Voltage Stability – Voltage control and stability problems are very much familiar to the electric utility industry but are now receiving special attention by every power system analyst and researcher. With growing size along with economic and environmental pressures, the possible threat of voltage instability is becoming increasingly pronounced in power system networks. In recent years, voltage instability has been responsible for several major network collapses in New York, France, Florida, Belgium, Sweden and Japan. Research workers, R and D organizations and utilities throughout the world, are busy in understanding, analyzing and developing newer and newer strategies to cope up with the menace of voltage instability/collapse.
Voltage stability covers a wide range of phenomena. Because of this, voltage stability means different things to different engineers. Voltage stability is sometimes also called load stability. The terms voltage instability and voltage collapse are often used interchangeably. The voltage instability is a dynamic process wherein contrast to rotor angle (synchronous) stability, voltage dynamics mainly involves loads and the means for voltage control. Voltage collapse is also defined as a process by which voltage instability leads to very low voltage profile in a significant part of the system. Voltage instability limit is not directly correlated to the network maximum power transfer
A CIGRE Task Force has proposed the following definitions for voltage stability.
Small-disturbance voltage stability
A power system at a given operating state is small-disturbance voltage stable if, following any small disturbance, voltages near loads do not change or remain close to the pre-disturbance values. The concept of small-disturbance voltage stability is related to steady-state stability and can be analysed using small-signal (linearised) model of the system.
A power system at a given operating state is voltage stable if on being subjected to a certain disturbance, the voltages near loads approach the post-disturbance equilibrium values.
The concept of voltage stability is related to transient stability of a power system. The analysis of voltage stability normally requires simulation of the system modelled by non-linear differential-algebraic equations.
Following voltage instability, a power system undergoes voltage collapse if the post-disturbance equilibrium voltages near loads are below acceptable limits Voltage collapse may be total (blackout) or partial.
Voltage security is the ability of a system, not only to operate stably, but also to remain stable following credible contingencies or load increases.
Although voltage stability involves dynamics, power flow based static analysis methods often serve the purpose of quick and approximate analysis.
Comparison of Angle and Voltage Stability
The problem of rotor angle (synchronous) stability is well understood and documented. However, with power system becoming over stressed on account of economic and resource constraint on addition of generation, transformers, transmission lines and allied equipment, the voltage instability has become a serious problem. Therefore, voltage stability studies have attracted the attention of researchers and planners worldwide and is an active area of research.
Real power is related to rotor angle instability. Similarly reactive power is central to voltage instability analyses. Deficit or excess reactive power leads to voltage instability either locally or globally and any increase in loading’s may lead to voltage collapse.
Voltage Stability Studies
The voltage stability can be studied either on static (slow time frame) or dynamic (over long time) considerations. Depending on the nature of disturbance and system/subsystem dynamics voltage stability may be regarded a slow or fast phenomenon.
Static Voltage Analysis
Load flow analysis reveals as to how system equilibrium values (such as voltage and power flow) vary as various system parameters and controls are changed. Power flow is a static analysis tool wherein dynamics is not explicitly Considered. Many of the indices used to assess voltage stability are related to NR load flow study.
Some Counter Measures
Certain counter measures to avoid voltage instability are:
- generator terminal voltage increase (only limited control possible)
- increase of generator transformer tap
- reactive power injection at appropriate locations
- load-end OLTC blocking
- strategic load shedding (on occurrence of undervoltage)
Reactive Power Flow and Voltage Collapse
Certain situations in power system cause problems in reactive power flow which lead to system voltage collapse. Some of the situations that can occur are listed and explained below.
- Long Transmission Lines: In power systems, long lines with voltage uncontrolled buses at the receiving ends create major voltage problems during light load or heavy load conditions.
- Radial Transmission Lines: In a power system, most of the parallel EHV networks are composed of radial transmission lines. Any loss of an EHV line in the network causes an enhancement in system reactance. Under certain conditions the increase in reactive power delivered by the line(s) to the load for a given drop in voltage, is less than the increase in reactive power required by the load for the same voltage drop. In such a case a small increase in load causes the system to reach a voltage unstable state.
Shortage of Local Reactive Power: There may occur a disorganised combination of outage and maintenance schedule that may cause localised reactive power shortage leading to voltage control problems. Any attempt to import reactive power through long EHV lines will not be successful. Under this condition, the bulk system can suffer a considerable voltage drop.