Distribution Generator Fed By Three Level Inverter Used To Compensate Voltage Unbalancung Effect Under Svpwm Strategy

Transcription

1 Distribution Generator Fed By Three Level Inverter Used To Compensate Voltage Unbalancung Effect Under Svpwm Strategy M.CHANDU (PG Scholar) 1 A.KRISHNAIAH (Ph.D) 2 1 Department of EEE, AVANTHI S SCIENTIFIC TECHNOLOGICAL & RESEARCH ACADEMY, JNTU (HYD) 2 Assistant Professor, AVANTHI S SCIENTIFIC TECHNOLOGICAL & RESEARCH ACADEMY, JNTU (HYD) ABSTRACT-By eliminating would like of separate controllers or crucial islanding detection, this paper proposes a fuzzy primarily based three-phase electrical converter in distributed generation which may be enforced for each islanded and grid-tied operations. within the planned strategy the three-phase electrical converter is regulated as simply current supply by inner electrical device current loop in grid-tied and for islanding mode a voltage loop within the synchronous arrangement can mechanically regulates the load voltage. This paper proposes a unified load current feedforward to keep up the grid current waveforms in grid-tied mode and cargo voltage waveforms in islanding mode to be artless even below nonlinear native load. The effectiveness of the planned strategy is valid by simulation. Index Terms Fuzzy Logic Controller, unified control, islanding, load current, seamless transfer, Distributed generation (DG), three-phase inverter, unified control. I. INTRODUCTION The distributed generation (DG) thought emerged as how to integrate totally different power plants, increasing the decigram owner s responsibility, reducing emissions, and providing further power quality edges [4]. the price of the distribution power generation system victimisation the renewable energies is on a falling trend and is predicted to fall additional as demand and production. DG delivers power to the utility and native crucial hundreds in gridconnected mode. Upon outage of any generator connected to the utility the islanding is made. below these things, decigram should be tripped and should stop to energise in keeping with IEEE customary so as to still feed the native crucial load by disconnecting DG s and a few native load so as to boost the facility responsibility. Load voltage is fastened by the decigram within the islanded mode and by the utility within the grid mode operation. So, maintaining the load voltage is vital. so as to scale back transients within the load decigram should take over the load as presently as attainable that is difficult operation for the decigram. During this paper voltage management mode is nothing however Droop-based management is employed wide for the sharing of power among parallel inverters and might be applied to decigram to appreciate power sharing between decigram and utility in grid-tied mode [11-12]. below this operation, load voltage is secure throughout transitions of operation modes and electrical converter is regulated as voltage supply by voltage loop is sweet solely steady-state performance whereas dynamic performance is poor as a result of information measure of voltage loop is over of the external power loop, realizing droop management. additionally to the (phase fastened loop) PLL and therefore the virtual inductance, the influx grid currents throughout transition from islanded mode to grid-tied mode invariably exists it means that grid current isn't controlled directly [13]. Hhigher dynamic performance is achieved by hybrid voltage and current mode sort management for decigram. In which electrical converter is controlled as current supply by one sets of controller in grid-tied mode, and as a voltage supply by alternative sets of controller within the islanded mode. influx grid currents ar virtually eliminated within the output by directly dominant the output current in grid-tied mode. there's no got to modification the switch of the controller once the operation mode of decigram is modified, with the employment of hybrid voltage and current management mode. With the incidence of utility outage the interval throughout to vary it to voltage mode, the load voltage is neither regulated by decigram nor fastened by the utility however the length of the interval is decided by the islanding detection method..second issue is below nonlinear native load with same approaches is that the quality undulation of the grid current and cargo voltage. The output current of DG is generally desired to be pure in grid-tied mode [13]. The harmonic component will fully flow into the utility when nonlinear load is fed. The harmonic components of the grid current can be mitigated by harmonics

2 Fig. 1. Schematic diagram of the DG based on the proposed control strategy. injected by single-phase DG in [4]. DG will emulate a resistance at harmonic frequency is being controlled by voltage mode control and then the harmonic current flowing into the utility can be mitigated. In the islanding mode, the nonlinear load may distort. With the use of multi-loop control method, resonant controllers, sliding mode control and many control schemes have been proposed to improve the quality of the load voltage. Existing control strategies, DG with nonlinear local load will mainly concentrate on grid current in the grid-tied mode and on load voltage in island mode and improving both of them for unified strategy is rarely used. This paper discusses about unified control strategy that avoids the aforementioned shortcomings. With a given reference in the synchronous frame (SRF) the three-phase inverter is controlled in DG act as a current source using traditional current loop. A novel voltage controller is presented to supply reference for the inner inductor current loop in D-axis and Q-axis proportional-plus-integral (PI) compensator and a proportional (P) compensator are employed. The load voltage is dominated by the utility and the voltage compensator in D-axis is saturated, while the output of the voltage compensator in Q-axis is forced to be zero by the PLL. The reference of the inner current loop cannot be regulated by the voltage loop. With the occurrence of grid outage, the load voltage is no more determined by the utility. The voltage controller is automatically activated to regulate the load voltage. Hence proposed control strategy does not need a forced switching between two different sets of controllers. So, there is no need of detecting islanding quickly and accurately is no more critical in approach. For better dynamic performance, the proposed control strategy utilizes the feedback control for both current and voltage compares to voltage control mode. And paper is enhanced by introducing a unified load current feedforward, is implemented by adding the load current into the reference of the inner current loop in order to deal with the issue caused by the nonlinear local load. The benefits of the proposed load current feedforward can be extended into the islanded operation mode, due to the improved quality of the load voltage. This paper is arranged as follows. Section II discusses about Distributed generation (DG) and its applications. Section III describes the proposed unified control strategy for three phase inverter in DG which includes the power stage, the basic idea and control diagram. Section IV discuss about fuzzy logic controller. The parameter design and small signal analysis of the proposed control system are given in Section V. The simulation results for the proposed system are shown in Section VI. Finally, the conclusion and remarks are given in section VII. II. DISTRIBUTED GENERATION (DG) AND IT S APPLICATIONS Distributed generation (or DG) generally refers to small-scale (typically 1 kw 50 MW) electric power generators that produce electricity at a site close to customers or that are tied to an electric distribution system [12]. Distributed generators include, but are not limited to synchronous generators, induction generators, reciprocating engines, microturbines (combustion turbines that run on high-energy fossil fuels such as oil, propane, natural gas, gasoline or diesel), combustion gas turbines, fuel cells, solar photovoltaic, and wind turbines. There are many reasons a customer may choose to install a distributed generator. DG can be used to generate a customer s entire electricity supply; for peak shaving (generating a portion of a customer s electricity onsite to reduce the amount of electricity purchased during peak price periods); for standby or emergency generation (as a backup to Wires Owner's

3 power supply); as a green power source (using renewable technology); or for increased reliability. In some remote locations, DG can be less costly as it eliminates the need for expensive construction of distribution and/or transmission lines. Islanding: Islanding occurs when a DG system is still generating power to the distribution system when the main breaker from the Wires Owner is open. In this case, the DG system would be the sole supplier of electricity to the distribution system. This is a concern for several reasons. i. Safety concern for system maintenance if the Wires Owner's line workers are not aware that the DG system is still running, they may be electrocuted working on the line or other equipment connected to the line. ii. Equipment damage to other Wires Owners customers. If the DG is still generating while the main breaker from the wire owner is open, the voltage and the waveform from the DG may fluctuate and may not meet the acceptable standard. Existing customers who are connected to the distribution line are then fed by very poor quality of power from the DG. As a result, their light fixtures, motors and other electric equipment may be damaged or its life may be shortened. If the situation persists unnoticed for an unacceptably long time, a fire hazard may exist. iii. Damage to the DG owner's generator if the DG is still generating while the main breaker from the wires owner is open, the DG equipment may be damaged when the wires owner s main breaker is closed due to closing out of synchronism. III. SYSTEM PROPOSED CONTROL STRATEGY A. Power Stage: To operate in both grid-tied and islanded modes this paper proposes unified control strategy for threephase inverter in DG. The DG is equipped with a three-phase interface inverter with a LC filter. The energy from prime mover is converted in electrical energy and then into DC by front end power converter, the DC voltage is regulated represented by V as shown in figure. Local grids are directly connected in the ac side of the inverter. The two switches S and S functions are different. DG will control the inverter transfer switch S and the utility will control the utility protection switch S. Under normal operation, the DG in the grid-tied mode injects power to the utility and both S and S switches are ON. When the utility is in fault, the utility instantly trips the switch S and then the islanding is formed. S Switch will be disconnected after the islanding has been detected by the DG and DG will be transferred to islanded mode from grid- tied mode. The DG will be resynchronized with the utility only after when the utility is restored and the switch S will be turned ON to connect the DG with the grid. B. Basic Idea: with the proposed control modes (hybrid voltage and current mode) the inverter is controlled as a current source to generate reference power P + jq in grid-tied mode, output power P + jq should be the power injected into the grid P + jq and load demand can be expressed as follows by assuming the load is represented as a parallel RLC circuit: P = 3 2 V R (1) Q = 3 2 V 1 ωl ωc (2) Where V the amplitude of load voltage and f is is the frequency of load voltage. Considering the fundamental component still equivalent to the parallel RLC circuit when the nonlinear local load is fed. The load voltage will neither be fixed by the utility nor regulated by the inverter during the time interval the moment of switching the control to the instant of islanding mode. The load voltage may drift from the normal range. The inverter will still controlled as current source and kept it output power almost unchanged during this time interval. The power injected to utility decreases to zero rapidly, and then the power consumed by the load will be imposed to the output power of DG. If considered both active power and reactive power injected into the grid is positive in the grid-tied mode, then P and Q will increase the after the islanding mode. The amplitude and frequency of the load voltage will rise and drop according to equations (1) and (2). Comparing to the traditional analysis, the output power of inverter P + jq can be regulated to match the load demand by changing the current reference before islanding is confirmed. The load voltages excursions will be mitigated which is implemented in this paper. By regulating the threephase inductor current i only the output power of the inverter is controlled in the proposed control strategy, while the magnitude and frequency of the load voltage v are monitored. While islanding is about to operate, the magnitude and frequency of the load voltage may drift from normal range and then they are controlled automatically and recovered to normal range by regulating the output power of the inverter.

4 C. proposed Control strategy: Figure 2 shows the proposed unified overall control block diagram. The sensed values from the block diagram are the utility voltage v, the inductor current i and the load current i. The threephase variables of the three-phase inverter will be represented in dc quantity is controlled in the SRF. The main modes of the control diagram are the inductor current loop, the PLL, and the current reference generation module. In order to mitigate the couplings due to the inductor, is implemented by the PI compensator in both D- and Q-axes and decoupling of the cross coupling ω L /k. Decoupling capacitor 1/k and output of inner current loop d sets the reference for the standard space vector modulation (SVM) that control the switches of the three-phase inverter. Where k denotes the voltage gain of the inverter which equals to half of the dc voltage in this paper. The widely used SRF PLL in three-phase power converter to estimate the utility frequency and phase is also proposed in the control strategy [15], in order to hold the frequency of the load within the normal range in the islanded operation a limiter is inserted between the PI compensator G and integrator. From figure it can be concluded that the inductor current is regulated to follow the current reference i and the current phase is synchronized to the grid voltage v. If current reference is constant, the inverter is just controlled to be a current source, which is same with the traditional grid-tied inverter. The new thing in this paper is the current reference to guarantee the power match between the DG and local load and enables to operate in islanded mode. In this module even unified load current feedforward to cope with nonlinear local load is implemented. Figure 3 provides the current reference for the inner current loop in both grid-tied and islanding modes. An unsymmetrical structure is used in D- and Q-axis where PI compensator in D-axis with an extra limiter and P is employed in Q-axis. Load current i is being added to the final inductor current reference i by the load current feedforward. The benefits from figure 3 are represented by two parts: 1) without critical islanding detection seamless transfer capability; and 2) in both grid-tied and islanded operations improving the power quality. In D and Q- axes the current reference i composes of four parts namely: 1) controller output voltages i ; 2) the reference grid current I ; 3) the load current i and 4) the current through filter capacitor C. In grid-tied mode, the load voltage v is decided by the utility. The load voltage and current reference are irrelevant due to saturation of PI compensator in D-axis and the output of P compensator being zero in Q-axis. Thus, the inverter operates as a current source. Voltage controller takes automatically to control the load voltage by regulating current reference when islanding occurs and makes the inverter to operate as a voltage source to provide stable voltage to the local loads. The advantage of this control scheme is that it relieves from different control architecture. The other distinguished function of the current generation module is the load current feedforward. In order to compensate the harmonic component the sensed load current is added as a part of the inductor reference current i in the grid current under the nonlinear local load. But in the islanded mode still the load current feedforward operates and the disturbance caused by the nonlinear load can be suppressed by the fast inner inductor current loop and finally the quality of the load voltage is improved. In [18] the inductor current control shown in Fig 2 was proposed for grid-tied operation of DG. Inspired from [18] this paper proposes a unified control strategy for DG in both grid-tied and islanded modes can be represented by the current reference generation module in figure 3.This module can be summarized in two aspects for this contribution. First, PI compensator in D and P compensator Q-axis respectively, upon occurrence of islanding voltage controller is activated automatically and maintained inactive during grid-tied mode. There is no need for switching different controllers and load voltage quality during transition from grid-tied mode to the islanded mode can be improved. Another contribution of this module is to provide load current feedforward to deal with the issue caused by the nonlinear local load, by which load voltage quality in islanded mode is enhanced and the grid current waveform in grid-tied can also improved. It should be noted that the unbalance three-phase local load currents cannot be fed by the DG with the proposed control strategy, because there is no flow path for the zero sequence current of unbalanced load, and the regulation of zero sequence current is beyond the scope of the proposed control strategy. IV. FUZZY LOGIC CONTROLLER The error value of the dc-bus voltage Δv dc = v dc v dc is passed through a Fuzzy-type compensator to regulate the voltage of dc bus (v dc ) at a fixed value. The operation of FLC is as follows. FLC contains three basic parts: Fuzzification, Base rule, and

5 Defuzzification. FLC has two inputs which are: error and the change in error, and one output. The Fuzzy Controller structure is represented in fig.6. The role of each block is the following: Fig 4. Membership function of voltage error Fig 5. Membership function of output field voltage Fig 2: The general structure of Fuzzy Logic Controller Fuzzifier converts a numerical variable into a linguistic label.. In a closed loop control system, the error (e) between the reference voltage and the output voltage and the rate of change of error (del e) can be labeled as zero (ZE), positive small (PS), negative small (NS), etc. In the real world, measured quantities are real numbers (crisp). The FLC takes two inputs, i.e., the error and the rate of change of error. Based on these inputs, The FLC takes an intelligent decision on the amount of field voltage to be applied which is taken as the output and applied directly to the field winding of generator. Triangular membership functions were used for the controller. Rule base stores the data that defines the input and the output fuzzy sets, as well as the fuzzy rules that describe the control strategy. Mamdani method is used in this paper. Seven membership functions were used leading to 49 rules in the rule base. Table 1: Rule base for fuzzy controller Inference engine applies the fuzzy rules to the input fuzzy variables to obtain the output values. Defuzzifier achieves output signals based on the output fuzzy sets obtained as the result of fuzzy reasoning. Centroid defuzzifier is used here. Fig 3. Membership function of voltage V. PARAMETER DESIGN AND SMALL SIGNAL ANALYSIS OF THE PROPOSED CONTROL SYSTEM The fuzzy based proposed unified control strategy with operating principle of DG is illustrated in detail in this section. The four states of DG are as follows:1) grid-tied mode, 2) transition from the gridtied mode to islanded mode, 3) the islanded mode, 4) from islanded mode to the grid-tied mode. i. Grid-Tied mode: under normal case of utility, by inductor current loop the DG is controlled as current source and will supply active and reactive power through current D- and Q- axis independently. For

6 that utility voltage phase angle is obtained through PLL by park transformation, PI controller, a limiter and an integrator. 2 x x = 2 cos θ cos θ π cos θ π x sin θ sin θ 2 3 π sin θ + 2 x 3 π x (3) An inductor current reference i seems little complex and compared with the instantaneous filter inductor current which is transformed into SRF by the park transformation. The inductor current is regulated to track the reference i by the PI compensator G. The utility is assumed stiff, the three-phase utility voltages are expressed as v = V cos θ v = V cos(θ 2π 3 ) V cos θ 2π (4) The SRF transformation of the utility voltage is expressed as v = V cos(θ θ) v = V sin(θ θ) (5) Where V = magnitude of the grid voltage, θ = the actual phase angle. v is regulated to zero by the PLL, so v equals the magnitude of the utility voltage V. As the filter capacitor voltage equals the utility voltage in the gird-tied mode, v equals the magnitude of the utility voltage V, and v equals zero. In the D-axis, the inductor current reference i can be expressed by (6) according to Fig. 3 i = I + i ω C v (6) In steady state, the given voltage reference V is larger than the magnitude of the utility voltage v and the first part is the output of the limiter. So the PI compensator, denoted by GVD in the following part will saturate and the limiter outputs its upper value Igrefd. The second part is that the characteristics of local load will determine D- axis i load current. The third is the proportional part ω C v, where ω is the rated angle frequency, and C is the capacitance of the filter capacitor. It is fixed as v depends on the utility voltage. The given reference I and the load current i is being imposed by the current reference i and independent of the load voltage. In the Q-axis, the inductor current reference i consists of four parts as i = v k + I + i + ω C v (7) Where k = parameter of the P compensator, denoted by G in the following part. The first part is the output of G, which is zero as the v has been regulated to zero by the PLL. The second part is the given current reference I, and the third part represents the load current in Q-axis. The final part is the proportional part ω C v, which is fixed since v depends on the utility voltage. Therefore, external voltage drop will not influence the current reference i.but, the current reference i will determine the given reference I and the load current i.the control diagram of the inverter is simplified n grid-tied mode, with the analysis of previous cases and the inverter is controlled as a current source with inductor current reference I and the load current i determined by the inductor current loop will track the current reference and the load current. I represents the grid currents if steady state error is zero will be explained in next section. ii. Transition mode from grid tied mode to the islanded mode: By opening utility switch S, the islanding mode begins; frequency and load voltage will drift because of active and reactive power mismatch between DG and the load demand. The transition is divided into two time intervals where first is from the instant of turning off S to the instant of turning off S when islanding mode is confirmed. The second one starts from instant of turning off inverter switch S. As switch S is in ON state, in first interval the utility voltage v will be same as load voltage v because dynamic of the inductor current loop and the voltage loop is much faster than the PLL [15] but load voltage and current are varying dramatically considering load voltage angle frequency to be not varied. In the grid-tied mode, it is assumed that the DG injects active and reactive power into the utility, which can be expressed by (8) and (9), and that the local critical load, shown in (10), represented by a series connected RLC circuit with the lagging power factor Fig. 6. Operation sequence during the transition from the grid-tied mode to the islanded mode.

7 Fig. 7. Transient process of the voltage and current when the islanding happens. P = 3 2 v i + v i = 3 2 v i (8) Q = 3 2 v i + v i = 3 2 v i (9) Z = R + j ωl 1 ωc = R + j ωl 1 ωc = R + jx (10) In islanding mode, i will decrease from positive to zero, and i will increase from negative to zero. During this time load current will vary in the opposite direction. From equations (11) and (12) it can be concluded that D- and Q-axes load voltage each consists of two terms. The load voltage in D-axis v will increase as both terms increase. But in Q-axis v is uncertain because the first term decreases and the second term increases, and it is not concerned for a while v = i R i X (11) v = i R i X (12) The input of PI compensator G will become negative with the increase of the load voltage in D- axis v, when it reaches and exceeds V so its output will decrease. Then the output of limiter will not imposed to I any longer and the current reference i will drop. In the regulation of the inductor current loop, D-axisi load current will decrease. The load voltage in D-axis v will drop and recover to V. If i has almost fallen to the normal value, the load voltage in Q-axis v will drop according to (12). The PI compensator G will going to be negative if v is decreased from zero to negative and its output will drop. The angle frequency ω will be reduced. If it falls to the lower value of the limiter ω, the angle will be fixed at ω. At the end of the first time interval the load voltage in D-axis v will increase and fix at V and angle frequency of the load voltage ω will also drop.pll can still operate normally if the value is higher than the lower value of the limiter ω, and the load voltage in Q-axis v will be zero. If ω is fixed, load voltage in Q-axis v will be negative. With the help of power relationship the variation of frequency and amplitude can be understandable. When the islanding happens, the local load must absorb the extra power injected to the grid, as the output power of inverter is not changed instantaneously. From (1) the magnitude of load voltage V will rise with the increase of P. In meanwhile the angle frequency ω should drop, in order to consume more power with (2). Results from power relationship coincide with the previous analysis. The second time interval transition begins from the instant when the switch S opens after the islanding detection method. If switch S opens the load voltage v is independent with the grid voltage v. In the mean time v will reduce to zero theoretically as the switch S has opened. The angle frequency is invariable and then, input of the compensator G becomes zero and fixed to the end of the first time interval. The inverter is controlled to be a voltage source when v is regulated by the voltage loop. Under islanding operation, the load voltage is restricted to particular range to drift the amplitude and frequency and the inverter is transferred from the current source operation mode to the voltage source operation mode autonomously. With the increase in the time of delay, the drift becomes worse in the hybrid voltage and current mode control. So, the time delay of islanding detection is critical to drift of the frequency and magnitude in the load voltage. In proposed control strategy this phenomenon is avoided. iii. Islanded Mode: in this state switching S and S both in OFF state. The PLL cannot track the utility voltage normally, and angle frequency is fixed. Since voltage compensator G and G can regulate the load voltage v, the DG is controlled as a voltage source. In D-axis the voltage reference is V and in Q-axis voltage reference is zero respectively and the magnitude of the load voltage equals to V approximately, elaborated in next section. The control diagram of three-phase inverter for islanded mode can be simplified and is shown in figure 7. If there is any disturbance in the load current, it will be suppressed quickly by the inductor current loop and a stiff load voltage can be achieved. Finally, the load current i is partial reference of the inductor current loop. iv. Transition from the Islanded Mode to the Grid- Tied Mode: a. If the utility switch S made ON after the restoring the utility, the DG should be connected with utility

8 by turning ON switch S. There are several steps before preparation before turning on switch S.as soon as utility voltage is restored, the PLL will track the phase of the utility voltage which results that the phase angle of utility voltage v will follow the grid voltage v. If the load voltage v is in phase with the utility voltage, according to equation 5 v will equal the magnitude of the utility voltage. Fig.8. Simplified block diagram of the unified control strategy when DG operates in the islanded mode. b. The magnitude of the load voltage V is larger than the utility voltage magnitude V, the reference voltage V will be changed to V by toggling the selector S from terminals 1 to 2. The load voltage will equal t the utility voltage in both phase and magnitude. c. The switch S is turned on, and the selector S is reset to terminal 1 where load voltage is held by utility. As V = V which is larger than the magnitude of the utility voltage V, so PI compensator G will saturate, and the output of limiter is its upper value I meanwhile v is regulated to zero by the PLL from equation 5. The output of G will be zero. By inactivating G and G, DG is controlled as a current source by inductor current loop. Analysis and Design: This section briefs about the proposed fuzzy based control strategy is analyzed and designed in both steady state and transient state along with three-phase inverter. In the steady state, the operating points of both gridtied and islanded modes of DG are analyzed where limiters and references are selected. Whereas in transient state, compensators in both inductor current loop and the external loop are designed based on the small-signal model and the effect of load current feedforward is also analyzed as well. A. Steady State 1) Analysis of Operation Points: 2) Selection of References and Limiters 1) Analysis of Operation Points: in the grid-tied mode, the inverter is controlled as a current source, and the current reference for the inductor current loop i is expressed according equation (6) and (7). The steady-state error will be zero with the Fuzzy Logic Compensator in the inductor current loop, so the inductor current in steady state can be expressed as follows: i = I ω C v + i i = v k + ω C v + I + i (13) In the SRF, the relationship between the voltage and the current of the filter capacitor in steady state can be expressed by i = v ωc i = v ωc (14) Where ω represents the angle frequency of the DG and C denotes capacitance of the filter capacitor. As a result, the output current of the inverter i can be gained i = i i = I (ω ω) i = i i = v k + I +(ω ω) C v + i (15) As angle frequency ω is very close to the rated angle frequencyω, it can be found that the output current followsi and the load current i, as v equals zero in the grid-tied mode. The active and reactive power injected into utility can be obtained as follows. Consequently, the active power and reactive power flowing from the inverter to utility can be given by I and I, respectively P = 3 2 v (i i ) + v i i = 3 2 v I Q = 3 2 v (i i ) v i i = 3 2 v I (17) In the islanded mode, the inverter is controlled as a voltage source by the external voltage loop. In the D- axis,v is regulated by the Fuzzy Logic compensator G, so the steady state error will be zero and v can be expressed as follows: v = V (18) Where V is in D-axis. In the Q-axis, the regulator G is P compensator, so the steady state error may not be zero. As the load current is added to the inductor reference, the condition shown as below can be achieved

9 v k + I = (19) And then, the load voltage in Q-axis can be expressed by (20). It should be noted that the absolute value of v rises with the increase of the current reference I which is related to the reactive power injected into the utility v = I k (20) The magnitude of the load voltage V can be represented as follows. It equals to V approximately, because v should be much lower than V with proper current reference I V = V + I V k (21) During islanding operation, the angle frequency is restricted in the given range by the limiter. During transition from grid-tied mode to the islanded mode, In first-time interval only the angle frequency is determined. If current reference I is set to zero, then v is zero. It means that the angle frequency ω does not vary in the first time interval of the transition, and it should equal the angle frequency of the utility before islanding happens ω. the angle frequency of the load voltage ω in the islanded mode is determined by the current reference I, where ω represent the upper values of the limiter and ω represent the lower values of the limiter shown in fig 2 ω, I > 0 ω = ω, I = 0 ω, I < (22) 2) Selection of References and Limiters: In the gridtied mode, through the current reference I the active power is injected into the grid P. Therefore, the selection of I depends on the power rating of the inverter. According to equation 17, the current reference I, first determines the amount of reactive power to be injected into utility Q in the grid-tied mode and even affects the magnitude of the load voltage in the islanded mode according to equation 21. Finally, the reactive power Q cannot be very large, In order to maintain load voltage within the normal range in the islanded mode. In grid-tied mode, V should be maintained larger than the utility voltage V. At the same time, load voltage will be determined by V in the islanded mode according to equation 21. So V should not be much larger than V. For this case only it is selected as the maximum magnitude of the utility voltage in this paper. As per IEEE standard the range of the normal grid voltage p.u. so V can be selected as V = V (23) Where V =The RMS value of the rated phase voltage. In order to guarantee that the PLL operates normally in the grid-tied mode, the utility angle frequency ω should not touch the upper value ω or lower value ω of the limier in the PLL. Besides, the angle frequency ω is restricted between ω and ω in the islanded mode, and it should not drift from the normal value too far. So, ω and ω are selected as the maximum and minimum angle frequencies allowed by the utility standard. B. Transient State 1) Small-Signal Model of the Power Stage 2) Design and Analysis of the Current Loop 3) Design and Analysis of the Voltage Loop 4) Impact of Load Current Feedforward 1) Small-Signal Model of the Power Stage: The transient performance is analyzed; the three-phase inverter in the DG needs to be modeled. According to the power stage shown in Fig. 1, by front-end converter in DG the DC-link voltage V is regulated. By eliminating its dynamic performance in this paper the dc voltage V is assumed very stiff. The average model of the power stage can be described by V i i v d 2 d = L d dt i + R i + v d i i v v i (24) i i = C d i i dt v + i + i v i i (25) In (24),d,d, and d = the average duty cycle of each leg varying from 1 to 1,and R = the equivalent series resistance of the filter inductor. Then, the average model in the SRF can obtained with the Park transformation shown in (3), which is represented by V 2 d = L d d dt i i + 0 ωl ωl 0 i i + R i i + v v (26)

10 i i = C d dt v + 0 ωc ωc 0 v + i i + i. i (27) Considering the dc voltagev as stiff, the smallsignal model will be same as the average model. In SRF model between D and Q-axes the inductor L and capacitors C couplings are introduced and these couplings can be mitigated by the decoupling components and ω C in Fig. 3. Therefore, the small-signal model can be simplified into two identical SISO systems, which is represented by (28) ignoring the subscript d and q V dc 2 d = Lf d dt i L + R l i L + v C i L = C f d dt v C + i LL + i g. (28) TABLE 1 PARAMETERS OF THE POWER STAGE Parameters value DC voltage V 400V Filter inductor L 3.5mH Filter capacitor C 15μF Switching frequencyf 10kHz Sampling frequency f 20 khz Rated power of DG P 3000W Rated RMS phase voltage V 115V 50 2π Rated utility angle frequency ω rad/s Rated linear local load R _ 60Ω Rated nonlinear local load R _ 120 Ω 2) Design and Analysis of the Current Loop: In both islanded and grid-tied modes to regulate the inductor current loop it should operate normally. From 28 equation, the small-signal model of the control-to-current can be obtained. Which is shown as G (s) = i L(s) = V d (s) 2 sc s L C + sr C + 1 (29) However, In the grid-tied mode, because of the stiff utility the dynamics of the capacitor C is ignored, and the small-signal model of the control-to-current is described by G (s) = i L(s) = V d (s) 2 1 sl + R (30) The required parameters of the power stage implemented in this paper shown in Table I. In fig.8 for both operation modes the bode plot of the control- to-current transfer function can be obtained. It can be found that huge difference appears in the low and medium frequency range and it is difficult to design the compensator G to achieve good performance in both of operation modes. It is because of the inductor current is coupled with the capacitor voltage in the islanded mode. This difference can be mitigated by the capacitor voltage is fed forward with the coefficient in Fig. 2, by decoupling the inductor with the capacitor voltage. Fig. 9. Bode plot of the loop gain of the inner current loop. Fig. 10. Block diagram of the simplified voltage loop. The transfer function of control to current in the islanded mode is changed to be close to the one in the grid-tied mode, and the current compensator G can be designed based on unified transfer function shown by (30). The loop gain of the current loop is shown in Fig. 9, with the crossover frequency of 1100 Hz, and the phase margin of s ωg G (s) = k s (31) 3) Design and Analysis of the Voltage Loop: To regulate the load voltage, the voltage loop just operates in the islanded mode can be seen in the simplified block diagram Fig. 10. Where G (s) andg (s) denote the closed-loop transfer function of an inductor loop and C =the impedance of the filter capacitor, respectively. These two compensators are designed, and the loop gain of the current loop is

11 shown in Fig.11.results in little difference in the low frequency range. The phase margin is set to 55 and crossover frequency is around 600Hz in both D- and Q-axes. Fig. 10. Bode plot of the loop gain of the voltage loop in D-and Q-axes. 1 + s ωg G (s) = k s (32) closed-loop transfer function G (s) to unity in the bandwidth of the current loop. With two conditions, the output impedance of the bode plot is shown in Fig. 12. The output impedance is reduced from dc to 600Hz with the load current feedforward. At the same time, the quality of the load voltage v will be improved with the load current feedforward. The inductor current loop is regulated directly by inductor current in grid-tied mode. The incurrent reference is mainly composed by the current reference I, and the load current i. The output current i of the inverter will be fixed byi. The disturbance of the load current will be fully injected into the utility, which can be represented by feedforward, when DG operates in islanded mode. ı (s) ı (s) = (36) G (s) = k (33) 4) Impact of Load Current Feedforward: the disturbance from the load current can be suppressed by the inductor current reference and the load current ı is a part of the inductor current reference. The transfer function of the output impedance is derived to estimate the response of the load current feedforward in the islanded mode. The output impedances with and without load current feedforward are expressed by Fig. 11. Bode plot of the output impedance with and without the load current Z (s) = v (s) ı (s) = G (s) [1 G (s)] 1 + G (s) G (s) G (s) (34) Z (s) = v (s) ı (s) = G (s) 1 + G (s) G (s) G (s) (35) With load current feedforward an extra factor [1 G (s)] appears in the output impedance. The magnitude of output impedance will be reduced in the low frequency range because the gains of the

12 result for the proposed control strategy is represented in fig 14(b). Here the time interval of the dynamic process is less than 5ms. From the Comparison of the simulation results above. It can be seen that the dynamic performance of the proposed fuzzy based unified control strategy is better than the conventional voltage mode control. Fig. 12. Bode plot of the transfer function from load current to grid current With and without the load current feedforward DG operates in the grid-tied mode. The disturbance of the load current can be compensated by the inverter and the transfer function from load current to grid current can be explained from equation 37. The bode plots of transfer function and the gain is mitigated upto 1050Hz with the load current feedforward and therefore, the quality of the grid current can be improved ı (s) TABLE II PARAMETERS IN THE CONTROL SYSTEM Parameters Value Voltage reference V 179V Rated current reference I 9A Rated current reference I 0A Upper value of the limiter ω π rad/s Lower value of the limiter ω π rad/s ı (s) = G (s) 1 (37) VI. SIMULATION RESULTS The proposed control strategy is investigated in MATLAB simulink and simulation results are verified. For simulation purposes the three-phase inverter power rating is considered as 3kW. The parameters in the simulation are compared with Tables I and II. The RMS rated phase voltage is 115V ad the voltage reference V is set as 10%higher than the rated value. The utility rated frequency is 50Hz, and the upper and the lower values of the limiter in the PLL are given as 0.2Hz higher and lower than the rated frequency, respectively. By stepping down the grid current reference from 9A to 5Ain the grid-tied mode the conventional voltage mode control and proposed fuzzy based unified control strategy are compared. The simulated results for the voltage mode control are shown in fig. 14(a). at the moment of 14s the current reference is changed. It is found that dynamic process lasts until around 15.2s. The simulation Fig. 13. Simulation waveforms of load voltage v, grid current i, and inductor current i when DG is in the grid-tied mode under condition of the step down of the grid current reference from 9 A to 5 A with: (a) conventional voltage mode control, and (b) proposed unified control strategy

13 Fig. 14. Simulation waveforms of load voltage v, grid current i,and inductor current i when DG is transferred from the grid-tied mode to the islanded mode with: (a) conventional hybrid voltage and current mode control, and (b) proposed unified control strategy. Under transition states the grid-tied mode to the islanded mode, the unified control strategy is compared with the hybrid voltage and current mode control, and the simulation scenario is shown as follows: Initially, the utility is normal, and the DG is connected with the utility; At 0.5s, islanding happens; and At 0.52s, the islanding is confirmed. Simulated results of hybrid voltage and current mode control can be seen in figure 15(a). It can be found that the grid current drop to zero at 0.5s, and load voltage is seriously distorted from 0.5 to 0.52s. The load voltage is recovered to normal value after 0.52s. Fig 15(b) presents the simulated of proposed fuzzy based control strategy. The magnitude of the grid current is 9A and follows the current reference I. The load voltage magnitude and frequency is held by the utility. When islanding happens, to follow the voltage reference V amplitude levels of load voltage will increase little more whereas the output current of DG decreases autonomously to match the load power demand. The voltage quality in the proposed control strategy in three states (two modes and two transition states) is no more critical. CONCLUSION Fuzzy primarily {based} unified management strategy was planned for decigram based three-phase inverters to work in each islanded and grid-tied modes, with no would like for change between 2 totally different management architectures or for crucial islanding detection. a completely unique fuzzy based mostly voltage controller was conferred. once grid-tied mode is inactivated the decigram operates as a current supply with quick dynamic performance. In outage conditions, the controller will mechanically be activated to control the load voltage and even a load current feedforward propos wherever it will improve the wave type quality of the each the grid current within the gridtied mode and cargo voltage in islanded mode. The planned fuzzy based mostly unified management strategy was verified through the MATLAB simulation. REFERENCES [1] R. C. Dugan and T. E. McDermott, Distributed generation, IEEE Ind. Appl. Mag., vol. 8, no. 2, pp , Mar./Apr [2] R. H. Lasseter, Microgrids and distributed generation, J. Energy Eng., vol. 133, no. 3, pp , Sep [3] C. Mozina, Impact of green power distributed generation, IEEE Ind. Appl. Mag., vol. 16, no. 4, pp , Jul./Aug [4] IEEE Recommended Practice for Utility Interface of Photovoltaic(PV) Systems, IEEE Standard , [5] IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, IEEE Standard , [6] J. Stevens, R. Bonn, J. Ginn, and S. Gonzalez, Development and Testing of an Approach to Anti- Islanding in Utility-Interconnected Photovoltaic Systems. Livermore, CA, USA: Sandia National Laboratories, [7] A. M. Massoud, K. H. Ahmed, S. J. Finney, and B. W. Williams, Harmonic distortion-based island detection technique for inverter-based distributed generation, IET Renewable Power Gener., vol. 3, no. 4, pp , Dec [8] T. Thacker, R. Burgos, F. Wang, and D. Boroyevich, Single-phase islanding detection based on phase-locked loop stability, inproc. 1st IEEE Energy Convers. Congr. Expo., San Jose, CA, USA, 2009, pp

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