An Enhanced Control Strategy for the Stable Operation of Distributed Generation during Grid-connected and Islanded Mode

Received Apr 14, 2018 Revised May 20, 2018 Accepted May 29, 2018 The incident of islanding occurs while the distributed generator fed constant power into the grid after power flow on the source of essential utility has been intermittent. If islanding is not rapidly detected, it can reason serious security and harmful condition. This paper presents coordinated manage of solar photovoltaic unit through maximum power point tracking technique to make available voltage and frequency support for an islanding condition. In normal condition, structure works as constant current control mode subsequent to islanding the structure switched to voltage control mode. The projected technique is capable to discriminate involving an islanding and a non-islanding occurrence. In this paper phase locked loop detection technique designed for detecting islanding situation is conceded out. The authentication of proposed system is established efficiently using MATLAB/ Simulink environment. The simulated results exhibit that the islanding detection technique has zero nondetection zone property and be able to detect islanding within few seconds.


INTRODUCTION
The inverter-based distributed generation (DG) system incorporated by renewable energy such as photovoltaic, wind power and fuel cell etc. toward supply power intended for the network and local load. Photovoltaic energy have turn out to be more prominent for DG system when compare to other renewable energy resources [1][2]. It is being extensively applied to shelter environment and makes the power industry enlargement sustainable. In regulate to formulate certain the safe operation in cooperation the arrangement and the DG, the DG has to be operational with islanding detection utility [3][4]. Islanding is the condition of creating Power Island like a segment of the utility scheme in case of a widespread interruption in the central power grid. All through islanding conditions constant power provide to load is maintained smooth in a case of main grid stoppage and DG source energizes the load pending the main grid is resynchronized through DG [5]. A maximum disruption of 2 seconds is essential for the islanding detection study wherever scattered network, RLC load and DG system are associated at the point of common coupling (PCC) [3][4]. The study of islanding circumstances can be sub-divided into three sections i.e. the arrangement of islanding, operation during islanding and resynchronization [6][7][8][9][10]. The technique of islanding detection classified mainly remote and local. The local methods auxiliary classify in passive or active. Passive method has huge nondetection zone (NDZ) and depending on the situation will not be accomplished to identify islanding. Active methods have smaller NDZ, except it causes extra current harmonics problems [11]. Remote method support detection has a just right concert, although the system and process cost is extremely prominent, required telecommunication strategy and sensors installed on the side of utility. According to the drawbacks of remote and local methods, the hybrid method is a better performance, reduce NDZ, superior power quality and system cost are cheaper [12].
This paper presents islanding detection technique of DG system through phase locked loop (PLL). PLL consists of two loops i.e. an inner loop scheming the DC link voltage and an outer loop scheming the active (Id) with reactive (Iq) orthogonal current components. The object of the islanding detection technique is to eliminate the NDZ and detection time meets the requirement set out in the standards. The used of the DC-DC boost converter to eliminating the additional controller for steadiness the DC link voltage, where a appropriate assortment of inductor and capacitor is important.
The description of this paper classified in following section: section 2 provides problem formulation of islanding detection and overview of the islanding recognition techniques with system study of islanding detection illustrate in section 3. Section 4 describes modeling of PV module, maximum power point tracking (MPPT) technique, a design of the boost converter and inverter control strategy with a discussion of PLL and proportional-integral (PI) regulator. MATLAB based simulation results for predictable technique is presented in section 5 and overall conclusion of this paper is presented in section 6.

PROBLEM FORMULATION
The voltage and frequency are coordinated by the electric power system, when a main grid is working in healthy condition. However, when a fault occurs to the utility grid, these values are no longer recognized and they are strongly dependent on the local load associated in the direction of the PCC. The locally distributed generator and its interaction with the local load, on the other hand may cause dangerous situations not only to the equipment but to personnel. The IEEE standard 929-2000 was created to provide strategy, which frequency and voltage restrictions the distribution grid [3][4]. Table 1 show the frequency and voltage limits intended for the utility grid. The main situation is correlated to the unceasing power insertion by the DG after the main grid is not there. This happens when the voltage and frequency limits correspond with those determined through standard at the instant of islanding. Algorithm presents delineate of the pseudo code intended for islanding detection.

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The islanding detection techniques have been developed to speed up the islanding detection so as to meet the detection situation in time of the standards.

ISLANDING DETECTION TECHNIQUES 3.1. Islanding Detection Techniques: Overview
Islanding detection methods can be commonly categorized into two types: local and remote method. Local methods be further sub-divided into the passive, active and hybrid method. Remote method is further subdivided into utility and communication-based method [13][14][15][16]. Categorization of islanding detection techniques describes in Figure 1.  Table 2 shows the various islanding detection methods and their performances during operational principle, the size of NDZ, detection time, power quality disturbances, system cost, multiple DGs operation, an effect on the distribution system and influence by the number of connected inverters [17][18][19]. The remote method of islanding detection is better than local islanding detection methods, except at the increased cost and complexity [20][21][22][23]. Local methods are more preferable because they are dependent on the measurement of local parameters, free from noise in communication, cheaper etc.

System Study for Islanding Detection
Recommended test study of islanding detection arrangement consists of an inverter-based DG, parallel RLC load and the grid represented through a source following impedance. The equipped mode of the DG depends on whether the circuit breaker is closed or not [3][4]. The inverter-based DG such as photovoltaic structure is usually configured with the MPPT algorithm. Since islanding detection time is extremely small, the output power is able to measure unvarying through the detection. As a result, by a constant DC source following a three-phase inverter, the DG is considered as a unvarying power source. Figure 2 shows the islanding detection system which consists of two modes of operations i.e., grid-connected mode and islanded mode. During the standard operation or grid-connected mode, the DG within the microgrid will be operational in unvarying current control mode and within islanded mode, the current control approach switched to voltage control approach [24]. describes the active and reactive power enthusiastic by the load when DG is associated toward the utility grid in grid-connected mode [23][24][25].
The resonant frequency (fLC) and quality factor (Qf) of parallel RLC load are defined like: Islanded mode incidental from eq. (1) that if the active power disparity (∆P) =PLoad -PDG=PGrid is not identical to zero. The PCC voltage resolve drop or raise it does not depends on DG operates on unity power factor or not. The quantity of voltage divergence depends upon on the assessment of ∆P. If the active power orientation of the DG is set to be constant, ∆P can be expressed as follows [24][25]: where, ∆V represents the voltage disparity and it can be expressed as: where VPCC and VPCC.i describes the PCC voltage before and after islanding.

PROPOSED TECHNIQUE
Three-phase two-stage grid-connected inverter is collected of two conversion stages, the first-stage is DC-DC boost converter i.e. used to boost the PV section output voltage and organize the PV voltage which regulates the operation of a module at maximum power point (MPP). The regulated voltage of DC-DC boost converter is attended to the DC link, which is an input of the DC-AC inverter. The DC-AC inverter suitable for maintaining the DC link voltage and control the output current. PLL intended for AC power output of the inverter is synchronized with the grid and islanding detection purpose when grid frequency and voltage is below and above IEEE standards limits. The explanation of proposed technique is categorized in following i.e. modeling of PV module, maximum power point tracking (P&O), a design of boost converter and inverter control strategy.

Modeling of PV Module
A photovoltaic module is created by concerning many solar cells in series and parallel, it consists of a current source, a diode, and series-parallel arrangement resistors as seen from Figure 3.

Maximum Power Point Tracking
Perturb and observe (P&O) algorithm is extensively used in MPPT because of their simple structure and real-time implementation is easy. By the help of MPPT controller, produce duty cycle to trigger the boost converter switch [30][31]. The mathematical illustration of P&O algorithm [32] is given as follows The operating point is perturbed from time to time by shifting the voltage in PV system. The working voltage of PV section is disconcerted by a small rise and the consequential change in power (∆P). If ∆P is positive, the working point reached secure to MPP. If ∆P is negative, the operating point has moved distant from the MPP and direction of perturbation encouraged inverted reverse towards MPP [32][33].

Design of Boost Converter
The boost converter is step up the value of input DC voltage and gives by an output. The main components of a boost converter are an inductor, a diode, a capacitor and high-frequency IGBT switch [34][35]. In this paper, the duty cycle of converter switch is generated from MPPT technique and operating switching frequency at high to generate maximum power. Representation circuit diagram of boost converter as depicted in

Inverter Control Strategy
The control system of an inverter is based on PLL for determination of frequency and voltage to islanding detection. PLL consists of two loops i.e. an outer loop scheming the DC link voltage and an inner loop scheming the active (Id) and reactive (Iq) orthogonal current components. The orientation setting for the Id component is determined by an external voltage controller at the same time as the value of the Iq component is set to zero to keep up unity power factor. The output power of three-phase inverter is coordinated through grid using a PLL.
PLL is designed for the mainly determination of the frequency and angle orientation at the PCC, it consists of four components i.e. Clark's transformation, Park's transformation, PI regulator and an integrator. dq0 to abc transformation is called Clark's transformation and abc to dq0 transformation is called Park's transformation. These transformations in the PLL method will generate a three-phase balanced waveform through a 120 o phase shift which can be given as input to the inverter. PI regulators are used for scheming the errors in the output of the grid frequency and grid voltage magnitude.
Park's transformation: Where, Va, Vb, Vc are the constant quantity and Vd, Vq, V0 are stipulated quantity of three-phase voltage.
The block diagram of inverter control for projected islanding detection process is specified in Figure  5. It includes three-phase reactive current components, outer voltage loop intended for power control and inner loop intended for grid voltage control.  Figure 5. Diagram of inverter control The projected control system requires two types of measurements i.e. three-phase output currents of the DG (IDG) and a voltage at PCC (VPCC). The IDG and VPCC are changed into d-q mechanism by the park's transformation. In Figure 6 d-q mechanism of currents are denoted by id and iq and d-q mechanism of voltage are denoted by ud and uq. The idref control the active power supplied through the DG while iqref controls the reactive power output of the DG, with iqref set equal to zero rejection reactive power is supplied through the DG and the DG operates at unity power factor. Then by transitory these reference currents during the phase angle transformation i * dref and i * qref are obtained by Equation.
éù é ùé ù êú ê úê ú êú ê úê ú êú ê úê ú ë ûë û ëû (10) i * dref and i * qref are compared with id and iq, which are inverter output currents and pass through a current regulator to the outputs Vdref and Vqref. Vdref and Vqref transform to ed and eq given with Equation (11 (11) The inverter terminal voltages ed and eq are used to evaluate the modulation index and phase angle which are considered from Equation (12). 22 dref qref m = V +V When the DG is associated to the utility grid, Equation (13) and (14) describe the power flows and the active and reactive power consumed by the load.

PCC Load
DG Grid V P = P + P = 3 R  (14) It can be incidental from Equation (13) that if PGrid is not zero earlier than the grid detachment, the PCC voltage will collapse or go up during islanding because of the active power disparity ∆P (∆P = PLoad -PDG). Thus, the voltage dissimilarity can be utilized to detect the islanding and the quantity of voltage divergence depends on the value of active power mismatch. If the active power of DG is set constant, the active power disparity can be expressed as follows:  (15) and ∆V can be expressed as Similarly, it can be seen from Equation (12) that the amount of disparity in the frequency depends on the significance of both active and reactive power disparity previously islanding occurs. If the DG operates at a unity power factor and there is rejection active power disparity throughout islanding, the reactive power divergence ∆Q (∆Q = QLoad -QDG) can be written in expressions of the frequency disparity as follows: and ∆f can be expressed as f f SPWM is implemented to establish the on-off signals of inverter switches. This category of interface can organize the DG active and reactive power output.

RESULTS AND DISCUSSIONS
In this section of the paper, the result of projected method to detection islanding mode is designed for the MATLAB platform. For checking the islanding condition circuit breaker on the utility side is twisted to the open condition. The projected islanding detection technique is implemented with a PV-based inverter and intended to reenergize the system when islanding is detected. The main system parameters are given in Table 3. The amount of output power generate by PV module depends on operating voltage of the array. The output voltage and current of the MPPT based PV structure devoid of boost shown in Figure 6.
The generated duty cycle from the MPPT algorithm width of the pulse is modified according to desired output, generated output duty cycle pulse through MPPT which gives to converter switch is shown in Figure 7.
To an increase the desired level of PV output voltage it interfaces with a DC-DC boost converter. The voltage should be increase from 80V to 317V approximately. The boosted output voltage from DC-DC converter shown in Figure 8. Figure 9 shows the voltage and current of the inverter and Figure 10 show the supply voltage of the main grid. Initially the system in grid-connected mode, at 0.1 second there is a disparity in the main grid through changing the voltage. In the same way there is a change in inverter part which is instead of the islanding. The voltage change deliberately creating an annoyance in the main grid. This disturbance is detecting by the breaker switch by the help of interval investigation in the inverter elevation which is detecting main grid frequency and voltage. At 0.1 second the limits of voltage are exceeding. So, the breaker switch is shedding the microgrid from the main grid. There will be a disparity occurs in all parameters like voltage, current, and power of the scheme. This disparity in voltage, current, and power is balanced with the help of the DG structure for the duration of the islanding mode of operations.
Harmonic patterns under grid-connected mode i.e. the grid supports local load and exports power toward the grid as shown in Figure 11. From Figure 11 it is observed that the low order harmonics (THD=1.04%) are presented in FFT plot.
Simulation FFT investigation for islanded mode i.e. the inverter supports local load and exports zero power to the grid, are shown in Figure 12. Figure 13 shows simulation results of inverter voltage at PCC when the islanding detected within 0.002 second. It is established that the islanding can be detected effectively and the detection time is in 2 seconds when the projected technique is applied.  Table 4 shows the comparison of values during grid-connected and islanding mode based on different parameters.
The comparison between passive, active and projected islanding detection procedure, it is observed that the passive technique have short detection time but large NDZ, active technique small NDZ but long detection time. The projected technique has no NDZ and medium detection time. Table 5 shows the comparative investigation of passive, active and proposed technique.  The comprehensive investigation of passive, active and projected technique based on NDZ and detection time. The proposed technique is appropriate for islanding detection for the reason that of no NDZ and very short detection time.

CONCLUSION
In this paper, voltage and frequency control for islanding detection scheme has been projected for the operation of the islanded and grid-connected mode is demonstrated through MATLAB Simulink. In ordinary condition system supports constant control mode of current after islanding, scheme switched to voltage control mode. The PLL is intended for the determination of PCC reference frequency and angle. In a PLL, PI regulator is used for controlling the output inaccuracy of frequency and voltage magnitude of a grid. The comparative analysis based on passive, active and proposed approach, in proposed technique islanding detection time is 0.002 second which is suitable for IEEE standard and NDZ is zero.