Stability analysis of photovoltaic system under grid faults

Received May 26, 2019 Revised Oct 22, 2019 Accepted Dec 5, 2019 This work includes the establishment of a Photovoltaic system connected to the grid by means of an inverter. The fundamental goal of the work is to incorporate an advanced active power flow management scheme in order to adopt load at any weather condition along with the advantage of maximum active power flow and zero harmonics from PV inverter to the grid. The outcome of analysis and control design of grid connected PV inverter using a Proportional-Integral (PI) control technique is based on synchronous dq rotating reference frame so as to achieve maximum output voltage and record the active power. It has been observed that the model provides a better rate of stability as compared to the existing topology.


INTRODUCTION
As fossil fuels are depleting day by day, more research is going on harvesting electrical energy from renewable energy. Among all other renewable energy sources, Solar and wind energies, are the maximum accessible and dispersed all over the universe. Utilization of photovoltaic (PV) has been rising notably by virtue of the fast advancement of power electronic techniques. Here the objective of the study is to get the maximum energy efficiency under the different operating condition and to approach the design of a robust controller having the ability to reject the noises. The main objective of the study is to coordinate the voltage, the power injected into the utility. Many earlier works have inspected different control aspects of Inverter that are connected to the grid. Most of the analysis is done to reduce the THD of the system. But very few of them focus on stability issues of grid coonected PV system. Different Power quality issues of the grid connected PV system are described by Vikas khare [1]. The author herself explained about the different types of fault analysis in grid connected photovoltaic system [2]. M. G. Molina and L. E. Juanico explained the designing of the photovoltaic structure connected to the grid [3]. The dynamic behavior of grid connected Photovoltaic system is studied under the LLLG fault by author K. Manohar [4] and [5]. The proposed work gives the information about a three-phase Pulse Width Modulated inverter modeling and its control strategy. The single staged grid connected inverter gets its input from a dc-dc converter fed through a photovoltaic input [6]. Particularly, the control design of grid connected inverter using the PI control technique has been proposed in the dq rotating frame in order to achieve maximum output voltage and maximum active power. A current control PWM technique has been suggested to provide the pulse for voltage source inverter. By virtue of the huge amount of interconnection of dispersed power generation to the grid in some regions, uncertainty and fluctuation of a different parameter of the power system may appear which requires proper design of controllers for stability analysis under variable conditions [7]. more importance is put on the capability of a Distributed Power Generation System (DPGS) to overcome sharp grid disruptions such as variations in voltage and frequency. This paper focuses on some desirable control techniques that DPGS can follow during damaged grid circumstances. First, a grouping of different kinds of grid faults is illustrated, and it ensured by some applications of the control technique during damaged grid circumstances. A. V. Timbus [8] and [9]analyzed some possible control techniques that DPGS can follow during faulty grid conditions followed by the grouping of different kinds of grid faults in this paper .The primary objective of the control technique is reproduction of power, and also the generation of the currents referred which can meet the grid power requirement. M.Cai [10] adopted a shunt controller for voltage dip compensation when a fault is introduced in the grid. In [11,12] dynamic stability of a Photovoltaic structure connected with a distribution system is analyzed. Reference [13] and [14] has adopted a control technique to analyze the dynamic stability of the PV System.

PROPOSED ACTIVE POWER FLOW CONTROL SCHEME FOR STABILITY ANALYSIS
This section describes the internal structure proposed control design which consists of an external voltage circuit and an internal current circuit [15]. The proposed model is discussed in Figure 1. It consists of a Solar (PV) system, step up converter, which is then interfaced with an inverter (VSI), a distribution network, load and grid with a power flow controller. Figure 1 describes the circuit diagram of the proposed structure. The suggested structure is built to control the various parameters like current, voltage and power injected into the utility [16]. The 305 Solar Panel is being selected for designing of PV system using MATLAB. MPPT approach is employed for enhancing the efficiency of the solar panel. The Perturb & Observe (P&O) method is selected as the MPPT tracker whose primary goal is to obtain rapid and correct tracking act and reduces oscillations with respect to changeable weather conditions. P&O method implementation is quite cheap. The output of the boost converter which is1200 V DC is converted into 800V AC by a voltage source inverter, then that voltage is increased to 11 KV by a step-up delta/star transformer. Figure 2 represents the circuit diagram representation of the control structure. It includes the abc-dq transformation block, PLL block, PI controller and the PWM generator. The step-up converter to steps up the voltage of the solar MPPT system while the MPPT generates the pulse for the converter.

DC-link voltage control scheme
The active power is restrained by the voltage at the DC-link with its desired value. Figure 3 shows the internal structure of the dc-link voltage management strategy. It has six components: MPPT, voltage compensator, limiter, current components, and integrator. Here dref u is a reference DC voltage recorded by the maximum power point technique [17].  Where C is the capacitor, ic is current through the capacitor, ipv is the PV current, udc is the voltage across DC link, udref the DC voltage reference, id is d-axis component of grid current, idref is it's a reference value.

Internal current loop
The d−q transformation matrix is used for the conversion of three-axis grid voltage and current to the dq reference frame. The AC grid voltage and current are converted into DC values with the help of abc → dq transformation matrix, by regulating the variables. The phase angle is calculated with the help of PLL (Phase Lock Loop) [18] which is further required for synchronization of grid voltage with controlled current. To get the unity power factor, iqref*(reference reactive current) is set to zero. The Proportional-Integral controllers provide the voltage outputs (Vd*, Vq*) which fed to the PWM generator to generates the pulses used to for Inverter [19].
By using the above formula given in (6), the abc-dq transformation is made where T is the transformation matrix.
As the voltage of grid is cannot be managed, the ultimate desirable choice of governing the action of the arrangement is by regulating the Id and Iq that is injecting to the grid. Figure 4 depicts output of a compensator (Ud),is the control signal focusing on minimizing error as: (7) Correspondingly, the error signal that another regulator processes, The error commands are fed to a PI regulator. The PI regulator constants are selected so that, (10) where is the time constant of the circuit. The internal control structure of the system is shown in Figure 4.
Where are d-axis and q-axis factors of modulating commands, Vdc is the voltage at DC link, are control signals of d-axis and q-axis, Usd, Usq are refined voltage of VSI, is the grid frequency. The pulses are developed by conversion of d and q signals into a,b,c signals. By regulating md and mq, the id and iq quickly follow their corresponding reference signals idref and iqref [20]. Then active and reactive power of Photovoltaic arrangement are given as: As the voltage at the grid is unmanageable, the ultimate practicable approach of monitoring the arrangement is by regulating grid currents Id and Iq. As active power flow is reliant on the current Id, and reactive power inserted to the grid is dependent on the current Iq and thus Iqref = 0 is set forcefully to make zero reactive power [21,22].

Simulink environment
The validity of the system is checked with the help of computer simulations. The MATLAB/SIMULINK software is used to simulate the final system and for unusual circumstances, the analysis is done. The symmetrical and unsymmetrical faults are created on the distribution network, for the analysis of system behavior under fault [23].

SIMULATION OUTCOMES OF FAULT ANALYSIS
The simulation is conducted in order to achieve stability of the photovoltaic system under different conditions of grid faults. The transient state stability of the system may be analyzed by many methods; here some aspects are taken into account. The faults on the grid can be divided into two main groups. a) Symmetrical fault: When an equal decrease in the magnitude of the grid voltage occurs in all three phases whereas the system remains balanced. Symmetrical faults are not uncommon (they do happen often: bus faults, lightning strikes in substations etc). b) Unsymmetrical fault: When an unequal decrease in grid voltage occurs with alternation in phase in between. This category of fault arises as more than one phase is shorted to ground or to one another.
The effectiveness of the control system is checked by considering three cases of grid faults. In each of the cases, the result analysis is carried out that has been recorded for stability analysis [24]. case-1: Three phase to ground fault (LLLG fault): Three phases to ground fault is a familiar fault in the industrial climate. The outcome of the PV circuit with a LLLG fault of 0.3 sec to 0.4 sec is analyzed in this section. case-2: LLG fault in distribution network: In this case, a double line to ground fault is simulated from 0.3 sec to 0.4 sec. When this type of fault occurs a sudden power surge is experienced in the converter. The voltage of the converter rises rapidly and then recovers with an oscillation that lasts for a few cycles. This response is generated with the PI controller. case-3: LG fault in distribution network: In this case, a single line to ground fault is simulated.
The Figure 5 illustrates the Simulation model of the grid connected PV system. The sub-system of the PV source includes a PV block, MPPT controller along with step up converter interfaced to the grid via voltage source inverter. Table 1 gives system parameters data and specifications [25]. Here the system is certified under the least fault circumstances.

Three phase to ground fault (LLLG fault)
The output waveforms of proposed structure with a LLLG fault are illustrated below. At t=0.3 sec to 0.4 sec, LLLG fault is inserted into the system.      Figure 7(a) shows the DC voltage generated by the boost converter is disrupted more than LLLG fault. With 0.3 sec DC voltage oscillates between 1200 to 1600V and then decreases to 1200V after the fault. Later at 0.41 sec, voltage settles to its steady-state value.   940 achievement of the system. In this paper, the structure is certified under the least damaged circumstances i.e. the symmetrical as well as unsymmetrical fault. Just after the fault, system parameters retain their original values because of the robustness of the controller. The correctness of the current control scheme is demonstrated by the result analysis. The output waveform compiled the validity of the used control techniques since the delivered powers pursue the regular values, provided by the MPPT controller of the PV system, both in a normal and abnormal condition.