Current Matching Control System for Multi-Terminal DC           Transmission to Integrate Offshore Wind Farms             ...
Fig.1 The test MTDC system configurationover use of communications, modern telecommunication              paper focuses on...
Fig.3 MTDC control strategies: (a) voltage margin (b) voltage droop                                                       ...
mode. GCI3 and GCI4 will operate using the proposed CMCin order to address the shortcomings of the VMC and VDCcontrol, reg...
5 Performance Evaluation                                                        Table.1: Simulation event description and ...
being conducted to analyse the performance of this system                                                                 ...
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Current matching control system for multi-terminal dc transmission to integrate offshore wind farms


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Current matching control system for multi-terminal dc transmission to integrate offshore wind farms

  1. 1. Current Matching Control System for Multi-Terminal DC Transmission to Integrate Offshore Wind Farms J. Zhu, C. Booth, G.P. Adam Department of Electrical and Electronic Engineering, University of Strathclyde, Glasgow G1 1XW, UK. Email: HVDC, voltage source converter, multi-terminal used for many HVDC installations [3], but is not well suitedDC, control. to MTDC, in comparison to VSC. A summary of the advantages of VSC over LCC is listed below:Abstract  VSC has a smaller footprint which facilitates offshore installations of reduced platform size [3];The inherent features of Voltage Source Converters (VSCs)  LCC requires large filtering components;are attractive for practical implementation of Multi-Terminal  VSC provides additional reactive power support and ACHVDC transmission systems (MTDC). MTDC can be used voltage regulation for wind farms connected to weak ACfor large-scale integration of offshore wind power with systems, and possesses black-start capability;onshore grids. However, many of the control strategies for  VSC improves wind farm AC fault ride-throughMTDC that have been proposed previously for offshore wind capability and facilitates Grid Code compliance atfarm integration depend on local control of the wind turbine reduced costs [4];generators.  Power reversal can be achieved in VSC without changing the DC voltage polarity, facilitating the realisation of aThis paper proposes a new control strategy, termed Current flexible MTDC transmission system.Matching Control (CMC), which can be used with any MTDC transmission systems have attracted much attentionnumber of converter terminals, and is independent of the for wind farm integration [5][6][7]. Firstly, MTDC reducestypes of wind turbines used within each wind farm. The converter numbers when compared to numerous point-to-proposed CMC matches the current reference of the grid side point HVDC solutions. Secondly, it is conceivable that, due toconverter to that of the wind farm side converters. In order to limited correlations of weather systems in geographicallyachieve such current matching, a telecommunication system dispersed wind farm locations, the overall variability of windwill be required to facilitate calculations of the grid side power may be reduced by interconnecting manycurrent references to be carried out in real time. To validate geographically-dispersed wind farm systems via a large-areathe performance of the proposed control strategy, a generic MTDC system, thus increasing the overall availability offour-terminal MTDC network, which integrates two offshore energy. In future, energy storage devices may be integratedwind farms with two mainland grids, is simulated and results with MTDC system [1], which further supports energyrelating to several steady state and transient scenarios are availability and quality of supply. MTDC is also beingpresented. proposed as the means of interconnecting independent large- scale AC power systems, (e.g. European super-grid proposal1 Introduction [5] to promote the interconnection of Norwegian hydro, French nuclear, Sahara solar and North sea wind power into aThere has been a tremendous pace of development of large- common MTDC) to resolve local power shortages orscale offshore wind farms in recent years. It is anticipated that congestion, to enable international power sharing, and tothere will be an approximate increase of 26.6 GW in provide an excellent level of overall power system reliability.aggregate generation capacity over the period from 2009/10 As discussed in [5], there are many challenging obstacles toto 2016/17 in the UK, 11.7 GW of which will be contributed the introduction of MTDC. The control system for an MTDCby wind power [1]. More broadly, the European Wind Energy must be robust, coordinated and reliable, as problems withAssociation has, in its “high wind” scenario, a target of 180 one terminal have the potential to affect the entire MTDCGW from wind energy sources in by 2020, of which 35 GW network. A number of control strategies are proposed [6]will be sourced from offshore wind installations. This [8][9] which will be introduced later. These strategies remaincapacity target for offshore wind increases to 120 GW by at the modelling and testing stages of development. Critical2030 [2]. concerns about these strategies are the controllability andThese targets, if they are achieved, will have great impact on reliability of MTDC systems, as most of the proposedpower transmission design, planning, construction and strategies manage an MTDC system without use ofoperation. Many offshore wind farms will require long- communications between terminals. The proposed currentdistance power transmission systems. AC may not be suitable matching control strategy employs minimal (in terms ofdue to high power losses over longer distances. Classical line- required traffic and bandwidth) telecommunications betweencurrent-commutated (LCC) HVDC transmission has been terminals in an MTDC network. While there may be concerns
  2. 2. Fig.1 The test MTDC system configurationover use of communications, modern telecommunication paper focuses on the dynamics of AC and DC interaction,technologies are increasingly highly developed, reliable and which is dictated by the converter control. As the controlredundant [10]. Furthermore, risk can be mitigated by system for VSC employs vector control in the synchronousemploying redundancy, through continuous monitoring of rotating reference frame dq, the current references id_ref andtelecommunications channels, and ensuring operation can iq_ref which are derived by the commanded active power Pcommcontinue, albeit in a less efficient fashion, if and reactive power Qcomm, are given in equation (1). In thetelecommunications is lost. Operation of the scheme is based rotating reference frame, the d-axis voltage Vd, aligned withon measurement (discretely, with a step of 1-2ms in this one of three phases, is equal to the magnitude of AC voltage,example) of the total DC current provided by wind farm side and the q-axis voltage Vq is zero.rectifiers (i.e. WFR1 and WFR2 in Fig.1) and allocation(matching) of this current across the inverters, according to a Pcomm Qpre-determined sharing factor. This is described in more idref  , iqref  comm (1) Vd Vddetail in Section 4. The scheme also provides furtherprotection for the entire MTDC system by monitoring the DCvoltage. Finally, the system can operate if the Once the current references id_ref and iq_ref are generated, thetelecommunication system fails, but accurate sharing of the inner current control loops adjust the actual id and iq values toinverter currents may not be possible. be in accordance with the computed reference values. This process takes a short period to complete and is determined by the natural frequency of the converter control dynamic in2 MTDC system configuration Laplace equation (2), which contains the proportional gainMTDC topology design may vary depending on specific (kp) and the integral gain (ki) of the proportional-and-integralsituations (e.g. the locations of grid connection points and (PI) controller, reactor inductance (L) and resistance (R):offshore wind farms, available undersea cable routes). Fig.1presents a four terminal MTDC system for wind farm idq kp  Li kintegration, which utilises bi-polar cables R5 with nominal DC  L Rk p (2) s2  s idqref kivoltage of 200 kV (± 100 kV). On the offshore side, two wind L Lfarms are connected via two voltage source neutral-pointclamped rectifiers (WFR1, WFR2). On the onshore side, two From the DC side perspective of the VSC in Fig.2, the DCgrid-connected inverters (GCI3, GCI4) feed power to two voltage udc across the converter or the output capacitors, theindependent 2000 MVA equivalent AC power systems. All DC current idc injected by the converter, and the current icVSCs are rated at 200 MVA. Targets of converter control conducted by the DC cables, are related as shown in equationdiffer for WFR1 and WFR2, implementing frequency and AC (3). The capacitors are charged (or discharged) to possess avoltage control at the point of common connection (PCC) certain DC voltage. The current idc injected to the MTDCwith wind farms, while GCI3 and GCI4 are equipped with a network by the converter is calculated from AC side PCCcurrent controller and DC voltage regulator respectively, in currents id and iq, pulse-width-modulation index M andaddition to AC voltage/or reactive power control. converter terminal voltage angle  with respect to the PCC voltage, using equation (4):2.1 AC/DC interaction for a VSC dudc C  idc  ic (3)Instead of presenting an in-depth study of the VSC control dtsystem formulae, such as those presented [9] and [7], this idc  Mid cos   Miq sin  (4)
  3. 3. Fig.3 MTDC control strategies: (a) voltage margin (b) voltage droop namely voltage margin control [9] and voltage droop control [8] [6]. 3.1 Voltage margin control (VMC)Fig.2 DC equivalent circuit for the MTDC In VMC control, one node’s DC voltage is controlled by a DC voltage controller (DCVC). This effectively acts as a DC2.2 Equivalent MTDC circuit voltage slack bus, with other VSCs operating in currentAs demonstrated in Section 2.1, the DC property of individual control mode as illustrated in equations (1) and (2).VSCs in the MTDC can be represented as a “controlled”current source, shown in the equivalent circuit in Fig.(2). The VMC equips all converters with DCVCs, but the DCVC ofextremely small inductance and capacitance of the DC only one converter station must be activated. Considering thenetwork with respect to direct current are neglected. V-I characteristic of Fig.3(a), GCI4 has its DC node voltage controlled at udc4 ref by the activated DCVC, represented as theAs the focus of this section is on the analysis of DC network solid line. This acts to balance the current flows frombehaviour, it is essential to analyse the effect of the variation rectifiers WFR1 and WFR2 to inverter GCI3, by automaticallyin DC current idc from one converter station, on either its DC “sliding” its current output along the constant DC voltagevoltage and/or the DC voltage at other stations. Taking WFR1 udc4_ref. Inverter GCI4 has an inherent current limit. If thisas an example, a control action to increase WFR1 current idc1 limit is exceeded (e.g. strong wind pushing more currentwill quickly charge its DC capacitors and boost its DC though rectifiers into the MTDC), GCI4 will not be able tovoltage udc1 to a higher value, based on equation (3). The maintain the DC voltage, and will operate at its maximumhigher udc1 with respect to other DC node voltages acts to current output. According to the analysis in Section 2.2, thesupply the conducted current ic1 into the MTDC network. The DC network voltage will continuously rise in line with currentincreased current ic1 charges capacitors at other nodes, until “surplus” in the MTDC network. The voltage will ultimatelythe voltage levels at all the nodes reach a new higher rise to a new level (udc3_ref) that activates the back-up DCVRequilibrium value. The rectifier DC voltage is slightly higher in the other inverter GCI3. In this case, GCI3 beginsthan the inverter voltage so that current flows from rectifier maintaining the MTDC voltage at udc3_ref, the dashed line innode(s) to inverter node(s). The magnitude of individual Fig.3(a). The term “voltage margin” refers to the differenceconverter DC voltage depends on two elements: (a) the between udc4 _ref and udc3_ref in Fig.3(a).conducted current though the node; (b) the resistancesbetween the nodes. 3.2 Voltage droop control (VDC) VDC basically has multiple activated DCVCs (in both of theThus, it can be concluded that a temporary current mismatch inverters in this example). The two DCVCs are controlled atbetween rectifier and inverter in the MTDC results in an different levels for inverter current dispatch, as shown inoverall DC voltage variation. As the converter control Fig.3(b). The V-I droop characteristic is obeyed by GCI3 tosystems use DC voltage information to function, it is share the total current with GCI4. To demonstrate the droopdesirable to quickly address any DC current surplus (by operation, for example, in order to increase the current ofincreasing exported current) or DC current shortage (by GCI3 and decrease GCI4 based on the droop characteristic, thereducing exported current), so that a stable DC voltage DCVC in GCI3 converter control must lower the voltageoperating point for the MTDC system will be realised. reference udc3_ref and then its current output “slides” along theCommunications between rectifier and inverter nodes in the droop to the right hand side, to output more current.system is therefore critical to the operation of this scheme.3 Previously reported MTDC control 4 Proposed current matching control strategy strategies As discussed in Section 2.2, a stable DC network operating point can be achieved by quickly acting to reduce anyHistorically, there have been two distinct control strategies mismatch between rectifier and inverter DC currents. As bothused to facilitate power dispatch from DC to AC systems, WFR1 and WFR2 inject all the power generated by wind farms into MTDC network, they will operate in current control
  4. 4. mode. GCI3 and GCI4 will operate using the proposed CMCin order to address the shortcomings of the VMC and VDCcontrol, regarding DC current mismatch that may arise duringchanges in wind power generation. The detailed operation ofthe scheme is now presented.4.1 Converter operating statesTo facilitate the development of the proposed controlstrategy, it is important to understand VSC operating stateswith their control references in the MTDC system. The Fig.4 The Central CMC with communicated variablesfollowing equations (5) and (6) are given, referring to Fig.2: uS  udc1  R1ic1  udc 2  R2ic 2 (5) uS  udc3  R3ic3  udc 4  R4ic 4 (6)uS is the sending end voltage and uR is the voltage at thereceiving end of the DC link. ic1 to ic4 are the rectifier andinverter currents as shown in Fig.2. R5 is given by: uS  uR  R5ic5 (7)ic5 is the current through the DC link as shown in Fig.2.Kirchhoff’s current law dictates that: Fig.5 CMC and the additional protection loop idc1  idc 2  idc3  idc 4  0 (8) idc3  (1  KS )(idc1  idc 2 ) (11)As demonstrated in VMC and VDC, GCI4 has its DCVCactivated to control DC voltage at udc4; the other converters DC current reference for idc3 is transmitted from the CMC toWFR1, WFR2 and GCI3 control their currents at idc1 idc2 and the GCI3 converter control system, to produce a commandedidc3 respectively. Therefore, by combining equation (5), (6), active power reference, given in equation (12):(7) and (8), the following converter operating state matrix, Pcomm3  idc3udc3 (12)which incorporates DC network resistance, can be derived: In this way, GCI4 with activated DCVC maintains the current  udc1   R1  R4  R5 R4  R5  R4 1   idc1  balance in the MTDC network, but GCI3 also acts to u      dc 2    R4  R5 R4  R5  R4 1   idc 2   effectively preserve the current matching by adjusting its  udc3   (9) R4 R4 ( R3  R4 ) 1   idc3  output active power, using the data relating to the total       ic 4      1 1 1 0  udc 4    rectified current. By setting a proper sharing factor KS, accurate current allocation between GCI3 and GCI4 is4.2 Current matching control principle achieved. For example, a setting of KS=0.4 will allocate 40% of the total current to GCI4, with the remaining 60% allocatedFig.4 shows the communicated variables of the proposed to GCI3.CMC for an MTDC. The green blocks in Fig.1 and Fig.4 aretelecommunication feedback signals “idc1”and “idc2” from the Additionally in Fig.5, there is an over-voltage and under-rectifiers WFR1 and WFR2, based on equation (4). Feedback voltage protection function placed within the main controlsignal “udc4” from GCI4 is used for over- or under-voltage loop in Fig.5. It will detect MTDC over-voltage or under-protection. The current reference for GCI3 converter is voltage by monitoring the feedback signal DC voltage “udc4”continuously updated by the proposed CMC and transmitted at GCI4, and will trigger the back-up DCVR in GCI3 if udc4to the GCI3 vector control, via the communicated control exceeds an upper or lower constraint value (set to 180 kV andsignal “idc3_ref”. 220 kV in this simulation).Modern wireless communication system has been proposed in In the event of telecommunication failure, which could beHVDC application to secure power reliability [12] and it is detected by the loss of data, or by use of a standardhere used here to favour the CMC strategy for the MTDC communications health monitoring signal, GCI3 also adopts asystem. Fig.5 illustrates the CMC inner control logic, where triggering voltage which is higher (230 kV) than the higherthe total rectifier current from WFR1 and WFR2 is the sum of DC voltage protection constraint of GCI4 (220 kV). If thisfeedback signals “idc1”and “idc2”. Rectified current is divided voltage is exceeded, the converter control system in GCI3 willbetween the inverters GCI3 and GCI4 by applying a sharing trigger its back-up DCVR in any case. With the CMC strategy,factor KS. KS represents the portion of the expected power to the converter operating states can be ascertained withbe exported from the MTDC network through GCI4, given by reference to equation (13):equation (10). Accordingly, the reference current idc3 for GCI3  udc1   R1  R4  R5 R4  R5  R4 1  idc1  u    is given by equation (11):  dc 2    R4  R5 R4  R5  R4 1   idc 2   udc3    (1  K S )(idc1  idc 2 )  (13) idc 4  KS (idc1  idc 2 ) (10)    R4 R4 ( R3  R4 ) 1    ic 4      1 1 1 0   udc 4  
  5. 5. 5 Performance Evaluation Table.1: Simulation event description and timescales Time (s) EventsFor the performance evaluation of the proposed CMC 1 Sharing factor KS changes from 0.6 to 0.4strategy, a generic four-terminal MTDC network with eachconverter station rated at 200MVA is simulated in Matlab PWFR1 changes from 0.3 to 0.5 pu 3 PWFR2 changes from 0.4 to 0.7 puSimPowerSystems [14], as shown in Fig.1. The central CMC 5 3-ph-earth fault at GCI4 (100 ms)unit is placed in an independent block from the converter 7 Permanent trip of GCI4current control systems of each of the four converters. DCcable resistances are obtained from typical HVDC cableparameters [15] and copper resistivity at 0 in [16]. Thisresults in modelled resistance values of 1.61 for R2 and R4,0.32 for R1 and R3, and 1.94 for R5. The performance ofthe MTDC using the proposed CMC is examined, understeady state and fault conditions. Several events have beensimulated, and details are listed in Table 1.Fig.6 shows the direct current injected into the DC networkfrom the wind farm rectifiers WFR1 and WFR2, while Fig.7illustrates the direct current flow from the DC network intothe grid connected inverters GCI3 and GCI4 (the red dashed Fig.6 Direct current idc1 and idc2 from WFR1 and WFR2line represents the reference current idc3ref for GCI3, calculatedby the CMC). Initially, GCI3 and GCI4 share the current flowbased on the specified sharing factor KS=0.6, that is 60% forGCI4, and 40% for GCI3. At t=1s, when KS is changed from0.6 to 0.4, a new current reference is assigned to GCI3 toincrease its DC current, and the current quickly tracks thereference change. GCI4 is observed to decrease its currentfrom 60% to 40%.At t=3s, due to the simulated increase in wind powerproduction (a gust simulated by a step change in wind speed),the active power references for WFR1 and WFR2 change and,as shown in Fig.7, their DC current input to the MTDC rises Fig.7 Direct current idc3 and idc4 from GCI3 and GCI4to 0.7 and 0.5 pu respectively. This increased input current isexported and shared correctly by GCI3 and GCI4, based on KS.Fig.8 presents the DC voltage of WFR1, WFR2, GCI3 andGCI4. At t=5s, there is a severe AC voltage dip due to a100ms duration three-phase-to-earth fault at PCC4. In thiscase, GCI4 contributes limited current to the fault to supportthe grid voltage at PCC4 until the fault is cleared. It can benoticed that a transient DC over-current occurs not only atGCI4 but also at WFR1, WFR2 and GCI3. This is due to thetemporary reduction in the power transfer capability of GCI4as the voltage magnitude at PCC4 collapses. That is becauseof DC voltage interactions across all converters. The DC Fig.8 DC voltage udc of WFR1,WFR2,GCI3 and GCI4over-current is effectively limited by the converter current t=7.3s. Immediately, the CMC triggers the back-up DCVC incontrol system to no greater than 1.8 pu; this current is GCI3’s converter controller via communicating the controlexported by the CMC and returns to normal values as soon as signal “Trigger_DCVC_3” (highlighted in orange in Fig.1the fault is cleared. and Fig.5), and GCI3 begins controlling the DC voltage to a higher target level using its DCVC (220 kV in this case). ThisAt t=7s, inverter GCI4 is tripped, and the total rectifier current is to allow the DC capacitors to absorb the additional powermismatches the inverter GCI3 current output (sharing only that cannot be temporarily transferred to the AC side through60% of total current based on KS=0.6) during a short period, GCI3. The CMC therefore can continue to operate the MTDCleading to significant over-voltage in the MTDC network as after tripping of inverter GCI4.shown in Fig.8. The protection control loop, depicted in thelower part of Fig.5, detects the over-voltage when feedback It should be noted that inverter GCI3 is directly controlled bysignal udc4 reaches the upper constraint level (220 kV) at the CMC, so plays an important role in continuously adjusting
  6. 6. being conducted to analyse the performance of this system under other scenarios, with different control targets (e.g. to provide voltage support to connected grid AC systems) and to more extensively compare performance with other existing and emerging MTDC control strategies. Acknowledgement The authors gratefully acknowledge the kind support of the Engineering and Physical Sciences Research Council and Rolls-Royce plc.Fig.9 AC current output at PCCs of WFR1,WFR2,GCI3 and GCI4 Referencesits DC current export to ensure the DC network current [1] GB National Grid, Seven Year Statement. Available:balance, as shown by the red dashed line in Fig7. Inverter, with its DCVC activated, acts as the DC side “slack 8E66-6F698D429DC5/41470/NETSSYS2010allChapters.pdf. [2] European Wind Energy Association (EWEA), Wind Enery Scenariosbus” to maintain voltage stability in the MTDC system, and it up to 2030. Available:also contributes to power dispatch in conjunction with GCI3. 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"Application of SMES Unit in Improving theThis paper has presented the theory and examples of Performance of an AC/DC Power System,", IEEE Transactions onoperation of a novel current matching control (CMC) scheme Sustainable Energy, vol.2, no.2, pp.109-121.for MTDC networks, where the total rectified (input) current [12] Jiuping Pan; Nuqui, R.; Srivastava, K.; Jonsson, T.; Holmberg, P.;to the MTDC network is used as basis for actively controlling Hafner, Y.-J.; , "AC Grid with Embedded VSC-HVDC for Secure and Efficient Power Delivery," Energy 2030 Conf., 2008. ENERGY 2008.the inverted (output) current from the network to supplied AC IEEE , vol., no., pp.1-6, 17-18 Nov. 2008.grid systems. Power sharing and ability to protect against [13] S. Cole, J. Beerten, R. Belmans. "Generalized Dynamic VSC MTDCvoltage violations are also features of the scheme. The Model for Power System Stability Studies," IEEE Transactions onscheme requires communications, but can still operate in the Power Systems, vol.25, no.3, pp.1655-1662.event of loss of communications facilities. The CMC scheme [14] MathWorks. "VSC-Based HVDC Link," Available: be used with any number of converter terminals, and 9059.html.independent of the types of the wind turbines used within [15] ABB, HVDC Light Cables- Submarine and land power cables,each wind farm. Available: 98f62e5c1257154002f9801/$file/hvdc%20light%20power%20cables.pThe theoretical study and simulation results prove that, with df.coordination between the CMC and local converter control [16] D. R. Lide. CRC Handbook of Chemistry and Physics 75th ed. Bocasystems, the passiveness of previous control strategies – Raton, CRC Press.voltage margin and voltage droop – is avoided. This enables astable and secure DC network operating environment, allowsflexibility of power allocation across inverters, and providesan effective restriction of DC over-voltages. Future work is