Power Flow Control in Meshed DC Grids 29_03_17 Dr Jun Liang Cardiff University
Contents Challenges in DC Grid Development Inter-line DC Current Flow Controllers (CFCs) Recent Developments Experimental Results and Analysis Coordinated Control Conclusions 2
Challenges in DC Grid Development Part I
Why DC Grid? Why offshore wind Source: MEEPS newsletter University of Manchester Low carbon foot print Europe 2030-126GW wind Cross border energy exchange Transform wind energy into a stable power source Increasing power demand Increase the reliability 4
Challenges in DC Grid Development DC Breakers are still in conceptual stage DC grid protection Standardisation issues Project cost Multi-vendor Future interoperability Flexible power flow between dc nodes 5
Challenges in Line Power Flow Control Passively determined by the resistance between the nodes Can only be partially controlled via adjusting converter voltage set points Line overloading could damage the cable and leads to cascaded failure affecting other DC lines Transmission bottlenecks How can we change the line power flow? Installations of new auxiliary HVDC cables. However cables are expensive and have high capital and environmental cost Installation of a passive or Power electronics devices?? 6
Current Flow Controllers (CFCs) Part II
Types of Power Flow Control Devices Current Control Devices Series Connection Shunt DC-DC Converter DC chopper Controllable Voltage Source Series Resistive Shunt Connection AC-DC Nodal Thyristor CFC DC-DC Nodal Inter line DC CFCs IGBT CFC 8
Shunt devices Advantages: Great controllability Fault blocking capability Challenges: Large footprint Power losses- same as VSC Expensive Solution : Series devices Source: T. Lüth "High-Frequency Operation of a DC/AC/DC System for HVDC Applications," Aug. 2014. 9
Line Power Flow Control Methods No power flow control Control through Power dissipation P cfc = r cfc. (I ij ) 2 Control through Power transfer P cfc = v cfc. I ij DC node 1 DC node 1 Grid T VSC DC node 2 Grid T VSC DC node 2 AC node AC node DC node n-1 DC node n-1 V cfc typically around 5 % rated system voltage DC node n Power must be transferred or dissipated DC node n 10
AC-DC Nodal devices Challenges: Thyristor based CFC Large footprint Galvanic isolation transformers Vulnerable to ac faults Expensive An ideal device? IGBT based CFC What are device design requirements? 11
CFC Requirements Inexpensive Small footprint Low power losses High Controllability High reliability Solution : Line series Inter line devices 12
Resistive CFC (R-CFC) Advantages: Small footprint Inexpensive Challenges: High power losses Limited controllability Line current reversal Can the controllability be increased? -> Infinite resistance? 13
RC-CFC Switching States of RC-CFC State 1: Charge State 1 State 2 Consists of anti-series connected IGBT switch in parallel with an RC circuit State 2: Discharge Current control through energy dissipation Improved controllability CFC can provide current nulling on the compensated dc line Can the losses be minimized?? -> transferring the power to another dc line Zero line current current nulling 14
Inter Line Power Exchange Line current control via power transfer between dc nodes Require at least one energy storing device such as capacitor Power taken from one line is equal to power added to the other line U 2 I 2 = U 3 I 3 15
Capacitive CFC (C-CFC) Consists of two anti-series connected IGBT switches (B 1 and B 2 ) and a capacitor State 1 State 2 Switching States of C-CFC Objective: Current reduction on line 12 V BB = V C (1-D) V B2 = V C D V BB. I L12 = V B2.I L13 State 1-Charge Operation is limited to two quadrants quadrants 1 & 3 ( i.e. when line currents are in same direction) Can the control range be improved? By introducing additional semiconductor switches State 2-Discharge 16
Four Quadrant CFC An ac powered IGBT based CFC Current control through energy exchange between AC (P DC,1 )-DC nodes (P AC,1 ) B2 can realised with 3-phase or single phase VSC H-bridge B1 provides variable DC voltage Energy exchange between two dc nodes- same as C- CFC B2 realised with a H-bridge connected to a neighbouring line Doesn t require ac connection/ac transformer Current control through energy exchange between DC (P DC,1 )-DC nodes (P DC,2 ) 17
Dual H-bridge CFC Consists of two back-to-back connected H-bridges and a DC capacitor A dual H-bridge CFC acts like a point-to-point HVDC link inside a DC grid The capacitor is used as an energy storage device to selectively provide a voltage source Four quadrant operational capability 18
Dual H-bridge CFC State 1: Charge Both switches are OFF State 3 State 2 State 1 Switching States V BB = V C (1-D 1 ) V B2 = V C (1-D 1 -D 2 ) V BB. I L12 = V B2.I L13 State 2: Charge Switch S 12 OFF and Switch S 21 ON State 3: Discharge Both switches are ON 19
Dual H-bridge CFC Reduced Model Switches S 11 and S 23 are connected between T 1 and capacitor positive terminal Switches S 12 and S 24 are connected between T 1 and capacitor negative terminal Switches S 23 and S 24 can be removed Removal could reduce the reliability Each half-bridge represent one line/port 20
Recent Developments Part III
Recent Developments 3 patents by Alstom Alternative CFC Topologies An apparatus for controlling the electric power transmission in a HVdc power transmission system (patent by ABB) An arrangement for controlling the electric power transmission in a HVdc power transmission system (patent by ABB) Chen, W., Zhu, X., Yao, L., Ning, G., Li, Y., Wang, Z., Gu, W. and Qu, X., A Novel Interline DC Power Flow Controller (IDCPFC) for Meshed HVDC Grids. 22
Recent Developments Comprises two three-phase modular multilevel converters (MMCs) where ac outputs are connected to an ac transformer MMC based inter-line CFC Module A can be realised with Half-bridge/ Full-bridge modules No filtering requirement on dc side -> ripples on lines currents are reduced M. Ranjram and P. W. Lehn, "A multiport power-flow controller for DC transmission grids," in IEEE Transactions on Power Delivery, vol. 31, no. 1, pp. 389-396, Feb. 2016. Require larger footprint compare to other inter-line CFCs 23
Experimental Results and Analysis Part IV
Test Network The VSCs have been arranged in a symmetrical monopole configuration Experimental Validation The DC cables are represented by emulated L circuits 25
Multi-terminal HVDC test rig at Cardiff University Unidrive PMSM M Mechanical Conection PMSG G VSC3 dspace Optic link ~ VSC1 R 1 R 3 DC network CANcommunication DSP ~ VSC2 R 2 VSC4 R 4 26
Experimental Setup Devices Specifications Equipment ratings Operating rating rated power 10 kw 2 kw Voltage source rated ac voltage 415 V 145 V converters rated DC voltage 800 V 250 V topology Two-level, three-phase DC capacitors C g1, C g2, C g3 1020 µf Control system dspace DS 1005 / ControlDesk 3.2 ( SIMULINK interface) dspace 1005 system is used to control the test-rig Devices Specifications Equipment ratings Operating rating CFC topology Dual H-bridge PFC and R PFC DC capacitors C CFC 4400 µf Resistor R CFC 1.2Ω Switching frequency f sw 2000 Hz Control system dspace DS 1005 / ControlDesk 3.2 ( SIMULINK interface) 27
R-CFC Results i 12 i 13 VSC 2 - VDC control (250V) i 23 VSC 1 is requested to inject an additional power of 1.2 kw into the dc grid CFC is activated after overload detection v cfc 1.2Ω resistance is used to provide the required series voltage injection I 12,ref = 4A i 12 Limited line current control R-CFC has reached its maximum controllability before achieving the desired line current (i 12 ) of 4A 28
C-CFC Results i23 i 13 i 12 VSC 2 - VDC control (250V) VSC 1 is requested to inject an additional power of 1.2kW into the dc grid CFC is activated after overload detection I 12,ref = 4 A Pulsed dc voltages V B1 and V B2 are injected in series with dc lines 29
Dual H-bridge CFC Results VSC 2 - VDC control (250V) I 13,ref = 4 A Small current reduction/increment, thus small capacitor voltage (compared previous test case with C-CFC) Pulsed dc voltages V B1 and V B2 are injected in series with dc lines 30
Coordinated Control Part V
Coordinated Control Grid Dispatch Centre U1 + Ux - P12 U2 System Monitor Power Flow Computation Control Order Setting VSC1 DCCFC R VSC2 Line power flow&line resistance Update measurements Telecommunication DC Grid Telecommunication... DC-CFC Update Control order Rx U 1:k...... P 12 = U 2 U 1 U 2 U x R U x = I R x U k 1 U 1 32
Coordinated Control MTDC Meshed Transmission System Px Ui Rij Uj DCPFC Ux Nj Ni VSC VSC... VSC... VSC... VSC P = U G G x P x U x 0 G x T U U x 33
Coordinated Control What is the impact of changing orders on DC power flow? Impact of Changing Orders U order P order Analytical Expression f(x) U dc P dc Droop System admittance 34
Coordinated Control Impact of Changing Orders Quick glance of the analytical expression f(x) U CC U BB = U CC P CC U BB P CC U CC P BB U BB P BB P CC P BB + U CM U CC U BB U CC 0 0 U CC 0 U CC P CC U BB P CC U CC P BB = U BB P BB J m/m J m/n J n/m J n/n diag k 0 0 0 1 P CC U CC P BB U CC 0 0 = diag k 0 0 0 J m/m J m/n J n/m J n/n 1 diag k 0 0 0 35
Coordinated Control Cable (0.0095Ω/km,Max. current:1962a) Overhead Line (0.0144Ω/km,Max. current:3000a) GS4 N4 200km WF1 N5 Power Losses Optimization GS3 VSC4 200km N3 GS5 N7 200km 200km N6 VSC5 WF2 VSC3 500km GS2 200km VSC2 N2 VSC6 200km DC-CFC 300km N8 VSC7 WF3 GS1 VSC1 N1 400km 200km 200km VSC8 VSC9 WF4 Constant wind power generation Change in onshore VSCs power orders Inserted dc voltage Power losses reduced by 14.27 MW 36
Coordinated Control Optimization Maximise the wind power delivered to onshore system by redispatching control orders No Specify numbers of converters and DC-CFCs; Specify system topology Solve Power Flow Equality Constraints (i+1) (i) u - u <µ? Yes Initialise control orders, system conductance (PCM-PT)<ζ? Yes No Obtain and Update: (i+1) (i+1) (i) u = u +u Obatin new control orders: u (i+1) Linearise the expression of controlling power flow Obtain new h(x,u); h(x,u)/ u Solve all Inquality Constraints Optimisation achieved 37
Coordinated Control Available wind power : P WF1 = 2200 MW P WF2 = 1800 MW P WF3 = 1700 MW P WF4 = 1500 MW Wind Curtailment Reduction With CFC less wind power curtailment line power losses are minimized less power injection from onshore converter VSC 4 exports more power 38
Conclusions Inter line CFC devices provide better solution in terms of cost and power losses. A CFC can be used increase the reliability and efficiency of the DC grid by rescheduling the grid power flow and maintaining the line current below thermal limit. CFC can be used to minimize the line power losses and wind power curtailment 39
Thanks for your attention Any Questions? 40