Electric circuits and power systems incorporating the same

Abstract

The invention relates to a power system with an electric circuit connected between a power grid and a power source. The electric circuit includes a main power converter having main input terminals connected to the power source 16 by a DC link and output terminals. The main power converter is controlled by a controller. The electric circuit includes a main transformer having a primary winding 8a and a secondary winding, the primary winding being connected to the output terminals of the main power converter. Main switchgear is connected between the secondary winding of the main transformer and the power grid. An auxiliary transformer has a primary winding connected to the power grid in parallel with the main switchgear and a secondary winding connected to the controller. A pre-charge circuit is connected between the auxiliary transformer and the DC link.

Claims

1. An electric circuit connectable between a power grid and one or more power sources or one or more loads, the electric circuit comprising: a main power converter including a DC/AC power converter having first main terminals connectable to the one or more power sources or the one or more loads by a DC link and second main terminals, the main power converter being associated with one or more electrical components including a controller configured to control the main power converter; a main transformer having a main primary winding and a main secondary winding, the main primary winding being connected to the second main terminals of the main power converter; a main switchgear connected to the main secondary winding of the main transformer and connectable to the power grid; an auxiliary transformer having an auxiliary primary winding connected directly to the power grid and in parallel with the main switchgear and an auxiliary secondary winding connected to the controller, a pre-charge circuit connected between the DC link and the secondary winding of the auxiliary transformer, and comprising an AC contactor selectively operable to isolate the pre charge circuit from the auxiliary transformer and a DC contactor selectively operable to isolate the pre-charge circuit from the DC link, wherein during a pre-charge process the AC contactor and DC contactor are closed, wherein the controller is configured to control operation of the main power converter and opening and closing of the main switchgear during normal operation and during a pre-charge process and a pre-magnetization and synchronization process, the electric circuit, using the controller is configured for: with the main switchgear open, supplying power from the power grid to the one or more electrical components associated with the main power converter through the auxiliary transformer; with the main switchgear open, suppling power from the power grid to the DC link to with the main switchgear open, operating the main power converter to supply reactive power to the main transformer to at least partially magnetize the main transformer to derive a voltage at the secondary winding of the main transformer; and closing the main switchgear when the voltage at the secondary winding of the main transformer has substantially the same parameters as a power grid voltage, and wherein when operating normally, power is transmitted from the one or more power sources to the one or more electrical components via the main power converter during daylight hours, and during night, power supply is switched from the one or more power sources wherein a contactor at the one or more power sources is open and the auxiliary transformer is configured to supply power from the power grid to the one or more electrical components including the controller and to the one or more loads.

2. The electric circuit according to claim 1, wherein the main power converter including a plurality of controllable semiconductor switches, wherein the first main terminals of the DC/AC power converter are DC terminals that are connectable to the one or more power sources or the one or more loads by means of the DC link, and wherein the second main terminals of the DC/AC power converter are AC terminals that are connectable to the power grid by means of the main transformer and the main switchgear.

3. The electric circuit according to claim 2, wherein the DC link includes a second DC contactor operable to selectively isolate the one or more power sources or the one or more loads from the DC link.

4. The electric circuit according to claim 2, wherein the DC link includes one or more capacitors.

5. The electric circuit according to claim 1, wherein the pre-charge circuit includes a pre-charge power converter.

6. The electric circuit according to claim 5, wherein the pre-charge power converter is an AC/DC power converter including a second plurality of controllable semiconductor switches.

7. The electric circuit according to claim 5, wherein the pre-charge power converter is a passive rectifier.

8. The electric circuit according to claim 1, wherein the pre-charge circuit includes a resistor.

9. The electric circuit according to claim 1, wherein the pre-charge circuit includes a DC fuse.

10. The electric circuit according to claim 1, wherein the main transformer is a step-up transformer with a first voltage on the main primary winding and a second voltage higher than the first voltage on the main secondary winding.

11. The electric circuit according to claim 1, further comprising one or more filters.

12. The electric circuit according to claim 1, wherein the auxiliary transformer is a step-down transformer with a first voltage on the auxiliary primary winding and a second voltage lower than the first voltage on the auxiliary secondary winding.

13. A power system comprising: the one or more power sources or the one or more loads; the power grid; and an electric circuit according to claim 1, wherein the first main terminals of the main power converter are connected to the one or more power sources or the one or more loads, the main switchgear is connected to the power grid, and the auxiliary primary winding of the auxiliary transformer is connected to the power grid in parallel with the main switchgear.

14. A method of pre-charging and pre-magnetising the power system according to claim 13, wherein operating the main power converter to supply reactive power to the main transformer to at least partially magnetise the main transformer further comprises: measuring, calculating or otherwise deriving parameter(s) of the power grid voltage to provide one or more parameter references; measuring, calculating or otherwise deriving corresponding parameter(s) of the transformer voltage at the main secondary winding of the main transformer; and operating the main power converter to supply reactive power to the main transformer in a controlled manner until an error between the measured parameter(s) of the transformer voltage and the one or more parameter references are reduced below a predetermined level.

15. The method of operating the power system according to claim 13, the method comprising: with the main switchgear closed, operating the main power converter to supply reactive power to the main transformer to achieve substantially unity power factor at the main switchgear; opening the main switchgear; operating the main power converter to decrease the supply of reactive power to substantially zero; and shutting down the main power converter.

Description

DRAWINGS

(1) FIG. 1 is a schematic diagram of a power system according to an embodiment; and

(2) FIG. 2 is a schematic of a controller for the main power converter of the power system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

(3) With reference to FIG. 1, a power system 1 according to the present invention includes an electric circuit connected between an AC power grid 4 carrying a medium voltage (MV) distribution voltage and a DC power source in the form of the solar farm 16. Although not shown, it will be readily understood that the electric circuit can also be connected to other types of power source or to one or more loads, e.g., as a drive circuit.

(4) The electric circuit includes a main AC circuit 2 that is connected to the AC power grid 4. The main AC circuit 2 is a three-phase circuit and includes MV switchgear 6, a main transformer 8 having a primary winding 8a and a secondary winding 8b, and a DC/AC power converter 10. The main transformer 8 is a step-up transformer which receives LV electrical power from the DC/AC power converter 10 at the primary winding 8a and derives MV electrical power at the secondary winding 8b which is supplied to the AC power grid 4.

(5) The main AC circuit 2 include filters 12 that are connected between the main transformer 8 and the DC/AC power converter 10.

(6) The DC/AC power converter 10 includes main AC output terminals that are connected to the main AC circuit 2.

(7) The electric circuit also includes a DC link 14 that is connected between main DC input terminals of the DC/AC power converter 10 and the solar farm 16. The solar farm 16 includes a plurality of photovoltaic (PV) panels that convert solar energy into DC electrical power. The DC link 14 includes one or more DC capacitors and a DC contactor 20 (optionally a motorised switch) that can be opened to selectively isolate the solar farm 16 from the DC link. The DC contactor 20 will normally remain closed, even during the night.

(8) It will be readily understood that the normal power flow direction through the main AC circuit 2 and the DC link 14 (represented by arrow A) is from the solar farm 16 to the AC power grid 4. The DC/AC power converter 10 can have any suitable construction and will typically include a plurality of controllable semiconductor switches that can be opened and closed under the control of a controller 18. During daylight hours, when the solar farm 16 is converting solar energy into DC electrical power, the DC/AC power converter 10 will normally be operated as an inverter to control power flow from the solar farm to the AC power grid 2.

(9) The controller 18 can be one of a plurality of electrical components that are used to control the DC/AC power converter or are directly associated in some way with its operation, including inter alia other controllers, relays, testing equipment, communication equipment, supervisory control and data acquisition equipment etc. In some arrangements, these electrical components can receive electrical power directly from the DC/AC power converter 10 when it is operating normally, i.e., to control power flow from the solar farm 16 to the AC power grid 2. But it will be readily appreciated that the solar fain 16 will only provide electrical power during daylight hours and that it is advantageous to open the switchgear 6 during the night to avoid no-load transformer load losses. Consequently, during the night, the electrical components 18 cannot receive electrical power from either the solar farm 16 or the AC power grid 4; in the latter case because the main AC circuit 2 is isolated from the AC power grid by the open switchgear 6.

(10) In order to provide electrical power to the controller 18 and the other electrical components during the night, the electric circuit includes an auxiliary AC circuit 22. The auxiliary AC circuit 22 is a three-phase circuit and includes an auxiliary transformer 24 with a primary winding 24a and a secondary winding 24b. The auxiliary transformer 24 is a step-down transformer which receives MV electrical power from the AC power grid 4 at the primary winding 24a and derives LV electrical power at the secondary winding 24b for the electrical components 18. The auxiliary AC circuit 22 is connected to the common bus of the switchgear 6 (but in other arrangements could also be connected separately to the AC power grid 4 or even to a separate power source) and to the controller 18. In some arrangements the auxiliary AC circuit 22 can be connected to an auxiliary AC input terminal of the DC/AC power converter 10 which allows LV electrical power to be provided to an integrated controller or other associated electrical components.

(11) During the night, electrical power is therefore provided to the controller 18 and the other associated electrical components by means of the auxiliary AC circuit 22 so that operation-critical or essential functions for the DC/AC power converter 10 such as control, communication, data acquisition etc., can be maintained at all times.

(12) The auxiliary AC circuit 22 can also supply LV electrical power to other electrical loads which are indicated generally in FIG. 1 by reference sign 26. Electrical loads 26 can include service loads for operating the heating, cooling, lighting and ventilation for a pre-assembled power station skid or electrical house (E-house) that houses the circuit components, for example.

(13) It will be readily understood that the normal power flow direction through the auxiliary AC circuit 22 (represented by arrow B) is from the AC power grid 4 to the controller 18 and the other electrical loads 26.

(14) The electric circuit also includes a pre-charge circuit 28 that is connected between the auxiliary AC circuit 22 and the DC link 14. The pre-charge circuit 28 includes a diode rectifier 30. In some arrangements the pre-charge circuit can include an AC/DC power converter with controllable semiconductor switches instead of the diode rectifier. The pre-charge circuit 28 also includes an AC contactor 32 for selectively isolating the diode rectifier 30 from the auxiliary AC circuit 22 and a DC contactor 34 for selectively isolating the diode rectifier from the DC link 14. A fuse 36 provides protection against failure of the DC/AC power converter 10. A resistor 38 is provided between the AC contactor 32 and the diode rectifier 30 to limit the pre-charge current. Although not shown, the pre-charge circuit can also include a pre-charge transformer to step-up the voltage for the purposes of charging the DC link to the level necessary for pre-magnetising the main transformer. The pre-charge transformer could optionally be implemented as an additional winding on the auxiliary transformer 24.

(15) It will be readily understood that the normal power flow direction through the pre-charge circuit 28 (represented by arrow C) is from the AC power grid 4 to the DC link 14 by means of the auxiliary AC circuit 22.

(16) The switchgear 6 and the various contactors (or switches) can be controlled to open and close by a controller that can optionally be integrated or coordinated with the controller 18.

(17) Voltage sensors 40 are located on the LV-side of the main transformer 8 and measure the voltage U.sub.LV at the primary winding 8a. Voltage sensors 42 are located on the LV-side of the auxiliary transformer 24 and measure the voltage U.sub.Aux at the secondary winding 24b. Current sensors 44 measure the output currents I.sub.v, I.sub.w and I.sub.w of the DC/AC power converter 10. Voltage sensors 46 measure the DC link voltage U.sub.DC. The voltage and current measurements from the various sensors are provided to the controller 18 for the DC/AC power converter 10 to control the switching of the semiconductor switches. Although not shown, sensors can also be located on the MV-side of main transformer and/or the MV-side of the auxiliary transformer 24 or at any other suitable location for providing AC power grid parameter measurement.

(18) Operation of the power system during pre-charge and pre-magnetisation will now be described in a situation where the PV panels cannot be used to pre-charge the DC link.

(19) The main switchgear 6 will be open during the pre-charge process.

(20) The DC contactor 20 is opened to isolate the solar farm 16 from the DC link 14.

(21) The AC and DC contactors 32, 34 in the pre-charge circuit 28 are closed.

(22) The DC link 14 is pre-charged by supplying power to the DC link from the power grid 4 by means of the auxiliary AC circuit 22 and the pre-charge circuit 28.

(23) Once the DC link 14 is pre-charged to the desired level, the main power converter 10 is operated to supply reactive power to the main transformer 8 to at least partially magnetise it. A suitable control strategy for the main power converter 10 during the pre-magnetisation and synchronisation process is described with reference to FIG. 2. The control strategy is used to control active and reactive power flow with respect to the direct (d) axis and quadrature (q) axis, respectively. It will be readily understood that other control strategies can be used.

(24) The controller 18 for the main power converter 10 includes an auxiliary observer 50 which uses the parameterised typical load current of the auxiliary transformer 22 and the measured voltage U.sub.Aux at the secondary winding 22b of the auxiliary transformer (i.e., on the LV-side) to derive a voltage magnitude .Math..sub.MV1 of the grid voltage at the primary winding 22a of the auxiliary transformer and its phase angle ΦU.sub.MV1.

(25) The controller 18 also includes a coordinate transformation module 52 which uses the measured output currents I.sub.v, I.sub.w and I.sub.w of the DC/AC power converter 10 to calculate actual d-axis and q-axis currents I.sub.d and I.sub.q in the dq reference frame.

(26) A main observer 54 uses the measured voltage U.sub.LV at the primary winding 8a of the main transformer 8 (i.e., on the LV-side) and the actual d-axis and q-axis currents I.sub.d and I.sub.q to derive a voltage magnitude .Math..sub.MV2 at the secondary winding 8b of the main transformer and its phase angle ΦU.sub.MV2.

(27) The auxiliary and main observers 50, 54 are derived from the equivalent circuits of the auxiliary and main transformers 22, 8, respectively.

(28) The difference Δ.Math. between the voltage magnitudes U.sub.MV1 and U.sub.MV2 is calculated and is used to calculate feed forward voltage control references U.sub.d and U.sub.q in the dq reference frame. In particular, the difference Δ.Math. is provided to a voltage controller 56 which also receives a voltage magnitude .Math..sub.LV, i.e., the magnitude of the measured voltage U.sub.LV. The voltage controller 56 uses the difference Δ.Math. and the voltage magnitude .Math..sub.LV to calculate voltage control references U.sub.d and U.sub.q.

(29) The difference Δ.Math. between the voltage magnitudes U.sub.MV1 and U.sub.MV2 is also provided to a synchronisation magnitude controller 58 which provides an output ΔI.sub.qref. A reference module 60 provides a d-axis reference current I.sub.dRef and an initial q-axis reference current which is summed with the output ΔI.sub.qref of the synchronisation magnitude controller 58 to derive a q-axis reference current I.sub.qRef. It will be understood that the synchronisation magnitude controller 58 is used to modify the reference current for the q-axis to order to achieve reactive power control. Active power control can be carried out on the basis of the d-axis reference current I.sub.dRef provided by the reference module 60 without such modification.

(30) The difference between the d-axis reference current I.sub.dRef and the actual d-axis current I.sub.d calculated by the coordinate transformation module 52 is calculated and provided to a current controller 62. Similarly, the difference between the q-axis reference current I.sub.qRef and the actual q-axis current I.sub.q calculated by the coordinate transformation module 52 is calculated and provided to the current controller 62. The current controller 62 calculates voltage control references ΔU.sub.d and ΔU.sub.q.

(31) The respective voltage control references for the d-axis (i.e., U.sub.d and ΔU.sub.d) provided by the voltage and current controllers 56, 62 are summed together and provided to a pulse pattern generator 64. Similarly, the respective voltage control references for the q-axis (i.e., U.sub.q and ΔU.sub.q) provided by the voltage and current controllers 56, 62 are summed together and provided to the pulse pattern generator 64. The pulse pattern generator 64 also received a measured voltage U.sub.DC at the DC link 14 and a corrected reference angle Φ.sub.Ref the derivation of which will now be described.

(32) A phase locked loop (PLL) 66 uses the measured voltage U.sub.LV to derive a reference angle Φ.sub.Ref0 for the rotating reference frame. The PLL 66 can be initialised using the phase angle ΦU.sub.MV1 calculated by the auxiliary observer 50.

(33) The difference between the phase angles ΦU.sub.MV1 and ΦU.sub.MV2 is calculated and provided to a synchronisation angle controller 68 which provides an output ΔΦ. The output ΔΦ of the synchronisation angle controller 68 is summed with the reference angle Φ.sub.Ref0 to derive the corrected reference angle Φ.sub.Ref that is provided to the pulse pattern generator 64 and the coordinate transformation module 52 to calculate the actual d-axis and q-axis currents I.sub.d and I.sub.q.

(34) The pulse pattern generator 64 uses the measured DC link voltage U.sub.DC to control modulation. The firing angle is calculated by the pulse pattern generator 64 using the summed control references for the d-axis and q-axis (i.e., (U.sub.d+ΔU.sub.d) and (U.sub.q+ΔU.sub.q), and the corrected reference angle Φ.sub.Ref.

(35) The synchronisation magnitude controller and synchronisation angle controller 58, 68 operate to bring the magnitude and angle of the voltage U.sub.MV2 at the secondary winding 8b of the main transformer 8 to be substantially the same as the magnitude and angle of the voltage U.sub.MV1 at the primary winding 22a of the auxiliary transformer 22 (which in turn is the same as the grid voltage).

(36) Once the voltage over the main switchgear 6 is within acceptable limits (but preferably as close to zero as possible), the main switchgear can be controlled to close. The synchronisation controllers can be locked and the current control parameters can be adjusted for normal operation of the power system 1, i.e., where power is supplied from the solar farm 16 to the power grid 4.