Transient stability can be improved either by using machines of higher inertia or by connecting the synchronous motors to heavy flywheels. This method, however, cannot be used in practice because of economic reasons and reasons of excessive rotor weight.
On contrary, the modern trend in generator design is to achieve more power from smaller machines and hence lighter rotors. Improved methods of generator cooling have helped in this process. However, this trend is undesirable from the point of view of stability. A salient-pole generator operates at lower load angles and is, therefore, preferred over cylindrical rotor generators for consideration of stability.
Now more emphasis and reliance is placed on controls to provide the required compensating effects with which we may be able to offset the reduction in stability margins inherent from above trends in generator design. With the advent of high-speed circuit breakers, high-speed excitation systems and fast valving the loss in stability margin has been made up.
The methods often employed in practice to improve system stability are:
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1. Increasing System Voltage:
Transient stability is improved by raising the system voltage profile, (i.e., raising E and V). Increase in system voltage means the higher value of maximum power, Pmax that can be transferred over the lines. Since shaft power, Ps = Pmax sin δ, therefore, for a given shaft power initial load angle δ 0 reduces with the increase in Pmax and thereby increasing difference between the critical clearing angle and initial load angle.
Thus machine is allowed to rotate through large angle before it reaches the critical clearing angle which results in greater critical clearing time and the probability of maintaining stability.
2. Reduction in Transfer Reactance:
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Transient stability can also be improved by reducing the transfer reactance,. The effect of reducing the transfer reactance means increase of Pmax resulting in increase in transient stability.
The line reactance can be reduced by using more lines in parallel instead of a single line. In general, more power is transferred during a fault on one of the lines if there are two lines in parallel, than that would be transferred over a single faulted line.
Increased power transfer means less available accelerating power, because the accelerating power is the difference between power input and power transfer. Lower accelerating power reduces the risk of instability. The use of bundled conductor lines also helps in reducing line reactance and improving stability.
The compensation of line reactance by series capacitors is another method of improving stability. For lines longer than 325 km this method has proved economical as a means of increasing stability.
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The use of shunt reactors will reduce the danger of instability on lightly loaded lines. If saturable reactors are used, the regulation can be maintained steady over a wide range of loads.
Increasing the X/R ratio increases the power limit of the line itself, which in turn aids stability.
3. Using High Speed Circuit Breaker:
The best method of improving transient stability is the use of high-speed circuit breakers. The quicker a breaker operates, the faster the fault is removed from the system and better is the tendency of the system to restore to normal operating conditions. The use of high-speed breakers has materially improved the transient stability of the power systems and does not require any other method for the purpose.
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Use of high-speed breakers increases the decelerating area A2 and decreases the accelerating area A1 and so improves the stability.
4. Automatic Reclosing:
As the majority of faults on the transmission lines are transient in nature and are self-clearing, rapid switching and isolation of faulty lines followed by reclosing are quite helpful in maintaining stability. The modern circuit breaker technology has made it possible for line clearing to be done as fast as in 2 cycles.
On occurrence of a fault on a transmission line, the faulted line is de-energized to suppress the arc in the fault and then the circuit breaker recloses, after a suitable time interval. Automatic reclosing increases the decelerating area A2 and thus helps in improving stability.
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5. Transient Stability:
It can be improved by reducing the severity of faults which can be achieved by using lightning arresters for protection of the lines.
6. In systems where the stability is of prime importance, high neutral grounding impedance may be used.
The grounding is effective only for unbalanced faults. Zero-sequence impedance comes into picture to restrict the fault current only in case of faults like line-generator to-ground or line-to-line-to-ground. For an electrical system depicted in Fig. 7.33 (a), the equivalent impedance diagram is shown in Fig. 7.33 (b).
The power transfer is given by the expression:
Where Xf is equivalent fault-shunt impedance.
Physically the resistance in the neutral of the transformer represents an absorption of electrical energy which in turn reduces the accelerating energy and thus improves the transient stability.
The grounding resistor consumes power during a ground fault and thus exerts braking effect on the synchronous machine which is greater, the closer the fault is to the resistor and closer the machine is to the fault. A grounding resistor located near a generator is, therefore, beneficial.
However, a grounding resistor should neither be employed near an actual or equivalent synchronous motor nor it should be employed near a synchronous condenser, as such machines already are retarded by faults. In a two machine system, it is, therefore, advisable to have resistance grounding at the sending end and reactance grounding at the receiving end.
7. Turbine Fast Valving:
One reason for power system instability is the excess energy supplied by the turbine during the disturbance period. Fast valving is a means of reducing turbine mechanical input power when a unit is under acceleration due to a transmission system fault. This can be initiated by load impedance relays, acceleration transducers or by relays that recognize only severe transmission system faults.
For maximum stability gains with fast valving, the turbine input power should be reduced as fast as possible. During a fast valving operation, the interceptor valves are rapidly shut (in 0.1 to 0.2 second) and immediately re-opened. This procedure increases the critical switching time long enough so that in most of the cases the unit will remain stable for faults with stuck-breaker clearing times. Presently some stations in USA have been put to use fast valving schemes.
8. Application of Braking Resistors:
An alternative or supplement to fast turbine valving action is the application of braking resistors. Braking resistors, as employed in the context of electric power system stability, is the concept of applying an artificial electric load to a portion of the generator-transmission load complex to correct a temporary imbalance between power generated and power delivered.
During a fault the resistors are connected to the terminals of the generator through circuit breakers by means of an elaborate control scheme. The control scheme determines the amount of resistance to be connected and its duration. The braking resistors remain on for a matter of cycles both during fault clearing and after system are restored to normal operation.
A few cycles after the clearance of fault, the same control scheme disconnects the braking resistors. However, the control schemes available are not very reliable. Control schemes using thyristors have recently been suggested. The noteworthy point is that a possible failure or mal-operation of control scheme can make the matter still worse.
9. Single Pole Switching:
Single pole switching or independent pole operation of a circuit breaker refers to the mechanism with which the three phases of the breaker are closed or opened independently
of each other. The failure of any one phase does not automatically prevent any of the two remaining phases from proper operation.
However, for a 3-phase fault, the three phases are simultaneously activated for operation by the same relaying scheme. The three phases are mechanically independent, such that the mechanical failure of any one pole is not propagated for the remaining poles.
Single pole switching is used at locations where the design criterion is to guard against a three-phase fault compounded with breaker failure. The successful independent pole operation of the failed breaker will reduce a three-phase fault to a single L-G fault (if one pole of the breaker is stuck), or to L-L-G fault (if two poles of the breaker are stuck). This criterion can be applied to the substation of a generating plant with multiple transmission outlets.
The advantages of single poles switching under three phase fault breaker failure contingency are twofold. First, they are among the cheapest stability aids. Single pole switching operation is most efficient at high transmission voltages where equipment’s are costlier.
Successful single pole switching may allow the critical clearing time of a plant circuit breaker to increase by as much as 2 to 5 cycles. Second, it is relatively easy to install. Most EHV circuit breakers are equipped with separate pole mechanism due to the large component size and wide phase space requirements at high working voltages. The only additional complexity is to provide separate trip coils to activate each pole.
10. Use of Quick-Acting Automatic Voltage Regulators:
The satisfactory operation of synchronous generators of a complex power system at high power (or load) angles and during transient condition is very much dependent on the source of excitation for the generators and on the automatic voltage regulator.
The power output of a generator is proportional to internal voltage E. Under fault conditions the terminal voltage V falls. A quick-acting voltage regulator causes increase in E so that the terminal voltage V remains constant. A higher value of E means a higher generator output.
It has already been shown that the maximum value of a power angle curve is proportional to the per unit excitation. Field forcing can, therefore, cause the machine to operate on a higher power-angle curve thereby allowing it to swing through a larger angle from its original position before it reaches the critical clearing angle.
As an example of the effectiveness of modern high-speed automatic voltage regulators, consider Fig. 7.34. Two power-angle curves a and b have been drawn representing two different excitations. Curves of terminal voltage V against rotor angle for the same excitations have also been drawn. Consider working at point B on power angle curve a, where the voltage rotor angle relation is normal.
If we now wish to increase the power input and maintain the same rotor angle, we can do so by increasing excitation, i.e., by working on power angle curve b at point C. Although C is in the static stability limit, the terminal voltage is above normal. However, the same power can be supplied by working at point D, in the inherently unstable region.
If the field current is constant, the slightest difference would cause instability. However, because of the high inertia, the swings are slow, and if an automatic voltage regulator is fitted, it can sense the drop in terminal voltage as the rotor angle is increased. Before the generator has swung an appreciable amount, sufficient boost is given to the field to make the power output greater than the input, thereby arresting the swing and maintaining stability.