WO2019053447A1

WO2019053447A1 - Sub-station transformer load balancing system

Info

Publication number: WO2019053447A1
Application number: PCT/GB2018/052621
Authority: WO
Inventors: Ian Mcdonald; Nigel Jakeman; Fainan Hassan; David Gurwicz
Original assignee: Turbo Power Systems Ltd
Current assignee: Turbo Power Systems Ltd
Priority date: 2017-09-15
Filing date: 2018-09-14
Publication date: 2019-03-21
Anticipated expiration: 2020-03-15
Also published as: GB201714874D0; BR112020005181A2; EP3682523A1; GB2566933A; GB2566933B

Abstract

An electrical power distribution network comprises at least two current carrying feeders; at least two loads, each associated with a respective one of the feeders; at least one module electrically connected to two of the at least two feeders and including: a bi-directional auxiliary converter; DC bus including a capacitor; and a bi-directional balancing converter connected in parallel. The module further includes means to change the current and voltage between the feeders and the bi-directional auxiliary converter and a differential electrical connection between the two feeders and connected in series with bi-directional balancing converter and a controller configured to control at least one parameter of an electrical current flowing through the or each module, the voltage across the or each module being related to the maximum voltage difference between two of the at least two feeders.

Description

Sub-Station Transformer Load Balancing System

Field of the Invention

The present invention relates to electrical power distribution networks and in particular to balancing loads experienced by different parts of such networks.

Background of the Invention

There is an increasing drive towards distributed power generation, for example many buildings are now equipped with solar power systems which generate modest amounts of energy and which is often fed back into the distribution grid. Small scale wind power systems work on a similar basis. However, the network for distributing electrical power in many countries was designed to distribute centrally generated electrical power to many dispersed users, not for such users to generate electrical power for input into the distribution network.

There is an increasing need to adapt electrical power distribution networks to

accommodate distributed power generation.

Electrical power distribution networks comprise numerous feeders. As the number of power generating installations on the network increases the load on the feeders to which such power generating installations are connected increases. This is exacerbated by renewable electricity sources, the power generation of which is unpredictable. Where there are many solar installations, even when each installation is small, on a sunny day the feeder to which those solar installations are connected may be significantly overloaded.

A feeder can also become overloaded when there is a significant amount of consumption by users connected to a particular feeder. As more equipment becomes electrified, vehicles being a prime example, it is likely that electricity will displace other forms of energy, thereby increasing demands placed on networks and individual feeders thereof.

With current network arrangements the load on one electrical feeder may be changed by changing the configuration of feeders in a network from a radial configuration to a meshed configuration by tying the ends of neighbouring feeders together directly using a mechanical bus- tie breaker. However, a number of factors can make the use of mechanical bus-tie breakers difficult. For example, the voltage difference between feeders, the length of feeders, the total load on a feeder, fault levels on feeders, imbalances of phase where the feeder is a three phase feeder, and the load distribution along a feeder.

In some instances power electronics solutions have been used to shift additional capacity from an adjacent feeder to an overloaded feeder. Where such power electronics are provided at the same location a bus-tie breaker would be provided, the arrangement is commonly known as a Soft Open Point.

The power electronics currently used in Soft Open Points are rated to handle the full line voltage. For low voltage applications the power electronics used in Soft Open Points are bulky. For medium and high voltage parts of electricity distribution networks the size of power electronics systems would make their use unlikely.

The Applicant has developed an active balancing conversion system which is illustrated in Figure 1 and is for use in low voltage networks. The Soft Open Point illustrated in Figure 1 is situated in a network which comprises a three-phase high voltage supply to which two

transformers (which step the voltage down to a low voltage and which may be considered as feeders) are connected, a load being connected to each of the transformers. Each of the transformers is also connected to a respective bi-directional converter. The bi-directional converters are attached to a capacitor C1 which forms a DC bus. The bi-directional converters and the capacitor C1 are rated to transfer the maximum out-of-balance load between the two transformers.

The DC bus must exceed the peak line voltage and the maximum balancing current could be equal to the rated line current and hence each active converter must have the same power rating as one of the low voltage distribution transformers. If the same arrangement were used to share load between medium and high voltage networks the physical size of components required would increase and greater power losses would be incurred.

DE10103031 describes a system for a high power network which uses two bi-directional converters as modular multi-level converters.

US20160141876 describes an alternate arm converter for use in high voltage direct current power transmission and reactive power compensation and a method of controlling such an alternate arm converter. The alternate arm converter includes a combination of series connected semi-conductor switches and series connected full-bridge converter modules. The series switches direct the power flow from one branch of the network, while the full-bridge modules provide multilevel operation.

US2017/0141694 describes a power management system utilising a low voltage pre- charge. The low voltage source provides a charging current to electrically isolated stacks.

US2013/0024043 describes a computer implement method which configures and reconfigures parameters of distribution automation devices forming part of a grid topology.

US2015/0314696 describes a co-ordinated control method and system that

can accommodate distributed energy resources and electric vehicle charging.

However, all of the above-mentioned solutions have drawbacks associated with them. For example, in the arrangements described in DE10103031 and US20160141876 the bi-directional power converters are rated to the same power as the feeder therefore limiting their use in high power distribution networks.

It would be desirable to provide a control system which provides for the load between transformers of neighbouring feeders to be balanced. Such a system would allow conventional distribution networks to be improved so that they may cope with the new demands that are being placed on them.

Summary of the Invention

According to the invention there is provided an electrical power distribution network comprising:

at least two current carrying feeders;

at least two loads, each associated with a respective one of the feeders;

at least one module electrically connected to two of the at least two feeders and including: a bi-directional auxiliary converter; DC bus including a capacitor; and a bi-directional balancing converter connected in parallel; the module further including means to change the current and voltage between the feeders and the bi-directional auxiliary converter and a differential electrlcai connection between the two feeders and connected in series with bi-directional balancing converter;

a controller configured to control at least one parameter of an electrical current flowing through the or each module, the voltage across the or each module being related to the maximum voltage difference between two of the at least two feeders.

Preferably, the bi-directional balancing converter is single phase.

Advantageously, the at least two feeders are three phase feeders;

three modules are connected to respective phases of the feeders;

and the bi-directional auxiliary converter is a three phase converter.

The at least one module may comprise a plurality of sub-modules connected in parallel, each sub-module comprising a bi-directional auxiliary converter, a DC bus including a capacitor, and a bi-directional balancing converter connected in parallel, the sub-module further including means to change the current and voltage between the feeders and the bi-directional auxiliary converter.

Preferably, a step down transformer is the means through which the input current and voltage of the bi-directional auxiliary converter are changed.

The step down transformer may comprise single delta primary with three isolated star connected secondaries, each secondary feeding a respective one of the three modules.

The step down transformer may comprise a single transformer and three isolated high frequency bidirectional DC/DC converters, each DC/DC converter associated with a respective one of the three phases.

Preferably, a DC/DC converter is the means through which the input current and voltage of the bi-directional auxiliary converter are changed.

The auxiliary converter and the balancing converter may include forced eommutated switching semiconductor devices.

The forced-commutated switching devices may comprise semiconducting material such as silicon carbide. Preferably, the forced commutaied switching devices are selected from the group comprising: insulated gate bipolar transistors (IGBTs), integrated gate commutated thyristors (GTOs) and metal oxide semi-conductor field effect transistors (MOSFETs).

Preferably, the auxiliary converter and the balancing converter are configured to operate at frequencies of up to 100 kHz.

Advantageously, the electrical connection between the two feeders includes a fault condition detection and protection means.

It is preferred that the fault condition detection and protection means comprises a saturable reactor or a plurality of series connected high voltage semiconductor switches.

The at least one controllable parameter may be selected from the group comprising:

voltage magnitude, phase angle, active power and reactive power.

in an embodiment of the invention the electrical power distribution network configured as a STATCOM compensator and including a P/Q (active power/reactive power) controller 60.

In operation the bi-directional converter may operate initially in boost mode charging the bus capacitor 16 to a DC potential in excess of the peak supply potential, setting the P/Q controller 60 such that P parameter is zero.

The STATCOM compensator may further comprise a transformer situated between the three phase supply and the bidirectional converter.

Preferably, in operation reactive energy can be exchanged between the source and the bus capacitor 16 (STATCOM operation) by changing the phase angle with the P/Q controller.

Brief Description of the Drawings

In the Drawings, which illustrate both the prior art and preferred embodiments of the invention, which are by way of example:

Figure 1 is a schematic representation of a Soft Open Point arrangement of the prior art;

Figure 2 is a schematic representation of a simple load balancing system according to the invention;

Figure 3 is a schematic representation of a load balancing system according to the invention with a fault handling and protection arrangement; Figure 4 is a schematic representation of a load balancing system integrated into an electrical power distribution network;

Figure 5 illustrates balancing voltages and balancing currents for the load balancing system illustrated in Figure 3;

Figure 6 is a schematic representation of a load balancing system according to the invention for connection to more than two feeders;

Figure 7 is a schematic representation of an alternative load balancing system according to the invention for connection to more than two feeders;

Figure 8 illustrates a part of the load balancing system including a STATCOM compensator;

Figure 9 illustrates a part of the load balancing system including a STATCOM compensator;

Figure 10 is a circuit diagram of the configuration illustrated in Figure 8; and

Figure 11 is circuit diagram illustrating a controller for the load balancing system of the invention.

Detailed Description of the Preferred Embodiments

Referring now to Figure 2, a part of a three-phase electrical power distribution network 1 is shown. The network comprises a high voltage feeder 2 whose output is split into three phases 3a, 3b and 3c. The three phases 3a to 3c are connected to medium voltage feeder transformers 4, 5, the feeder transformers having three-phase outputs 4a-4c and 5a-5c. Loads 6, 7 are connected to the feeder transformers 4, 5 respectively.

In order that the loads 6, 7 may be balanced across the feeder transformers 4,5 respective pairs of phases 4a, 5a, 4b, 5b, 4c, 5c are connected to each other by modules A-C. Each of modules A - C is connected to the output phases 5a-5c of the feeder transformer 5 by respective step-down auxiliary transformers 9, 10, 11 each of which forms a part of one of the modules A-C, and to the output phases 4a-4c of the feeder transformer 4 by LC circuits 12, 13, 14 each of which forms a part of one of the modules A-C and comprises an inductor 12a, 13a, 14a and a capacitor 12b, 13b, 14b. Each capacitor 12b, 13b, 14b is connected on the module side to a respective one of the output phases 5a to 5c of the feeder transformer 5. The inductor side of each capacitor 12b, 13b, 14b is connected to a corresponding respective one of the output phases 4a-4c of the feeder transformer 4, that is for module A one side of the capacitor 12b is connected to the output phase 5a of the feeder transformer 5 and the other side of the capacitor 12b is connected to the output phase 4a of the feeder transformer 4.

In Figure 2, only module A is shown in detail. However, each of the Modules A - C is the same, save for the output phases of the feeder transformers 4, 5 to which they are connected.

Module A comprises a bi-directional three phase converter (auxiliary converter) 15 connected in parallel with a capacitor 16, which forms a DC bus and a bi-directional single phase converter (balancing converter) 17. The DC bus voltage marginally exceeds the voltage difference between the corresponding phases of the feeder transformers 4, 5. If the voltage difference between corresponding phases of the feeder transformers 4, 5 does not exceed 12% of the peak line to line system voltage, the DC bus will be at about 1 .5kV on a 11 kV medium voltage system. The DC bus voltage feeds the bi-directional single phase converter 17 which either boosts or bucks the differential voltage between corresponding phases 4a-4c, 5a-5c to feed a balancing current between the phases.

In Figure 2 each module is shown as comprising its own step-down transformer 9-11 . However, these could be replaced by a single transformer using a configuration comprising a single delta primary coil with three isolated star connected secondary coils, each secondary coil feeding one module. Alternatively, the separate step down transformers could be replaced by a single transformer feeding three isolated high frequency bi-directional AC/DC converters each feeding one of the balancing modules.

In a low voltage system the step-down transformers can be omitted and the feed for each module derived from isolated AC/DC converters.

Figure 3 illustrates a more sophisticated arrangement which provides for fault condition detection and protection. Whilst the configuration illustrated in Figure 2 will balance the loading on the feeder transformers 4, 5, and avoids circulating currents, in the event of a ground fault on a feeder the potential fault current will be double that of a single transformer. This is because the voltage rating of the devices making up the bi-directional single phase converter 17 are not adequate to limit the fault current and would be bypassed in the event of a fault. Figure 3 illustrates two possible configurations for managing the fault condition.

The first configuration provides a saturable reactor comprising an inductor 21 and a capacitor 22 in series between the capacitor of the LC circuit 12 and the output phase 5a of the feeder transformer 5. In normal operation the inductor 21 and capacitor 22 are resonant offering negligible impedance to the alternating balancing current (alternating at 50Hz in the United Kingdom). When a fault occurs inductor 21 will saturate and capacitor 22 will limit the current flowing between the feeder transformers 4, 5.

The second configuration provides a full phase blocking circuit 30 comprising a plurality of series connected high voltage IGBTs 31 between the capacitor 12b of the LC circuit 12 and the output phase 5a of the feeder transformer 5.

The IGBTs 31 are in conduction mode during normal operation but are switched off in the case of an over-current and provide full phase blocking capability for the circuit, that is when a fault condition is detected the converser is disconnected.

Example

The configuration illustrated in Figure 3 was modelled using PLECS power simulation software with the following system conditions:

SIMULATION 1

Nominal feeder voltage - 11 kV;

Rated load per transformer - 5MVA;

Assumed transformer regulation - feeder transformer 4 1 .2%, feeder transformer 5 2.4%; Applied loads - feeder load 6, 0.4MW, feeder load 7, 9.6MW.

SIMULATION 2

Simulation 2 comprised re-running Simulation 1 with the loading reversed. Results

The results for Simulation 1 and Simulation 2 were almost identical. Traces of the balancing voltage and balancing current for simulations 1 and 2 are illustrated in Figure 5. Values for feeder transformer currents and voltages, auxiliary transformer loading and energy dissipation for Simulation 1 are set out in the table below:

Figure 4 illustrates an installation where the load balancing system is integrated into an electrical power distribution network. Three phase vacuum circuit breakers 40, 50 are placed between the three phase outputs 4a-4c and 5a-5c of the feeder transformers 4, 5 to isolate the balancing modules A-C from the feeder outputs. Rather than using one three phase vacuum circuit breaker on each side of the modules A-C, three separate single phase circuit breakers could be used.

The modules A-C in Figure 4 include two sub-modules 8' connected in series. By connecting more than one module in series feeders with greater voltage differences may be connected together for balancing therebetween. Where each sub-module is specified to handle a voltage difference between the corresponding feeder phases of approximately 1 .5kV, using two sub-modules increase the possible voltage difference between feeder phases to 3kV. Each sub- module includes a single phase bi-directional converter 15 and a single phase bi-directional converter 17 both connected to a DC bus 16. Fault protection for the two sub-modules 8' is provided by the phase blocking circuit 30 comprising a plurality of pairs of IGBTs 31 , each pair of IGBTs provided with a voltage dependent resistor 32 in parallel therewith. In the illustrated embodiment the IGBTs are configured to switch off when the current exceeds 300Arms and/or when the voltage exceeds 1600 volts.

The sub-circuits 9', 12' each comprise a voltage dependent resistor 9a, 12a which limits voltage spikes and transients, voltage transducers 9b, 12b and current transducers 9c, 12c. The function of the voltage transducers 9b, 12b and the current transducers 9c, 12c is to provide information relating to the operating condition of the converter to the control system. The sub- circuits 9', 12' also comprise an inductor 9e, 12e and a capacitor 9d, 12d which together form a low pass filtered input to the bi-directional converters 15, 17 respectively. The low pass filters avoid the transfer of high frequency noise from the auxiliary transformer 9. In fact, the low pass filter removes a high frequency carrier signal from the sinusoidal output of the single phase converter 17. The high frequency carrier signal is introduced during pulse width modulation of a signal derived from the DC bus 16.

Figures 6 and 7, which illustrate alternative arrangements for balancing three (or more) feeders A, B and C. In Figure 6 the three feeders A, B and C are interconnected by two modules D, E. The modules D, E each include auxiliary converters 15 and balancing converters 17 and a DC bus capacitor 16.

Figure 8 illustrates a part of a module configured as a STATCOM compensator and includes a P/Q (active power/reactive power) controller 60. The bi-directional converter initially in boost mode charges the bus capacitor 16 to a DC potential in excess of the peak supply potential, setting the P/Q controller 60 such that P parameter is zero, that is no active power exchange and the Q parameter to either a leading or lagging value. Reactive power can be exchanged between the three phase supply and the bus capacitor within the limits of the inverter, input inductor and capacitor current capability.

Figure 9 illustrates a part of a module configured as a STATCOM compensator similar to that shown in Figure 8 but with the addition of a transformer 70 between the three phase supply and the bidirectional converter 17 which simply changes the operating voltage levels of the auxiliary converter.

Figure 10 is a circuit diagram corresponding to the part of the module illustrated in Figure 8. The auxiliary converter 15 comprises six semi-conductor switches 15'. The DC bus capacitor 16 is charged to a voltage which exceeds the peak value of the three-phase source. To transfer current to the capacitor 16 from the source the six semi-conductor switches 15' act as boost converters. To transfer current from the capacitor 16 to the source, the semi-conductor switches 15' act as buck converters (or inverters). Reactive energy can be exchanged between the source and the bus capacitor 16 (STATCOM operation) by changing the phase angle with the controller 60. Note, the controller 60 corresponds to the control elements 104, 105 described below with reference to Figure 11 .

Figure 11 illustrates a controller operationally connected to the active balancing converter illustrated in Figure 3. The controller provides for the active balancing converter to operate in two modes, a standby mode and a current control mode.

In the standby mode the two interconnected feeders 4, 5 are monitored continuously by measuring the current and voltage of each feeder phase and calculating the power flow. If the power flowing through each feeder 4, 5 does not exceed a threshold power value PREF, the active balancing converter remains in standby mode, that is current does not flow between the feeders 4, 5.

If the power flowing through one of the feeders exceeds PREF then current control mode is initiated.

The current controller 100 comprises a current reference calculator 101 which receives input currents from current transducer 102, 102' each associated with one of the phases of the two feeder transformers 4, 5 (note, current transducers 102, 102' are shown for only one of the three phases of the feeders 4, 5 whereas in practice each phase would be provided with a current transducer).

When current control mode is initiated power is exchanged between the feeders 4, 5 through switching of the auxiliary converter 15 and the balancing converter 17. The controller 100 controls the auxiliary converter 15 as follows:

The DC-link voltage (that is the voltage across the capacitor 16) is measured by voltage transducer 103 and compared with a set value VdcRef (for example 1 .5kV) in the DC voltage regulator 104. The output of the DC voltage regulator is a reference voltage waveform. If the measured voltage deviates from the set value VdcRef, the DC voltage regulator 104, which may consist of a proportional-integral (PI) controller, will output a reference voltage waveform with an amplitude proportional to the error of the voltage (Vmeasured - VdcRef) . This reference voltage waveform is compared in pulse width modulator 105 to a high frequency (>5kHz) sawtooth modulating waveform that produces a duty ratio which turns on the switches of the auxiliary converter 15. The increase in the duty ratio results in the time during which the capacitor 1 6 is connected to the feeder being increased and controls which switch (the auxiliary converter 15 may comprise a plurality of switches 15' as shown in Figure 10) of the auxiliary converter is on and which is off and directs the current either on the changing direction or discharging direction through the capacitor 16. This means that the pulse width modulator 105 controls both the duty ratio (the charging or discharging period for capacitor 16) and the direction of current (up or down the capacitor 1 6) and by doing this increases or decreases the DC link voltage amplitude, to keep it regulated around the set value VdcRef .

The controller 100 controls the balancing converter 17 as follows:

Currents measured by the current transducers 102, 102' and voltages measured by voltage transducers 103, 103' (note - each phase of each feeder would be provided with current and voltage transducers) are used to calculate the power flow on each phase of the feeders 4, 5.

These values are compared to a set reference value in a current reference calculator 101 which generates an error_CUrrent signal. This error_CUrrent signal forms an input to current limiter 107. The current limiter 107 checks that the error_CUrrent signal is within the current limitation of the balancing converter 1 7 in the current limit block 107. If the error_CUrrent signal is within the afore-mentioned current limitation error_CUrrent is used as an input for the current controllers 108, 109 and 110 along with the output of voltage difference waveform generator 111 to calculate a reference voltage waveform VREF. This waveform can be calculated for the nominal 50Hz power reference, and also some higher harmonic components can be extracted from the 50Hz waveform using a phase locked loop 112, 113, which both provide inputs to the voltage difference waveform generator 111 , to calculate a voltage reference in each of the harmonics reference frames of interest.

The generated voltage reference outputs from the current controllers 108 to 110 are then summed up to produce a single reference sinusoidal 50Hz voltage waveform with the

superimposed harmonic components that the controller is designed to compensate for.

This reference voltage waveform is then compared in the pulse width modulator 106 with a high frequency (>5kHz) sawtooth modulating signal that generates the duty ratio for each device in the balancing converter 17, and selects the devices in a way to control the power flow in the required direction. The duty ratio generated determines the duration for which capacitor 16 appears to be connected across the feeders (charging and discharging), and the choice of the switches determines in which polarity of the DC-link voltage appears across the feeders 4, 5. If capacitor 16 discharges through the balancing converter, it charges back through the auxiliary converter and vice versa so that its voltage is regulated around VdcRef during the current control period.

The electrical power distribution network of the invention allows loads to be shared across feeders. Reactive energy can be extracted from one feeder and fed into the corresponding phase of another feeder. The device may also be configured to provide STATCOM compensation. This is achieved without subjecting the components connecting the feeders to the full current and voltage carried to the load by the feeders.

Claims

1 . An electrical power distribution network comprising:

at least two current carrying feeders;

at least two loads, each associated with a respective one of the feeders;

at least one module electrically connected to two of the at least two feeders and including: a bi-directional auxiliary converter; DC bus including a capacitor; and a bi-directional balancing converter connected in parallel; the module further including means to change the current and voltage between the feeders and the bi-directional auxiliary converter and a differential electrical connection between the two feeders and connected in series with bi-directional balancing converter;

2. An electrical power distribution network according to Claim 1 , wherein the bi-directional balancing converter is single phase,

3. An electrical power distribution network according to Claim 1 or 2, wherein:

the at least two feeders are three phase feeders;

three modules are connected to respective phases of the feeders;

and wherein the bi-directional auxiliary converter is a three phase converter.

4. An electrical power distribution network according to any preceding claim, wherein the at least one module comprises a plurality of sub-modules connected in parallel, each sub-module comprising a bi-directional auxiliary converter, a DC bus including a capacitor, and a bi-directional balancing converter connected in parallel, the sub-module further including means to change the current and voltage between the feeders and the bi-directional auxiliary converter.

5. An electrical power distribution network according to any preceding claim, wherein a step down transformer is the means through which the input current and voltage of the bi-directional auxiliary converter are changed,

6. An electrical power distribution network according to Claim 5 when dependent on Claim 3, wherein the step down transformer comprises single delta primary with three isolated star connected secondaries, each secondary feeding a respective one of the three modules,

7. An electrical power distribution network according to Claim 5 when dependent on Claim 3, wherein the step down transformer comprises a single transformer and three isolated high frequency bidirectional DC/DC converters, each DC/DC converter associated with a respective one of the three phases.

8. An electrical power distribution network according to any of Claims 1 to 3, wherein a DC/ DC converter is the means through which the input current and voltage of the bi-directional auxiliary converter are changed.

9. An electrical power distribution network according to any preceding claim, wherein the auxiliary converter and the balancing converter include forced commutated switching

semiconductor devices.

10. An electrical power distribution network according to Claim 9, wherein the forced- commutated switching devices comprise semiconducting material such as silicon carbide.

11 . An electrical power distribution converter according to Claim 9 or 10, wherein the forced commutated switching devices are selected from the group comprising: insulated gate bipolar transistors (IGBTs), integrated gate commutated thyristors (GTOs) and metal oxide semi-conductor field effect transistors (MOSFETs).

12. An electrical power distribution network according to any preceding claim, wherein the auxiliary converter and the balancing converter are configured to operate at frequencies of up to 100 kHz.

13. An electrical power distribution network according to any preceding claim, wherein the electrical connection between the two feeders includes a fault condition detection and protection means.

14. An electrical power distribution network according to Claim 13, wherein the fault condition detection and protection means comprises a saturable reactor or a plurality of series connected high voltage semiconductor switches.

15. An electrical power distribution network according to any preceding claim, wherein the at least one controllable parameter is selected from the group comprising: voltage magnitude, phase angle, active power and reactive power.

16. An electrical power distribution network according to any preceding claim configured as a STATCOM compensator and including a P/Q (active power/reactive power) controller.

17. An electrical power distribution network according to Claim 16, wherein in operation the bidirectional converter operates initially in boost mode charging the bus capacitor to a DC potential in excess of the peak supply potential, setting the P/Q controller such that P parameter is zero.

18. An electrical power distribution network according to Claim 16 or 17, further comprising of a transformer situated between the three phase supply and the bidirectional converter.

19. An electrical power distribution network according to any of Claims 16 to 18, wherein in operation reactive energy can be exchanged between the source and the bus capacitor

(STATCOM operation) by changing the phase angle with the P/Q controller.