Abstract

This study investigates the effects of applying Static Synchronous Compensator (STATCOM) to electric power systems with losses. The northeast and part of northwest of the Nigerian grid system is chosen for a case study in order to characterize the performance of the entire power systems and infer at what point or under what condition, the system can suffer from emergency voltage instability. The intent is to examine whether such FACTS (Flexible AC Transmission System) device can be effective in minimizing the transmission losses and improve the voltage stability. Simulation results carried out in the PSCAD/EMTDC environment are presented to show the significant reduction of power losses in transmission lines and improvement of voltage stability with STATCOM.

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INTRODUCTION

With the ever increasing complexities in power systems across the globe and the growing need to provide stable, secure, controlled, economic and high-quality electric power, it is envisaged that Flexible AC Transmission System (FACTS) controllers are going to play a critical role in power systems (Arnold, 2001). FACTS devices can be used to influence power flows to support voltages to increase transmission capacity over an existing transmission corridor to damp system oscillations and to increase stability margins. Due to the fast control characteristics and continuous compensating capability of these FACTS devices, it is imperative that they should be made with high precision to ensure correct behaviour on power systems.

Otherwise, they could quickly initiate system instabilities which can lead to power system outages, loss of generation or unacceptable voltage fluctuations (Gyugi, 1994). Basically, Static Synchronous Compensator (STATCOM) is a voltage-sourced converter which is connected in shunt with the transmission line through a shunt transformer.

It is voltage favourable and can generate full capacitive output at low voltage (Canizares, 2000; Sadikovic et al., 2004; Song and Johns, 1999).

Usually, a long and loaded overhead transmission line is characterized as an inductive reactance (i.e., consumes reactive power as it transmits the active power) resulting in power losses. But the connection of STATCOM to the transmission line will increase, its transmission capacity and will produce more reactive power to provide voltage and transient stability and hence reducing the power losses on transmission lines.

Many studies have been carried out and reported in literature on the use of STATCOM in improving voltage and transient stability (Canizares et al., 2003; Mathur and Varma, 2002; Scarfone et al., 2003). However, not much has been reported on the minimization of power losses on transmission line by the use of STATCOM. This study studies the effects of applying STATCOM to electric power systems with the intent to minimize the transmission losses and improve voltage stability under contingency conditions.

MATERIALS AND METHODS

STATCOM modeling and its control system: In order to investigate the impact of STATCOM on power systems effectively, appropriate STATCOM model is very important.

STATCOM is a voltage sourced converter which is connected in shunt with the transmission line through a shunt transformer.

Thus, it is modeled using a voltage source V_STATCOM and V_shunt. During the steady state and dynamic analysis, the control variables V_STATCOM and V_shunt are transformed into injected nodal currents according to Eq. 1. The mathematical model of STATCOM is shown in Fig. 1.

Fig. 1:	STATCOM mathematical model

 

Fig. 2:	Basic STATCOM pulse width modulation voltage control

 

 

Fig. 3:	The voltage/current characteristics of a STATCOM

 

 

(1)

 

A simple pulse width modulation voltage controller of STATCOM is shown in Fig. 2 (Uzunovic et al., 1999; Reeve et al., 1999). The ac bus voltage magnitude is controlled through the modulation index, m since this has a direct effect on the ac side Voltage Sourced Inverter (VSI) voltage magnitude. The phase angle α, determines the active power flowing into the controller which charges and discharges the capacitor.

Fig. 4:	Northeast and part of Northwest Nigeriar grid system

This phase angle is used to directly control the dc voltage magnitude. The controller limits are defined in terms of the controller current limits (i.e., maximum and minimum converter currents I_max(I_Lmax) and I_min(Ic_max), respectively.

Figure 3 shows the controller current limits in the voltage/current characteristic of the STATCOM which illustrates how the STATCOM offers better voltage support and improve voltage stability margin by providing more reactive power at lower voltages (Garcia-Gonzalez and Garcia-Cerrada, 1999; Wang et al., 1998).

Studied system: The northeast and part of Northwest of the Nigerian grid system (330 kV) is used to validate the STATCOM model discussed in this study. The network is modeled with 5 buses, 4 branches and 1 hydro-unit as shown in Fig. 4. The parameters of the system are obtained from Power holding company of Nigeria. STATCOM is optimally sited at bus 5.

RESULTS AND DISCUSSION

Simulation results: At steady state, the network in Fig. 4 was evaluated and observed to be prone to voltage instability at loads above 200 MW. This network is used to validate the STATCOM model into the PSCAD (Power System Computer Aided Design) based on detailed EMTDC (Electromagnetic transients) simulation results. The EMTDC is used to simulate the STATCOM operating under phase control.

A 3 phase to ground fault was initiated at bus 2 at 300 ms which triggers the circuit breaker at 400 ms to clear the fault without STATCOM and the voltages obtained were as shown in Table 1. Consequently, the power losses along the transmission line were also read and recorded as shown in Table 2.

Table 1:	Bus voltage profile with and without STATCOM

 

Table 2:	Transmission losses with and without STATCOM

 

 

Fig. 5:	Voltage output with and without STATCOM (kV)

 

 

Fig. 6:	Real power losses with and without STATCOM

 

 

Fig. 7:	Reactive power losses with and without STATCOM

 

 

Table 3:	Voltage output with variation in load demand at Gombe bus

 

 

Table 4:	Transmission losses with variation in load demand at Gombe bus

 

 

Fig. 8:	Voltage output at the load of; a) 67.9 MW; b) 215.0 MW (Gombe bus)

 

It is generally observed that the voltage at which bus 5 operated was very low but considerably improve at the connection of STATCOM (Fig. 5). However, the transmission line losses were reasonably high.

But with the STATCOM embedded on the line, the losses were drastically reduced as shown in Fig. 6 and 7. Furthermore, the loading of the system was varied at bus 5 in order to examine the robustness of the STATCOM in the reduction of the system losses and voltage stability enhancement.

Fig. 9:	Real power losses at; a) 67.9MW load; b) 215.0 MW; c) Reactive power losses at 67.9 MW; d) 215.0 MW load (Gombe bus)

The system was then results in system voltages and losses as shown in Table 3 and 4, respectively. The effects of STATCOM can be appreciated as it is connected at the variation of the load at Gombe (Fig. 8a, b and 9a, b).

CONCLUSION

Evaluations of the electric power network at steady state showed that Gombe in Northeast region of the Nigerian grid system is vulnerable to possible voltage collapse. Simulations confirmed that a dynamic reactive compensator such as a STATCOM could provide the fast acting voltage support necessary to prevent the possibility of voltage collapse and minimize the power losses along the transmission lines.

It is observed that the reduction of reactive power losses along the transmission lines is more pronounced than the active power losses with consequent improvement of the system voltage profiles as the STATCOM is connected to the system.

How to cite this article:

Nwohu Mark Ndubuka. Effects of Static Synchronous Compensator (STATCOM) on Voltage Stability and Transmission Losses of Electric Power Systems.
DOI: https://doi.org/10.36478/ijepe.2011.13.17
URL: https://www.makhillpublications.co/view-article/1990-7958/ijepe.2011.13.17