**1. Introduction**

156 Induction Motors – Modelling and Control

1989), p. 328 (in Russian).

Kostic, M. & Nikolic, A. (August 2010). Negative Consequence of Motor Voltage Asymmetry and Its Influence on the Inefficient Energy Usage, *Wseas Transaction On* 

Kostic, M. (1998). Reduction of loads and electric energy consumption by setting voltage

Kostic, M. (2001). Evaluation methods for load and efficiency of induction motor in the exploitation, *11th International Symposium Ee 2001*, Novi Sad, Serbia, pp.332-336. Kostic, M. (2010). Equivalent circuit parameters of the squirrel-cage induction motors in short circuit regime, *Tehnika*, *separate Elektrotehnika* 5/2010), pp. 7E-13E (in Serbian). Kostic, M.; Stanisavljevic, I.; Ivanovic, M.; Jankovic, R.; Mihajlovic, Lj. & Vasic, P. (2006). The Reduction of Own Electric Energy Consumption of Thermal Power Plants, *Symposium* 

Kravčik. A.E. (1982). *Induction Machines Handbook* (Moscow, 1982), p. 504, (in Russian). Linders, J.R. (July/August 1972). Effects of Power Supply Variations on AC Motor Characteristics, *IEEE Transaction on Ind. Applic.* Vol. IA-8", No 4, 1972, pp. 383-400. Radin, I.; Bruskin & Zorohovič, A.E. (1989). *Electrical machines: Induction machines* (Moscow,

Vukic, DJ. (1985). Time Harmonics Influence on Operating of Induction Motors, *Tehnika*,

*Circuits And Systems*, Issue 8, Volume 9, August 2010, pp. 547-556.

*Power Plants 2006*, Vrnjacka Banja, Serbia, 2006, No paper 59.

separate Elektrotehnika 12/1985), pp.11E-13E (in Serbian).

magnitude, *Elektroprivreda Magazine*, *No. 3, 1998*, pp 65-78 (in Serbian).

As the power system is being operated in an economic and environment friendly fashion, there is more emphasis on effective resource utilization to supply the ever increasing demand. Consequently system experiences heavy power transaction, and one of the very important stability phenomena, namely voltage stability, is capturing the attention of many power system engineers, operators, researchers, and planners. Concerns for voltage instability and collapse are prompting utilities to better understand the phenomenon so as to devise effective, efficient and economic solutions to the problem.

Past studies have investigated the intricate relationship that exist between insufficient reactive power support and unreliable system operation including voltage collapses [1, 2, 3, 4], as was observed in 2003 blackout of USA [5]. It is not just the amount of reactive support, but also the quality and placement of reactive support that matters. For instance, it is found that due to the presence of electric loads that are predominantly induction motors the voltage recovery of the system following a severe disturbance is delayed due to lack of fast responding reactive support, thereby threatening to have secondary effects such as undesirable operation of protective relays, electric load disruption, and motor stalling [6, 7]. While a number of techniques have been developed in the past to address the problems of voltage instability [8], there has been little work towards a long term reactive power (VAR) planning (RPP) tool that addresses both steady state as well as dynamic stability issues.

The available reactive power devices can be classified into static and dynamic devices [9]. The static devices include mechanically switched shunt capacitors (MSCs) and series capacitors that exert discrete open-loop control action and require more time delay for correct operation. The dynamic devices are more expensive power electronics based fastacting devices such as static VAR compensators (SVC), Static Synchronous Compensator (STATCOM), Unified Power Flow Controller (UPFCs) that exert continuous feedback

© 2012 Krishnan and McCalley, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Krishnan and McCalley, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

control action and have better controllability and repeatability of operation. While MSCs are able to strengthen a power system against long term voltage instability issues [1], transient voltage dip and slow voltage recovery issues (influenced by electric load dynamics) is most effectively addressed by fast responding dynamic VAR sources [10, 11, 12]. While, past methods allocate static and dynamic VAR sources sequentially for one contingency at a time, the key to solving transmission system problems in the most cost-effective way will be to coordinate reactive power requirements simultaneously under many contingencies for both static and dynamic problems. The RPP should therefore identify the right mix of VAR devices, good locations and appropriate capacities for their installation.

Role of Induction Motors in Voltage Instability and Coordinated Reactive Power Planning 159

A major factor contributing to voltage instability is the voltage drop that occurs when active and reactive power flow through inductive reactance of the transmission network; which limits the capability of the transmission network for power transfer and voltage support [13]. The power transfer and voltage support are further limited when some of the generators hit their field or armature current time-overload capability limits. Under such stressed conditions, the driving force for voltage instability is usually the inductive loads that try to recuperate after a disturbance. For instance, in response to a disturbance, power consumed by the induction motor loads tends to be restored by the action of motor slip adjustment, distribution voltage regulators, tap-changing transformers, and thermostats [6]. Restored loads increase the stress on the high voltage network by increasing the reactive power consumption and causing further voltage reduction. A run-down situation causing voltage instability occurs when load dynamics attempt to restore power consumption beyond the capability of the transmission network and the connected generation to provide

The publication [14] corroborates this voltage instability phenomenon by means of a powervoltage (PV) curve, as shown in Figure 1. For a particular system and loads considered, the normal system can be stable with both resistive and motor loads at points where load curves and system curves intersect. However, when the system becomes stressed, with increased system reactance, it can only have a stable operating point with a resistive load. Due to lack of reactive power support that limits the transfer capability or loadability of the system, there is no intersection of system and load curves for the induction motor load since there is

**2.1. Role of induction motors** 

the required reactive support.

no stable operating point.

**Figure 1.** Stability and Load Characteristics

**2.2. A typical scenario of slow voltage recovery leading to collapse** 

Heavily loaded transmission lines during low voltage conditions can result in operation of protective relays causing some transmission lines to trip in a cascading mode. A common

In this chapter, we present a long term VAR planning algorithm, which coordinates between network investments that most effectively address steady-state voltage instability and those which most effectively address transient voltage recovery problems under severe contingencies. The study takes into account the induction motor dynamic characteristics, which influence the transient voltage recovery phenomenon. The algorithm is applied on a portion of a large scale system consisting of 16173 buses representing the US eastern interconnection. The planning method is a mixed integer programming (MIP) based optimization algorithm that uses sensitivity information of performance measures with respect to reactive devices to plan for multiple contingencies simultaneously.

The remaining parts of this chapter are organized as follows. In section 2, we discuss the very important role played by induction motor loads in the voltage stability phenomena. Section 3 sheds focus on the models used to build a base case for voltage stability assessment, and on appropriate performance criteria and solution strategies used in this chapter to devise the proposed coordinated planning. A summary of proposed RPP algorithm that considers both static as well as dynamic reactive resources in a coordinated planning framework, together with the various stages of planning are presented in Section 4. Section 5 illustrates the influence of induction motors on voltage stability phenomenon under severe contingencies, and demonstrates application of the coordinated VAR planning method on a large scale system to effectively avoid induction motor trips. Section 6 presents the conclusions.
