APPLICATION OF INVERTER BASED SHUNT DEVICE FOR VOLTAGE SAG MITIGATION DUE TO STARTING OF AN INDUCTION MOTOR LOAD A. F. Huweg, S. M. Bashi MIEEE, N. Mariun SMIEEE Universiti Putra Malaysia - Malaysia norman@eng.upm.edu.my ABSTRACT by the motor. Abstract-- This paper shows a study of the static synchronous compensator (STATCOM) behaviors for voltage sag mitigation to an induction motor. The proposed of STATCOM stability model is justified based on the basic operation characteristics of phase control strategies. The simulation of 6 pulses STATCOM based on voltage source inverter (VSI) using Matlab Simulink is presented to show its good performance under balanced voltage sag condition due to the motor starting. Experimental testing has been made by using thyristor firing board control (FS36M). The STATCOM response of compensated reactive power to the system during voltage sag has been shown. Finally simulation results and experimental results have been described and compared once and good performance has been obtained. Introduction Power quality is a quite broad concept, summarizing different electrical power characteristics. Ideally, the delivered power should have perfect current and voltage wave shapes and hundred percent reliable. Over the last decade the power quality issue has become a matter of growing concern. There are several causes that have initiated and later stimulated this concern, however one other most important cause is the growing sensitive loads, (eg. Industrial plants) that use power electronics as a means of modernising their manufacturing processes [1]. As the sensitivity of the load increases the costs associated with the damage caused by voltage sags increase too. One of the most important voltage-related problems is voltage sag. Voltage sag is an rms reduction in the ac voltage at the power frequency, for duration from a half-cycle to a few seconds [2]. Magnitude and duration are its two most important characteristics. There are two causes of voltage sag, normally line to ground faults and motor starting. Line to ground faults result in voltage sags that are shorter (from approximately 10 ms) and have magnitudes down to 10%. Motor starting produces sags having smaller magnitudes but they last longer up to 600ms. Fig 1a,b shows voltage sag caused by line to ground faults and motor starting respectively. This paper focused on voltage sag caused by starting of induction motor. The induction motor are subjected to the voltage sag slow down but usually do not stop operating, if not tripped by contactors. Problems can occur due to torque oscillations that can be associated with very deep sags or to tripping of over current relays, due to the high current drawn (a) Fig.1. Typical waveform of voltage sag caused by (a) line to ground fault and (b) motor starting A solution to the problem of voltage sags can be introduced at several levels between the utility and the customer. Since the cost of the solutions increases as the mitigation equipment drifts a part from the customer, often it is more efficient to install the equipment closer to the customer [1]. Some solution for compensation of voltage sag has been presented earlier with two approaches, a shunt injection of reactive current and a series injection of voltage. Ampta Sannnino, et al [3] in their paper has carried out research to examine the operation of a series connected VSC for voltage sag mitigation static series compensator SSC depends on quite large dc source. However, the -right margins of display elements, such as the text headings, figure captions, table titles, table columns and references should be left ragged. Some application of STATCOM operation, analysis and mitigation have been discussed and presented in [4-5-6].
STATCOM Model In general, STATCOM is employed to generate or absorb reactive power. The active power generation or absorption capability of the STATCOM is normally used under special circumstances such as to enhance the steady state and transient voltage control, to improve the sag elimination capability. The STATCOM basically consists of a step-down transformer with a leakage reactance, a three-phase thyristor based voltage source inverter (VSI) or current source inverter (CSI), and a DC capacitor. A basic structure of the STATCOM is shown in Fig 2. The AC voltage difference across the leakage reactance produces reactive power exchange between the STATCOM and the power system, such that the AC voltage at the bus bar can be regulated so as to improve the voltage profile of the power system, which is the primary duty of the STATCOM. However, for instance, a secondary damping function can be added into the STATCOM for enhancing power system oscillation stability. Basic operation Fig 3 six pulses VSI Fig 4 shows the basic principle of STATCOM operation, it is as follows. The VSI generates a controllable AC voltage source behind the leakage reactance. This voltage is compared with the AC bus voltage system; when the AC bus voltage magnitude is above that of the VSI voltage magnitude, the AC system sees the STATCOM as an inductance connected to its terminals. Otherwise, if the VSI voltage magnitude is above that of the AC bus voltage magnitude, the AC system sees the STATCOM as a capacitance connected to its terminals. If the voltage magnitudes are equal, the reactive power exchange is zero. Fig 2 basic structure of STATCOM DC-AC Voltage Source Inverter The major aim of a VSI is to generate an AC voltage from a DC one; that is because of it is often referred to as a DC AC converter or inverter. It is able to generate a symmetric AC voltage with a desired magnitude and frequency, which can be fixed or varied according to the application. The voltagesource inverter is the building block of the STATCOM and other FACTS devices. The three-phase basic configuration is called six-pulse inverter, consisting of six asymmetric turn-off devices as shown in Fig 3; the SRC thyristor with line commutated and phase angle control has been used for this application. Fig 4 STATCOM operation Details of the implementation and use of the STATCOM models proposed here is discussed in regard to MATLAB program. However for this application the VSI is modeled to operate in steady state power system mode, which regulate the voltage during voltage sag event.
Principle of reactive power control The principle of control reactive power via STATCOM is well known that the amount of type (capacitive or inductive) of reactive power exchange between the STATCOM and the system can be adjusted by controlling the magnitude of STATCOM output voltage with respect to that of system voltage. The reactive power supplied by the STATCOM is given by VSTATCOM VL Q = VL X Where Q is the reactive power. V STATCOM is the magnitude of STATCOM output voltage. V L is the magnitude of system voltage. X is the equivalent impedance between STATCOM and the system. When Q is positive the STATCOM supplies reactive power to the system. Otherwise, the STATCOM absorbs reactive power from the system. Simulation results The system in Fig 5 has been modeled and simulated by using the MATLAB package program. The 415V grid voltage supply the 5KW induction motor load. Fig 6 shows the magnitude and duration of voltage sag due to starting of the induction motor before the installation of the STATCOM device. From this figure it is shown that the motor has switched on after 0.3sec of running the system, this time is required for charging the capacitor via the uncontrolled rectifier. The voltage sag magnitude is 0.25% lasted for 0.3 sec duration. Vrms Fig 7 voltage sag magnitude and duration after the instillation of the STATCOM device DC link capacitor voltage and current Figure 8 shows the dc capacitor voltage during the start-up of the STATCOM. In this figure during the interval from 0 to 0.3 sec the capacitor is charged. Then at 0.3 sec the motor is switched on, which causes the voltage sag in the main supply, in this instant the STATCOM will convert the dc power stored in the capacitor to ac power, this will cause the capacitor discharge as shown in the figure. Once the motor reaches the steady state the voltage sag will be cleared and the capacitor will start charging again to its nominal voltage. Figure 9 shows the pre-charging current and the discharging current with negative value, during voltage sag event. Pu(V) Vrms Fig 8 dc capacitor voltage Fig 6 voltage sag magnitude and duration before the instillation of the STATCOM device Fig 7 shows the rms voltage at load terminal after the installation of the STATCOM device. The sag shown in the rms voltage had duration of about 0.23sec and a depth of roughly 13.25%. The duration and magnitude of the voltage sag represents the response speed of the STATCOM to voltage sag. The improvement in the grid voltage at the motor terminal was approximately 11.75%. Pu(A) Fig 9 capacitor charging current
Experimental results Fig 10 STATCOM model The general structure of the laboratory STATCOM is illustrated in Fig 10. The inverter has a 6-pulse structure, and is composed of three phase SCR Thyristor Bridge. The control and triggering have been implemented by using thyristor firing board control (FS36M). The firing board is used to control the VSI by changing the thyristor firing angle. Fig 11a shows the rms voltage at the load terminal during the voltage sag before the STATCOM device was installed to the system. Fig 11b shows the rms voltage at the load terminal when the STATCOM device was connected to the system. The results show that the STATCOM improved the voltage sag from 28% to 18%, which mean the improvement was around 10%. It should be noted that the sag magnitude and duration in the simulation and hardware were not the same. This difference is due to the motor parameters, losing and the components were not ideal. Fig 11a the system voltage without STATCOM during voltage sag Fig 11b the system voltage with STATCOM during voltage sag
In the experimental testing, during the motor starting the voltage sag occurred and lasted for 0.35 second. During this interval the STATCOM inject the current to the system as shown in Fig 12, this figure show that the STATCOM is operating during voltage sag interval. [3] Wang, P., Jenkins, N., Bollen, M.H.J., Oct. 1998, Experimental investigation of voltage sag mitigation by an advanced static VAr compensator, Power Delivery, IEEE Transactions, vol.13, Issue 4, Pages 1461-1467. [4] Ricardo Da valos M., Juan M. Ramı rez*, Rube n Tapia O., January 2005, "Three-phase multi-pulse converter StatCom analysis", International Journal of Electrical Power & Energy Systems, Vol. 27, Issue 1, Pages 39-51. [5] X. -P. Zhang, E. Handschin and M. Yao, 15 December 2004, Multi-control functional static synchronous compensator STATCOM in power system steady-state operations, Electric Power Systems Research, Vol. 72, Issue 3, Pages 269-278. Fig 12 STATCOM compensator current during voltage sag event [6] Claudio A. Cañizares, Massimo Pozzi, Sandro Corsi and Edvina Uzunovic, July 2003, "STATCOM modeling for voltage and angle stability studies", International Journal of Electrical Power & Energy Systems, Vol. 25, Issue 6, Pages 431-441. Conclusion In this paper, the simulation model of the static synchronous compensator STATCOM based thyristor has been constructed on MATLAB program package. The laboratory STATCOM has been structured and tested as well. The influence of the dc capacitor to the voltage sag has been studied. The simulation and laboratory results have shown that they can completely mitigate the voltage sag by 10% to 11.75% in magnitude as well as improving the sag duration. The result presented in this paper shows that the STATCOM may significantly reduce the number of trips in the sensitive equipment. Acknowledgement The authors wish to thank Universiti Putra Malaysia (UPM) and the Ministry of Science, Technology and Innovation Malaysia (MOSTI) for the financial support to carry out the research work and CIRED Malaysia for the financial support to attend this conference. Reference [1] Knyazkin, V. Soder, L., 1-4 Oct. 2000, The use of coordinated control to mitigate the impact of voltage sags caused by motor start, Harmonics and Quality of Power, 2000. Proceedings. Ninth International Conference, vol.3, Pages 804 809. [2] Sannino, A., Svensson, J. Application of converterbased series device for voltage sag mitigation to induction motor load, Power Tech Proceedings, 2001 IEEE Porto, vol.2, Pages 6.