The basic power quality problems in the distribution network are voltage sag (dip), voltage flickering, and the service interruptions. STATCOM is a Flexible AC Transmission Systems (FACTS) technology device which can independently control the flow of reactive power. This paper presents the simulation and analysis of a STATCOM for voltage dip and voltage flickering mitigation. Simulations are carried out in MATLAB/Simulink to validate the performance of the STATCOM. A comparison between the six-pulse inverter and the five-level diode-clamped inverter is carried out for the performance of 66/11 KV distribution system.

STATCOM has got a much more widely application due to the advent of the concepts of the smart grid and the microgrid and the rapid development of new energy and the distributed generation. The Distributed Static Compensator (D-STATCOM) becomes the tendency of the reactive power compensation and the power quality control in distributing networks at the present time. It is of great significance to enhance the power quality and keep the relay protection devices working normally as it can make a comprehensive compensation to voltage fluctuation, voltage flicker, and three-phase unbalance. The output harmonic in D-STATCOM comes to our attention, as it is a member of the power electronic devices. The relationship between power quality and distribution system has been a subject of interest for several years.

The application of a high-voltage multilevel inverter in a 13.8 kV distribution system Static Synchronous (SSC) is examined in [

Topologies like diode-clamped inverter (neutral-point clamped), capacitor clamped (flying capacitor), and cascaded multicell with separate dc sources are presented in [

A STATCOM configuration is described in [

Variation on the typical MPC control scheme for a H-Stat Com which provides excellent current tracking performance while simultaneously trading off the voltage balancing characteristics with the switching losses is presented in [

Flickering curve.

In this paper, two D-STATCOM controllers based on six-pulse inverter and five-level diode-clamped inverter are proposed. Both strategies are simulated using MATLAB/Simulink models. Simulation results confirming the effectiveness of the control schemes to impose a linear STATCOM dynamics are presented.

D-STATCOM is one of the most recent FACTS devices for power transmission shunt compensation. It is shunt-connected device which was developed as a static VAR compensator where a Voltage Source Converter (VSC) is used instead of controllable reactors. The STATCOM can be seen as a current source since it is connected in shunt with the distribution system and the load. By controlling the magnitude and the phase angle of the output voltage of the VSC, both active and reactive power can be exchanged between the distribution system and the STATCOM. Being a shunt-connected device, the STATCOM mainly injects reactive power to the system [

D-STATCOM configuration on the radial system.

State vector diagram.

The stationary reference frame in terms of synchronously rotating reference frame is illustrated by the following equation:

This can be elaborated with angle

Corresponding current equation is given as

The relationship between current and voltage equation is shown by the following equation:

The value of

Linearization of (

The characteristic equation is not a function of firing angle. Hence, firing angle does not affect the position of characteristic roots on the complex plane.

The stability of STATCOM can be tested with Routh-Hurwitz criterion. By assigning

Examination of all elements in the first column of Routh's array reveals that all elements are positive, and the STATCOM is a stable system. Therefore, the values of resistors, inductors, and capacitors in the STATCOM equivalent circuit have no effect on stability.

Equation for the steady state operation of STATCOM can be obtained from the dynamic model by setting all derivative terms to zero. After transformation into

The equations of direct current, quadrature current, and capacitor voltage do not contain capacitor. Hence, the size of dc capacitor does not affect STATCOM steady state performance. Especially, the quadrature current, which is reactive current (

Steady-state response.

At steady state, the reactive current (

Verification of the proposed control strategies was accomplished through simulation studies using a detailed model of a three-phase STATCOM implemented in MATLAB/Simulink. The test data taken for simulation is given in the Appendix. For easy comparison of the performance of a STATCOM under the proposed control schemes, results concerning the six-pulse inverter are presented first.

In this case, the D-STATCOM is prepared with six-pulse inverter. The complete 66/11 KV utility distribution system is shown in Figure

(a) Circuit implementation of the D-STATCOM with the six-pulse inverter. (b) MATLAB Simulation model for D-STATCOM with 6-pulse converter for mitigation of voltage dip and voltage flickering.

Initially, the D-STATCOM is not connected to the system and the load of pure inductive of 10 MVAR is applied on the system in the time interval of 0.1 sec to 0.6 sec as shown in Figures

Reactive power drawn at the time of applying the inductive load.

Voltage dip representation because of the sudden switching of inductive loads.

Voltage dip mitigation because of the application of six-pulse inverter.

Comparison of with and without D-STATCOM with six-pulse inverter.

The flickering circuit prepared with the help of R-L load which is periodically operated on the system causes the voltage flickering at the PCC (bus1) as shown in Figure

Voltage flickering because of the inductive loads likes electric arc furnaces and rolling mills.

Voltage flicker mitigation by applying the D-STATCOM six-pulse inverter.

The instantaneous current of the D-STATCOM is obtained by

FFT transformation of six-pulse inverter.

In this case, the D-STATCOM is prepared with five-level diode-clamped multilevel inverter. The complete 66/11 KV utility distribution system is shown in Figure

(a) Voltage dip because of the sudden application of the inductive loads. (b) MATLAB Simulation modal for D-STATCOM with five-level inverter for mitigation of voltage dip and voltage flickering.

Initially, the D-STATCOM is not connected to the system and the load of pure inductive of 10 MVAR is applied on the system in the time interval of 0.1 sec to 0.6 sec as shown in Figure

Voltage dip because of the sudden application of the inductive loads.

The flickering circuit is prepared with the help of R-L load which is periodically operated on the system cause the voltage flickering at the bus1 (PCC). The magnitude of flickering level without the D-STATCOM in the circuit is 2.09% which is above the tolerable limits. The voltages dip mitigation is shown in Figure

Performance comparison between the six-pulse and the five-level diode-clamped inverters.

Type of inverter | D-STATCOM status | T.H.D. |
Voltage flickering | Voltage dip |
---|---|---|---|---|

Five-level diode-clamped inverter | Off | 12.16% | 2.09% | 17.50% |

On | 0.29% | 0.00005% | ||

Six-pulse inverter | Off | 36.35% | 2.09% | 17.50% |

On | 0.68% | 3.043% |

Voltage dip mitigation by D-STATCOM five-level diode clamped.

Comparisons of voltage dip mitigation with and without D-STATCOM having five-level inverter.

Reactive power drawn because of flickering loads.

Voltage flickering.

Voltage flicker mitigation because of five-level diode-clamped inverter D-STATCOM.

Comparison of voltage flicker mitigation with and without D-STATCOM.

FFT transformation of five-level diode-clamped inverter.

Voltage dip and voltage flickering are the two major power quality problems which are frequently seen in the distribution systems. These power quality problems in 66/11 KV distribution system are investigated in this paper. The analysis and simulation of a D-STATCOM application for the mitigation of power quality problems are presented and discussed. Here, the D-STATCOM was prepared with the six-pulse inverter and the five-level diode-clamped multilevel inverter. A systematic approach for designing a nonlinear internal controller for the converter has been developed. For the STATCOM with five-level inverter, the mitigation of the power quality problems is effective. The strategy has been validated with extensive MATLAB Simulation results.

The following test data is considered for MATLAB Simulation.

100 MVA, 66 KV,

100 MVA, 66/11 kV,

+/− 10 MVAR, 11 kV on secondary side of transformer.