WO2008153376A1

WO2008153376A1 - Power flow controller

Info

Publication number: WO2008153376A1
Application number: PCT/NL2007/050289
Authority: WO
Inventors: Johannes Abraham Ferreira; Zhihui Yuan; Sjoerd Walter Hero De Haan
Original assignee: Technische Universiteit Delft
Priority date: 2007-06-15
Filing date: 2007-06-15
Publication date: 2008-12-18

Abstract

The invention relates to a power flow controller, comprising at least one first converter coupled with a power transmission line, and at least one second converter coupled with a power source, wherein said power source operates at a predeter-mined first frequency and connects to the power transmission line, said controller further comprising means for exchanging power between the first converter and the second converter, wherein the transmission line forms part of the only means for exchanging power between the first converter and the second con- verter, wherein the first converter and the second converter each have a power storage device, and wherein the second converter is arranged to continuously feed the power transmission line with power at a second frequency which is a higher harmonic of the power source' s first frequency, for transferring power from the second converter to the first converter.

Description

Power flow controller

The invention relates to a power flow controller comprising at least one first converter coupled with at least one power transmission line, and at least one second converter coupled with a power source, wherein said power source operates at a predetermined first frequency and connects to the power transmission line, said controller further comprising means for exchanging power between the first converter and the second converter .

Such a power flow controller is known from US-A- 5,343,139.

The known power flow controller is shown in the simplified diagram that is provided in Fig. 1 hereof, and the operation of this known power flow controller can be clarified as follows . Fig. 1 shows a prior art unified power controller

(UPFC) comprising a converter 1 connected in series with the transmission line 4 and a converter 2 connected in shunt with the transmission line 4. Converter 1 is used to generate a voltage δv, on winding 3, of controllable amplitude and phase that is added to the system voltage of the transmission line 4. The injected voltage δV is used for voltage control and power flow control by phase control of the transmission line 4. Converter 2 serves as power source for converter 1, and in addition it is also used to inject reactive power Q in the transmission line 4. The power balance between the two converters is achieved by regulating the voltage on the dc bus capacitor 7. The current i_dc,_A in line 5, that converter 1 draws is determined by the power requirement for establishing δV. The current i_dc,_B in line 6, that converter 2 feeds into the capacitor 7, is controlled to main- tain the dc bus capacitor voltage at a preset level. This implies that current i_dc,_B of converter 2 has the same average value as current i_dc,_A of converter 1.

The known power flow controller has had limited impact because of the high cost and low reliability. The relative high cost is mainly due to the high voltage insulation requirements (up to 40OkV), fault protection and the low production numbers.

The invention has as objective to resolve these problems of the prior art and to provide an alternative solution in which the high voltage installation requirements can be relaxed. To this end the power flow controller of the invention is characterized by one or more of the appended claims.

In a first aspect of the invention the transmission line forms part of the only means for exchanging power between the first converter and the second converter, that the first converter and the second converter each have a power storage device, and that the second converter is arranged to continuously feed the power transmission line with power at a second frequency which is a higher harmonic of the power source's first frequency, for transferring power from the second converter to the first converter.

In a second aspect a current loop for the power at the second frequency is arranged.

By these measures the power flow controller of the in- vention provides transmission of power from the second converter to the first converter while the second converter can electrically float at line potential. Due thereto provisions for high voltage insulation can be dispensed with for the second converter. The second converter can act as a generator of a rela- tively small harmonic voltage causing a corresponding current in the power transmission line that flows to the first converter that in turn generates a corresponding harmonic voltage at its terminals, in order to extract the required amount of power for storage in its power storage device and conversion into the re- quired power of the first frequency.

An effective and adequate manner of construing the power flow controller of the invention is characterized in that the first converter and the second converter each are of the type AC/DC converter and that the power storage devices of said converters are capacitors at the converter's DC/sides.

In order to effect that effectively the first converter and the second converter are operated at a balanced energy level in respect of their respective power storage devices, it is preferable that the first converter and the second converter are connected to a dedicated first respectively second waveform- controller so as to shape the power converted by said converters depending on at least a desired predetermined level of energy in the respective power storage devices of said converters.

In order to promote that the power flow controller of the invention adequately operates at all circumstances, it is further preferable that each waveform controller is connected to and operates depending on a dedicated power-controller, the con- trol actions of which are derived from predetermined electrical parameters, viz. for the power controller of the first converter: the power consumption of a load connected to the power transmission line, and for the power controller of the second converter: the power provided at the exit side of said second converter .

It is further desirable that a pass-filter for said higher harmonic second frequency is connected to the power transmission line, for instance at the side of the load. This arranges for a suitable current loop for the harmonic frequency power that the second converter induces into the power transmis^¬ sion line.

In still a further aspect of the invention it is proposed that a number of first converters and/or a number of sec- ond converters are placed in series. In this manner it is possi^¬ ble to apply relatively small converters which is cost- effective, and to simultaneously increase reliability because the higher number of converters introduces redundancy.

Hereinafter the invention shall be further elucidated with reference to a non-limiting example and with reference to the drawing showing in:

- Fig. 1 a schematic diagram of a prior art power flow controller ;

- Fig. 2 a schematic diagram of a power flow controller in accordance with a first embodiment according to the invention;

- Fig. 3 some exemplary time domain waveforms occurring in the power flow controller of the invention;

- Fig. 4 a schematic diagram of a converter circuit to- gether with its waveform controller and power controller;

- Fig. 5 a schematic diagram of a second embodiment of a power flow controller according to the invention;

- Fig. 6 a schematic diagram of a third embodiment of a power flow controller according to the invention; - Fig. 7 shows a schematic diagram of a fourth embodiment of a power flow controller according to the invention.

Fig. 1 shows the prior art power flow controller and has been discussed hereinabove.

Fig. 2 shows a simplified circuit diagram wherein the first converter 1 and the second converter 2 are shown to be disconnected from each other. The first converter 1 is coupled in series with the power transmission line 4, whereas the second converter 2 is coupled parallel with a power source 10. The current balance to stabilize the voltage on the capacitors of said converters 1,2 is achieved by generating two current components. A first component, idci,A/ ^and i_<ici,B at lines 8 and 9, respec- tively, is generated by the power flowing through both converters 1,2 at a predetermined fundamental frequency of the power source 33. The second component, idc3,A (line 8) and idc3,B (line 9) relates to power at a harmonic frequency. In this embodiment it concerns the 3^rd harmonic, but the principle is also applicable to other harmonic frequencies. The fundamental power flows through the power transmission line 4 running from the sourcing part of the grid 10 to the load part of the grid 11. The power and the current at the harmonic frequency flow through a local loop 12. A harmonic pass filter, 13, is added to the power transmission line 4 to close the current loop for this third harmonic current. In a three-phase system this third harmonic voltages in each of the phases are generated with the same phase to avoid circulating currents.

The first converter 1 and the second converter 2 each are of the type AC/DC converter as Fig. 2 shows, and the figure shows further that the power storage devices 14', 14'' are capacitors at the converters' DC-sides.

Fig. 2 and Fig. 4 further show that the first converter

I and the second converter 2 are connected to a dedicated first (16') respectively second (16'') waveform controller so as to shape the power converted by converters 1 and 2, depending on at least a desired predetermined level of energy in the respective power storage devices 14', 14'' of said converters 1,2.

Also each waveform controller 16' , 16' ' is connected to and operates depending on a dedicated power controller 18',

18'', the control actions of which are derived from predetermined electrical parameters viz. for the power controller 18' for the first controller 1: by the power consumption of a load

II connected to the power transmission line 4, and for the power controller 18'' of the second converter

2: by the power provided at the exit side of said second converter 2. A further detailed explanation shall be offered hereinafter in relation to Fig. 4.

Fig. 4 shows schematically the power electronic circuit commonly devised for and separately applied to both converter 1 and converter 2. Switching devices in the form of insulated gate bipolar transistors (IGBT) 15 or similar power semiconductors are switched with pulse width modulation control 16' , 16' ' to generate a voltage, e, on transformer winding 17', 17'' . The control reference is generated by an external power controller 18', 18''. The measurements and references, 19, depend on the configuration and parameters of the grid, and are as discussed above with reference to Fig. 2. The voltage, e, is a pulse width modulated square wave and is filtered by the inductance L, 20' , 20' ' . The inductance L could be from an inductor or from the leakage inductance of the transformer. The terminals, 21, 22 are connected to the grid. Each converter 1,2 contains its own capacitor 14', 14'' to provide a stable dc voltage. The voltage is stabilized by controlling the two current components, i_dc,i and i_{dC/ 3} in lines 8 and 9 respectively, so that they have the same average value. For the purpose of illustration reference will now be made to Fig. 3. Fig. 3 illustrates an example of the voltage and current waveforms in the system of Fig. 2 with the active power exchange between converter 1 and 2 at harmonic frequency according to the invention. Assume that, for reasons of power control, δVi is the voltage that converter 1 injects into grid at the fundamental frequency, and that Iχ is the current that is flowing at the fundamental frequency (Fig. 3a) . The voltage δVi is the fundamental component of the voltage δV, 3. The resulting power injected to grid at the fundamental frequency is P_iA (Fig. 3b) . To compensate the power Pi_A that is extracted from the capacitor 14' a power which has the same mean value should be absorbed by converter 1 at a different frequency, for instance the 3^rd harmonic. For the case of a third harmonic V_3A and I₃ are the third harmonic voltage and corresponding third harmonic current at the ac terminals of converter 1 (Fig. 3c) . Neglecting losses, the mean value of power in the 3^rd harmonic frequency P_3A should be equal to the power at the fundamental frequency Pi_A. The power P_3A that is absorbed by converter 1 is generated by converter 2 acting as a source of the third harmonic. The voltage generated by con- verter 1 is the sum of δV, _λ and V_3A, (Fig. 3d) . The associated combined power P_iA + P_3A has a zero mean value.

Turning now to Fig. 5 a very schematic diagram is shown of a second embodiment of a power flow controller according to the invention. The power flow controller of Fig. 5 differentiates from the power flow controller according to a first embodiment shown in Fig. 2 in that the first converter 1 and the second converter 2 are placed in series with the power transmission line 4. Again, also in this embodiment there is provision for a current loop 30 to transfer the power that the second converter 2 generates at the second frequency in order to allow same to arrive at the first converter 1. Note in this connection that provision is made for suitable pass filters 13 to close the current loop 30.

Reference is now made to Fig. 6 showing schematically a third embodiment of the power flow controller of the invention showing as an example that both a number of first converters 1, 1' , 1' ' can be placed in series and/or a number of second converters 2, 2', 2'' can be placed in series. In this manner for both the first converters 1, 1', 1' ' and the second converters 2, 2', 2'' the reliability of the system can be improved due to the redundancy that is introduced into the system. When one of the first converters or one of the second converters fails, then the task of the failing converters is assumed by the remaining converters .

Fig. 7 shows a fourth embodiment of the power flow controller of the invention. In the schematic diagram shown it con- cerns a three-phase system having at the side of the power sup^¬ ply 10 a delta-wye transformer 33 and at the side of the load 11 a wye-delta transformer 32.

As in the embodiment shown in Fig. 6 each line of the thiee-phase system of Fig. 7 is provided with a series of first converters 1, 1' and 1' ' .

The second converter 2 is connected to the starpoint of the transformer 33 and provides power to the first converters 1, 1' and 1'' at the second frequency which is a higher harmonic of the power source's first frequency. Differently from the other embodiments shown and discussed here above the second converter 2 of the embodiment shown in Fig. 7 receives its power from a third converter 22 which provides an intermediate connection to the power source 10.

Finally, it is remarked that neither of the Figs. 5, 6 and 7 show the waveform controller 16', 16'' and the power controller 18', 18'' as shown in Fig. 2. Similar waveform controllers and power controllers are however to be applied in connec^¬ tion with the converters shown in the embodiments of Fig. 5, 6 and 7 consistent with the embodiment shown in Fig. 2.

Claims

1. Power flow controller, comprising at least one first converter (1) coupled with a power transmission line (4), and at least one second converter (2) coupled with a power source (10), wherein said power source (10) operates at a predetermined first frequency and connects to the power transmission line (4), said controller further comprising means (4) for exchanging power between the first converter (1) and the second converter (2), characterized in that the transmission line (4) forms part of the only means for exchanging power between the first converter (1) and the second converter (2), that the first converter (1) and the second converter (2) each have a power storage device (14', 14''), and that the second converter (2) is arranged to continuously feed the power transmission line (4) with power at a second frequency which is a higher harmonic of the power source's first frequency, for transferring power from the second converter (2) to the first converter (1) .

2. Power flow controller according to claim 1, wherein the first converter (1) and the second converter (2) each are of the type AC/DC converter, and that the power storage devices

(14', 14'') of said converters (1, 2) are capacitors at the converter's DC-sides.

3. Power flow controller according to claim 1 or 2, wherein the first converter (1) and the second converter (2) are connected to a dedicated first (16') respectively second (16'') waveform controller so as to shape the power converted by said converters (1,2) depending on at least a desired predetermined level of energy in the respective power storage devices (14', 14") of said converters (1,2).

4. Power flow controller according to claim 3, wherein each waveform controller (16',16'') is connected to and operates depending on a dedicated power-controller (18', 18''), the control actions of which are derived from predetermined electrical parameters, viz. for the power controller (18') of the first converter (1) : the power consumption of a load (1) connected to the power transmission line (4), and for the power controller (18'') of the second converter (2) : the power provided at the exit side of said second converter (2) .

5. Power flow controller according to any one of claims 1-4, wherein at least one pass-filter (13) for said higher har- monic second frequency is connected to the power transmission line (4) .

6. Power flow controller according to anyone of claims 1-5, wherein a number of first converters (1, 1', 1'') and/or a number of second converters (2, 2' , 2'') are placed in series.

7. Power flow controller according to any one of claims 1-6, wherein the second converter (2) connects with the power source (10) via a third converter (22).