Chapter-1INTRODUCTIONThe introduction of liberisation into the power system and the deregulating process across the world mean that operators are faced with new challenges in power system operation. Power system operation is becoming more competitive and is dictated heavily by market conditions [1]. This has led to power systems frequently being run closer to their limits of both steady state and transient stability. The Power Transfer Limit (PTL) dictates how much power can be sent through a transmission corridor without making a system unstable. This limit is typically defined as an active power limit which is a static limit defined for a worst-case scenario voltage condition, 0.9 p.u. With the growing trends in Smart Grid technology and the introduction of Wide-Area-Measurement Systems (WAMS) and Phasor Measurement Units (PMUs, there is now more availability of accurate measurement of system variables. The use of these technologies in power systems provides operators with dynamic operation data that was previously more difficult to attain. Work such as [2] used these data to perform complex stability analysis to locate sources of oscillation within a system and identify key generators that are contributing to unstable oscillations. Transient stability is important as it is the ability of the power system to maintain synchronism when subjected to a severe transient disturbance such as a fault on transmission lines, loss of generation, or loss of a large load. To prevent transient instability, variouscounteracting measures can be taken. These measures can include changing voltage phase angles through a phase-shifter [3], changing network impedance through series compensation [4], dynamic braking [5] or online instability detection [6]. There is also a need to look at expanding the transmission system capacity, but there are limits that restrict how far asystem can be pushed in order to retain stability. Work such as [7] looks at enhancing the approach to the PTL by using an algorithm to reallocate generation based on a Transient Time Margin (TTM). The TTM provides a standard for comparing the stability degree for different contingencies. The minimum TTM of critical contingencies corresponds to the stability level of a power transmission corridor. By using these sensitivityfactors, generation can be reallocated in order to increase the stability level of a power transmission corridor. Work in [8] looks at using fuzzy logic intelligent systems combined with OPF to calculate the transmission capability between two areas. Using the OPF, the interchange between the two areas can be maximised and constraints are added until both dynamic and static stability limits are met. The methodology used in this paper looks to improve on the active power limit that is currently used as a constraint for transient stability. In system operation, for the same power flow through a line, the angle difference between the two buses will be greater at lower voltages. This also means that it will be closer to its transient stability limit. The limit is typically defined in terms of the active power, which is defined at the worst case voltage profile, when voltage is low. By this reasoning, when voltage support is high, a stronger voltage profile, the angle difference will be much less for the same power flow. Yet, the constraint stays constant as it is defined in terms of the active power. More power could be sent through a transmission corridor when voltage support was higher, and therefore, a higher active power limit could be realised. There has been work covered in the areas of increasing transmission capacity. [9] looks at the use of superconducting generators inthe network to increase transmission capacity, and by using them in critical locations can also add to system stability. In [10], the authors develop a tool for transient stability that can assess the online transient stability margin using potential energy or potential energy boundary surface methods

CHAPTER-2BACKGROUNDThe theory behind this concept can be illustrated using the Equal Area Criterion (EAC) on a Single Machine Infinite Bus (SMIB) system, shown in Fig. 1. The equal area criterion is a well understood concept which simply states that stability is maintained only if an area A2 at least equal to A1 can be found above the input mechanical power, Pm [12] shown in Fig. 2. If the area A1 is greater than A2, for the post fault period, the rotor angle, _rotor > _u, the unstable rotor angle and stability is lost.

Fig. 1. Single machine infinite bus

Fig. 2. Equal Area Criterion

It is well known that higher voltage helps transient stability where the electrical power can be increased and reduces the accelerating power. This approach is investigating the effect of adequate voltage support which would correspond to a higher voltage at the generator terminal, Et. Operating limits for voltage in the UK are _10% of 1 p.u. Pmaxe =EtEBX(1)The maximum electrical power, Pmax e , shown in Fig. 2 occurs at an angle _ = 90_, and is shown in Equation (1). Et is the generator terminal voltage, EB is the voltage at the infinite bus, shown in Fig. 1 and X is the total reactance between the generator and the infinite bus, which can include the transient reactance of the generator, x0d.To show the effect of higher voltage support, the generator terminal voltage, Et, is set at 1.1 p.u. According to equation (1) an increase in Et would correspond to a larger value of Pe. This would correspond to a decrease in accelerating power and an increase in the decelerating power, A2. Therefore, a larger potential energy is beneficial to preserve stability. This means that if there is high voltage support, there is a higher potential energy to stabilise the system so that the system can be pushed harder and it will remain stable.Fig.

Fig. 3. Comparison of decelerating area

Fig. 3 shows a comparison of the post fault P_ curve showing the change in the decelerating area A2 and the unstable rotor angle, _u. This angle shows that when there is higher voltage support, the rotor angle can be increased further without going unstable. This is quite obvious, so the aim of the paper is to show how to exploit this benefit for increased capacity in transmission corridors

CHAPTER-3METHODOLOGY OF INCREASING TRANSIENT STABILITYnderstood. The methodology can be carried out with a twolevel approach. The methodology relies on the operation of WAMS in a power systems for utilisation of an angle based constraint over the traditional active power constraint. The introduction of an angle based constraint on transient stability can be looked at in two ways using: (1) a static and (2)a dynamic angle constraint. This can be illustrated using the SMIB system of Fig. 1. A. Using a static angle constraint As the limit is currently defined by a fixed active power limit, the corresponding critical clearing angle for the worstcase scenario is found at 0.9 p.u. which is used as the active power limit. Fig. 4 shows this active power limit as Plimit calculated for the system of Fig. 1. In current operation, this is a static value, hence independent of the current voltage status of the system. For the extreme case when voltage on the system is 1.1 p.u., a new active power limit can be used, shown here as Pu limit. Rather than set a new active power limit based on this value, WAMS allow for angle measurement, so the constraint can be defined as per _limit shown in Fig. 4. The advantage to this method is that the angle limit can be found in a similar way to how the active power limit is currently defined. This will allow increased transmission through a corridor based on the voltage of that network which would be an increase of Pu limit - Plimit.

Fig. 4. Extra capacity by using static angle constraint

B. Using a dynamic angle constraintThe previous static angle constraint can also be viewed as conservative in the way that it used the worst case angle. An increased benefit can be achieved if the angle constraint that is used is dynamic. This would mean that the critical clearing angle is dependent upon system conditions instead of being set at the base case. It was seen in section II that for increased voltage support, the unstable angle increases. If this can be taken into account then the transmission capacity can be increased further.

Fig. 5. Extra capacity by using dynamic angle constraint

Fig. 5 shows the critical clearing angle for the 0.9 p.u. and 1.1 p.u. voltage cases. These were calculated using the SMIB system shown in Fig. 1. As critical clearing angle is dependent on the voltage support, the corresponding power output can be enhanced further. The angle shown as _cl 0:9 is the same as the static angle constraint shown in Fig. 4. Using the critical clearing angle for the higher voltage case of _cl 1:1, which as expected is higher, would correspond to the value of Pdyn limit which is an increase from the power defined using a static constraint of Psta limit. This dynamic angle limit can be utilised with dynamic measurements offered by PMUs and WAMS. Such a dynamic constraint would allow the system to be run more effectively depending on the voltage conditions of the system and maximise throughput of a transmission corridor based on the real time status. Theseresults correspond to the benefit when using 1.1 p.u. but having normal operating voltages of 1.0 p.u. would still allow extra power to be transmitted when compared to using the conservative active power constraint

TRANSIENT STABILITYGenerators are connected to each other by a network that behaves much like weights interconnected by rubber bands (see Figure 5). The weights represent the rotating inertia of the turbine generators and the rubber bands are analogous to the inductance of the transmission lines. By pulling on a weight and letting go, an oscillation is setup with several of the weights that are interconnected by the rubber bands.The result of disturbing just one weight will result in all the weights oscillating. Eventually the system will come to rest, based on its damping characteristics. The frequency of oscillation depends on the mass of the weights and the springiness of the rubber bands. Likewise, a transient disturbance to the generator/network can be expected to cause some oscillations due to the inability of the mechanical torque to instantaneously balance out the transient variation in electrical torque. Both components of torque act on each generator in the system. A lack of sufficient synchronizing torque will result in loss of synchronism. Such loss of synchronism can only be prevented if sufficient magnetic flux can be developed when a transient change in electrical torque occurs. This is facilitated by a high initial response excitation system having sufficient forcing capability and sufficiently fast response to resist the accelerating or decelerating rotor. In order to be effective for both accelerating and decelerating rotor response, the exciter must be capable of forcing both positively and negatively. When the rotor is accelerating with respect to the stator flux, the rotor angle is increasing due to mechanical torque higher than electrical torque. The exciter system must increase excitation by applying a high positive voltage to the alternator field as quickly as possible. Conversely, when the rotor angle is decreasing due to mechanical torque less than electrical torque, the exciter system must decrease excitation by applying a high negative voltage to the alternator field as quickly as possible

Fig:-Transient Stability Illustration

Transient stability is primarily concerned with the immediate effects of a transmission line disturbance on generator synchronism. Figure 6 illustrates the typical behavior of a generator in response to a fault condition. Starting from the initial operating condition (point 1), a close-in transmission fault causes the generator electrical output power Pe to be drastically reduced. The resultant difference between electrical power and the mechanical turbine power causes the generator rotor to accelerate with respect to the system, increasing the power angle (point 2). When the fault is cleared, the electrical power is restored to a level corresponding to the appropriate point on the power angle curve (point 3). Clearing the fault necessarily removes one or more transmission elements from service and atleast temporarily weakens the transmission system. After clearing the fault, the electrical power out of the generator becomes greater than the turbine power. This causes the unit to decelerate (point 4), reducing the momentum the rotor gained during the fault. If there is enough retarding torque after fault clearing to make up for the acceleration during the fault, the generator will be transiently stable on the first swing and will move back toward its operating point. If the retarding torque is insufficient, thepower angle will continue to increase until synchronism with the power system is lost. Power system stability depends on the clearing time for a fault on the transmission system. Comparing the two examples in Figure 7 illustrates this point. In the example of slower fault clearing (left figure), the time duration of the fault allows the rotor to accelerate so far along the curve of PE, that the decelerating torque comes right to the limit ofmaintaining the rotor in synchronism. The shorterfault clearing time (right figure) stops the acceleration of the rotor much sooner, assuring that sufficient synchronizing torque is available to recover with a large safety margin. This effect is the demand placed on protection engineers to install the fastest available relaying equipment to protect thetransmission system. Figure 7: Effect of Fault Clearing Time

Fig:-Effect of Fault Clearing Time

3Power System Stability Improvement with (FACTs)Controller

T HE electrical power demand is gradually increasing with increase in load demand. This requires the highest reliability and security with minimum transmission expenditure. Power system engineers are currently facing challenges to increase the power transfer capabilities of existing transmission system with above constraints. However it becomes much difficult to construct new transmission lines Vasundhara Mahajan is with Department of Electrical Engineering, because of the geographical and environmental conditions. For solving these problems, it is significant to use existing power system networks more effectively up to their maximumcapability. In such situations it is necessary to improve the performance and stability of the power system network. Due to these conditions the stability margin of power systems has decreased significantly. Thus, new techniques are required to improve the performance and stability. Flexible AC Transmission system (FACTs) controllers can balance the power flow and thereby using the existing power system network most efficiently. Because of their fast response FACTs controllers can also improve the stability of an electrical power system by helping critically disturbed generators to give away the excess energy gained through the acceleration during fault. Thyristor controlled series compensator (TCSC) is an important device in FACTs family and is widely recognized as an effective and economical means to solve the power system stability problem. TCSC controller can control the line impedance through the introduction of a thyristor-controlled capacitor in series with the transmission line [1-10].

THYRISTOR CONTROLLED SERIES COMPENSATOR (TCSC)

Series capacitors offer certain major advantages over the shunt capacitors. With series capacitors, the reactive power increases as the square of line current, whereas with shunt capacitors, the reactive power is proportional to the square of bus voltage. For achieving same system benefits as those of series capacitors, shunt capacitors required are three to six times more reactive power rated than series capacitors. Furthermore shunt capacitors typically must be connected at the midpoint, whereas no such requirement exists for series capacitors. A series capacitor is capable of compensating for the voltage drop of the series inductance in a transmission line. During low loading the system voltage drop is lower and at the same time the series compensation voltage is lower. When loading increases and the voltage drop becomes higher, the contribution of series compensation increases and therefore system voltage will be regulated as desired.

Basic Module The basic module has a fixed series capacitor C, in parallel with a thyristor-controlled reactor, L, as shown in Fig. l(a). A metal oxide varistor (MOV) is connected across series capacitor to prevent the occurrence of high capacitor over voltages. MOV allows the capacitor to remain in circuit even during fault conditions and hence improves transient stability. A circuit breaker (CB) is installed across capacitor for controlling the insertion of capacitor. If TCSC valves are required to operate in fully on mode for prolonged duration, the conduction losses are minimized by installing an ultra high speed contact (UHSC) across the valve. This offers a loss less switching operation similar to that of circuit breakers. The metallic contact is closed shortly after the thyristor is turned on, and is opened shortly before the valve is turned off. During a sudden overload of the valve, and also during fault conditions, the metallic contact is closed to minimize the stress on the valve as shown in Fig. 1(b) [11-19].

Fig. 1. TCSC module (a) A basic module (b) a practical module.

IMPROVEMENT OF TRANSIENT STABILITY OF THE POWER SYSTEM USING SMALLMAGNETIC ENERGY STORAGE

SMES unit typically consists of a superconducting coil, thyristor controllers and refrigerator. The coil is kept below the critical temperatures. The power system constitutes of a single machine connected to an infinite bus bar through double transmission line as shown in Fig I is considered for study [II]. The system parameters are given in the appendix. The disturbance of 3