Análisis Cuantitativo

The reference signals used by the dual mode controller are different depending on the active mode of operation. In grid connected mode, the controller is operating in current control mode and requires an input of the desired power (Pref, Qref) to

calculate the desired reference currents (Ipdref, Ipqref). While in island mode, the

controller is operating in voltage control mode and requires an input of the desired voltage reference (Vpdref, Vpqref) and the reference frequency (ωref). The operation of

the grid connected current controller can be found in Section 6.6.

6.9.1 Reference Currents

With the assumption that both ipd and ipq can quickly follow their corresponding

reference signals (ipdref, ipqref), it follows that the power (P, Q) similarly follows the

desired power reference (Pref, Qref). Therefore, the output current (ipdq) can be

expressed with respect to the output power when Vpq = 0 as:

M/% =_{3 E/%}2 ! (6.37)

M/& =_{3 E/%}−2 # (6.38)

When a PLL is in steady state Vpq = 0, which shows the real and reactive power

components are proportional to id and iq. The power equations from Appendix A

can be simplified to:

! = 3_{2 E/% M/%} (6.39)

# = −3_{2 E/% M/&} (6.40)

To consider fully the real and reactive power references the corresponding reference currents can be found [6.15]:

%-B0C = E/%_{+ E/& (6.41)}

M/% ,-+ = 3 2 !,-+ E/% − Q,-+ E/&%-B0C (6.42) M/& ,-+ = 3 2 !,-+ E/& + Q,-+ E/%%-B0C (6.43)

With the assumption that the system performs well and quickly to track the references, the following substitutions can be made: idref for id, iqref for iq, Pref for P

and Qref for Q for the dq power equations from Appendix A. As well, vpd can be

treated as a DC variable in steady state. The resulting reference currents can be determined in terms of the commanded reference power delivered by the inverter as shown in Figure 6-7 giving the reference current as:

M/% ,-+ = _{3 E/%}2 !,-+ (6.44)

M/& ,-+ = _{3 E/%}−2 #,-+ (6.45)

6.9.2 References for Island Operation

The basic idea for the generation of island operation references is to recreate the behavior of a synchronous generator driven by a turbine regulated through a speed governor. The governor reacts to changes in frequency. A decrease in frequency results in an increase in real power production. An exciter on the synchronous generator is used for voltage regulation. A decrease in voltage amplitude results in a reactive power increase. For voltage control, a dq frame rotating at ω _{speed is} orientated such that the d-axis is aligned with the grid voltage vector. The output power

from the inverter is required to follow the load for a fixed voltage level. Within the Island Voltage Control VSC (see Section 6.7), the controller generates a desired current (idref and iqref). While in island operation, the frequency is fixed except

during the re-synchronization period when transitioning from island operation back to grid connected operation is occurring.

6.10 References

[6.1] M. Kazmierkowski, L. Malesani, Current Control Techniques for Three-Phase Voltage-Source PWM Converters: A Survey, IEEE Trans. on Industrial Electronics, Vol. 45, No. 5, October 1998, pp. 691-703

[6.2] M. Marwali, A. Keyhani, Control of Distributed Generation Systems – Part I: Voltage and Currents Control, IEEE Trans. on Power Electronics, Vol. 19, No. 6, November 2004, pp. 1541-1550

[6.3] M. Marwali, J. Jung, A. Keyhani, Control of Distributed Generation Systems – Part II: Load Sharing Control, IEEE Trans. on Power Electronics, Vol. 19, No. 6, November 2004, pp. 1551-1561

[6.4] M. Prodanovic, T. Green, Control and Filter Design of Three-Phase Inverters for High Power Quality Grid Connection, IEEE Trans. on Power Electronics, Vol. 18, No. 1, January 2003, pp. 373-380

[6.5] R. Teodorescu, F. Blaabjerg, Flexible Control of Small Wind Turbines with Grid Failure Detection Operating in Stand-Alone and Grid Connected Mode, IEEE Trans. on Power Electronics, Vol. 19, No. 5, Sept. 2004, pp. 1323-1332

[6.6] P. Loh, D. Holmes, Analysis of Multi-loop Control Strategies of LC/CL/LCL-Filtered Voltage-Source and Current-Source Inverters, IEEE Trans. on Industrial Applications, Vol. 41, No. 2, March/April 2005, pp. 644-654

[6.7] V. Blasko, V. Kaura, A Novel Control to Actively Damp Resonance in Input LC Filter of a Three-Phase Voltage Source Converter, IEEE Trans. on Industry Applications, Vol. 33, No. 2, March/April 1997, pp. 545-551

[6.8] N. Pogaku, M. Prodanovic, T. Green, Modeling, Analysis and Testing of Autonomous Operation of an Inverter-Based Microgrid, IEEE Trans. on Power Electronics, Vol. 22, No. 2, March 2007, pp. 613-625

[6.9] J. Lopes, C. Moreira, A. Madureira, Defining Control Strategies for MicroGrids Island Operation, IEEE Trans. on Power Systems, Vol. 21, No. 2, May 2006, pp. 916-924 [6.10] A. Yazdani, R. Iravani, A Unified Dynamic Model and Control for the Voltage-

Sourced Converter Under Unbalanced Grid Conditions, IEEE Trans. on Power Delivery, Vol. 21, No. 3, July 2006, pp. 1620-1629

[6.11] O. Wasynczuk, S. Sudhoff, T. Tran, D. Clayton, H. Hegner, A Voltage Control Strategy for Current-Regulated PWM Inverters, IEEE Trans. on Power Electronics, Vol. 11, No. 1, January 1996, pp. 7-15

[6.12] K. Ahmed, S. Finney, B. Williams, Passive Filter Design for Three-Phase Inverter Interfacing in Distributed Generation, Journal Electrical Power Quality and Utilization, Vo. XIII, No. 2, 2007

[6.13] A. Yazdani, R. Iravani, Voltage-Sourced Converts in Power Systems, Wiley IEEE Press, 2010

[6.14] L. Huy, R. Yacamini, Calculation of harmonic interference in HDVC system with unbalance, Inst. Elect. Eng. Int. Conf. AC and DC Transmission, Sept. 17-20 1991, pp 390-394

[6.15] Q. Yu, S. Round, L. Norum, T. Undeland, Dynamic Control of a Unified Power Flow Controller, IEEE Power Electronics Specialists Conference, 1996, Vol. 1, pp 508 – 514

[6.16] A. Yazdani, Control of An Islanded Distributed Energy Resource Unit with Load Compensating Feed-Forward, IEEE PES General Meeting, August 2008

7 TRANSITION BETWEEN MODES OF

OPERATION

One of the main difficulties of operating a dual state controller is the management of the transitions between connection states and controller modes of operation. In addition, control modes can also be affected by changes in control parameters and other operator actions. The principle concern is the transition from island operation to grid connected operation. In order to reconnect to the grid, the island system must meet strict voltage, frequency and phase requirements before the protection system will permit the island system to reconnect with the grid. As well, reference signals used by the island and grid connected controllers are different. Finally, the control algorithms themselves face the challenge of how one controller’s independent actions interact with the other controller since the algorithms in each controller are attempting to control the system to meet their independent control objectives.