WHAT is CASCADING BLACKOUT and REASONS of BLACKOUT

  CASCADING BLACKOUT

1. A power outage (also called a power cut, a power blackout, power failure or a blackout) is a short-term or a long-term loss of the electric power to a particular area.
2. There are many causes of power failures in an electricity network. Examples of these causes include faults at power stations, damage to electric transmission lines,   substations or other parts of the distribution system, a  short circuit, or the overloading of electricity mains.
3. Power failures are particularly critical at sites where the environment and public safety are at risk. Institutions such as hospitals, sewage treatment plants, mines, shelters and the like will usually have backup power sources such as standby generators, which will automatically start up when electrical power is lost. Other critical systems, such as telecommunication, are also required to have emergency power. The battery room of a telephone exchange usually has arrays of lead-acid batteries for backup and also a socket for connecting a generator during extended periods of outage.
4. Power outages are categorized into three different phenomena, relating to the duration and effect of the outage:
5. A permanent fault is a massive loss of power typically caused by a fault on a power line. Power is automatically restored once the fault is cleared.
6. A  brownout is a drop in voltage in an electrical power supply. The term brownout comes from the dimming experienced by lighting when the voltage sags. Brownouts can cause poor performance of equipment or even incorrect operation.
7. A blackout is the total loss of power to an area and is the most severe form of the power outage that can occur. Blackouts which result from or result in power stations tripping are particularly difficult to recover from quickly. Outages may last from a few minutes to a few weeks depending on the nature of the blackout and the configuration of the electrical network.

Figure 3.2 shows the photographs of blackout when cascading failure is affected.

 Figure 3.2 The photographs of blackout when cascading failure is affected.

3.3  REASONS OF BLACKOUT

Cascade tripping occurs due to unbalance in the grid, mainly due to under frequency situation. Under frequency in power system occurs in the following circumstances.
1. During a steady condition when an online generating capability is inadequate to meet the load requires a steady decline of frequency occurs.
2. When a sudden loss of large generating unit occurs or there is a sudden tripping major of a transmission line carrying the bulk load in a system, a sudden decline in frequency is observed.
For all practical purposes, electricity flowing through the grid cannot be stored. Once it is generated electricity must flow somewhere. If there is not enough demand it will cause voltage spikes, if it is too little, it will cause voltage dips.
For high voltage power to remain stable synchronism must be maintained when the synchronism is disturbed by inevitable local events such as sudden loss of major transmission line or generator power can begin to flow in an uncontrolled manner causing automatic safety devices to trip and isolate parts of the system to prevent damage to equipment.
Blackout result when generation is separated from the load. The grid typically will withstand any single event (single generator failure or single transmission- line failure) under worst case conditions. This called “N-1” contingency planning. But the system can collapse if several failures take place in rapid succession when the grid is already stressed. Such events include.
- Multiple lightning strikes
- Falling trees
- Equipment failure
- Human error
- Wires sagging into the underbrush
- Overloads, voltage sags, frequency deviations.
- Sabotage.
- Fire.

  Unseal operation of grid occurs when:
- Under frequency condition in power grid causes unusual operation of machines in the grid. This kind of situation causes automatic safety devices to trip to prevent damage to equipment.
- When demand is more than the power generated, the machine will overload. So speed will decrease. So frequency goes down. Sometimes it may happen that demand is very much less than the generation, it will also cause unbalance condition. If there is not enough demand it will voltage spikes, if it is too little, it will cause voltage dips.

  In thermal plants under frequency causes the following effect in the generator:
- Higher flux density resulting in machine saturation and higher field requirement.
- Excessive core losses.
- Heating of core and other parts.
- Reduced speed reduced cooling effect.
- The reduced reactance of the machine resulting in higher fault currents.


In hydropower plants under frequency causes the following effect in the generator.
- Increases flux level and magnetic saturation. - More iron losses.
- Overheating and overloading.
- Decreased speed and poor ventilation and hence overheating.