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CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. I, NO. I, MARCH

36

2015

An Integrated Control and Protection System for Photovoltaic Microgrids

Laijun Chen,

Member, IEEE

and Shengwei Mei,

Abstract-The microgrid has shown to be a promising solution for the integration and management of intermittent renewable energy generation. This paper looks at critical issues surrounding microgrid control and protection. It proposes an integrated control and protection system with a hierarchical coordination control strategy consisting of a stand-alone operation mode, a grid-connected operation mode, and transitions between these two modes for a microgrid. To enhance the fault ride-through capability of the system, a comprehensive three-layer hierarchical protection system is also proposed, which fully adopts different protection schemes, such as relay protection, a hybrid energy storage system (HESS) regulation, and an emergency control. The effectiveness, feasibility, and practicality of the proposed systems are validated on a practical photovoltaic (PV) microgrid. This study is expected to provide some theoretical guidance and engineering construction experience for microgrids in general.

Index Terms-Control strategies, integrated protection, micro­

grid, operation modes.

I. INTRODUCTION HOTOV OLTAIC (PV ) technologies have received widespread attention in recent years owing to their ability to reduce fossil energy use and provide positive impacts to the environment. Photovoltaic generation in the form of distributed photovoltaic microgrids that are integrated into the power system rely on efficient use of solar energy [ 1], [2]. When compared to traditional distribution networks, photovoltaic microgrids are distinctly different in terms of their control strategies and protection methods [3], [4]. Specifically, when PV microgrids are being operated in isolated mode, improving peer-to-peer control strategies are considered as critical factors for supporting islanded microgrid operations [5]. In [6], the authors have presented a coordinated voltage/frequency (V /F) and active power and reactive power (PQ) control system for both islanded and grid-connected mode in a PV microgrid [6] that shows effective coordination between inverter V /F (or PQ) controls.

P

Manuscript received November 2S, 2014; revised January 2S, 201S and February 9, 201S; accepted February 13, 201S. Date of publication March 30, 201S; date of current version March 4, 201S. This work was supported by the National High Technology Research and Development of China 863 Program under Grant 2012AAOS0204, China, and the National Natural Science Foundation of China under Grant S132100S, S1207076. L. J. Chen is with State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China (e-mail: [email protected]). S. W. Mei is with State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China (e-mail: [email protected]). Digital Object Identifier 1O.1777S/CSEEJPES.20IS.0000S

Fellow, IEEE

Several other strategies have been proposed for seamless transfer between different microgrid operation modes [7]­ [9]. They include a seamless control methodology for a PV-diesel generator microgrid that can operate both in the grid-connected and islanded modes, and at the same time does not require any islanding detection mechanism [7]. Similarly, in [8], a control strategy has been proposed that contains the control state/reference compensation algorithm to effectively reduce the impact caused by microgrid operation mode transitions on critical loads and distributed generators (DGs). The control strategies mentioned above provide excellent solutions for microgrid operational control. However, these strategies are relatively independent having poor flexibility and weak expansibility, which may lead to collapse when the microgrid contains multiple distributed generators. Therefore, there is a need to integrate microgrid operational control technologies at steady and transient states in more practical ways. In addition to advanced control technologies, microgrids also require effective protection systems [ 10]-[ 12]. The maximum short-circuit current in a microgrid is generally limited to less than two times the rated current because of a large number of DGs configured with power electronic interface devices [l3]. Power flow and short-circuit capacity changes are significant under different microgrid operation modes. As a consequence, conventional protection methods in large-scale power grids are not able to effectively meet the needs of an inverter-dominated microgrid. A range of advanced methodologies is available in the literature for microgrid protection. They include a simple three-phase four-wire system with differential current and zero sequence current used to detect faults in a microgrid [ 13]; a protection scheme that uses both modes of operation for optimally setting direction over current relays [ 14]; as well as other protection schemes with voltage restraint algorithms or inverse time characteristics [ 15], [ 16]. All these systems can be adapted to address the frequent changes in a microgrid. However, the main focus of these aforementioned methods is on relay protection, and they tend to neglect regulation resources and means available in the microgrid, e.g., energy storage systems. As a result, there is a need to further explore and develop integrated microgrid protection systems. Many recently built microgrid laboratory systems have been based on the above mentioned control strategies and protection

2096-0042 © 201S CSEE

CHEN el al.: AN INTEGRATED CONTROL AND PROTECTION SYSTEM FOR PHOTOVOLTAIC MICROGRIDS

schemes [ 17]-[ 19]. However, microgrids that are integrated with advanced control and protection systems have received relatively less attention. This paper addresses key issues of PV microgrid control and protection. The rest of the paper is organized as follows: Section II presents the hierarchical coordination control sys­ tem. The integrated and layered protection system is described in detail in Section III. The field applications and test results are demonstrated in Section IV, and the conclusion is given in Section V. II. HIERARCHICAL CONTROL SYSTEM A. System Structure

Grid-connected and stand-alone operations are the two typical operation modes in a microgrid. The requirements of PV microgrid operation modes include: 1) The rnicrogrid voltage and frequency should be stable and the power flow should be balanced, so as to realize the independent operation in different modes; 2) The two modes can transfer smoothly from one to the other, which can help avoid transient surge in the microgrid. The proposed hierarchical coordination control architecture is shown in Fig. 1.

the peer-to-peer and the master-slave controls. The second part mainly refers to the idiographic control techniques, used in the local controllers, such as VIF control, PQ control, and droop control. The proposed structure is flexible so that different control strategies and basic techniques can be applied to realize different microgrid operation modes. This paper mainly discusses the master-slave control combined with VIF control and PQ control, which is demonstrated in the following sections. B.

Stand-alone Operation

When the microgrid operates in stand-alone mode, the Li­ battery energy storage system (BESS) is the main power source for providing stable voltage and frequency with the VIF control [20]. To improve the practical application of this system, the proposed VIF control block diagram is shown in Fig. 2.

SVPWM

generator

Self-synchron iz"tion Self-isolation

Layer I: Operation modes

Layer 2 :

{

; :�!��!s

s r

,:�:;�::{

Fig. 1.

I I Droop control II I I V/F control II contr I: : I I I Devi c e-level control --==-==-==-=.==.: =;::;.==-= '-.:...- L"7"o;::-_ P_Q_ _ _ _ _ O __I1 --' L _ r--------,

________

L...-_---'

Fig. 2.

Control structure of PY microgrid.

Fig. 1 shows the three-layered hierarchical control archi­ tecture of a PV rnicrogrid in which the microgrid operation modes are interchangeable based on the control strategies and some basic techniques. The microgrid control strategies mainly include two parts: the system-level control modes and the device-level control strategies. The first part includes

PWM pulse

YF control block diagram.

In Fig. 2, varef, Vbref, and Vcref denote the reference of the three-phase output voltage of BESS. Vd and Vq represent the d axis and q axis component of the measured three phase voltage ( va, Vb, and vc ) based on the dq coordinate transform, respectively. Vdref and Vqref are the d axis and q axis component of the reference voltage ( Varef, Vbref, and Vcref ),

respectively. The proposed VIF control mainly corresponds to layer 2 in Fig. 1, which is based on the coordinate transform and proportion integration (PI) regulation with some basic techniques, such as magnitude and phase angle calculation and space vector pulse width modulation (SVPWM). The proposed VIF control has good dynamic response though it only adopts voltage loops. Moreover, only two PI regulators are included in the VIF control, which is more effective than the traditional VIF control. C.

L-________________________________�

37

Grid-connected Operation

The BESS is controlled as a power buffer to provide power flow with PQ control when the microgrid operates in grid­ connected mode. Based on the fact that the current can be obtained from power and voltage, a simplified PQ control is proposed as in Fig. 3. In Fig. 3, the quantities of current reference are obtained by utilizing the instantaneous power theory, and the current loops are used to regulate the output value with PI regulators. In addition, the SVPWM technique is also adopted in generating

CSEE JOURNAL OF POWER AND ENERGY SYSTEMS. VOL. I. NO. I. MARCH

38

SVPWM generator

Fig. 3.

PWM pulse

PQ control block diagram.

PWM signals. The proposed PQ control strategy, as shown in Fig. 3, with two PI control units reduced and the decoupling control of active and reactive power realized, has more applicability in engineering and is equivalent to the traditional control methods in both power loops and current loops.

2015

Stage 3: Control modes transition. The VIF control mode is immediately switched to PQ control as the contactor is turned off. At this point, the transition from stand-alone mode to grid-connected mode is achieved. 2) Transition from Grid-connected Mode to Stand-alone Mode: In general, self-isolation control is utilized to ensure the smooth transfer from grid-connected mode to stand-alone mode. However, to describe briefly, this paper mainly takes the intentional islanding as an example. The stages are described as follows. Stage 1: Preparation. The power flow regulation is the first stage, which can be achieved by adjusting the power reference of the BESS. The corresponding control block diagram is depicted as in Fig. 5.

D. Operation Modes Transition 1) Transition from Stand-alone Mode to Grid-connected Mode: Self-synchronization control is adopted to realize the seamless transfer from stand-alone mode to grid-connected mode. The detailed stages are demonstrated as follows. Stage 1: Preparation-voltage regulation. The microgrid voltage is regulated through VIF control with the calculated voltage reference. Take the voltage magnitude control scheme as an example; the control block diagram is shown in Fig. 4.

PQ control

Fig. 5.

The power flow regulation control.

PPCC-ref Ppcc

Fig. 4.

{

I!rnie -!gridl :s; !lirnit ((!Vgridl -!Vrniel)/!Vgridl) IBrnie -Bgrid I :s; Blirnit

x

100% :s; Viirnit

( 1)

where !, V , and B represent the frequency, voltage magnitude, and voltage phase, respectively. The subscript "mic" and "grid" denote the microgrid and power distribution grid. The subscript "limit" means the threshold of those variables. Stage 2: Contactor states transition. After the voltage is synchronized with the main grid, the grid-connected contactor should be turned off by the BESS, in which an I/O output signal for turning off the contactor installed on the point of common coupling (PCC) is generated.

Qpcc

(P;ef' Q�ef)

{ Ppee

The voltage magnitude regulation control.

In Fig. 4, Vi-grid represents the power grid voltage magnitude of phase AlBIC, and Vi is the voltage of phase AlBIC of the microgrid, respectively. In the control block, only proportion control is utilized due to the fact that the voltage is of AC sinusoidal quantity. The regulated voltage CV;') is applied to VIF control. Then, the voltage magnitude, phase angle, and frequency are adjusted consistently with those of the main grid, as described in ( 1).

QPCC-ref

In Fig. 5, and means the expected active power and reactive power on the tie line, which are preset as zero. and are the measured power on the tie line. The reference power values put into the PQ control module are obtained from proportion control and difference calculation, which are described in Fig. 5. Then, the power flow on the tie-line should be regulated to satisfy (2). :s; Plirnit Qpee :s; Qlirnit

(2)

where llirnit and Qlirnit denote the threshold of the active power and reactive power, respectively. Stage 2: Contactor states transition. After performing the power flow regulation control, the BESS can rapidly turn on the grid-connected contactor by delivering an 110 input signal to open the PCC contactor. The power interaction between the microgrid and the power distribution grid is so little that there is very little transient surge when the contactor opens. Stage 3: Control modes transition. The PQ control mode should be quickly changed to the VIF control mode as the contactor is turned on. The transition from grid-connected mode to stand-alone mode is then completed. III. HIERARCHICAL PROTECTION S YSTEM A. System Structure

The microgrid has many advantages when compared to the bulk power grid because of its smaller scale and fewer feeders. More information can be acquired through reasonable configuration of measurements and establishment of a data

CHEN el al.: AN INTEGRATED CONTROL AND PROTECTION SYSTEM FOR PHOTOVOLTAIC MICROGRIDS

center, which provide possibilities for designing a compre­ hensive protection system. A hierarchical protection system is proposed as shown in Fig. 6. The proposed hierarchical protection system, consisting of relay protection, HESS regulation, and emergency control, is aimed at improving microgrid security. Compared to a traditional protection scheme having only relay protection, the hierarchical protection system not only clears the fault, but also deals with adjustments of HESS and load shedding. Therefore, it has many advantages. First, it is straightforward with three layers of hierarchical structure; second, many techniques, such as relay protection, hybrid energy storage system (HESS) regulation, and emergency control are integrated to improve the reliability of the PV microgrid. The details of the three­ layer hierarchical protection system is depicted in Section III, B-D.

Layer I: Relay protection

1-------1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

I I �_��i� __ J � _ �e��s__ J

1

n ---' t -o n -.-d F-u-n c-i '--

Methods

Fig. 6. B.

Structure of the hierarchical protection system.

Relay protection in PV microgrids can be classified into three categories: system protection, feeder protection, and element protection. Among these, feeder protection is a re­ search priority for PV microgrid protection. In general, the microgrid feeders can be divided into three types where the feeder contains only either the source or the load, or the feeder contains both source and load. Different relay protection schemes should be applied on different feeders due to their different characteristics. 1) Feeder Only Contains Source: For this kind of feeder, the power flows from the feeder to the bus. It could be determined that the short circuit fault occurs in the feeder only when the bus voltage and feeder current meet the following inequality [2 1]:

Uk cos(arg-. h

+

0:

)