Fuzzy-PI Controllers for Grid-Connected and Islanded Operation of DG in a Microgrid

Fuzzy-PI Controllers for Grid-Connected and Islanded Operation of DG in a Microgrid

Sangeetha R Nair

Department of EEE Saintgits College of Engineering

Jaison Cherian

Department of EEE Saintgits College of Engineering

Abstract

Islanding describes the condition in which a microgrid or a portion of the power grid, which consists of a load and a distributed generation (DG) system, is isolated from the remainder of the utility system. In this situation, it is important for the microgrid to continue to provide adequate power to the load. To demonstrate the operation of microgrid in grid connected mode and islanded mode, a simulink model has been designed with necessary parameters by connecting with the main grid allowing the sharing of different loads with reference to grid connection and disconnection. An islanding detection algorithm has been used to act as a switch between the two controllers so that the system operates under intentional islanded mode. This paper also proposes an algorithm of synchronization for grid reconnection. In addition, fuzzy logic controller and SVPWM have been used to reduce the THD of the inverter output.

Index terms — Distributed generation (DG), grid- connected operation, intentional-islanding operation, islanding detection, synchronization.

1. INTRODUCTION

   Distributed generation is now becoming a popular power scenario in a de-regulated environment. Integration of distributed generation and incorporation of controllers lead conventional power network to operate as active power networks. Under disturbances these power network split and if a split part contains generators and loads and if the load demand can be matched with supply a power island is established [1].

   Islanding is a condition in which a microgrid or a portion of the power grid, which contains both load and distributed generation, is isolated from the remainder of the utility system and continues to operate.

   The disconnection of the DG once it is islanded is required by the IEEE Std. 929-2000 [2] and by the IEEE Std. 1547-2003 [3]. With the increasing competition among the power companies to secure more and more customers, the pressure to maintain a high degree of uninterrupted power service quality and reliability is felt by the utility companies [4]. Thus, in a deregulated market environment, current practices of disconnecting the DG following a disturbance will no longer be a practical or reliable solution.

   Figure 1. Schematic diagram of the grid-connected

   inverter system

   During the grid-connected operation, each DG system is usually operated to provide or inject preset power to the grid, which is the current control mode in stiff synchronization with the grid [5], [6]. When the microgrid is cut off from the main grid, each DG system has to detect this islanding situation and has to be switched to a voltage control mode to provide constant voltage to the local sensitive loads [7], [8]. This paper describes a control strategy that is used to implement grid-connected and intentional-islanding

   operations of microgrids. Specifically, this paper proposes an islanding detection algorithm [9] for identifying the islanded condition and an algorithm for synchronization for grid reconnection [9]. For reducing the THD of the inverter output fuzzy logic controller [10], [11] and SVPWM has also been used.

2. ISLANDING DETECTION ALGORITHM

   Islanding is a condition where the DG remains operative in the distribution system with utility disconnected. In the past years several islanding detection methods have been proposed and the detection methods can be categorized into two main groups: passive and active method. Passive method depends upon measuring system parameters and thresholds are set to these parameters to differentiate between an islanding and a non islanding condition. Active methods directly interact with the power system operation by introducing perturbation in the inverter output. But the disadvantage with active method is that the harmonic distortion introduced is more.

   Figure 2. Islanding detection Algorithm

   The islanding detection algorithm is used to act as a switch between the two controllers so that the algorithm effectively detects islanding situation and is able to switch between the two controllers.

3. CONTROLLER

   Figure 1 shows the main circuit topology. This

   point of common coupling (PCC) before and after the grid is disconnected.

   Under normal operation, each DG system in the microgrid usually works in a constant current control mode in order to provide a preset power to the main grid. When the microgrid is cut off from the main grid, each DG inverter system must detect this islanding situation and must switch to a voltage control mode. In this mode, the microgrid will provide a constant voltage to the local load.

   1. Grid-Connected Operation Mode

      For grid-connected operation, the controller shown in Figure 3 is designed to supply a constant current output [14]. A phaselocked loop (PLL) is used to determine the frequency and angle reference of the PCC [15], [16].

      Figure 3. Block diagram for current controller for grid connected mode

      To simplify the design and operation of the controller, the control of the system is designed in a synchronous reference frame. Figure 4 shows this control topology employing synchronous frame current control.

      Figure 4. Block diagram of current

      controlled inverter

      The inverter currents are transformed into a synchronous frame by Parks transformation (1) and regulated in dc quantity is fed back and compared with

      system consists of the microsource that is represented

      the reference currents I

      DQ

      ref. This generates a current

      by the dc source, the conversion unit which performs the interface function between the dc bus and the three-phase ac world, and the LCL filter that transports and distributes the energy to the end use and the load [12], [13]. The controller presented provides a constant DG output and maintains the voltage at the

      error that is passed to the current regulator (FUZZY controller) to generate the voltage references for the

      inverter. In order to get a good dynamic response, V

      DQ

      is fed forward. This is done because the terminal

      voltage of the inverter is treated as a disturbance, and the feedforward is used to compensate for it [17].

      The voltage references in dc quantities VDQ ref are transformed into a stationary frame by the inverse of Parks transformation (2) and are utilized as command voltages in generating high frequency pulsewidth- modulated voltages.

      acts as the voltage reference signal that is fed to the sinusoidal pulsewidth modulator to generate the high frequency gating signals for driving the three-phase voltage source inverter. The current loop is included to stabilize the system and to improve the system dynamic response by rapidly compensating for near- future variations in the load voltages. In order to get a

      cos cos + 2 3 cos 2 3

      2 sin sin + 2 sin 2

      good dynamic response, V is fed forward. This is

      DQ

      done because the terminal voltage of the inverter is

      = 3

      3 3 treated as a disturbance, and the feedforward is used to

      1

      1 1

      compensate for it [17].

      2 2

      2

   2. Synchronization for grid reconnection

(1)

Where = and is the frequency of the electric system

cos sin 1 2

When the grid-disconnection cause disappears, the transition from islanded to grid-connected mode can be started. To avoid hard transients in the reconnection, the DG has to be synchronized with the grid voltage [18]-[20]. The DG is operated in the synchronous island mode until both systems are

2 2

2 1

synchronized. Once the voltage in the DG is

3

3

=

cos

3 sin

2

3 2

2 1

synchronized with the utility voltage, the DG is reconnected to the grid, and the controller will pass

cos +

3 sin +

3 2

from the voltage to the current control mode. This

2

0

3.1. Grid disconnected mode

The voltage closed-loop control for intentional- islanding operation is shown in Figure. 5. The control works as voltage regulation through current compensation. The controller uses voltage compensators to generate current references for current regulation.

synchronization is achieved by implementing the following algorithm.

1. Assume that the phase difference between the grid and inverter voltages is given by

   = (3)

2. In order to obtain the information of , two sets of voltage values are used

= + +

= 3 cos (4)

2

= + +

= 3 cos + 3 sin (5) 4

Using the variables k and g, sin can be found as

4 + 2

sin = 3 3

3

(6)

Figure 5. Block diagram of the voltage controller for grid disconnected operation

Figure 6 shows how sin is used to obtain the new phase angle for which the grid and inverter voltages are synchronized.

As shown, the load voltages (V

D

and V ) are forced

Q

to track its reference by using a PI compensator (voltage regulator). The outputs of this compensator

(I ref and I ref) are compared with the load current

D Q

(I and I ), and the error is fed to a current regulator

D Q

(PI controller). The output of the current compensator

Figure 6. Synchronization controller

1. SIMULATION RESULTS

   The converter fed microgrid consisting of a dc source, LCL filter and RLC load is modeled and analyzed in MATLAB/SIMULINK. This is shown in Figure 7. The RLC load is modeled so as to consume

   Voltage

   Voltage

   125kW active power and zero reactive power. The system is tested under the following conditions.

   1. Output frequency: 60 Hz

   2. Filter inductor L1: 0.4mH

   3. Filter inductor L2: 0.4mH

   4. Filter capacitor C: 2mF

   5. DC voltage: 600 V

   6. Breaker operation: [0.4 0.7]

   7. Output capacity: 125 kW

      S1

      -T- Vabc_PV

      S2

      Time(s)

      Vabc_PV

      S3

      Figure 9. Inverter voltage

      -T-

      Iabc_PV

      -T-

      Vabc_S

      Iabc_PV

      S4

      S5

      Vabc_S

      S6

      PWM

      Discrete, Ts = 1e-006 s.

      Discrete, Ts = 1e-006 s.

      A a A A a

      B1

      A A A

      S1

      1. A a

         m S

         m S

         g D

         g D

         m S

         m S

         g D

         g D

         S3 S5

         PV module

         m S

         m S

         g D

         g D

         Current (A)

         Current (A)

         +1

         1. b B

         2. c C

            Three-Phase Source B3

            B bC c

            Breaker

      2. B B

         C

         C

         Bb

         Bb

         c

         c

      3. C C L1

      1. B b

      2. C c

      L2 B2

      A B C

      A B C

      g D

      g D

      g D

      g D

      g D

      g D

      -1

      Aa

      Aa

      B4

      MESURMENT

      A B C

      A B C

      Three-Phase Series RLC Load

      A B C

      A B C

      Three-Phase Parallel RLC Brancp

      m S

      m S

      m S

      m S

      m S

      m S

      S2 S4 S6

      Figure 7. Overall simulated system

      The RLC load was adjusted to be resonant at 60 Hz and to consume 125kW. The DG system was designed to supply 125 kW active power and zero reactive power. The system was initially operated in grid

      Figure 10. Inverter current

      Time(s)

      connected mode. The grid was disconnected at 0.4 sec and this event was detected by the islanding detection algorithm at 0.41 sec. The grid is again reconnected at

      0.7 sec. Let us first consider the case when there is no voltage controller.

      CASE 1: WITH CURRENT CONTROLLER ALONE AND NO VOLTAGE CONTROLLER

      During the grid connected mode, the current controller supplies the load. Since we are analyzing the case with no voltage controller, from 0.41 sec to

      e

      ag t l

      Vo

      e

      ag t l

      Vo

      0.7 sec, the inverter ceases to energize the RLC load. The simulation results are as below.

      CASE 2: WITH BOTH THE CONTROLLERS

      The grid was disconnected at 0.4 sec and this event was detected by the islanding detection algorithm at

      0.41 sec. After 0.41 sec the control mode was changed from current to voltage controlled operation. The grid is again reconnected at 0.7 sec. The proposed synchronization algorithm successfully forces the voltage of the DG to track the voltage of the grid. After synchronization with the grid, the microgrid will supply 125 kW of active power into the grid. So from

      0.7 sec onwards the active power is again increased to

      Voltage

      Voltage

      250 kW. Once the synchronization is complete the controller was switched from voltage to current control mode. The simulation results are shown below.

      Figure 8. Grid voltage

      Time (s)

      Figure 11. Grid voltage

      Time (s)

      Voltage

      Voltage

      Time(s)

      Current (A)

      Current (A)

      Figure 12. Inverter voltage

      Time (s)

      Figure 13. Inverter current

2. FFT ANALYSIS OF THE INVERTER

   Figure 14. FFT analysis of the inverter

   FFT analysis of the inverter shows that the inverter has operated with a THD of 1.72 %.

3. CONCLUSION

Through this paper, the control, islanding detection, and reclosure algorithms have been proposed for the operation of grid-connected and islanding mode of DGs. A controller was designed with two interface controls: one for grid-connected operation and the other for islanding operation. An islanding-detection algorithm, which was responsible

for the switch between the two controllers, was presented. The simulation results showed that the detection algorithm can distinguish between islanding events. The reclosure algorithm causes the DG to resynchronize itself with the grid. The experimental results showed that the proposed control schemes are capable of maintaining the voltages within the standard permissible levels during grid-connected and islanding operation modes. In addition, it was shown that the use of fuzzy logic controller and SVPWM has reduced the THD of the inverter output.

REFERENCES

1. P. Fuangfoo, T. Meenual,W.-J. Lee, and C. Chompoo- inwai, PEA guidelines for impact study and operation of DG for islanding operation, IEEETrans. Ind. Appl., vol. 44, no. 5, pp. 13481353, Sep./Oct. 2008.

2. IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems, IEEE Std 929-2000, 2000, p. i. [3]IEEE Standard for Interconnecting Distributed Resources With Electric Power Systems, IEEE Std 1547-2003, 2003, pp. 0_116.

1. H. Zeineldin, E. F. El-Saadany, and M. M. A. Salama, Intentional islanding of distributed generation, in Proc. IEEE Power Eng. Soc. Gen. Meeting, 2005, vol. 2, pp. 14961502.

2. G. Franceschini, E. Lorenzani, C. Tassoni, and A. Bellini, Synchronous reference frame grid current control for single-phase photovoltaic converters, in Conf. Rec. IEEE IAS Annu. Meeting, 2008, pp. 17.

J. Selvaraj and N. A. Rahim, Multilevel inverter for grid-connected PV system employing digital PI controller, IEEE Trans. Ind. Electron.,vol. 56, no. 1, pp. 149158, Jan. 2009.

3. T. C. Green and M. Prodanovic, Control of inverter- based micro-grids, Electr. Power Syst. Res., vol. 77, no. 9, pp. 12041213, Jul. 2007.

4. M. E. Haque, M. Negnevitsky, and K. M. Muttaqi, A novel control strategy for a variable-speed wind turbine with a permanent-magnet synchronous generator, IEEE Trans. Ind. Appl., vol. 46, no. 1, pp. 331339, Jan./Feb. 2010.

5. Balaguer-Alvarez, Q. Lei, S. Yang, U. Supatti, and Z. Peng, Control For Grid-Connected and Intentional Islanding Operations Of Distributed Power Generation, IEEE Trans. Ind. Electron., 2011.

6. P.Pradeep Kumar, B.Kiran Babu, Power-Management Strategies for a Grid-Connected PV-FC Hybrid System by Using Fuzzy Logic Controller, international journal of modern engineering research (ijmer),2012.

7. Guohui Zeng, Xiubin Zhang, Junhao Ying,Changan Ji, A Novel Intelligent Fuzzy Controller for MPPT in Grid- connected Photovoltaic Systems, Proc. of the 5th WSEAS/IASME Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Tenerife, Spain, December 16- 18, 2005 (pp515-519)

8. K. Hemmes Towards multi-source multi-product and other integrated energy systems, Int. J. Integr. Energy Syst., vol. 1, pp. 115, 2009.

9. D. C. Patel, R. R. Sawant, and M. C. Chandorkar, Three-dimensional flux vector modulation of four-leg sine-

   wave output inverters, IEEE Trans. Ind. Electron., vol. 57, no. 4, pp. 12611269, Apr. 2010.

10. S. Alepuz, S. Busquets-Monge, J. Bordonau, J. A. Martinez-Velasco, C. A. Silva, J. Pontt, and J. Rodriguez, Control strategies based on symmetrical components for grid-connected converters under voltage dips, IEEE Trans. Ind. Electron., vol. 56, no. 6, pp. 21622173, Jun. 2009.

11. J. C. Vasquez, J. M. Guerrero, A. Luna, P. Rodríguez, and R. Teodorescu, Adaptive droop control applied to voltage-source inverters operating in grid-connected and islanded modes, IEEE Trans. Ind. Electron., vol. 56, no. 10, pp. 40884096, Oct. 2009.

12. Y. Srinivasa Rao and M. C. Chandorkar, Real-time electrical load emulator using optimal feedback control technique, IEEE Trans. Ind. Electron., vol. 57, no. 4, pp. 12171225, Apr. 2010.

13. J. M. Espi Huerta, J. Castello, J. R. Fischer, and R. Garcia-Gil, A synchronous reference frame robust predictive current control for threephase grid-connected inverters, IEEE Trans. Ind. Electron., vol. 57, no. 3, pp. 954962, Mar. 2010.

14. H. Gaztanaga, I. Etxeberria-Otadui, S. Bacha, and D. Roye, Real-time analysis of the control structure and management functions of a hybrid microgrid system, in Proc. IEEE 32nd IECON, 2006, pp. 51375142.

15. G. Iwanski and W. Koczara, DFIG-based power generation system with UPS function for variable-speed applications, IEEE Trans. Ind. Electron., vol. 55, no. 8, pp. 30473054, Aug. 2008.

16. D. N. Gaonkar, G. N. Pillai, and R. N. Patel, Seamless transfer of microturbine generation system operation between grid-connected and islanding modes, Electr. Power Compon. Syst., vol. 37, no. 2, pp. 174 188, Feb. 2009.