Comparison between Different Control Strategies of a Z-Source Inverter Based Dynamic Voltage Restorer

In this paper, dynamic voltage restorer (DVR) compensation methods are compared to each other for the load side connected shunt converter topology of z-source inverter based DVR to choose the best method. Four different topologies are recognized for DVR that two of them have energy storage devices, and two topologies have no energy storage that take energy from the grid during the period of compensation. Here the load side connected shunt converter topology that takes necessary energy from the grid is used. Pre-sag compensation, in-phase compensation, energy-optimized methods are the three DVR compensation methods that studied and compared. A deep analysis through different diagrams would show the advantages or disadvantages of each compensation method. Equations for all methods are derived and the characteristics of algorithms are compared with each other. The simulation results done by SIMULINK/ MATLAB shows compensating by this topology based on the compensation methods.


Introduction
Power quality has obtained more attention in recent years due to growth in using industrial sensitive loads [1]. Voltage sag is one of the most important power quality issues that occur in the shape of sudden voltage fall, which happens greatly due to short circuit fault. There are several solutions to this problem [2] and DVR is the most effective of them (for load voltage control and compensation). DVR consists of a converter, energy storage device and a transformer to inject the appropriate voltage to clear the power quality problems. The compensation ability of the conventional DVR (consists of the voltage source inverter) depends on the maximum value of energy storage. Due to the limitation of energy storage, a DC-DC boost converter is needed to use with VSI. In this paper, Z-source inverter, based DVR topology is used that has several advantages regarding cost, reliability and simplicity. In this topology, VSI and DC-DC boost converter is replaced by Z-source inverter [3]. Boosting the ability of Z-source inverter is about a switching state called shoot-through that is created by turning on both upper and lower switches of one phase [4], [5], [6] which is very important in reliability. Shoot-through is a unique switching state of Z-source inverter that is an unallowable state for conventional inverters that causes several disturbances [7].
Based on the type of energy storage, four different topologies of DVR are proposed. Two topologies can be realized with a minimum amount of energy storage that take energy from the grid during the compensation period and other topologies, take energy from internal energy storage devices [8]. Load side connected shunt converter topology of z-source inverter based DVR and the control method [9], [10] are discussed in section 2.
There are three compensation strategies for conventional DVR in literatures [11], [12]. Comparisons between them are done in for a general type of DVR. All compensation methods include pre-fault, in-phase, and energy optimized compensation algorithm are analyzed and compared in this paper for the Z-source inverter based DVR. Equations for each method are derived. Furthermore some parameters to characterize the behavior of each method for this special kind of DVR are defined. Finally, comparisons are done in various aspects to show the characteristics of each compensating algorithms.
In section 3 three compensation methods are discussed for the topology of the shunt converter connected to the load side of the z-source inverter based DVR and the equation helps to compare these compensation methods and choose one of them to show the operation of this topology to clear the power quality problem.

2.
Proposed Topologies for Z-Source Inverter based DVR Different topologies of DVR are applied in two parts. One uses energy storage to supply the delivered power and the other one has no energy storage device and uses the grid supply during the compensation period.
Here, closed-loop control is used for this topology of DVR [10] as it is shown in Fig. 1. Here current mode close loop control is used to determine the appropriate value of shoot-through time (D s ). Current loop control can get a higher speed response by comparing the instantaneous inductor current and the reference current signal. Reference current signal is operated by passing the error signal of capacitor voltage through the PI controller. Reference signals for the DVR are calculated from the ideal supply reference voltage and the actual state of the supply voltage. Then these signals are turned to dq0. The error signal, between DVR reference voltage and the actual voltage of the DVR is minimized using PI controllers. This signals, generate dq0 signals to DVR, then transform to the three phase stationary reference frames which are in turn PWM modulated.

System with no Energy Storage
In this type, the requested energy, came from the grid supply by a shunt converter and will charge the DClink capacitor to the actual state of the supply voltage. Based on the location of connecting the shunt converter to the grid supply, two topologies are considered for this type of system. Source side connected shunt converter and load side connected shunt converter, the second one is the best topology according to [8] and section 3 that explained the vantage of this topology for this operation. So here load side connected shunt converter topology of Z-source inverter based DVR is used for voltage harmonic compensation.

Load Side Connected Shunt Converter
As it is shown in Fig. 2, shunt converter is connected to the load side of the grid. The input voltage to the shunt converter is controlled and the DC-link voltage can be held constant by injecting sufficient voltage. This topology has the disadvantage of larger currents to be handled by the series converter than another topology.

Compensation Strategy DVR
Load compensation could be done through active or reactive power injection by DVR. There are three compensation techniques based on using active, reactive, or both power by DVR. These methods are recognized as pre-fault compensation method, in-phase compensation method, and energy optimized compensation method. These strategies are introduced in this section followed by derivation of their respective equations. Some simulations would show the characteristics of each method individually [11], [12].

Pre-Fault Compensation Method
This method is mainly used for loads that are sensitive to phase jump such as thyristor controlled drives. In other word, both magnitude and phase of the load voltage are compensated in this strategy such that the load voltage during disturbances is as same as the one before disturbances. Fig. 3 shows this kind of compensation in a per-unit plane. In Fig. 3, I P re−f ault load = 1 0 is the load current before disturbance that is considered as the reference vector. The load is selected as a general type so the load voltage could be defined as I P re−f ault load = 1 ϕ. The disturbance might change the source voltage magnitude from 1 to k with δ degree phase jump. Therefore the source voltage after disturbance can be defined as V P re−f ault s = k (ϕ + δ). DVR should inject a voltage with appropriate magnitude and phase to compensate this disturbance. The DVR injected voltage is defined as V DV R = x β. Also the load voltage and current after compensation are the same as their values before disturbance which means V P ost−f ault load = V P re−f ault load and I P ost−f ault load = I P re−f ault load . Moreover the resultant phase jump between load voltage before and after disturbance, γ, is zero in this compensating method. By these definitions, the DVR injected voltage can be calculated as follows: Equation (1) shows that the magnitude of the injected voltage only depends on the degree of distortion in the input voltage waveform. Fig. 4 shows this parameter versus the magnitude of the input voltage for different values of δ.

In-Phase Compensation Method
In this method, DVR only compensates the load voltage magnitude so the phase jump would be remained uncompensated. Fig. 5 shows this control strategy. It is obvious that there is a phase difference between the load voltages before and after compensation. By using the same definitions in the previous section, the DVR injected voltage is as follows: Therefore the load voltage after compensation can be written as V P ost−f ault load = 1 (ϕ + δ), so that the load current would be I P ost−f ault load = 1 δ. Moreover there is a phase jump equal to γ = δ in the load voltage. The magnitude of the injected voltage for any disturbance's phase jump and any load power factor is shown in Fig. 5 that satisfies Eq. (3).

Energy Optimized Compensation Method
It should be noticed that both previous strategies need a storage system to operate. However in energy optimized method, the capacity of this storage system is minimized. Furthermore in shallow sags, the compensation could be done only with reactive power and the amount of active power injection is zero. This strategy is based on injecting voltage in such a way that the injected voltage vector would be at almost 90 • to the resultant load current vector. This will minimize the injection of active power and even it may be zero. Fig. 7 shows the basics of this strategy.
According to Fig. 8 the injection voltage phase angle is β = π/2 + γ. By replacing this value in Eq. (6), the load phase jump can be calculated after some manipulations: To ensure that Eq. (7) is feasible, 1 k cos(ϕ) ≤ 1 has to be established. So the source amplitude must be more than the load power factor or k ≥ cos(ϕ). If this situation is not yielded, γ should be calculated through Eq. (8): Figure 8 and Fig. 9 show the load phase jump and the injected voltage magnitude for different values of δ. The load power factor is chosen as 0,7.  In Fig. 10 and Fig. 11, δ = 30 • is constant and the load phase jump and the injected voltage magnitude are plotted for different load power factor. There are some notices in these figures. It should be noted that the injected voltage magnitude is independent of δ. Moreover the break-points in these figures are relevant to the load power factor. In other word, the load can be compensated only through the injection of reactive power for k ≥ cos(ϕ); otherwise there would be an active power injection too.

Analysis of the Input Source Current
According to Fig. 12, the Z-source inverter based DVR has no energy storage and the required power for compensation is taken from the grid itself. So the source current can be a comparative characteristic of different compensation techniques for this kind of DVR.
As it has been mentioned, Z-source converter is able to control its input power factor. The shunt converter input voltage is the load voltage which is V P ost−f ault load = 1 (ϕ + γ) after compensation. By considering a unity power factor for the input of Zsource converter, the current of shunt converter would be I P ost−f ault M C = y (ϕ + γ). Also the source current is defined as I P ost−f ault s = z (ξ). Because there is no energy storage in the structure, the input and output active power of the Z-source converter (with shunt converter) should be equal. Therefore: now by using KCL we have: after some manipulation, Eq. (11) yields to:  (14) can be applied to all of the compensation strategies. It should be noted that for the pre-fault method γ = 0, for the in-phase method γ = δ, and for the energy optimized method can be calculated by Eq. (7) or (8).

Comparison Between Compensation Strategies
In this section, all compensation strategies are compared with each other. There are different criteria for comparison. Suppose that a disturbance with δ = 45 • occurs and the load power factor is cos(ϕ) = 0, 6. In Fig. 13 to Fig. 15 the magnitude of the injected voltage, the phase jump of the load voltage, and the magnitude of the input source current are shown, respectively. It could be observed that: • The in-phase method has the least injection voltage magnitude.
• For shallow disturbances, energy optimized method has less injection voltage magnitude than the pre-fault method.
• For large disturbances, the pre-fault compensation strategy has the most magnitude of the DVR injected voltage. This means that in this compensation method, DVR components should have a higher voltage rating compared with the other methods.
• The energy optimized method has the most load phase jump angle followed by the in-phase method. This parameter can be a criterion of the load voltage distortion especially at the start and the end of the disturbance occurring time.
• The pre-fault method compensates the load voltage to its pre-fault value so it makes no phase jump in the load voltage.
• The energy optimized method and the pre-fault method have the most and the least magnitude of the input current, respectively. The higher value of the input current makes a more severe disturbance for the upstream loads because it makes more voltage drop. Moreover the DVR components should have a higher current rating.

Simulation
Load side connected Shunt converter topology of Zsource inverter based DVR that operate based on prefault, in phase and energy optimized compensation methods are simulated in MATLAB/ SIMULINK to study the results of simulation tests. Table 1 shows the characteristic of Z-source inverter, DC-link and also load parameters. The injection transformer ratio is one. Phase a and c are stricken to the same fault that cause to reduce the peak voltage from 300 V to 120 V and 32 • shift phase also phase b is stricken to ξ = tan −1 sin(γ) 1 + tan 2 (γ) + x cos(β) [sin(ϕ + γ) − tan(γ) cos(ϕ + γ] cos(γ) 1 + tan 2 (γ) + x sin(β) [sin(ϕ + γ) − tan(γ) cos(ϕ + γ] . (13)  the different fault that causes to reduce the amplitude of voltage to 81 V and 54 • shift phase. Fig. 16 a), b) describes this image fault.

6.2.
In-Phase Compensation Method Study Figure 18 b), d) shows the effectiveness of thementioned topology usingthe in-phase method as its compensation strategy. As previously said, this method compensatesjust the amplitude of the voltage and the phase jump remains in the load voltage after compensation. According to Tab. 2 the injected voltage has a peak value of 178 V withthe angle of 31 • for phase a and c, and 217 V withthe angle of 55 • for phase b. So the injected voltage is almost in-phase with the supply voltage. It is expected thatthis method has the least amplitude of the injected voltage compared with other strategies. Moreover, its delay time is about half a cycle. The injection voltage from the 20 V link-DC makes the load voltage amplutide increasing to about 300 V but the phase jump is not compensated.  Figure 19 a), b) shows the load current during the fault. There is 33 • phase jump for phase a and c whilea 60 • phase jump occuredat phase b. Therefore the injected voltage angle should be about 120 • for phase a and c, and 150 • for phase b. The effectiveness of this compensation method is shown in Fig. 19 c)

Conclusion
Load side connected shunt converter topology is the best topology of DVR according to [8] that used VSI as a series converter. In this paper, Load side connected shunt converter topology of DVR with Z-source inverter as a series converter is used. In this paper different compensation strategies to provide the injec- tion voltage for a Z-source converter based DVR are analyzed. Different comparison criteria are studied. Results show that if the load is sensitive to voltage phase jump, pre-fault method should be used. However this method draws more current from the source which needs DVR components with higher current rating. If the load is insensitive to phase jump either in-phase or energy optimized methods can be used to compensate it. Here pre-fault compensation method is used for the identified topology of DVR. Simulation results show the benefit of this compensation method for this topology of Z-source inverter based DVR. The in-phase method has the least injection voltage magnitude that makes the components cheaper. Moreover for a similar disturbance compensation, the in-phase strategy needs less transformer voltage ratio compared to other methods. The energy optimized method needs the least input current magnitude to compensate disturbances. This will create less voltage drop for upstream loads and also the possibility to design a DVR with less current rating components.