Keywords

19.1 Introduction

Power quality of the power supply has become a significant factor to attract customers in the deregulated electricity market [1, 2]. Power quality problems are addressed in many standards to specify the acceptable levels of power quality indices [3]. Utilities keep power quality indices within standard limits using different power conditioning techniques such as passive filters, active filters [4, 5] and hybrid filters [6–9].

However, the fast and steady increase in implementing distributed generation units (DGs) in power distribution system [10] due to its positive impacts [11] such as voltage support, improved reliability, small size and losses reduction. With widespread of DG implementation, it is essential that applying DG in a manner that avoids degradation of power quality or negatively affecting the operating conditions of the existing distribution network. This can be achieved by proper control and regulation of the DG grid interfacing converters to improve the distribution system efficiency and power quality [12, 13].

Moreover, energy storage systems such as batteries, ultra capacitors and flywheels are used to improve power system performance. This paper presents a strategy based on using portion of the stored energy and feed this energy through a series compensator to mitigate power quality problems in order to improve power supply to critical loads. The proposed scheme based on interfacing energy storage units using voltage source inverter (VSI). The VSI supplied from the energy storage unit is connected through a transformer to inject voltage in series with the sensitive load. Where, Voltage-control method is utilized in this paper to control the VSI as this method is more flexible and simpler than the conventional current-control method and has similar compensation performance [14].

The power system is modeled and simulated using MATLAB/SIMULINK. In this model voltage-control algorithm is used to generate the firing signals for the VSI. The main supply voltage is considered as the reference voltage. In addition, a nonlinear load is connected to point of common coupling (PCC) at which the critical load is also connected. This nonlinear load will distort the voltage at the PCC due to—the system impedance Moreover, voltage Sag is simulated by creating three phase fault at the PCC and voltage swell occurs after the clearing of the fault.

19.2 System Block Diagram

The system block diagram is shown in Fig. 19.1. The voltage reference is selected to be the voltage at the supply terminal and this voltage is measured and fed to the controller to be compared with the distorted voltage at the PCC. The error signal is then fed to the controller to generate the gating signals for the 3-phase VSI.

Fig. 19.1
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Block diagram of series compensator fed from energy storage

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The controller used to generate the gating signals is a PI controller. PI controller is selected in this application due to its simplicity, flexibility and its ability to achieve zero steady state error. Moreover, the tuning of the PI controller can be done through Ziegler-Nichols tuning technique or simply by try and error which is utilized in this paper.

19.3 Simulation Results

The proposed model shown in Fig. 19.2 is simulated using MATLAB/SIMULINK. And the data used to simulate the system are presented in Table 19.1 [14].

Fig. 19.2
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MATLAB/SIMULINK model

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Table 19.1 Simulation model parameters
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19.3.1 Simulation Results at Normal Conditions

The model at Fig. 19.2 is simulated using MATLAB/SIMULINK with normal operating conditions. And, the voltage at the point of common coupling obtained from simulation is shown at Fig. 19.3.

Fig. 19.3
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Three phase voltages at point of common coupling

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As shown in Fig. 19.3, the voltage at the PCC is distorted with high order harmonics. Moreover, the voltage suffers from voltage notches due to the commutation overlap occurs within the full bridge rectifier (nonlinear load).

Curves of Fig. 19.4, Show the voltage at the sensitive load terminals during normal operating conditions.

Fig. 19.4
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Three phase voltages at load terminals in per unit

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After applying the proposed series compensator fed from the energy storage device, the voltage at the sensitive load is greatly improved and the voltage notches are completely removed. This proves the effectiveness of the control algorithm in removing the superimposed harmonics at the PCC.

Hence, keeping the supply at the sensitive load terminals nearly pure sinusoidal independent of the non-linear load connected to the same PCC.

Figure 19.5 shows the harmonic spectrum for the load voltage in the presence of the series compensator. Total harmonic distortion (THD) is calculated for the load voltage and found to be 0.3179 %. The THD calculated is significantly small due to the effectiveness of proposed series compensation.

Fig. 19.5
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Voltage harmonic spectrum at load terminals

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19.3.2 Simulation Results When the System Is Subjected to Voltage Sag

In order to investigate the effectiveness of the proposed voltage control in mitigating the voltage sag problem, a three phase short circuit is created at the PCC at t = 0.05 s and removed at t = 0.1 s and the response of the controller is investigated. The capability of the compensator to mitigate such voltage sag problem is checked by monitoring the load voltage during fault period and the following results were obtained:

The voltage at PCC is shown in Fig. 19.6.

Fig. 19.6
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Three phase voltages at point of common coupling during fault

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The voltage at the PCC is reduced to a certain value depends on the impedance of the fault created. As shown in Fig. 19.6 the voltage sag is 50 % during the simulated fault.

Curves of Fig. 19.7, Show the voltages at load terminals in per unit.

Fig. 19.7
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Three phase voltages at load terminals during fault

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From the simulation results shown in Fig. 19.7, the compensator has kept the voltage at the load almost constant during the fault with very small drop about 2 % (the voltage at load terminal during fault is 0.982 per unit).

Also, it is noticeable that the voltage after fault clearing has some transients and overshooting until the compensator restores the pre-fault voltage conditions at load again. However, the compensator successfully limits the voltage swell follows fault clearing in addition to preventing voltage sag during fault duration.

The compensator effect on reducing harmonics even during disturbances (with PI tuned to mitigate sags and swells) is studied by calculating harmonic spectrum as shown in Fig. 19.8 with and without the compensator.

Fig. 19.8
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Comparison between voltage harmonic spectrum with and without the compensator

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Based on results shown in Fig. 19.8, adding the compensator reduced the voltage harmonic contents at load terminals. Comparison of some important individual harmonic values for both cases is shown in Table 19.2.

Table 19.2 Comparison between individual harmonics values with and without the compensator
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From the results, it is obvious that the compensator had significant effect on reducing the values of each individual harmonic as well as the THD at load terminals with same PI values tuned to mitigate voltage sags/swells.

19.3.3 Simulation During Supply Voltage Interruption

To study the ability of this configuration to act as an uninterruptable power supply (UPS) to secure the supply for the sensitive load even if the main supply is interrupted.

This case is studied by creating a bolted short circuit at the PCC which results in almost zero voltage at the PCC. This is simulated by applying the bolted fault at t = 0.05 s and clearing the fault at t = 0.1 s.

Voltage at PCC during fault is shown in Fig. 19.9. It is clear that the voltage at the PCC is dropped to very small value (nearly zero) as the impedance of the bolted fault created is zero.

Fig. 19.9
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Three phase voltage at point of common coupling with bolted fault

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Voltage at the load terminals with bolted fault is shown in Fig. 19.10. From the obtained results, the compensator has almost kept the voltage at the load constant during the fault with very small drop about 3 % (the voltage at load terminal during fault is 0.9698 per unit). So that the proposed compensation is capable of acting as UPS during supply interruption for certain duration depends on the energy stored without retuning of PI parameters.

Fig. 19.10
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Three phase voltage at load terminals with bolted fault

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19.4 Conclusions

This paper has discussed the effectiveness of using the energy storage units which are used in power systems for power quality improvement. Portion of the energy stored is utilized through a series compensator connected to a sensitive load in order to keep the power quality at load within the acceptable limits. Also, energy storage units are being extensively used with renewable energy resources to improve their operation (e.g. smoothing output power of wind energy). Hence, this paper introduces the utilization of stored energy to improve power quality for sensitive loads.

A control scheme using a voltage controlled method is used in this paper. The proposed technique is proved to be capable of reducing the harmonics at the PCC and to mitigate the voltage sag/swell problems. This flexible and simple voltage controlled method results in an excellent performance. The proposed technique produces significant reduction in harmonic contents in addition to effective mitigation of voltage sag/swell problems. This was proved by keeping the load voltage around 0.97 per unit during three phase dead short circuit on the PCC and after fault clearing.

The proposed scheme is simulated and investigated using MATLAB/SIMULINK and the results are presented to show the effectiveness of the proposed harmonic mitigation technique.