Cascade tripping is the tripping of power grid in unbalanced condition or it is the tripping of safety devices and isolation of the part of the system to prevent damage to the equipment. This tripping occurs during under frequency or other unbalanced condition.
When demand is more than power generated the frequency of machine goes down. When this frequency is less than certain limit, the grid will trip. This will result in certain loss of power. If demand is less than generation it will also cause such situation. Which is also called blackout or grid failure. To avoid this synchronism must be maintained in the grid.
Synchronous generators thousands of kilometers apart must operate stably and in synchronism during infinitely many load and power transfer conditions, equipment outages, and power disturbance following a short circuit or other disturbance, one grope of generators could accelerate relative to another grope causing instability and loss of synchronism. Implication of blackout is clearly enormous for the electrical power industry. Parts of grid are being stretched to their limits.
1.1 CASCADING FAILURE IN GRID SYSTEMMany of the world’s grids are heavily loaded and operating close to their maximum capacity. When sudden bulk transfer occurs, the grid becomes unstable and vulnerable to system wide disturbances such cascade tripping or blackout. Cascade tripping occurs when demand is more than power generated the frequency of machine goes down, When this frequency is less then certain limit, the grid will trip This will result in sudden loss of power. Which is also called blackout or grid failure.
Frequency plays an important role for stability of grid. Generators separated by thousands of miles must rotate together with split cycle synchronization and the flow of power thousands of transmission lines must be coordination over large regions of country. Not only supplies have to be synchronized but also so do supply and demand and they also have to be synchronized everywhere.
Cascade on an electrical system is dynamic unplanned sequence of events that one started cannot be stopped by human intervention. Power swing voltage fluctuations cause sequential tripping of transmission lines, generators and automatic load shedding in a widening geographic area. The fluctuations diminishes in amplitude as the cascade spreads eventually equilibrium is restored and the cascade stops.
Cascade tripping of safety devices and isolation of the part of the system to prevent damage to equipment under low frequency or unbalanced condition. For balanced operation of generating units in power grid, frequency plays an important role. Severe frequency swing due to large unbalance between generation and load may force load
shedding or even result in system failure, thus affecting the continuity and reliability of supply. So, synchronous generators thousands of miles apart must operate stability and in synchronism during infinitely many load and power transfer conditions, equipment outages and power disturbances.
2.1 WHAT IS POWER GRID SYSTEM
An electrical grid interconnected with network is known as a power grid. National grid may be perceived as a mesh of interlinked transmission lines, interconnecting different electrical regions, viz., Northern, Eastern, Western, southern and North-Eastern region of country. All the regional grids, accept Eastern and North-Eastern region, operate independently with only a limited exchange of power across the region prior to independence, small generating stations where used to supply power to local loads through small radial transmission system, which gradually progressed towards the formation of state grids in 60’s, regional grids in mid 70’s. progressively moving on along to provide the formation of “National Grid” by way of integration of the existing regional grids. The figure 2.1 shows the basic structure of a power grid
Figure 2.1 National Grid System
Advantages of Grid System
- Exchange of peak loads
- Use of older plants
- Ensures economical operation
- Increases diversity factor
- Reduces plant reserve capacity
- Increases reliability of supply
- Let us discuss each of the advantages in detail
(i) Exchange of peak loads
- If the load curve of a power station shows a peak demand that is greater than the rated capacity of the plant, then the excess load can be shared by other stations interconnected with it.
(2) Use of Older Plants:
- The interconnected grid system makes it possible to use the older and less efficient plants to carry peak loads of short duration.
(3) Ensures economical operation:
- The interconnected grid system makes the operation of concerned power stations quite economical.
(4) Increases diversity factor:
- The load curves of different interconnected stations are generally different.
- In other words, the diversity factor of the system is improved, so increasing the effective capacity of the system.
(5) Reduces plant reserve capacity:
Every power station is required to have a standby unit for emergencies.
(6) Increases reliability of supply:
- If a major breakdown occurs in one station, continuity of the supply can be maintained by other healthy stations.
Disadvantages Of Grid System
- Grid software and standards are still evolving
- Learning curve to get started
- Non-interactive job submission
CASCADE TRIPPING IN POWER SYSTEM
3.1 CASCADE TRIPPING
Cascade tripping is the tripping of power grid in unbalanced condition or it is the tripping is a tripping of safety devices and isolation of part of the system to prevent damage to equipment during low frequency or unbalanced condition.
Cascading failure is common in power grids when one of the elements fails (completely or partially) and shifts its load to nearby elements in the system. Those nearby elements are then pushed beyond their capacity so they become overloaded and shift their load onto other elements. Cascading failure is a common effect seen in high voltage systems, where a single point of failure (SPF) on a fully loaded or slightly overloaded system results in a sudden spike across all nodes of the system. This surge current can induce the already overloaded nodes into failure, setting off more overloads and thereby taking down the entire system in a very short time.
This failure process cascades through the elements of the system like a ripple on a pond and continues until substantially all of the elements in the system are compromised and/or the system becomes functionally disconnected from the source of its load. For example, under certain conditions a large power grid can collapse after the failure of a single transformer.
Monitoring the operation of a system, in real-time, and judicious disconnection of parts can help stop a cascade. Another common technique is to calculate a safety margin for the system by computer simulation of possible failures, to establish safe operating levels below which none of the calculated scenarios is predicted to cause cascading failure, and to identify the parts of the network which are most likely to cause cascading failures.
One of the primary problems with preventing electrical grid failures is that the speed of the control signal is no faster than the speed of the propagating power overload, i.e. since both the control signal and the electrical power are moving at the same speed, it is not possible to isolate the outage by sending a warning ahead to isolate the element.
Cascading failure caused the following power outages:
- Blackout in northeast America in 1965
- Blackout in Southern Brazil in 1999
- Blackout in northeast America in 2003
- Blackout in Italy in 2003
- Blackout in London in 2003
- European Blackout in 2006
- Blackout in northern India in 2012
Cascading failures can also occur in computer networks (such as the Internet) in which network traffic is severely impaired or halted to or between larger sections of the network, caused by failing or disconnected hardware or software. In this context, the cascading failure is known by the term cascade failure. A cascade failure can affect large groups of people and systems.
The cause of a cascade failure is usually the overloading of a single, crucial router or node, which causes the node to go down, even briefly. It can also be caused by taking a node down for maintenance or upgrades. In either case, traffic is routed to or through another (alternative) path. This alternative path, as a result, becomes overloaded, causing it to go down, and so on. It will also affect systems which depend on the node for regular operation.
The symptoms of a cascade failure include: packet loss and high network latency, not just to single systems, but to whole sections of a network or the internet. The high latency and packet loss is caused by the nodes that fail to operate due to congestion collapse, which causes them to still be present in the network but without much or any useful communication going through them. As a result, routes can still be considered valid, without them actually providing communication.
If enough routes go down because of a cascade failure, a complete section of the network or internet can become unreachable. Although undesired, this can help speed up the recovery from this failure as connections will time out, and other nodes will give up trying to establish connections to the section(s) that have become cut off, decreasing load on the involved nodes.
A common occurrence during a cascade failure is a walking failure, where sections go down, causing the next section to fail, after which the first section comes back up. This ripple can make several passes through the same sections or connecting nodes before stability is restored. The cascading failure in power grid cause cascading blackout or power outage in the grid system.
Figure 3.1 shows the cascade tripping triggering events, protection relay operations and impacts on the grid system.
Figure 3.1 Cascade tripping triggering events, protection relay operations and impacts
3.2 CASCADING BLACK OUT
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.
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.
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.
Power outages are categorized into three different phenomena, relating to the duration and effect of the outage:
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.
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.
A blackout is the total loss of power to an area and is the most severe form of 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 BLACK OUT
Cascade tripping occurs due to unbalance in grid, mainly due to under frequency situation. Under frequency in power system occurs in the following circumstances.
1. During steady condition when on line generating capability is inadequate to meet the load requirement a steady decline of frequency occurs.
2. When sudden loss of large generating unit occurs or there is a sudden tripping major of transmission line carrying bulk load in a system, a sudden decline in frequency is observed.
For all practical purposes, electricity flowing through 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 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 underbrush
- Overloads, voltage sags, frequency deviations.
Unseal operation of grid occurs when:
- Under frequency condition in power grid causes unusual operation of machines in grid. This kind of situation causes automatic safety devices to trip to prevent damage to equipment.
- When demand is more than 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 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 following effect in 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.
- Reduced reactance of the machine resulting in higher fault currents.
In hydro power plants under frequency causes following effect in generator.
- Increases flux level and magnetic saturation. - More iron losses.
- Over heating and over loading.
- Decreased speed and poor ventilation and hence overheating.
3.4 SOLUTIONS OF CASCADE TRIPPING
The network should have sufficient capacity to allow the unexpected loss of the most critical network element at any time, without any primary transmission plant being overloaded or any normal customer load being shed. This criterion has been adopted for the analysis of future requirements included in this review. To prevent black out following are the fundamental rules of grid operations.
Balance supply and demand
Balance reactive power supply and demand to maintain voltages
Monitor flows to prevent overloads and line overheating
Keep the system stable
Keep the system reliable, even after loss of a key facility
Plan, design and maintain the system to operate reliably
Prepare for emergencies Training Procedures and plans Back-up facilities and tools Communications
Each control area is responsible for its system.
Some advanced methods can be used to void blackout. To avoid blackout important point is to avoid frequency variations and unbalances in grid and to maintain synchronism in the grid. The figure 3.4 shows that how to synchonise one generator with another.
Figure 3.3 How to synchronize one generator with another
Paralleling of a Generator Set
A system that could measure and monitor voltage and current input through our the grid in real time has long been a priority for utilities of governmental organization and major industrial users such a system would enable operators to detect the first signs of instability and take appropriate action to stop the disturbance from spreading.
A new advanced Phasor measurement system has been developed which includes Global Positioning System (GPS) and application software to measure and monitor the status of a power grid. The PMUs (Phasor measurement units) are located at key points in the grid, such as in substations, to measure various types of input, such as synchronized Phasor measurements which allow to compute and to monitor different type of instabilities (e.g. voltage instability, frequency instability) on a System Monitoring Center.
To avoid cascade tripping we have to improve power grid also. ORNL (oak ridge national laboratory) researchers are helping industry develop and evaluate new technology that could improve the efficiency and reliability of existing transmission lines.
Today’s overhead transmission lines consist of aluminum conductor strands wrapped around a steel core. Because of the weight and properties of the steel, these cables will stretch and sag if they are heated up too much by carrying too much current. Sagging lines caused by excessive current and hot weather triggered a major power outage in 1996 in the northwestern United States. To overcome this limitation, 3M developed a composite consisting of Nextel ceramic fibers and an aluminum-zirconium alloy to make an advanced cable that can carry more current than current that steel aluminum lines without sagging at higher temperatures.
The power grid of the future will include high temperature superconducting (HTS) cables, with offer much less resistance to the flow of electricity than do copper lines. “A superconducting cable will conduct up to 5 times as much current as a copper cable of the same size. Because an HTS cable loses little energy as heat, it will cut electrical transmission losses in half, from 8% to 4% .An HTS cable is more environmentally friendly than a copper cable also because it is cooled with safe inexpensive liquid nitrogen rather than oil-impregnated paper insulation, which may leak oil.
Another technology is the flexible alternating current transmission system (FACTS), a combination of large scale power electronic devices that can control the flow of power through transmission and distribution lines.
FACTS can control the voltage magnitude and phase angle at both ends of the line, as well as the amount of real and reactive power that is passed through the line,” says Kirby. FACTS devices could greatly increase the power-flow capacity and stability of our existing transmission lines.
Another alternative is to use distributed generation and grid power alternative technologies. For years distributed generation technologies fuel cells, micro turbines, reciprocating generator sets, static turbine switches, and others have been considered ‘alternative’ and optional.
However Power system planning and operations aim to balance the risk of failures against an economical design & operation, and when problem arises, to have mitigating measures on hand. These measures are designed to minimize the cascading of failures and the size of area affected.
Maintenance of synchronism in power grid
Now days several ways to maintain the synchronism like 3-dark lamp method, 2-bright lamp and one dark lamp method etc. There were also some conditions for maintain the synchronism that are .
Same phase angle
Magnitude of voltage should be same
Phase sequence should be same
Three dark lamp method is shown in figure 3.4
Figure 3.4 Three dark lamp method
FACTS : Flexible Alternating Current Transmission System
GPS : Global Positioning System
HTS : High Temperature Superconducting
ORNL :Oak Ridge National Laboratory
PMU : Phasor Measurement Units
SPF : Single Point Of Failure
L : Lamp
S 1 : Switch No:1
S2 : Switch No:2
S3 : Switch No:3