Optimal Placement of STATCOMs Against Short-Term Voltage Instability

Abstract

Short-term voltage stability is an increasing concern in today’s power systems given growing penetration of induction motors. The instability can lead to catastrophic consequences such as cascading failures and/or wide-spread blackouts. STATCOMs are able to provide rapid and dynamic reactive power (VAr) support into the system and therefore improve system’s short-term voltage performance following a large disturbance. Importantly, the sizing and locating of the STATCOM integration determine how much the short-term voltage performance can be improved. This chapter presents a novel systematic method for optimal placement of STATCOMs against short-term voltage instability. The problem is formulated as a multi-objective optimization model minimizing two conflicting objectives: (1) total investment cost and (2) expected unacceptable short-term voltage performance subject to a set of probable contingencies. Indices for quantifying short-term voltage stability and the related risk level are proposed for problem modeling. Candidate buses for STATCOM installations are analytically selected based on trajectory sensitivity technique. Load dynamics are fully considered using a composite load model comprising induction motor and other typical components. For the proposed model, rather than a single solution, a set of trade-off solutions called Pareto optimal solutions can be obtained, and the decision-maker may select one from them depending on practical needs. A relatively new and superior multi-objective evolutionary algorithm called multi-objective evolutionary algorithm based on decomposition (MOEA/D) is introduced and employed to find the Pareto optimal solutions to the model. The proposed method is verified on the New England 10-machine 39-bus system using industry-grade simulation tool and system models. Simulation results have validated the effectiveness of the proposed method. The method can be practically applied to provide decision-support for STATCOM installations.

Appendices

Appendix 1

The block diagram of the “SVSMO3” model for STATCOM is shown in Fig. 14.13 [18].

Fig. 14.13

Block diagram of the SVSMO3 STATCOM model [18]

Appendix 2

Table 14.6 SVSMO3U1 model parameters used in this chapter [17]

Appendix 3

The IEEE/CIGRE Joint Task Force on Stability Terms and Definitions (2004) suggested a classification criterion for voltage stability, as shown in Fig. 14.14 [2].

Fig. 14.14

Classification of voltage stability

• Large-disturbance voltage stability refers to the system’s ability to maintain steady voltages following large disturbances such as system faults, loss of generation, or circuit contingencies. This ability is determined by the system and load characteristics, and the interactions of both continuous and discrete controls and protections. Determination of large-disturbance voltage stability requires the examination of the nonlinear response of the power system over a period of time sufficient to capture the performance and interactions of such devices as motors, underload transformer tap changers (ULTCs), and generator field-current limiters. The study period of interest may extend from a few seconds to tens of minutes.

• Small-disturbance voltage stability refers to the system’s ability to maintain steady voltages when subjected to small perturbations such as incremental changes in system load. This form of stability is influenced by the characteristics of loads, continuous controls, and discrete controls at a given instant of time. This concept is useful in determining, at any instant, how the system voltages will respond to small system changes. With appropriate assumptions, system equations can be linearized for analysis thereby allowing computation of valuable sensitivity information useful in identifying factors influencing stability. This linearization, however, cannot account for nonlinear effects such as tap changer controls (dead-bands, discrete tap steps, and time delays).

• Short-term voltage stability involves dynamics of fast acting load components such as induction motors, electronically controlled loads, and HVDC converters. The study period of interest is in the order of several seconds, and analysis requires solution of appropriate system differential equations; this is similar to analysis of rotor angle stability. Dynamic modeling of loads is often essential. In contrast to angle stability, short circuits near loads are important.

• Long-term voltage stability involves slower acting equipment such as tap-changing transformers, thermostatically controlled loads, and generator current limiters. The study period of interest may extend to several or many minutes, and long-term simulations are required for analysis of system dynamic performance. Stability is usually determined by the resulting outage of equipment, rather than the severity of the initial disturbance. Instability is due to the loss of long-term equilibrium (e.g., when loads try to restore their power beyond the capability of the transmission network and connected generation), post-disturbance steady-state operating point being small-disturbance unstable, or a lack of attraction toward the stable post-disturbance equilibrium (e.g., when a remedial action is applied too late). The disturbance could also be a sustained load buildup (e.g., morning load increase). In many cases, static analysis can be used to estimate stability margins, identify factors influencing stability, and screen a wide range of system conditions and a large number of scenarios. Where timing of control actions is important, this should be complemented by quasi-steady-state time-domain simulations.