Performance evaluation of different structures of power system stabilizers

Received Jan 6, 2019 Revised Jun 20, 2020 Accepted Jul 13, 2020 The electric power from the system should be reliable and economical for consumer’s equipment satisfaction. An electric power system consists of many generators, transformers, transmission lines, loads, etc. For the power system network, dynamic performance and stability are important. The system is lost its stability by some disturbances i.e., load variations, generator failure, prime mover failure, transmission line outage, etc. Whenever load variations in the system, generator rotor speed will vary, means oscillations in the rotor speed, which is deviating from rated speed. The excitation system will control the generator rated line voltage. When fault occurs at any equipment in the system, the system will unstable. If fault occurs at generator, the generator oscillates. To reduce the oscillations and to make the system stable used power system stabilizers (PSS’s). Here, three types of PSS’s are used i.e., PSS1B, PSS2B, PSS4B. Comparisons of three PSS’s are on the multi machine system under some disturbance. From the observations, concluding that PSS4B is quickly control the oscillations in the physical parameters of machine in the system than other power system stabilizers.

The Heffron-Phillips constants are dependent on operating conditions and machine parameters considered for the system. Here, K1, K2, K3, K4, K6 are positive and K5 is negative.

POWER SYSTEM STABILIZER
In an interconnected power system arises the low frequency oscillations with less time durations, then system will stops working completely. The system shut down due to unavailability to compress those oscillations in the system. In order to overcome this problem, PSS is tested and implemented to compensate those oscillations. PSS Figure 1 is a supplementary control connected to the excitation system. The feedback signal is connected in such a way that rotor electric torque in phase with speed variations. The PSS parameters are tuned with system conditions, otherwise system will be in unstable state. The Block Diagram of PSS is shown in Figure 1. The gain related to damping introduced by the stabilizer. Then increasing gain moves unstable mode to stable mode. Gain should be set corresponding to maximum damping. Washoutis high pass filter, this will block the offsets and responds for speed oscillations. This also controls the terminal voltage deviations from normal voltage of the generator. Phase Compensationis a Lead-lag compensator. The lead-lag compensator is tuned in such way that speed oscillations give damping torque on the rotor. When Terminal voltage is changed, the PSS affect the generator power flow. The terminal voltage is limited under AVR variations.

Types of power system stabilizers
Single input PSS: The inputs which are given to PSS are change in rotor speed (∆ῳ), the change in frequency (∆f) and the accelerating power (∆Pa). PSS1B has change in rotor speed as input. This stabilizer will reduce the damping of the low frequency oscillations. The structure of PSS is in the form as, for which the parameter such as stabilizer gain Kc, lead lag time constants T1 and T2 are to be computed such that overall system is stable when PSS included in feedback loop.
Dual input PSS(PSS2B): In this PSS, one input as change in rotor speed and another input as change in electric power. The change in electric power (∆Pe): Due to simplicity of measuring electric power and relationship to shaft speed, as an input signal to stabilizer. This stabilizer uses electric power (∆Pe) as input and it has stabilizer parameters K and T are to be computed, which are related to stability of the system. The block diagram of PSS2B is shown in Figure 2. 117 the generator of a system. Local oscillations are created by disturbances that occur between one stable generator and unstable generator in a power station. The local oscillations frequency range is 0.8Hz to 4.0Hz.
Inter-area oscillations are caused by a disturbance between two areas of generation satiations in the power system. The range of frequency is 0.2Hz to 0.8Hz. MB PSS oscillation damping three ranges of different frequencies to dampen entire spectrum frequency oscillation that can occur in the power system achieved. Low and medium frequency oscillations related to inter-area mode, high frequency oscillation related to local mode. . Under no-load conditions, the single machine system maintains rated speed as 1p.u. From Figure 3, when a three-fault created at load side from 0sec to 1.06sec, the generator becomes unstable. The generator speed oscillates (deviates from 1p.u.). In order to control the oscillations in generator speed used power system stabilizer (PSS). Here, using three different PSS's (PSS1B, PSS2B, PSS4B). In Figure 4, the power system stabilizer is connected to excitation system. The output of power system stabilizer is connected to input of excitation system. The PSS will control oscillations in physical parameters of machine in the system.

RESULTS
From the Figure 5, without PSS the generator rotor speed oscillates. The magnitude of rotor speed increased from 1p.u. to 1.017p.u. With PSS1B, The generator rotor speed decreased from 1.013p.u. To 1p.u. and reached its steady state within 6sec. With PSS2B, the generator rotor speed decreased from 1.007p.u. To 1p.u. and reached its steady state within 5.2sec. With PSS4B, the generator rotor speed decreased from 1.003p.u. To 1p.u. and reached its steady state within 5sec.So, PSS4B will reduce the Generator rotor speed oscillations in a less time compared to other PSS's.

Comparison of results
From the Table 1, Without PSS, the generator speed is increased to 1.017p.u., and it is settles at 7 sec. For different stabilizers, the generator rotor speed increased in magnitude with steady state time. PSS4B settles in less time.    15 In Figure 6 multi machine system consists of generators, transmission lines, transformers, loads, etc Figure 6. Simulation diagram of multi machine system In Figure 7, inside the area1, two generators, two transformers with load are connected. The threephase fault created at generator 1. Similarly generators, transformers, etc there in inside area 2. Figure 8 inside every machine is connected with excitation system and different power system stabilizers. The excitation system maintains the rated voltage of generator. The PSS will reduce the oscillations and retains the system stability. When three phase faults created at generator1 from 0.005 sec to 0.007sec. Then all generators in system became unstable. Means, initially the generator rotor speed is 12p.u. After the fault, all generator speeds deviate from 12p.u.
From Figure 9, the magnitude of rotor speed reduced to 0p.u.the generator becomes unstable. Form Figure 10, generator2 (without PSS) is unstable, rotor speed reduced from 12p.u. to 0p.u. With PSS1B, initially it is unstable. Rotor speed retains its steady state 12p.u. within 0.024sec. With PSS2B, Rotor speed retains its steady state 12p.u. within 0.022sec. With PSS4B, Rotor speed retains its steady state 12p.u. within 0.008sec.
From Figure 11, generator3 (without PSS) is unstable, rotor speed reduced from 12p.u. to 0p.u. With PSS1B, initially it is unstable. Rotor speed retains its steady state 12p.u. within 0.029sec. With PSS2B, Rotor  Figure 12, Generator 4 (without PSS) is unstable, rotor speed reduced from 12p.u. to 0p.u. With PSS1B, initially it is unstable. Rotor speed retains its steady state 12p.u. within 0.03sec. With PSS2B, Rotor speed retains its steady state 12p.u. within 0.026sec. With PSS4B, Rotor speed retains its steady state 12p.u. within 0.021sec. From the above results, we can observe that when a three-phase fault created at generator1, all the generator rotor speeds are decreased from 12p.u. to 0p.u. and Generators are stopped working, they can't supply power to load. It is also observed that generator1 still unstable by connecting different PSS stabilizers. Generator2 becomes stable with PSS4B in less time 0.008sec. Generator3 becomes stable with PSS4B in less time 0.021sec. Generator3 becomes stable with PSS4B in less time 0.021sec

CONCLUSION
PSS is additional controlretain the steady state of single machine infinite bus system under large disturbance from unstable state. From the simulation results, can observe that PSS will reduce the amplitude of damping oscillations, improve the transient stability of the power system. With PSS application on the multi machine system is under the disturbance, the results shown with various PSS's applied on the system. Compared the results of three PSS's under three phase faults in the system. Primarily the state of the system facing three phase faults at generator1, all the generators in the system are unstable. When PSS introduced in the system, it will dampen the oscillations of physical parameters of generators and make the generators 2, 3, and 4 are stable. PSS4B will reduce the oscillations in less time in all the generators of system compared to other PSS's (PSS1B, PSS2B)