and weight, reduced manufacturing costs, etc. can be achieved. Furthermore, the AC power supply circuit 2, and more specifically the semiconductor switches (e.g. IGBTs) of the AC power supply circuit, configured to control the output voltage frequency, may be positioned within the control cabinet in a protected environment, unlike prior known systems using AC feeding, exploiting higher frequencies, which in relatively harsh environments have to be placed in the vicinity of the transformer(s) arranged outdoors.

Fig. 2 is a schematic circuit representation of a high voltage power supply system 1 according to an exemplary embodiment of the present invention. With reference to this figure, further details of the sub-units of the system 1 and their functional aspects will be described. The high voltage power supply system 1 includes an AC power supply circuit 2 configured to generate a first AC power supply voltage and a second AC power supply voltage. In more detail, the AC power supply circuit 2 comprises a first power inverter 3 and a second power inverter 4 configured to convert the DC feed voltage into a first AC supply voltage and a second AC supply voltage, respectively. The DC supply voltage is generated by a DC power supply circuit 22 comprising a three-phase rectifier bridge connected to an AC mains (e.g. 380V/50 Hz). Naturally, within the general knowledge of the person skilled in the art, there are ways for providing a suitable DC supply voltage to the power inverters 3, 4 (e.g. using a single phase power supply instead of a three phase power supply, connecting the inverters directly to a DC source, etc.) and will therefore be omitted for the sake of brevity.

Each of the power inverters 3, 4 comprises a set of IGBTs arranged in a full-bridge configuration and an anti-parallel diode connected across each transistor. However, other topologies commonly used in high power applications are possible, such as half-bridge inverters. Even though IGBTs are shown in the illustrated example, other semiconductor switches, such as MOSFETs, BJTs, etc., may be applied.

Further, the high-voltage power supply system 1 has a DC power supply circuit 5 connected to the output terminal of the first power inverter 3. The DC power supply circuit 5 comprises a first transformer 7 and a first rectifier circuit 8 for transforming the first AC supply voltage and converting it to a DC base voltage (in the range of 20kV to 150 kV) for the ESP 10. The negative electrode of the first rectifier circuit 8 (i.e. having a negative potential U)_B) To the discharge/emitter electrode of the ESP and the positive electrode is grounded.

Furthermore, a pulsed power supply circuit 6 is connected between the output of the second power inverter 4 and the ESP10, wherein the pulsed power supply circuit has a second transformer 9 and a second rectifier circuit 11 for transforming the second AC supply voltage and converting it into a DC pulsed supply voltage (of magnitude, for example, in the range of 40kV to 120 kV). The positive terminal of the second rectifier circuit 12 is connected to ground, while the negative terminal of the second rectifier circuit 11 (having a negative potential U)_C) Is connected to the discharge/transmit electrodes of the ESP10 via a plurality of components 23, 24, 27 included in the pulse forming circuit 12. The pulse forming circuit 12 is then configured to generate high voltage pulses for ESP10 (thus, these pulses are superimposed on the DC base voltage U)_BAbove).

Still further, the system 1 comprises a pair of optional series capacitors 41, 42, namely a first series capacitor 41 connected between the first power inverter 3 and the first transformer 7 of the DC power supply circuit 5, and a second series capacitor connected between the second power inverter 3 and the second transformer 9 of the pulsed power supply circuit 6. The series capacitors 41, 42 together with the leakage inductances of the transformers 7, 9 and any potential primary chokes form a series resonant circuit, and the IGBT can be controlled in such a way that it is switched off at a lower current level, thereby reducing power losses and increasing the lifetime of the IGBT. Further, output ripple can be reduced by employing the series capacitors 41, 42.

Continuing, the pulsed power supply circuit 6 includes a pulse forming circuit 12 connected between the second rectifier circuit 11 and the ESP 10. The pulse forming circuit is configured to generate and forward high voltage pulses to the ESP 10. The pulse forming circuit may be configured such that the pulse repetition frequency is in the range of 2 to 200Hz, and the pulse width of each pulse is, for example, in the range of 50 to 150 μ s. The pulse repetition frequency is suitably controlled by a control circuit or firing circuit connected to the switching element(s) of the high voltage switching circuit 24, however, as will be discussed in more detail below.

The pulse forming circuit 12 has a storage capacitor 21 connected in parallel with the second rectifier circuit 11, i.e. between the negative (output) terminal and the positive terminal of the second rectifier circuit 11, or between the negative terminal of the second rectifier circuit and ground. Thus, the voltage across the storage capacitor 21 is charged to the same level as the DC output of the second rectifier circuit 11 (U in this case)_C). Connected in series between the negative terminal of the reservoir capacitor 21 and the ESP10 is a first series inductance 23 and a high voltage switching circuit 24. The high voltage switching circuit 24 comprises a thyristor 25 or chain of thyristors coupled in anti-parallel with a diode 26 or chain of diodes. In other words, the thyristor(s) and the diode(s) are connected in opposite conduction directions to each other, so that the diode(s) can have a blocking effect on the current flowing to the second rectifier circuit 11 when the thyristor(s) is turned off. The chain of components is used in order to be able to cope with high voltages in the circuit without burning or damaging the components.

A control circuit or triggering (firing) circuit (not shown) is used to trigger the thyristor(s) at a predefined frequency to monotonically form a series resonant circuit, causing a voltage V across the ESP_ESPSuddenly increases (i.e. negative potential of the discharge electrode increases)) And the voltage V across the storage capacitor 21_CAnd is reduced accordingly. This is schematically illustrated in the waveforms shown in fig. 3A and 3C, where fig. 3A shows the voltage across the reservoir capacitor 21 over time, and more specifically during the oscillation period, and fig. 3C shows the voltage across the ESP10 during the oscillation period. In addition, fig. 3B illustrates the current flowing through the pulse forming circuit 12 and into the ESP10 during the oscillation period.

Returning to fig. 2, the pulse forming circuit 12 also has a coupling capacitor 27 connected in series between the first series inductance 23 and the discharge electrode of the ESP 10. The coupling capacitor 27 assists in forwarding and adding the pulsed voltage above the DC base voltage and also mitigates the risk of short-circuiting of the DC power supply 5 by the pulsed power supply 6.

Further, the pulse forming circuit 12 includes an optional auxiliary circuit 30 connected in parallel with the high voltage switching circuit 24 and the reservoir capacitor 21. In other words, one terminal of the auxiliary circuit 30 is connected between the high voltage switch circuit 24 and the first series inductance 23, and the other terminal is grounded. Here, the auxiliary circuit 30 has two parallel branches 31, 34, wherein one terminal of each branch is connected to a node between the high voltage switching circuit 24 and the first series inductance 23 and the other terminal is connected to ground. One of the branches is represented as a protection branch 31 comprising a first series resistance 33 and a series diode 32 for limiting the voltage peak across the high voltage switching circuit 24. The auxiliary circuit 30 further has an optional recovery branch 34 comprising a second series resistance 36 and a second series inductance 35 for recovering the charge of the coupling capacitor 27 between the pulses. Preferably, the second series inductance has a relatively high inductance value, for example in the range of 0.1H to 10H (such as 1H).

Note that other examples of the auxiliary circuit 30 are possible. In particular, the auxiliary circuit may be simplified and for example comprise only a recovery branch, which may comprise only an inductance or only a resistor.

Fig. 4 shows an alternative embodiment of the pulse forming circuit 12'. The components are substantially the same as those in circuit 12 in fig. 2, but with some differences.

Here, the rectifier 11 is connected to provide a positive supply voltage. Further, the reservoir capacitor 21 'and the high voltage switching circuit 24' have changed positions such that the switching circuit 24 is connected in parallel with the rectifier 11, i.e. between the rectified outputs. With this solution an additional impedance 28 (here an inductance in series with a resistor) is required between the rectifier output and the reservoir capacitor 21'.

The protection branch 31 and the restoration branch 34 may be connected in the same manner as in fig. 2, i.e., in parallel with the storage capacitor 21 'and the switching circuit 24'.

Although the present invention has been described with reference to specific exemplary embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. For example, each of the power inverters 3, 4 may have its own separate feed through the rectifier circuit and the DC link capacitor. The DC feed circuit 22 may be powered by, for example, single-phase AC rather than three-phase AC. This and other obvious variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Furthermore, in the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.