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Control of DPGS Under Grid Faults Control of DPGS Under Grid Faults Marco Liserre [email protected] Marco Liserre [email protected] Control of DPGS Under Grid Faults Outline • Introduction • Crowbar protection for DFIG • Effect of voltage sag over the IG connected to grid • Effect of voltage sag over the wind farm • Solutions for dynamic reactive injections • LVRT with variable-speed wind turbines • Control strategies under unbalanced voltage • Conclusions Marco Liserre [email protected] Control of DPGS Under Grid Faults Introduction Control under grid faults has become very actual after new grid codes (Germany, Spain) has requested not only ride-through but also reactive power injection under fault Low voltage ride-through LVRT can be achieved by quickly limiting the active power using a crowbar and inject reactive power to support grid voltage restoring Reactive power injection during asymmetrical fault requires a positive sequence angle extraction Advanced control specially designed for unbalance voltage requires positive/negative sequence extraction and current controllers for both sequences P+Resonant current controllers can handle both sequences Using the instantaneous power theory different control strategies can be designed in order to minimize the dc voltage oscillations or output P and Q oscillations during unbalanced voltage New grid codes are expected to require even more controllability under grid fault in order to support the grid recovering (for ex. Non-oscillating P or Q) Marco Liserre [email protected] Control of DPGS Under Grid Faults Grid Faults and their consequences In general when a fault occurs in the electric grid and the fault is detected, the minimum possible portion of the grid containing the fault should be isolated and the neighboring parts should try to go back to normal operation. If a component fails then a larger area can be affected. Then the fault is removed and the damage is repaired, finally the normal service is restored. Faults can lead to both angle and voltage instabilities The angle instability is cause by active power imbalance and by the excitation of mechanical generator dynamics. As a consequence the generators lose synchronism and they are disconnected. Voltage instability arises due lack of reactive power and as a consequence leads to overload of power lines and as a consequence disconnection of loads. Marco Liserre [email protected] Control of DPGS Under Grid Faults Low Voltage Ride Through Two requirements can be defined for WTS during grid fault: to remain connected even during severe under-voltage to control reactive power to support the grid voltage These two requirements are associated with: Measure and maintain full control on the reactive and active power injected hence the capability of limiting it Limit the power quality decrement in terms of power fluctuations and harmonic distortion that may have severe effects on the grid. LVRT is strictly related to the capability of the WTS to preserve the converter safety and avoid overcurrent tripping. The semiconductor over-load capability can be increased only reducing the switching frequency Marco Liserre [email protected] Control of DPGS Under Grid Faults Crowbar protection Resistive crowbar Rotor short-circuited When the resistive crowbar is implemented, the stator and rotor transient current decay rapidly to value with amplitude lower than 1 p.u. Amplitude of transient electromagnetic torque is reduced when the resistive crowbar is activated. On the other hand the electromagnetic torque oscillates longer with the resistive crowbar. Marco Liserre [email protected] Control of DPGS Under Grid Faults Effect of voltage dips over the IG connected to grid During the fault period: Voltage at the wind turbine terminal drops as well as the air-gap flux in the generator. Reduction in the electromagnetic torque which causes the rotor to accelerate. After the clearance of the fault: Reactive power is supplied by the power system to recover the air-gap flux. This causes a high inrush current to be drawn by the wind turbine from the power system, which in turn causes a voltage drop at the wind turbine terminal. Effect of a sustained voltage drop after fault clearance: If the electromagnetic torque is not strong enough in comparison with the aerodynamic torque, the rotor speed will continue to increase and the induction generator could draw high inrush current from the external power system until appropriate protection devices trip it. In this condition, voltage at the wind turbine terminal dips and the output power of wind turbine drops. The system loses stability and the wind turbine has to be disconnected. Marco Liserre [email protected] Control of DPGS Under Grid Faults Effect of voltage dips over the IG connected to grid Electromagnetic torque Active power delivered by the SCIG-WT Marco Liserre Rotor speed Reactive power delivered by the SCIG-WT Positive-sequence voltage at the stator Negative sequence voltage at the stator [email protected] Control of DPGS Under Grid Faults Effect of voltage dips over the wind farm without STATCOM Source: American Superconductor Marco Liserre [email protected] Control of DPGS Under Grid Faults Effect of voltage dips over the wind farm With D-VAR STATCOM Source: American Superconductor Marco Liserre [email protected] Control of DPGS Under Grid Faults Dynamic reactive power injection LVRT retrofit solution for wind farms – D-VAR,Dynamic Capacitors Source: American Superconductor Marco Liserre [email protected] Control of DPGS Under Grid Faults LVRT with STATCOM STATCOM is a potential technology to retrofit LVRT to existing wind farms based on fixed-speed generators As LVRT is compulsory in some countries and will be spread worldwide a big market is open for STATCOM industry Along with providing LVRT, the STATCOM is providing also voltage regulation especially important for weak grids Reactive power grid support is an ancillary service that will be paid by TSO in the future making it more attractive Marco Liserre [email protected] Control of DPGS Under Grid Faults LVRT with DFIG. Stability issue DFIG stability and reactive power Conventional DFIG rotor current control based on synchronous d-q reference frames is prone to oscillations when reactive power is fed to grid from the rotor side under fault conditions The generator will be unstable if: v RS ird 2 irq Lmo LS o This is only twice the rotor current that is required for operation at unity power factor In voltage dips the stability limit is further lowered However, generators that used Direct Torque Control (DTC) instead of vector current control turned out to be reasonable stable allowing the rated reactive current production down to very low grid voltages Marco Liserre [email protected] Control of DPGS Under Grid Faults LVRT with DFIG example Control of reactive power injection on the synchronous d-q frame Stable, but dampnig is low Slow decay of the dc component Slow Source: ABB Marco Liserre [email protected] Control of DPGS Under Grid Faults LVRT with DFIG example Control of reactive power injection using DTC active crow bar Very stable dc voltage Fast decay of the dc component Fast 100ms Source: ABB Marco Liserre [email protected] Control of DPGS Under Grid Faults LVRT with DFIG example Control of reactive power injection using DTC active crow bar Boost, voltage level without reactive injection Fast reactive power support Marco Liserre Source: ABB [email protected] Control of DPGS Under Grid Faults LVRT with Variable Speed WT Two technologies: Full-scale converter with SG, PMSG or IG Double-fed IG Both are capable of injecting reactive power to the extent of the kVA rating Both are capable of quickly reduce the generated power (vector control or DTC of generator, chopper) Both can reduce the aerodynamic torque after fault by pitching control (few seconds) Full-scale converter can control faster the grid currents being directly connected Marco Liserre [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault ∙ Power oscillations occurs due to negative sequence ∙ These translates into dc voltage oscillations trip ∙ The aim of the control is to reduce these oscillations and inject positive sequence reactive power ∙ The following additional control issues are necessary to be addressed to the conventional control structure: ▫ Positive/negative sequence calculation ◦ ◦ ▫ Current control that can handle both sequences ◦ ◦ ▫ Marco Liserre Dual synchronous (PI-dq) reference frame Single stationary reference frame (P+Resonant) Current reference calculator to address a specific control strategy like: ◦ ◦ ◦ ▫ in synchronous dq frame (LP.BP, Notch filtering) In stationary frame with (DSC, SOGI-FLL) minimization of dc voltage oscillations maximizing the transferred power minimization of oscillations in the instantaneous P&Q Power reference calculator (P* and Q*) [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – instantaneous power theory Grid voltage under generic conditions: V V Vh V n 1 va va vh v v v b b h vc vc vh sin( n t n ) sin( n t n ) n n n n 2 2 V sin( n t 3 ) V sin( n t 3 ) sin( n t n 23 ) sin( n t n 23 ) vh V 0n sin( n t 0 n ) n 1 Generic current injected by the three-phase grid inverter: i a I ib ic Marco Liserre n 1 sin( n t n ) sin( n t n ) n n n n 2 2 I sin( t ) I sin( t ) n n 3 3 sin( t n 2 ) sin( n t n 23 ) n 3 [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – instantaneous power theory Instantaneous active power delivered to the grid: p3 3 V n I n cos( n n ) V n I n cos( n n ) 2 n1 p3 3 V m I n cos((m n )t m n ) 2 m1 n1 m n m n V I cos((m n )t m n ) m 1 n 1 m n V m I n cos((m n )t m n ) m 1 n 1 m n m n V I cos((m n )t ) . m 1 n 1 Marco Liserre [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – instantaneous power theory Instantaneous reactive power delivered to the grid: q3 3 V n I n sin( n n ) V n I n sin( n n ) 2 n1 q3 3 V m I n sin((m n )t m n ) 2 m1 n1 m n m n V I sin((m n )t m n ) m 1 n 1 m n V m I n sin((m n )t m n ) m 1 n 1 m n m n V I sin((m n )t ) . m 1 n 1 Marco Liserre [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – instantaneous power theory ∙ Experimental evaluation Source: [4,5] Simulated fault Pgreen DC power source vdc PWM inverter v pwm y v, i LC filter Grid simulator Transformer Local load s SVM modulator v* D Sensors i v Current controller i* Current reference .- Grid Code Requirements .- Fault strategy .- MPPT .- Protection Marco Liserre P* Q* v ,v Sequence detector Power reference POWER PROCESSOR CONTROL [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – P control laws Instantaneous Active Reactive Control (IARC) ∙Basis: p vi ∙Reference constrains: P v i*p ∙Control law: ; g P v 2 5 i [A] i gv * p 10 0 -5 • Distorted and unbalanced current • Instantaneous power perfectly controlled • Overcurrent trip risk Marco Liserre -10 2 p, q [kW, kvar] ∙Comments: q v i*p 0 1 p q 0 -1 [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – P&Q control laws Instantaneous Active Reactive Control (IARC) ∙Basis: 400 ∙Reference constrains: 0 -200 -400 Q v i 10 * q 5 i [A] P vi * p 200 v [V] q v i v i p vi v is a 90-degrees leaded version of the voltage vector v ∙Control law: 0 -5 i*q b v Marco Liserre ; g P 2 v ; b Q v -10 2 2 p, q [kW, kvar] i*p g v 1 0 -1 p q [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – P control laws Positive- Negative-Sequence Compensation (PNSC) ∙Basis: p v v i i 400 200 * p * p v i v i P v [V] ∙Reference constrains: * p * p v i v i 0 0 -200 -400 ∙Control law: ; 5 P g v 2 v i [A] i*p g v v 10 0 2 -5 -10 ∙Comments: v i*p v i*p v i*p v i*p 2 p, q [kW, kvar] q vi * p 1 p q 0 -1 0 q • Sinusoidal unbalanced currents • Oscillations in the instantaneous reactive power • Maximun value of current is a known function of power and sag deep Marco Liserre [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – P&Q control laws Positive- Negative-Sequence Compensation (PNSC) 400 p v v i i v [V] 200 q v v i i -200 -400 10 * p * p v i v i P v i v i 0 v i*q v i*q Q v i*q v i*q 0 * p * p i*q b v v Marco Liserre 0 -5 -10 ∙Control law: i*p g v v 5 i [A] ∙Reference constrains: 0 2 ; P g v ; 2 v 2 v 2 v 1 0 -1 Q b p, q [kW, kvar] ∙Basis: p q 2 [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – P control laws Balanced Positive Sequence (BPS) p v v i 400 200 v [V] ∙Basis: ∙Reference constrains: v i * p v P v i * p 0 v i * p p 0 -200 -400 ∙Control law: 10 ; G P v 0 2 ∙Comments: q v i*p v i*p v i*p -5 -10 2 p, q [kW, kvar] i G v * p i [A] 5 1 p q 0 0 q -1 • Sinusoidal balanced current • Oscillations in both instantaneous powers • Maximun value of current is easily calculated Marco Liserre [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – P&Q control laws Balanced Positive Sequence (BPS) 400 p v v i v [V] 200 q v v i -200 -400 -40 10 ∙Reference constrains: v i*p P v i * q Q v v v i*p 0 v i * q 0 0 v i*p p * q v i q i B v Marco Liserre ; G P v 0 10 20 30 40 50 60 20 30 40 50 60 20 30 40 50 60 t [ms] -10 -40 -30 -20 -10 0 10 t [ms] i G v * q -10 -5 ; B 2 2 1 0 -1 -40 Q v p, q [kW, kvar] -20 0 ∙Control law: * p -30 5 i [A] ∙Basis: p q -30 -20 -10 0 10 t [ms] 2 [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault e ow uip Eq line Current Reference Calculation – P control laws ∙ IARC allows full control of the power delivered to the grid at the expense of v v the current quality. This strategy should be used when the control of the dc-bus voltage is the main issue. v v ∙ PNSC cancels instantaneous active power oscillation keeping unbalanced i p ( IARC ) sinusoidal currents. However no full v i p ( BPS ) power can be delivered since q( BPS ) i p ( PNSC ) oscillations exist in the reactive power q( PNSC ) and the maximum value of the v v injected current should be limited. ∙ BPS achieves sinusoidal balanced currents and gives rise to oscillations T in the instantaneous powers. This 1 x y x y dt 0 strategy should be used when current T 0 quality is the main issue. r Marco Liserre [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Power Reference Calculation Active Power Delivery - APD PBS ∙Goal: deliver max. available P and no Q without exceeding nominal current ∙Control law: P 3 V I N * Q 0 * IARC ∙Comments: ∙V+-RMS value of pos seq. voltage ∙IN – RMS value of nominal current PNSC • PBS – sinusoidal, balanced, limited. Oscillations in P and Q • IARC – distorted hard to predict peak. No P and Q oscillations •PNSC – unbalanced,sinusoidal. Oscillations in Q only Source: [6] Marco Liserre [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Power Reference Calculation Grid Voltage Supporting - GVS PBS ∙Goal: deliver max. available Q and no P without exceeding nominal current ∙Control law: P 0 * Q 3 V I N * IARC ∙Comments: ∙V+-RMS value of pos seq. voltage ∙IN – RMS value of nominal current ∙GVS: ON when V+ <0.9VN, OFF V+ ≈ VN • PBS – sinusoidal, balanced, limited. Oscillations in P and Q • IARC – distorted hard to predict peak. No P and Q oscillations •PNSC – unbalanced,sinusoidal. Oscillations in P only Marco Liserre PNSC Source: [6] [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Power Reference Calculation Active Reactive Power Delivering- ARPD PBS ∙Goal: deliver P and Q according to E.ON guidlines. Deliver P & supporting grid with Q ∙Control law: P * 3 V I N Q 2 * 2 , V Q 6 V I N 1 VN * IARC ∙Comments: ∙V+-RMS value of pos seq. voltage ∙IN, VN – RMS value of nominal current(voltage ∙ARPD: ON when V+ <0.9VN, OFF V+ ≈ VN • PBS – sinusoidal, balanced, limited. Oscillations in P and Q • IARC – distorted hard to predict peak. No P and Q oscillations •PNSC – unbalanced,sinusoidal. Oscillations in P and Q Marco Liserre PNSC Source: [6] [email protected] Control of DPGS Under Grid Faults Experimental results Analyzing the reference current expressions it is possible to determine the maximum power to be delivered to the grid during unbalanced voltage sages without reaching the overcurrent limit in any of the phases at any time PNSC Marco Liserre IARC BPS [email protected] Control of DPGS Under Grid Faults P&Q Control Strategies Under Grid Fault Current Reference Calculation – P&Q control laws Sinusoidal balanced currents: High quality in the injected current Maximum current at the ac-side can be readily predicted Oscillations in injected active power moderated dc-voltage oscillations Sinusoidal unbalanced currents: Acceptable quality in the injected current, no high order harmonics. Maximum current at the ac-side can be predicted Depending on the selected strategy, active power oscillations can be cancelled when no reactive power is injected no dc-voltage oscillations Nonsinusoidal currents: Poor current quality in the injected current Maximum value of the injected currents can not be easily predicted No oscillations in the injected active power independently of the reactive power no dc-voltage oscillations Possible fault-tolerant scenario Marco Liserre Unbalance small (<2-5%) medium (5-15%) high (>15 %) Output power capability Full Full Partial Input currents Should be sinusoidal & balanced May be sinusoidal but unbalanced May be not sinusoidal & unbalanced [email protected] Control of DPGS Under Grid Faults Conclusions Marco Liserre LVRTcan be implemented with resistive crow-bar and reactive power injection to existing wind farms. Variable speed wind turbines can do LVRTas they are capable of quickly reduce the torque of the generator and inject reactive power. The capability of the system to allow ride-through grid disturbances is strictly related to the inverter rating (maximum voltage and current at the ac-side) as well as its dc-link energy storage (maximum/minimum voltage at the dc-side). A robust control strategy for the grid inverter should ride-through transient faults avoiding under-/over-voltage trip in the dc-bus and output over-currents due to distortion and imbalance Reactive power injection is helping the grid voltage to recover Several strategies for reference current generations. Overcurrent and under/over voltage should be avoided to ride-through transient faults Different power delivery strategies under fault are also presented New grid codes with even more sestrictions during grid faults are expected in the future an more advanced control strategies have to be developed. [email protected] Control of DPGS Under Grid Faults Bibliography 1. S. Seman, J. Niiranen, S. Kanerva, and A. Arkkio, ”Analysis of a 1.7 MVA Doubly Fed Wind-Power Induction Generator during Power Systems Disturbances”, 2. Molinas, Marta; Suul, Jon Are; Undeland, Tore , “Improved grid interface of induction generators for renewable energy by use of STATCOM” , International Conference on Clean Electrical Power, 2007. ICCEP '07., Vol., Iss., 21-23 May 2007 Pages:215-222 3. G. Saccomando, J. Svensson, “Transient Operation of Grid-connected Voltage Source Converter Under Unbalanced Voltage Conditions,” 2001 IEEE Industry Applications Conference, 36th IAS Annual Meeting, September 30 - October 5, 2001, Chicago, USA, 2001. 4. Rodriguez, P.; Timbus, A. V.; Teodorescu, R.; Liserre, M.; Blaagjerg, F, “Independent PQ Control for Distributed Power Generation Systems under Grid Faults”, IEEE Industrial Electronics, IECON 2006 - 32nd Annual Conference on, Vol., Iss., Nov. 2006 Pages:5185-5190 5. P. Rodriguez, A.V. Timbus, R. Teodorescu, M. Liserre, F. Blaabjerg, “Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults”. Industrial Electronics, IEEE Transactions on Volume 54, Issue 5, Oct. 2007 Page(s):2583 – 2592 6. P. Rodriguez, Luna, A., Teodoresc R., Blaabjerg F., - “Fault Ride-through Capability Implementation in Wind Turbine Converters Using a Decoupled Double Synchronous Reference Frame PLL” – Proceedings of EPE 2007, 2-5 sep. 2007 (on CD) 7. A. Luna, P. Rodriguez, R. Teodorescu and F. Blaabjerg, “Low voltage ride through strategies for SCIG wind turbines in distributed power generation systems,” in Proc. IEEE Power Electron. Conf. PESC 2008. Jun. 2008 8. F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of Control and Grid Synchronization for Distributed Power Generation Systems”, IEEE Trans. on Ind. Electronics, Vol. 53, Oct. 2006 Page(s):1398 – 1409 9. P. Rodríguez, A. Timbus, R. Teodorescu, M. Liserre and F. Blaabjerg, “Reactive Power Control for Improving Wind Turbine Systems Behaviour under Grid Faults,” IEEE Trans. on Power Electronics, Aug. 2008 (in press) Marco Liserre [email protected] Control of DPGS Under Grid Faults Acknowledgment Part of the material is or was included in the present and/or past editions of the “Industrial/Ph.D. Course in Power Electronics for Renewable Energy Systems – in theory and practice” Speakers: R. Teodorescu, P. Rodriguez, M. Liserre, J. M. Guerrero, Place: Aalborg University, Denmark The course is held twice (May and November) every year Marco Liserre [email protected]