The power system is the major part of the power transmission and this system is subjected to many disturbances. So when this system is affected by a disturbance then it must be able to regain its former state and this ability is called the stability of the power system. If the electric power system is not able to achieve its previous state then it would create a lot of power problems. Instability is caused because of loss of synchronization. The system integrity can be preserved when the entire power system would remain intact with no tripping, except for those which is tripped due to the faulted elements to protect the power system. The environment where the power system is situated is prone to many disturbances so the protection of the power system is really important. So in order to achieve the power transmission, we should improve the power system stability.
What are the major reasons for power blackout?
- In case if the HV line trips in an electromagnetically coupled network then a huge electric load will be transferred to the LV lines
- In case if large generators are used in small power grids then shutting down the generator could lead to instability
- Blackouts could also happen due to weak interconnection, if there is a sudden power interruption then it would cause the multiple tie lines to exceed their limit.
- Operational failures in the protective relays, power swing blocking, delayed tripping, etc
- Blackouts could happen if the power stability control is ineffective unreasonable remedial measurements, slow or no action against the power swing.
What are the major reasons for the instability in a power system?
- Short circuit
- Loss of tie circuit to the public utility
- Loss of portion on-site generator
- Switching operation
- Motor starting which is really huge to the generating capacity of the system
- Switching operation
- Motor impact loading
- A sudden decrease in the electrical load in the generator
What are the electrical characteristics of the power which would be affected according to the stability conditions?
- Motor internal voltage
- Machine and the transmission system
- The internal voltage of the generator
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What is power system instability?
The power system stability can be defined as how the power system could regain its characteristics after it is affected by a disturbance. The instability of the power system is the case in which the power system won’t be able to retain its operational characteristics. The power system instability could is due to the loss of synchronism.
What are the different types of power system stability?
STEADY-STATE STABILITY
This type of stability is related to finding the upper limit of the machine loading before it loses synchronization. So basically it is done to determine the higher loading limit of the machine. The loading will be small and it would be gradually increased. The load is increased slowly when it is compared with the natural frequency of the system’s oscillation. So while the load in the system is increased gradually, we can determine the maximum amount of power that can be transmitted without losing the synchronism and this is the steady-state stability of the power system.
TRANSIENT STABILITY
In case if there is a sudden disturbance occurs in a power system then the speed of the rotor, angular difference, and the power transfer would be affected by this disturbance or there will be a lot of changes and the magnitude of the changes are dependent upon the gravity of the disturbances. During certain kinds of disturbances, the above-mentioned changes would be really huge and this would cause the machine to fall out of step. So we can define transient stability as the maximum possible stability of the system during the maximum flow of power. The transient stability is a really fast process, this stability shows how much power a machine can handle before it losses synchronism.
Types of disturbances which is involved in the study of transient stability
- A quick change in the mechanical or electrical power
- The fault which is close to the generator
- Transmission line faults could be either in the end or in the middle
- Loaded generator tripping
- Sudden dropping of large load
How to achieve transient stability?
- The power transfer capability should be improved
- It can be achieved by utilizing a high-speed excitation system
- Single pole switching
- High speed of reclosing breakers
- Braking resistors
DYNAMIC STABILITY
In a power system, there are small disturbances such as slow and gradual load variation, variation in the speed of the turbine. All these would occur continuously in a power system. These changes are really small so it won’t affect the synchronism of the power system, but it would be enough to excite the system into the state of natural oscillations. The system can be considered dynamically stable if the oscillation won’t achieve more than a certain amount of amplitude and die out quickly. In case if the amplitude of the oscillation is large and exists for a long time then the system is not dynamically stable. So if the power system doesn’t have dynamic stability then there would be really difficult operating conditions. The dynamic stability checking is done for five to thirty seconds. We can improve the dynamic stability with the help of the power system stabilizer. Dynamic stability can also be improved by using a fast-acting voltage regulator.
How to solve the power system instability?
Improving the moment of inertia
The stability of the power system can be improved by increasing the moment of inertia per rated KVA of the synchronous machine in the system
Design of the system
If the impedance between the machines is greater, then the less severe disturbance could drive it out of step. The solution for dynamic stability is to construct new connecting circuits. In order to achieve maximum stability, all synchronous machines should be closely connected to a common bus.
Design and selection of the rotating equipment
By increasing the inertia of the mechanical system the stability of the system can be improved. The stability can also be improved by using synchronous machines with low transient reactance which permits the maximum flow of synchronizing power.
- The voltage regulator can be used to improve the stability of the system
- Power system stabilizers can be used to improve the stability, it would modulate the excitation of the generator to provide damping to the oscillation of the synchronous machine rotor
- We can use over current protection for short circuit
- In the case of a two-machine system if real power transfer between the two machines over a transmission link is high then it would remain synchronized during the transient disturbances
- The majority of the hazards in the asynchronous operation are due to high transient mechanical torque and current. In order to prevent this, we can use a pullout protector with a generator and motor.
What are the types of disturbances that could affect the power system?
The stability of the power system when it is susceptible to disturbances is dependent upon the disturbance nature and also the initial operating conditions. There are small and large disturbances in a power system small disturbances can be described as a change in load. A large disturbance would affect the power system badly such as a short circuit in a transmission line or in case if there is a loss of a large generator. There could be a lot of dynamic disturbance in a power system due to the interconnection of different elements. There are fast and slow dynamic disturbances in a power system, the fast disturbance is caused because of the fast wave in the HV transmission line. The slower disturbance is because of the electromechanical changes in the electrical machine windings.
The slower dynamic disturbances would be caused because of electromechanical rotor oscillation. This disturbance would badly affect the stability of the network.
