Voltage stability boundary and margin enhancement with FACTS and HVDC Priti Prabhakar a , Ashwani Kumar b,⇑ a Department of Printing Technology, GJU S&T, Hisar, India b Department of Electrical Engineering, NIT Kurukshetra, India article info Article history: Received 30 April 2014 Accepted 20 March 2016 Keywords: Voltage stability boundary Bus impedance matrix Thevenin’s equivalent network FACTS and HVDC abstract Voltage stability is a major concern of today’s power system, especially under heavily loaded conditions because of reactive power limits. FACTs devices are very effective solution to prevent voltage instability and voltage collapse due to fast and very flexible control. In this paper, the impacts of SVC, STATCOM, TCSC and HVDC on voltage stability boundary (VSB) in P–Q plane have been studied. The bus impedance matrix and load flow results are used to find the voltage stability boundary. The Z bus is modified to take into account the effect of FACTS on VSB. The variable susceptance model for SVC and variable series impedance power flow model for TCSC are used in Newton Raphson’s method. The STATCOM is modelled as variable voltage source connected in series with an equivalent impedance of the shunt connected transformer. Similarly HVDC is also modelled as two STATCOMs connected at each end of the line one as rectifier and another as inverter. Some important bus and line stability indices are evaluated to determine the most effective location for SVC/STATCOM and TCSC/HVDC respectively in order to achieve the maximum enhancement of voltage stability margin. The study has been carried out on IEEE-14 bus and IEEE-30 bus test systems using MATLAB programming. A comprehensive study is done to compare the effectiveness of FACTS devices and HVDC on voltage stability margins. Ó 2016 Elsevier Ltd. All rights reserved. Introduction The continuing interconnection, manifold increase in demand, restructuring, economic and environmental pressures led to a more complex and sensitive power system operating very close to its stability limits. The mainstream philosophy of restructured sector is to minimize investments and maximize the equipment utilization. This evolution of deregulated power system has increased the possible sources of system disturbances leading to a less robust, more unpredictable system as far as operation is concerned. In fact, what may seem stable in long term may not be stable in short term. As a result, in recent years since last three decades, several incidents of blackouts have been reported due to voltage instability [1,2]. Uncontrollable decay of the system volt- age at one or more load buses or even over a sufficient portion of the network as a response to load variation and generation or structure disturbances has been termed as voltage instability (VI). VI stems from the attempt of load dynamics to restore power beyond the capability of transmission and generation system [3]. Voltage collapse (VC) is the process of successive voltage decrease leading to blackout in significant parts of the system. The initial event may be due to a variety of causes – small gradual system changes, or large sudden disturbances such as loss of generating unit or heavily loaded line. Immediately following the loss of line there would be a considerable reduction of voltage at adjacent load centres due to extra reactive power demand. This would cause a load reduction and stabilizing effect. The actions of generator exciters and underload tap changer (ULTC) transformer to quickly restore voltages and hence loads worsen the situation. With each tap change operation, the resulting increment in load would increase line losses, which in turn cause a greater drop in load levels. As a result, the reactive power output of generators would increase and hit maximum excitation limit. The share of reactive loading would be transferred to nearby generators and thus leading to overloading of more and more generators. This cascade tripping of lines and generators would lead to major blackouts [2,3]. Several controls usually employed to mitigate VI and VC are proper adjustments of transformer tap settings, reactive power compensations (generators, synchronous condensers, shunt capac- itors, FACTs devices, etc.), active power transfer and load shedding. Voltage stability assessment and its enhancement have become the important aspects for power system operators and researchers. Many performance indices have been developed that can predict http://dx.doi.org/10.1016/j.ijepes.2016.03.038 0142-0615/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: ashwa_ks@yahoo.co.in (A. Kumar). Electrical Power and Energy Systems 82 (2016) 429–438 Contents lists available at ScienceDirect Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes