Transient Stability Simulation of 33 kV Power Grid

An example of Sandakan power grid problem is presented in this paper. Sandakan is a suburb in east coast of Sabah state of Malaysia. Stability problem occurs due to the increase in load demand, lack of generation sources and inadequate supply. The tripping disturbances occur frequently in the network which is contributing to voltage instability. In this paper dynamics stability of 33 kV power grid as related to Sandakan network is analyzed and simulated. The analysis is completed by modelling the network data in Power System Simulation for Engineering (PSS/E) software and simulate the transient stability of generator, exciter and governor during a three phase fault occurs on a far and close distance from a bus, and determine the critical clearing time as well as swing curve of rotor angle. The output values of electrical power, machine speed, rotor angle and bus voltage are observed.


Introduction
Generally, a power system under normal operating conditions may face a contingency such as transmission element outages, generator outages, loss of transformer, and sudden change in the load or faults [1]. Transient is an event occurs when a power system subjected to large disturbances under dynamic stability [2]. Disturbances include loss of synchronism, loss of generation, loss of load in transformers or faults on transmission element and lines. Transient stability is one of major analysis in the power system in order to ensure the system stability to withstand a major disturbance and to ensure that the transmission system is operated safely, steady state and contingency analysis must be performed [7].
The round rotor generator model (GENROU) represents as solid rotor generator at sub-transient level is used to produce machine rotor angle for transient stability. Rotor angle has the ability of interconnecting the synchronous machines with power system to remain in synchronism. Stability is related to generator electromagnetic torque and mechanical torque which cause the rotor to accelerate or decelerate [1]. Voltage stability is related to change in the load. This stability is the ability of a power system to maintain steady voltages at all busses from a given initial condition after being subjected to a disturbance [6]. Voltage instability increases by load demand or change in system condition which cause the incontrollable drop in voltage. With abnormal low voltage it is lead to voltage collapse also contributes to blackout of the grid system [2]. The problems were reported with power flow and contingency in terms of blackout after main grid supply outages with overload and high fault current on distribution system [4] [6]. Critical clearing time is known as maximum time duration that a fault may occurs in a power system without loss of stability. There are three type of fault condition which pre-fault system conditions, fault structure (type and location) and post fault conditions [3]. The three-phase fault is the most serious kind of fault and its critical clearing time can reflect the transient stability of power system. Critical clearing time (CCT) can be obtained by trial and error method [4].
Synchronous generator is the source of electrical energy where in the generator the mechanical energy usually transformed into electrical energy. This transformation is provided by excitation of synchronous generator and is regulated by excitation system. An IEEE Type 2 Excitation System (IEEET2) and turbine governor such as Gas Turbine (GAST) and Turbine IEEE Type 1 Speed Governing Model (IEEEG1) are used.
Generator excitation is defined as generator output voltage and output reactive power. It means that the excitation is actually output energy of generator regulation and this can impact the stability of the power system. The use of an excitation system is for maintaining the output voltage, control the shaft's speed and enhancing the generator performance.
In this paper, PSS/E will be used for characterizes the power system transmission network and generation performance for both load flow analysis and transient analysis [5]. All the sources referred from the books, articles, research papers and journals.
N. Barsoum et al. bus base is 33 kV and the lowest is 6.6 kV. Figure 1 represents the existing network.

Transient Stability Analysis
In order to achieve stability of power system, load flow study is an important tool that gives a numerical solution. In power flow analysis a per-unit system is used for voltage magnitudes and angles, real and reactive powers.

Load Flow Result
The diagram Figure 1 Table 6 and Table 7. These variations of voltages are normally applied for pre and post fault Table 8 shows all the voltages before load flow analysis are under normal condition which are not exceed than 105% (overvoltage) and not below 95% (under-voltage). Thus, the grid does not have any critical busses.

Bus Voltage before and after Load Flow Simulation
These parameters are found from Sandakan power grid of 26 buses. Load flow increases the voltage bus as shown.

Transient Stability Result
The transient stability analysis approach by applying a fault on a bus and run the

Generator
In the normal state, the machines in a power system network operate at equilibrium corresponding to the mechanical power input, Pm being equal to the electrical power output, Pe. When a fault occurs in the system at time = 1.0 seconds, the mechanical power input become greater than the electrical power output (Pm > Pe), then the speed of the machines increase as it will accelerate the rotor.         With the fault cleared at time = 1.18 seconds, the electrical power becomes low but the rotor still running above synchronous speed, hence the angle and the electrical power continue to increase. When the electrical power is greater than mechanical power (Pe > Pm), it causing the rotor to decelerate toward synchronous speed until the angle reaches its critical value. When the system reached the critical value, the rotor angle will continue to oscillate back and forth at its natural frequency until it becomes stable. In

Exciter
The results of the excitation system response in Figure 6 Based on the results in Figure 6 indicate that GN_11, BN_11, KB_6.6 and SB_6.6 exciters perform slower than LD1_11, LD2_11, LD3_11 and LD4_11 exciters which are this exciter meet the 1 pu the excitation system voltage requirement. These bus conditions are shown in Table 9. Figure 7 shows the variation of speed with time for governor IEEEG1 and GAST types. All final frequencies were determined by the droop, R of the responding governors, Table 10. The frequency drops depends upon the generator inertia values. The least frequency deviation occurs with high inertia and fast governors. Governor condition are given in Table 11.

Critical Clearing Time (Result)
To determine the critical clearing time when the fault occurs at bus that close and far from generator, refer to Figure 8. It shows that bus BS_33 is near to generator BN_11, while for bus SA_33 is 14 kilometers far from generator GN_11. Figure 9 and Figure 10 have been obtained by the technique of trial and error method in order to determining the critical clearing time of the system where the fault duration was increased gradually using the step time of Δt = 0.01 seconds until the system appears to be unstable by observing machine's rotor angle as a reference point. Figure 9 shows the variation of voltage with time for a three phase fault applied on bus BS_33 (bus near to generator). Since the three phase fault applied at time = 1.0 seconds, then the fault is clear at time = 1.18 seconds. Thus, the critical clearing time is 0.18 seconds and the system becomes stable. Figure 10 shows the variation of voltage with time for a three phase fault applied on bus SA_33 (bus far from generator). Since the three phase fault applied at time = 1.0 seconds, then the fault is clear at time = 1.23 seconds. Thus, the critical clearing time is 0.23 seconds and the system becomes stable.
It can be seen that transient stability is greatly affected by the location of a fault from bus to generator. Table 12 shows the critical clearing times in seconds determined for all the twenty-six buses on the Sandakan Power Grid.
Transient stability analysis is run starting with a clearing time of 0.01 seconds. If the system is proved a stable condition, another analysis run is made by increasing the clearing time higher than first run. If the second run is still in stable condition, then more runs are made until the system becomes unstable. If the run showed an unstable system, then the clearing time of previous run gives the desired result.

Conclusion
This paper presents a modeling and simulating case data of 26 buses, 8 generators and 22 loads 33 kV power grid that in service mode. Through load flow analysis, the voltage performance under different conditions can be determined. The desired analysis of the transient stability of the system based on output values such as machine rotor angle, electrical power, machine speed and bus voltage were found to be stable after fault is cleared. The rotor angle, power, speed and the voltage in the grid system is back to its normal condition where there is no generator set that will out of phase and when the fault is cleared. The theory of the critical clearing time (CCT) when the fault occurs close and far from the generator was proved in this paper by using trial and error method. Thus, as the