1. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@poliba.it

2. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Agenda Definition of ancillary services Active and reactive capability of DPGS Power Flow through a line Droop Control Services at Load Level Services at Electric Power System (EPS) Level

3. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org The IEEE 1547.3 IEEE Guide for Monitoring, Information Exchange, and Control of Distributed Resources Interconnected with Electric Power Systems defines ancillary services only those provided by DPGS at the Electric Power Systems Area They are: load regulation, energy losses, spinning and non-spinning reserve, reactive supply. However future ancillary services may include also power quality enhancement Moreover they can also be defined at Load Level (e.g. UPS functionality) Ancillary services contribute to a systemic approach to the management of the new power system characterized by an higher inflow from DPGS Anyway ancillary services could facilitate the penetration of RES in power systems Definition of ancillary services

4. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Ancillary services are based on the specific characteristic of inverter-based DPGS that can be used to inject active, reactive power and harmonics in the grid (the second and third even if the energy source is not available) Some of the ancillary services are similar to those that traditional power plant provide to contribute to the safe and stable system operation However DPGS ancillary services are mainly considered at a distribution level hence part of the features of active distribution grid (smart grid) and not at transmission level (where traditional power plants are usually connected) Moreover when a DPGS is LV-connected the grid frequency and the grid voltage cannot be controlled independently since low-voltage distribution lines have a not negligible resistance Hence these issues can not be considered as a mere transposition of known concepts at a different level since they are involved in a wider change of the power system Definition of ancillary services

5. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Active and reactive capability of the inverter-based DPGS Active and reactive power management of the DPGS depends on the grid converter full power converterhalf power converter (DFIG)

6. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org In case of an inductive line The previous equations can be used for: controlling the power sharing among different systems feeding/absorbing power (parallel operation of inverters for UPS applications or micro-grid) Support of voltage profile and frequency in the EPS area where the DPGS is connected Support of the load by DPGS (UPS functionality) Power Flow through a line The equations are the basis of the droop control that is the most straightforward way of relating P,f and Q,V

7. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Using short-line model and complex phasors, the analysis below is valid for both single-phase and balanced three-phase systems. At the section A For a mainly inductive line Droop control technique: inductive line

8. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org The angle δ can be controlled regulating the active power P whereas the inverter voltage V A is controllable through the reactive power Q. Control of the frequency dynamically controls the power angle and, thus, the real power flow. Thus by adjusting P and Q independently, frequency and amplitude of the grid voltage are determined From another point of view to share the P and Q among several units working in parallel Droop control technique: inductive line Q 1 Q 2 V P 1 P 2 f

9. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Implementation of the droop characteristics V c, I c voltage and current of the converter I g grid current G p (s), G q (s) compensators transfer functions

10. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Droop control technique: RL line Distribution lines have a not negligible resistive nature, hence more complex formula have to be considered In case the aim is to control the active power injected by the DPGS and the reactive power exchanged with the grid the following equations can be used However they are dependent on the grid impedance nature

11. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Droop control technique: P/Q decoupling In order to remove from active and reactive powers the dependence of line impedance the following transformation is proposed: that leads to hence the control laws can be formulated as In these formula active and reactive powers are clearly related to the phase (frequency) and amplitude of the voltage how it was in the pure inductive case that express the reference amplitude and phase of the voltage produced by the DPGS to have the desired P and Q

12. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Implementation of the droop characteristics with P Q decoupling

13. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Stability and robustness analysis Root locus for 0.00002 < mp < 0.001 and mi=0.0002 Root locus for 0.000002 < mi < 0.0018 and mp=0.00006 Root locus diagram for grid inductance variations: 8.5 mH < Lg< 5000 mH Using the small-signal analysis it is possible to check stability margin and robustness respect to parameter variation such as the grid inductance

14. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Voltage support provided by the DPGS at load level

15. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org In normal conditions the shunt controller provides a current I C = I load In case of voltage dips it provides the active power required by the load and it injects the reactive power needed to stabilize the load voltage The amount of reactive power is inversely proportional to the grid impedance A large inductance will help in mitigating voltage sags although it is not recommendable during normal operation Voltage support provided by the DPGS at load level

16. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org PV system with shunt-connected multifunctional converter The voltage sags compensation requires a large-rated converter However the PV shunt-connected converter is already rated for supplying full power An inductance L g * of 0.1 pu is placed on the grid line (inductive line) It is possible to control the voltage frequency and amplitude adjusting active and reactive power independently.

17. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org The droop controller provides the reference for the voltage control Multifunctional PV inverter

18. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Voltage control The PV inverter is voltage controlled The current injection is controlled indirectly The voltage error is pre-processed by the repetitive controller (the periodic signal generator of the fundamental component and of the selected harmonics) The PI controller improves the stability of the system The voltage in the PCC is constant and equal to the desired value In presence of a voltage dip I g is forced to be phase-shifted by almost 90° with respect to the corresponding grid voltage the 3rd and the 5th harmonics are compensated

19. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Simulation results: grid normal conditions active and reactive power provided by the PV inverter inverter current I c (top), load current I load (middle), grid current I g (bottom).

20. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Simulation results: voltage sag of 0.15 pu active and reactive power provided by the PV inverter E = grid voltage I c = inverter current I g = grid current I load = load current

21. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Voltage harmonic mitigation Experimental results obtained in case of distorting load and without shunt converter: A grid voltage [300V/div], C load voltage [300V/div], 1 load current [10V/div]. Experimental results in case of distorting load and shunt converter connected to the grid: A) grid voltage [300V/div], C) load voltage [300V/div], 1) load current [10V/div]. Without compensation (Black bar), with compensation (white bar)

22. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Experimental setup: Politecnico di Bari

23. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Experimental results: voltage dip duration equal to 1.5 s grid voltage E (top) : voltage dip of 0.15 pu, load voltage V load (middle), grid current I g (bottom) without droop control

24. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org High wind condition + reactive power compensation High wind condition upgrade for 600 kW WT systems using 300 kW back-to-back converter Reactive power compensation at the PCC reduction in mechanical stress reactive power control without capacitor banks

25. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Voltage support provided by the DPGS at EPS area The reactive power injection by grid-connected systems can enhance the voltage profile The goal is to reduce the active power supplied by the low-voltage feeder, injecting reactive power to support the voltage amplitude decreasing the current and as a consequence the losses without reactive power injection with reactive power injection gridinvertergridinverter f (Hz)50 E (V)228 230 P (kW)24.5021.30 Q (kVar)n.a.0 16.5

26. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Active filter operation Only current is compensated The WTs are cleaning the line current Power Quality Enhancement provided by the DPGS at EPS area

27. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org Ancillary services are considered as those services that the DPGS can provide to the grid in order to behave more similarly to a traditional power plant However DPGS are connected at distribution network level, hence new problems and new possibilities arises Moreover DPGS are usually installed to meet demand of local loads hence they could be used to provide ancillary services also to the loads In general if the full power of the DPGS is managed by a PWM inverter, full control on active and reactive power and on the harmonics can be used to make DPGS contributing to the stabile and safe operation of the EPS area where it is connected and even to allow micro-grid operation Since all the ancillary services are connected to the management of the power flow in the grid hence the power flow theory and the consequent droop control are useful to guarantee the desired dynamic performances and robustness against grid impedance variation Conclusions

28. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org References 1.IEEE 1547.3, IEEE Guide for Monitoring, Information Exchange, and Control of Distributed Resources Interconnected with Electric Power Systems, 2007 2.Tsai-Fu Wu, Hung Shou Nien, Hui-Ming Hsieh, Chih-Lung Shen PV power Injection and Active Power Filtering with Amplitude-Clamping and Amplitude-Scaling Algorithms, IEEE Transactions on Industry Applications, vol. 43, no. 3, May/June 2007. 3.Josep M. Guerrero, José Matas, Luis García de Vicuña, Miguel Castilla, Jaume Miret, Wireless- Control Strategy for Parallel Operation of Distributed-Generation Inverters, IEEE Transactions on Industrial Electronics, vol.53, no.5, Oct. 2006, pp. 1461-1470. 4.Josep M. Guerrero, José Matas, Luis García de Vicuña, Miguel Castilla, Jaume Miret, Decentralized Control for Parallel Operation of Distributed Generation Inverters Using Resistive Output Impedance, IEEE Transactions on Industrial Electronics, vol.54, no.2, April 2007, pp. 994-1004. 5.K. De Brabandere, B. Bolsens, J. Van den Keybus, a. Woyte, J. Driesen, R. Belmans, A Voltage and Frequency Droop Control Method for Parallel Inverters, IEEE Transactions on Power Electronics,vol.22, no.4,July 2007, pp.1107-1115. 6.P. Wang, N. Jenkins, M.H.J. Bollen, Experimental investigation of voltage sag mitigation by an advanced static VAR compensator, IEEE Transactions on Power Delivery, Vol.13, No. 4, October 1998. 7.P. Mattavelli, F. Pinhabel Marafao, Repetitive-Based Control for Selective Harmonic Compensation in Active Power Filter, IEEE Transactions on Industrial Electronics, vol. 51, no. 5, October 2004, pp. 1018-1024.

29. Ancillary Services of Distributed Power Generation Systems Marco Liserre liserre@ieee.org References 8.R. A. Mastromauro, M. Liserre, T. Kerekes, A. DellAquila, A Voltage Controlled Grid Connected Photovoltaic System with Power Quality Conditioner Functionality, accepted for publication on IEEE Transactions on Industrial Electronics, forthcoming issue. 9.M. Bollen, Understanding Power Quality Problems: Voltage Sags and Interruptions; Piscataway, NJ: IEEE Press, 1999. 10.M. Routimo; M. Salo; H. Tuusa; Current sensorless control of a voltage-source active power filter, Applied Power Electronics Conference and Exposition, 2005. APEC 2005. Twentieth Annual IEEE, vol.3, Iss., 6-10, March 2005 pp. 1696- 1702. 11.R. R. Sawant and M. C. Chandorkar, Methods for multi-functional converter control in three- phase four-wire systems, IET Power Electron., vol. 2, no. 1, Jan. 2009, pp. 52-66. 12.S.-J. Lee, H. Kim, S.-K. Sul, F. Blaabjerg, A novel control algorithm for static series compensator by use of PQR instantaneous power theory, IEEE Trans. Ind. Electron., vol. 19, no. 3, May. 2004, pp.814-827. 13.J. M. Guerrero, L. García de Vicuña, J. Matas, M. Castilla, J. Miret, Output impedance Design of Parallel-Connected UPS Inverters With Wireless Load-Sharing Control, IEEE Trans. Ind. Electron., vol.52, no.4, Aug. 2005, pp. 1126-1135. 14.J. M. Guerrero, L. García de Vicuña, J. Matas, M. Castilla, Jaume Miret, A Wireless Controller to Enhance Dynamic Performance of Parallel Inverters in Distributed Generation Systems, IEEE Trans. Power Electron., vol. 19, no. 5, Sep. 2004, pp. 1205-1213. 15.J. C. Vasquez, R. A. Mastromauro, J. M. Guerrero, M. Liserre, Voltage Support Provided by a Droop-Controlled Multifunctional inverter, accepted to be published on IEEE Transactions on Industrial Electronics, forthcoming issue. 16.C.-C. Shen and C.-N. Lu, A voltage sag index compatibility between equipment and supply, IEEE Trans. On Power Delivery, vol. 22, no. 2, April 2007, pp. 996-1002.