The process by which sequence of events associated with voltage instability leads to loss of voltage in a significant part of the system is called voltage collapse.
The phenomenon of voltage collapse is created when the demand for reactive power increases proportionate to active power. At this moment, a fully loaded transmission line generates extra inductive reactive power. Thus, capacitive reactive power from local sources becomes insufficient. Therefore, the reactive power will have to be delivered from more distant places, as a consequence transmission of more reactive power through the lines will further increase the voltage drop on the customer side. Local control of voltage by means of auto transformers will supply more reactive power, and this, in turn, will increase further voltage drops in lines. In one moment this process can go like an avalanche, thereby reducing the voltage to zero.
In the meantime, most of the generators in power plants will switch-off due to an unacceptably low voltage which of course will deteriorate the situation.
Possible Scenario of a Voltage Collapse
Possible scenario for voltage collapse are given below.
- Generating units near load centers are out of service.
• Heavily loaded lines having low Reactive Power Reserves (RPRs).
• Tripping of a heavily loaded line causes load increases over other lines and loss of reactive power and voltage.
• Load consumption would temporarily lower to stabilize. AVRs would act to restore generator voltages, but increased reactive power flow would lower voltages at consumer end or elsewhere.
• Under the capability curve, Generators would hit Var limits.
Blackouts in a power system
A power system undergoes the voltage collapse if the post-disturbance equilibrium voltages are below acceptable limits. This voltage collapse may be converted into a total or partial blackout. A blackout in an electric system means that the complete system collapses. It originates from several causes.
Overloading of generators and transmission lines creates a deficiency of reactive power which leads voltage collapse and resultant cascade tripping can cause a blackout.
One such example is the loss of generation, e.g. the tripping of a power plant leads to overloading and under frequency over another power plant. It may result in the further loss of other generators.
Another example, is bottlenecking of transmission lines, trips other overloaded power lines, results in cascade trips. Finally, power system undergoes the voltage collapse due to high impedance in the weakened grid.
In general, one initial minor event leads to a second event, a third and so forth. Due to increased stresses on the system, it finally collapses and leads to blackouts.
Voltage Collapse: Causes and Prevention
INTRODUCTION
The important operating work of power system utilities is to keep voltage within an allowable range limit in order to provide a high quality customer service. Day by day increase in power demand results in more and more pressurized transmission lines. Such systems are usually subjected to voltage instability and eventually a voltage collapse. Now-a- days voltage collapse is becoming an increasing threat to system security and stability. Power systems are expected
to become more heavily loaded in the next decade as the demand for electric power rises while economic and environmental concerns limit the construction of new transmission and generation capacity. Heavily loaded power systems are closer to their stability limits and voltage collapse blackouts will occur if suitable monitoring and control measures are not taken. It is important to understand the mechanisms of voltage collapse so that voltage collapse blackouts may be effectively prevented.
A.) Causes of voltage collapse
Increase in loading
Generators, synchronous condensers, or SVC reaching reactive power limits
Action of tap changing transformers
Load recovery dynamics
Line tripping
Generator outages
Fig. 1. Potential causes of Blackout
Fig. 2. Flow Chart for Cascading Failure Power System Model
LOAD SHEDDING
Load shedding is an option that is becoming more widely used a final means of avoiding system wide voltage collapse. This option is only considered when all other effective means of avoiding collapse are exhausted. This option may be the only effective option for various contingencies, especially if the collapse is in the transient time frame, and if load characteristics result in no effective load relief by transformer load taps changer control. Load shedding results in high costs to electricity suppliers and consumers; therefore, power systems should be designed to require such actions only under very rare circumstances. Load may be shed either manually or automatically depending on the rate of voltage drop.
