Keywords

1 Introduction

Power quality (PQ) is an important and serious issue in power distribution network system. In present day, the industrial equipment is primarily based on electronic components such as rectifier, thyristors together with programmable sensing controllers and power electronic drives [1]. These devices are very sensitive to instabilities and end up less tolerant to PQ issues [3]. In the industrial and commercial equipment, voltage sags, swell, voltage fluctuation are serious and most common problems [1].

D-STATCOM is a reactive power shunt jointed reimbursement device that is capable for generating and absorbing the reactive power and also provides voltage sustenance to a system bus [4]. In a renewable energy system, it is also utilized in improving the constancy of the power system [2]. The reactive power imbalances can be minimized through the compensation device [5]. D-STATCOM is also capable to solve fault problems and compensate harmonics through capability of the system [7].

The first D-STATCOM was introduced by Japan country and it transported to the UK, where it has been installed in southwest England to adjacent a wind farm.

The Flexible Alternating Current Transmission System (FACTS) offers a fast and consistent control mechanism over transmission line constraints, like voltage, phase angle between the sending end and receiving end voltage, line impedance [8]. Custom power devices (CPD) are designed for distribution of voltage supplies at low voltage and encroachment of PQ due to which the system becomes reliable [11]. Solid-state CPD is utilized in electrical power network to offer reimbursement and advance voltage profile [1]. Some CPD are D-STATCOM, unified power quality conditioner (UPQC) as a combination of series and parallel [6], Dynamic Voltage Restorer (DVR) is in series connection [9] among D-STATCOM which is a cost operational solution for reimbursement of reactive power [6]. A FACTS is a power semiconductor device which is utilized to sustain the PQ by preserving superior movement of power and controlling the dynamically stability of system through altering the system constraints like voltage, angle of phase, and impedance [13].

2 Configuration and Operation of D-STATCOM

A D-STATCOM is voltage source inverter (VSI)-fed semiconductor-based power electronic expedient [2] that is associated with shunt to the system to moderating harmonics and PQ problems [9]. Its performance depends upon different control algorithms, for taking out reference current to deliver pulses to gate-terminals of VSI [10]. The D-STATCOM is extremely effective to providing regulation at load voltage. It keeps rated voltage value at load point that has numerous undesirable things from customer point of view. Within voltage value of 1 per unit at load point, D-STATCOM force to load to drive continuously at rated value [3]. It can easily convert both active and reactive power within electrical power distribution network through flexible amplitude and phase angle of the voltage converter device [4].

2.1 MATLAB Simulink Model of D-STATCOM

In Fig. 1, three phase supply 20 kV is utilized in power distribution system and nonlinear load is connected as a load in power system network. Three phase distribution transformer is connected with the three phase supply voltage. D-STATCOM is a shunt-connected device which is in between the power supply and nonlinear load. A fault occurs in distribution line near a load in a transmission line network. Circuit breaker is used to isolate the faulty network in power system network which can observe the difference at the time of fault and after the compensation by D-STATCOM. The symmetrical and unsymmetrical fault study and performance is done by the given D-STATCOM Simulink model.

Fig. 1
figure 1

MATLAB simulink model of D-STATCOM

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2.2 Controlling Scheme of D-STATCOM

A controller is utilized to sustain a continuous voltage magnitude at the value of point where a sensitive load is associated under system disturbances [11]. The VSC switching stratagem is based on a sinusoidal pulse width modulation (SPWM). This technique compromises a more efficient flexible and reliable preference than the fundamental frequency switching (FFS) techniques preferred within FACTS applications [8]. High switching frequencies (HSF) are utilized to improve the effectiveness of the voltage converter without acquiring substantial switching losses [12]. The error signal is the input for controller which is acquired from the reference value and rms value of the voltage measured at the terminal point [16]. This error is processed through a PI controller that output is the angle \( \phi \) which is delivered to the PWM signal generator [13]. In case of indirectly controlled converter, power exchanges with the network simultaneously of the active and reactive power. Error signal is achieved through relating the reference voltage within rms measured value near the load point [15]. The PI controller develops the error signal and produces requisite and suitable angle to determine the error to zero; i.e., the load rms voltage is taken back to the reference voltage [14] (Fig. 2).

Fig. 2
figure 2

Mathematical model of pulse width modulation (PWM) generator

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The sinusoidal signal VControl is phase modulated by means of the angle

$$ V_{\text{A}} = \sin \omega t + \phi $$
(1)
$$ V_{\text{B}} = \sin \omega t + \phi - \frac{2\pi }{3} $$
(2)
$$ V_{\text{C}} = \sin \omega t + \phi + \frac{2\pi }{3} $$
(3)

The foremost constraints of SPWM technique are modulation of amplitude indeed of signal and (FMI) frequency modulation index (Mf) of the triangular signal. The amplitude index (AI) is reserved stable at 1 per unit in directive to achieve the component of maximum fundamental voltage at controller output. The switching frequency (FS) is set at the point 1080 Hz. The frequency modulation index is specified by

$$ \begin{aligned} M_{\text{f}} & = \frac{{F_{\text{S}} }}{{F_{\text{f}} }} = 1080/50 \\ M_{\text{f}} & = 21.6 \\ \end{aligned} $$
(4)

where \( M_{\text{f}} \) is modulation index, \( F_{\text{S}} \) is switching frequency, and \( F_{\text{f}} \) is fundamental frequency

3 Performance of D-STATCOM Simulink Model Under Different Fault Conditions

3.1 Single Line-to-Ground Fault (SLG Fault)

In case of SLG fault, phase A is faulty and connected to the ground and phase B and phase C are unfaulty phases. The fault has impedance which is known as fault impedance denoted by ZF. Phase B and phase C are like an open circuit, and there is no current flow through these phases.

In Fig. 3a, single line-to-ground (SLG) fault shows the result when D-STATCOM is not connected along with transmission line, and the rms voltage value is 0.80 per unit duration of 0.2–0.4 s.

Fig. 3
figure 3

a SLG fault waveform without D-STATCOM. b SLG fault waveform without D-STATCOM

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Figure 3b shows the rms value is 0.99 per unit when D-STATCOM is coupled within transmission line and regulates the voltage near about to 1 per unit for time period of fault and attains the sinusoidal waveform at the load terminal in power distribution system.

3.2 Line-to-Line Fault (LL Fault)

In case of LL fault, fault takes place between two conductors, phase B and phase C, and both are short circuit; current flows through both phases. Fault impedance is ZF.

Figure 4a presents the result without connected D-STATCOM along with distribution line, and rms voltage value is 0.75 per unit which is less than 1 per unit, and the sinusoidal waveform of system is not perfectly sinusoidal.

Fig. 4
figure 4

a LL fault waveform without D-STATCOM. b LL fault waveform with D-STATCOM

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Figure 4b shows the voltage rms value is 0.95 per unit when D-STATCOM is utilized along with distribution line and regulates the voltage near to 1 per unit for duration of fault and achieves the sinusoidal waveform at the load terminal.

3.3 Double Line-to-Ground Fault (LLG Fault)

In case of LLG fault, phase B and phase C are short circuit and have their fault impedance value ZF, and both conductors are connected to the ground after a fault through a common line and have the value of ground impedance Zg.

Figure 5a presents the result when D-STATCOM is not used in distribution system and shows the rms voltage value is 0.70 per unit and waveform is not sinusoidal.

Fig. 5
figure 5

a LLG fault waveform without D-STATCOM. b LLG fault waveform with D-STATCOM

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Figure 5b shows the rms value is 0.93 per unit when D-STATCOM is utilized along with system. D-STATCOM regulates the voltage 0.70–0.95 per unit Regulated voltage is 0.25 per unit duration of fault, and achieved waveform is sinusoidal at the load terminal.

3.4 Three Phase Fault

Three phase fault is known as symmetrical fault which involves all the three phases A, B, and C.

Figure 6a presents the rms voltage value that is 0.70 per unit duration of 0.4–0.6 s; this result is achieved when D-STATCOM is not connected along with distribution line. In this case, voltage sag takes place.

Fig. 6
figure 6

a Three phase fault without D-STATCOM. b Three phase fault with D-STATCOM

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Figure 6b shows the rms value is 0.94 per unit when D-STATCOM is connected with system. It regulates the voltage 0.7–0.94 per unit during fault occurring time (0.4–0.6 s). D-STATCOM is used to get sinusoidal balanced waveform at the load terminal in power distribution system.

3.5 Three Phase-to-Ground Fault

In three phase-to-ground fault, all the phases are involved with fault and connected with ground. This fault destroys the sinusoidal wave and causes to the unbalanced condition at the near nonlinear load.

Figure 7a presents the three phase-to-ground fault, and the result shows the rms voltage value is 0.60 per unit during the time 0.4–0.6 s; this duration is fault occurring time, and D-STATCOM is not linked within distribution line; the waveform is not sinusoidal at the load terminal.

Fig. 7
figure 7

a Three phase to ground fault without D-STATCOM. b Three phase to ground fault with D-STATCOM

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Figure 7b shows the rms value is 0.90 per unit when D-STATCOM is attached along with system; it regulates the voltage 0.60–0.90 per unit D-STATCOM regulate value is 0.30 per unit during fault condition, and achieved waveform is sinusoidal at the load terminal in power distribution system.

4 Results and Discussion

Fault condition

Without D-STATCOM voltage (RMS value) (per unit)

With D-STATCOM voltage (RMS value) (per unit)

Power quality improvement (per unit)

SLG fault

0.80

0.99

0.19

LL fault

0.75

0.95

0.20

LLG fault

0.70

0.93

0.23

Three phase fault

0.70

0.94

0.24

Three phase-to-ground fault

0.60

0.90

0.30

5 Conclusion

The PWM voltage control scheme of D-STATCOM is utilized to regulate the load voltage under the symmetrical and unsymmetrical fault, and the D-STATCOM can regulate the load voltage to the desired level which is described in this paper. The performance of D-STATCOM has been evaluated for linear loads and static nonlinear loads. In this paper, the problems associated with faults determination and proper D-STATCOM injected reactive power rating which causes improvement in profile of voltage, current and reduction of power-losses in electrical power system network has been presented. Pulse width modulation control scheme is appropriate for both unbalanced load compensation and balanced RC or RL load. In this paper, the exploration of the D-STATCOM is carried out to improve the power quality in power distribution networks within static linear and nonlinear loads. Proportional integral (PI) controller is utilized within device to improve system performance. The results show the satisfactory performance of D-STATCOM in the distribution system under different fault conditions, and it can be concluded that D-STATCOM effectively improves the power quality and voltage profile in power distribution system.