Power Transfer Between MV Feeders in a Power Distribution Network

Abstract

A method for transfer of power between medium voltage, MV, feeders via a MV direct current, MVDC, link in a power distribution network is presented. The method is performed in a controller in the power distribution network and includes setting an iteration step value for each of a set of power reference quantities of the MVDC link, and setting an initial value of each of the set of power reference quantities, iteratively changing values of each of the set of power reference quantities, and selecting one changed value of the set of power reference quantities by: changing a present value of each of the set of power reference quantities, one at a time, with the set iteration step value, respectively, into a new value, and measuring a total active power at a substation of the power distribution network for each of the new value, one at a time, and selecting the new value of the one of the set of power reference quantities that provides the lowest measured total active power at the substation, wherein a next iteration is performed with the selected new value as present value for the one of the set of power reference quantities and with the present value for the other of the set of power reference quantities. A controller for transfer of power between MV feeders via a MVDC link in a power distribution network is also presented.

Claims

1. A method for transfer of power between medium voltage, MV, feeders via a MV direct current, MVDC, link in a power distribution network, the method being performed in a controller in the power distribution network and comprising: setting an iteration step value for each of a set of power reference quantities of the MVDC link, and setting an initial value of each of the set of power reference quantities; iteratively changing values of each of the set of power reference quantities, and selecting one changed value of the set of power reference quantities by: changing a present value of each of the set of power reference quantities, one at a time, with the set iteration step value, respectively, into a new value, and measuring a total active power at a substation of the power distribution network for each of the new value, one at a time; and selecting the new value of the one of the set of power reference quantities that provides the lowest measured total active power at the substation, wherein a next iteration is performed with the selected new value as present value for the one of the set of power reference quantities and with the present value for the other of the set of power reference quantities.

2. The method as claimed in claim 1, further comprising: setting, after the setting step, an iteration stopping criterion for reduction of total active power; and determining, when the iteration stopping criterion has been fulfilled, after the changing step, a transfer of power between the MV feeders for the present values of the set of power reference quantities.

3. The method as claimed in claim 2, wherein the iteration stopping criterion is zero reduction.

4. The method as claimed in claim 1, wherein the iterative step values for the set of power reference quantities have the same absolute value.

5. The method as claimed in claim 1, wherein the power distribution network comprises two MV feeders, and the total active power is active power measured at primary substations for the two MV feeders added together.

6. The method as claimed in claim 1, wherein the set of power reference quantities comprise two or more of the following: voltage of a point of common coupling, PCC, for the MV feeders, active power injected by the MVDC link in the PCC for the MV feeders, and reactive power injected by the MVDC link in the PCC for the MV feeders.

7. The method as claimed in claim 1, wherein the set of power reference quantities comprises at least two of: voltage in a first side of a point of common connection, PCC, voltage in a second side of a PCC, active power in a first side of a PCC, active power in a second side of a PCC, reactive power in a first side of a PCC, and reactive power in a second side of a PCC.

8. A controller for transfer of power between medium voltage, MV, feeders via a MV direct current, MVDC, link in a power distribution network, the controller being configured to: set an iteration step value for each of a set of power reference quantities of the MVDC link, and set an initial value of each of the set of power reference quantities; iteratively change values of each of the set of power reference quanitites, and select one changed value of the set of power reference quantities by: changing a present value of each of the set of power reference quantities one at a time with the set iteration step value, respectively, into a new value, and measuring a total active power at a substation of the power distribution network for each of the new value, one at a time; and selecting the new value of the one of the set of power reference quantities that provides the lowest measured total active power at the substation, wherein a next iteration is performed with the selected new value as present value for the one of the set of power reference quantities and with the present value for the other of the set of power reference quanitites.

9. The controller as claimed in claim 8, wherein the controller is a converter controller configured to control the MVDC link.

10. The controller as claimed in claim 8, wherein the controller is a substation controller configured to control the substation.

11. The method as claimed in claim 2, wherein the iterative step values for the set of power reference quantities have the same absolute value.

12. The method as claimed in claim 2, wherein the power distribution network comprises two MV feeders, and the total active power is active power measured at primary substations for the two MV feeders added together.

13. The method as claimed in claim 2, wherein the set of power reference quantities comprise two or more of the following: voltage of a point of common coupling, PCC, for the MV feeders, active power injected by the MVDC link in the PCC for the MV feeders, and reactive power injected by the MVDC link in the PCC for the MV feeders.

14. The method as claimed in claim 2, wherein the set of power reference quantities comprises at least two of: voltage in a first side of a point of common connection, PCC, voltage in a second side of a PCC, active power in a first side of a PCC, active power in a second side of a PCC, reactive power in a first side of a PCC, and reactive power in a second side of a PCC.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which:

[0041] FIG. 1 is a diagram schematically illustrating a MV network with MVDC links installed between different feeders thereof;

[0042] FIG. 2 is a diagram schematically illustrating some converter quantities;

[0043] FIG. 3 is a diagram schematically illustrating a control structure for current vector control;

[0044] FIG. 4 is a diagram schematically illustrating an AP/DV control block of FIG. 3;

[0045] FIG. 5 is a diagram schematically illustrating measurement quantities according to a presented embodiment;

[0046] FIG. 6 is a diagram schematically illustrating different implementation configurations of a controller;

[0047] FIG. 7 is a flowchart schematically illustrating processing blocks according to a presented embodiment;

[0048] FIG. 8 is a flowchart schematically illustrating processing blocks according to a presented embodiment; and

[0049] FIG. 9 is a diagram schematically illustrating some components of a controller according to a presented embodiment.

DETAILED DESCRIPTION

[0050] The aspects of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown.

[0051] These aspects may, however, be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and to fully convey the scope of all aspects of invention to those skilled in the art. Like numbers refer to like elements throughout the description.

[0052] According to an aspect of the invention, an embodiment of a method for transfer of power between medium voltage (MV) feeders via a MV direct current (MVDC) link in a power distribution network is presented with reference to FIG. 7.

[0053] The method is performed in a controller 1 in the power distribution network. In processing block S100 an iteration step value is set for each of a set of power reference quantities of the MVDC link, and an initial value of each of the set of power reference quantities is set.

[0054] In processing blocks S120 and S140 values of each of the set of power reference quantities is iteratively changed, and one of the changed values of the set of power reference quantities is iteratively selected.

[0055] In processing block S120 a present value of each of the set of power reference quantities is changed, one at a time, with the set iteration step value, respectively, into a new value, and a total active power at a substation of the power distribution network is measured for each of the new value, one at a time.

[0056] In processing block S140 the new value of the one of the set of power reference quantities that provides the lowest measured total active power at the substation is selected. A next iteration is performed with the selected new value as present value for the selected one of the set of power reference quantities and with the present value for the other of the set of power reference quantities.

[0057] In optional processing block S110, after the processing block S100, an iteration stopping criterion is set for reduction of total active power.

[0058] In optional processing block S130, when the iteration stopping criterion has been fulfilled, after the processing block S120, a transfer of power is determined between the MV feeders for the present values of the set of power reference quantities.

[0059] The iteration stopping criterion may be zero reduction.

[0060] The iterative step values for the set of power reference quantities may have the same absolute value.

[0061] The power distribution network may comprise two MV feeders, and the total active power may be active power measured at the primary substation for the two MV feeders added together. The total active power may be measured at one or multiple measurement at the primary substation or below the primary substation, which can indicate the total loss reduction or increase with change in power reference quantity values.

[0062] The set of power reference quantities may comprise two or more of the following: voltage of a point of common coupling, PCC, for the MV feeders, active power injected by the MVDC link in the PCC for the MV feeders, and reactive power injected by the MVDC link in the PCC for the MV feeders.

[0063] The set of power reference quantities may comprise at least two of: voltage in a first side of a PCC, voltage in a second side of a PCC, active power in a first side of a PCC, active power in a second side of a PCC, reactive power in a first side of a PCC, and reactive power in a second side of a PCC.

[0064] Details of the presented embodiment is provided hereafter with reference to FIGS. 5 to 9.

[0065] An MVDC link connected between two MV feeders is presented with reference to FIG. 5. The MVDC link is connected between two different feeders, Feeder 1 and Feeder 2. The MVDC link is illustrated with a back-to-back MVDC link connected parallel to a normally open (NO) switch. However, there may not be a parallel NO switch, and the MVDC link may not be a back-to-back link but rather a point-to-point link with a DC cable in-between the converters.

[0066] The voltage in the primary substation, V.sub.PSS, is measured and so are also the active power fed into the top segment of the two feeders, i.e. P.sub.1, P.sub.2. Further, at each side of the PCC of the MVDC link, AC voltage, active power and reactive power are measured, i.e. V.sub.PCC1, P.sub.PCC1, Q.sub.PCC1 for Feeder 1 and V.sub.PCC2, P.sub.PCC2, Q.sub.PCC2 for Feeder 2.

[0067] In this example, converter 1 (i.e., the one connected to Feeder 1) is configured for active and reactive power control. Converter 2 (i.e., the one connected to Feeder 2) is on DC voltage control and reactive power control. To reduce power loss in the power distribution network reference values P.sub.PCC1.sup.ref, Q.sub.PCC1.sup.ref, P.sub.PCC2.sup.ref, Q.sub.PCC2.sup.ref, should be identified that minimizes losses while keeping other quantities (typically AC voltages and currents) within allowed limits.

[0068] DC side losses are typically minimized by keeping the DC voltage as high as possible. Thus, P.sub.PCC1.sup.ref, Q.sub.PCC1.sup.ref, P.sub.PCC2.sup.ref, remain to be determined.

[0069] The objective function

[00001]$\underset{P_{\mathrm{PCC}1}^{\mathrm{ref}},Q_{\mathrm{PCC}1}^{\mathrm{ref}},P_{\mathrm{PCC}2}^{\mathrm{ref}}}{\min }{P}_1+P_2$

[0070] should solved such that various (measured) AC quantities remain within limits.

[0071] The problem to solve is similar to what would be solved in the optimal power flow (OPF) problem described in the background. However, there are two major differences:

[0072] Instead of minimizing losses, which cannot easily be measured, the total power fed into the two feeders is minimized.

[0073] Instead of solving the minimization problem mathematically, it is done by actually changing the reference values in a structured fashion and observing the response in terms of the measured total power fed into the two feeders.

[0074] If the total active power, P.sub.tot=P.sub.1−P.sub.2, is reduced as P.sub.PCC1.sup.ref, Q.sub.PCC1.sup.ref, P.sub.PCC2.sup.ref, are changed there could be two reasons for this. Either the active losses or the active loads have been reduced. The active losses are mainly resistive losses in the transmission, i.e. I.sup.2R losses. The active loads may reduce as a consequence of dependence on voltage magnitudes. However, as long as the voltage magnitudes are kept within the stipulated limits, this should not be a problem. In other words, in practice the sum of active power fed into the feeders can be used as a proxy for active losses.

[0075] However, it follows that in particular the available voltage measurements should be observed to make sure that the voltage profile is kept in the agreed range. Further, also available current/power measurement should be observed to avoid overloading critical cable segments, in particular in situations when some cable segments are disconnected due to faults and some NO switches are closed to serve all customers.

[0076] FIG. 6 illustrates where controller 1 of the process may be implemented. The controller may be implemented in the primary substation control 1a, or in the converter control 1b. The controller 1 may yet alternatively be implemented in a distribution management system (DMS) control (not illustrated).

[0077] In a DMS implementation the procedure can be executed by the operator of the power distribution network, with measurement of P.sub.1, P.sub.2, V.sub.PSS, and V.sub.PCC and communicated (e.g., via SCADA) to a control room and that an operator in the control room then has the possibility to change the set-points or power reference quantities and see the response in total power while keeping voltages within a desired range. Measurements of the voltage in a subsidiary substation (V.sub.sss) may further be communicated to the controller.

[0078] For implementation in a primary substation control 1a, measurements in the PCC V.sub.PCC and also of V.sub.SSS may be sent through communication to the primary substation, and the primary substation control 1a is allowed to change the MVDC link set-points or power references quantities.

[0079] For implementation in a converter control 1b, measurements in the primary substation P.sub.1, P.sub.2, and V.sub.PSS, and also of V.sub.SSS may be sent through communication to the converter control 1b.

[0080] An embodiment of a method is presented with reference to FIG. 8, where one-by-one step change in active and reactive power reference quantities are illustrated in detail to check the change in total active power at the primary substation. The details of changing the reference values to reduce P.sub.tot is presented.

[0081] In processing block S100 a fixed step-size for each reference value, i.e. ΔP.sub.PCC1.sup.ref>0, ΔQ.sub.PCC1.sup.ref>0 and ΔP.sub.PCC2.sup.ref>0, is determined.

[0082] In processing block S110 an iteration stopping criterion ΔP.sub.tot.sup.ref<0, is established such that if ΔP.sub.tot>ΔP.sub.tot.sup.stop, then the minimum total active power has been reached (or at least close enough) for the give step size in reference values.

[0083] In processing block S120a the current operating point is started with the reference values P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, P.sub.PCC2.sup.ref,0 and a measured total power P.sub.tot.sup.0.

[0084] In processing block S120b an iteration step P.sub.PCC1.sup.ref1=P.sub.PCC1.sup.ref0+P.sub.PCC1.sup.ref is taken and note through measurement and comparison in processing step S120c ΔP.sub.tot.sup.1=P.sub.tot(P.sub.PCC1.sup.ref,1, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0)−P.sub.tot(P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0).

[0085] In processing block S120d return to P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0.

[0086] In processing block S120e take an iteration step P.sub.PCC1.sup.ref2=P.sub.PCC1.sup.ref0+ΔP.sub.PCC1.sup.ref and note through measurement and comparison in processing step S12f ΔP.sub.tot.sup.2=P.sub.tot(P.sub.PCC1.sup.ref,2, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0)−P.sub.tot(P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0.

[0087] In processing block S120g return to P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0.

[0088] Take an iteration block Q.sub.pCC1.sup.ref1=Q.sub.PCC1.sup.ref0+ΔQ.sub.PCC1.sup.ref and note through measurement and comparison ΔP.sub.tot.sup.3=P.sub.tot(P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,1, Q.sub.PCC2.sup.ref,0)−P.sub.tot(P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0).

[0089] Return to P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0.

[0090] In processing block S120h, check if all reference quantities have been processed. When all reference quantities have not been processed, proceed to processing block S120i.

[0091] Take an iteration step Q.sub.PCC2.sup.ref2=Q.sub.PCC1.sup.ref0+ΔQ.sub.PCC1.sup.ref and note through measurement and comparison ΔP.sub.tot.sup.4=P.sub.tot (P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,2, Q.sub.PCC2.sup.ref,0)−P.sub.tot(P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0.

[0092] Return to P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0.

[0093] Take an iteration step Q.sub.PCC2.sup.ref2=Q.sub.PCC2.sup.ref0+ΔQ.sub.PCC2.sup.ref and note through measurement and comparison ΔP.sub.tot.sup.5=P.sub.tot(P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,1)−P.sub.tot(P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0).

[0094] Return to P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0.

[0095] Take an iteration step Q.sub.PCC2.sup.ref1=Q.sub.PCC1.sup.ref0+ΔQ.sub.PCC2.sup.ref and note through measurement and comparison ΔP.sub.tot.sup.5=P.sub.tot(P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,1)−P.sub.tot(P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PCC2.sup.ref,0).

[0096] Return to P.sub.PCC1.sup.ref,0, Q.sub.PCC1.sup.ref,0, Q.sub.PPC2.sup.ref,0.

[0097] In processing block S120h, when all reference quantities have been processed, proceed to processing block S130.

[0098] In processing block S13o, when all ΔP.sub.tot.sup.1, ΔP.sub.tot.sup.2, ΔP.sub.tot.sup.3, ΔP.sub.tot.sup.4, ΔP.sub.tot.sup.5, ΔP.sub.tot.sup.6>ΔP.sub.tot.sup.stop stop the process, as the minimum total active power has been reached. Otherwise proceed to processing block S140 to select the reference value that resulted in the lowest power and proceed to processing block S120a with the selected reference value as the new starting point.

[0099] The step size in reference quantities and stopping criteria may be selected based on network structure, substation power, resolution, and accuracy of the measuring units. It may also depend on what can be detected at the substation based on connected load variations and power flow.

[0100] A large step size may result in significant changes in power flow while a very small step size may require several steps to achieve the desired value. A large stopping criterion may not reach a very suitable loss reduction while a very small stopping criteria can lead to numerous numbers of steps in reference changes.

[0101] Once the minimum has been reached, no further actions are required, however as the operating conditions may change, due e.g. to changing loads, the algorithm may be restarted with regular intervals (if the changes in operating conditions are small enough, no steps are actually taken).

[0102] If one or both converters is/are on AC voltage control the corresponding fixed step in reactive power reference ΔQ.sub.PCCx.sup.ref>0 should be replaced with a corresponding step in AC voltage reference ΔV.sub.PCCx.sup.ref>0.

[0103] There may be natural variations in load, making it harder to distinguish between load variations and the effect of a change in reference values. It is advantageous if the change in reference value and measurement of total power are coordinated in time, such that the step in reference value first is executed and then immediately afterwards, the measurement is performed.

[0104] Typical step values may be ΔP.sub.PCC1.sup.ref=300 [kW], ΔQ.sub.PCC1.sup.ref=300[kVar], ΔQ.sub.PCC2.sup.ref=300 [kVar], and may be ΔP.sub.tot.sup.stop=0, i.e. the algorithm will continue if there a reduction is possible. Initially, all references may be zero.

[0105] According to an aspect, an embodiment of a controller 1 for transfer of power between MV feeders via an MVDC link in a power distribution network is presented with reference to FIG. 9. The controller 1 is configured to setting an iteration step value for each of a set of power reference quantities, and setting an initial value of each of the set of power reference quantities, iteratively changing values of each of the set of power reference quantities, and selecting one changed value of the set of power reference quantities by: changing a present value of each of the set of power reference quantities, one at a time, with the set iteration step value, respectively, into a new value, and measuring a total active power at a substation of the power distribution network for each of the new value, one at a time, and selecting the new value of the one of the set of power reference quantities that provides the lowest measured total active power at the substation, wherein a next iteration is performed with the selected new value as present value for the one of the set of power reference quantities and with the present value for the other of the set of power reference quantities.

[0106] FIG. 9 is a schematic diagram showing some components of the controller 1. A processing circuitry 10 may be provided using any combination of one or more of a suitable central processing unit, CPU, multiprocessing circuitry, microcontroller, digital signal processing circuitry, DSP, application specific integrated circuit etc., capable of executing software instructions of a computer program 14 stored in a memory 12. The memory can thus be considered to be or form part of the computer program product 12. The processing circuitry 10 may be configured to execute methods described herein with reference to FIG. 7 or 8.

[0107] The memory may be any combination of read and write memory, RAM, and read only memory, ROM. The memory may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

[0108] A second computer program product 13 in the form of a data memory may also be provided, e.g. for reading and/or storing data during execution of software instructions in the processing circuitry 10. The data memory can be any combination of read and write memory, RAM, and read only memory, ROM, and may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The data memory may e.g., hold other software instructions 15, to improve functionality for the controller 1.

[0109] The controller 1 may further comprise an input/output (I/O) interface 11 including e.g., a user interface. The controller 1 may further comprise a receiver configured to receive signalling from other nodes, and a transmitter configured to transmit signalling to other nodes. Other components of the controller 1 are omitted in order not to obscure the concepts presented herein.

[0110] The aspects of the present disclosure have mainly been described above with reference to a few embodiments and examples thereof. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.