Simulation Modelling of Voltage Stability of an Interconnected Electric Power System Network
Chapter One
Purpose of the Study
From the points raised in section 1.3, it is clear that the state of the power system is influenced by both controllable and uncontrollable factors. For example, the system generation must increase as the load increases to keep power balance and maintain voltage stability. Often at high load levels, generators reach real and reactive power limitations and the power flows in lines exceed limits. Any of these events can initiate a change in the equations that model the power equation. This thesis is intended to achieve the following aims and objectives:
- To model an interconnected electric power system network that will continually track system changes and assess its voltage stability.
- To predict the possible causes of voltage collapse in an interconnected electric power systems like the Nigerian power system.
- To access stability margin and power transfer limit in the system
To indicate sensitivities and the major contributing factors that will provide insight into system characteristics to assist in developing remedial actions.
CHAPTER TWO
LITERATURE REVIEW
Electric Power System Stability
The continuing increase in the demand for electric power especially in developing countries has been projected to far exceed the planned generation of existing power systems in the coming decades. This has led to increasingly complex interconnected systems, which are forced to operate ever close to the limits of stability because of the high degree of coordination required to ensure system stability.
Traditionally, power system stability refers to the notion of whether synchronism of the generators can be maintained following a disturbance. Physically, this requires a balance between the mechanical power applied to each generator and its electrical power output.
Disturbances are typically categorized as small or large, depending on their severity. An example of a small disturbance is the usual load variations during the day. An example of a large disturbance is the outage of a major transmission line, or a sizable generator.
Analytically, small disturbance problems can be studied via linearization of the equations describing the dynamic behaviour of the network, large – disturbance problems, however, require nonlinear techniques. The system is said to be stable, if, following a disturbance, it is able to reach an acceptable steady state.
Electric Power System Stability is defined as the capability of a system to maintain an operating equilibrium point after being subjected to a disturbance for given initial operating conditions [12]. Electric Power System stability is primarily concerned with variations in speeds, rotor positions, and generator loads.
It focuses attention on the transmission network, since it is the network, more than power plant or system controls, which provides for the power shifts between generators required to maintain synchronism.
Stability considerations have been recognized as an essential part of electric power system planning for a long time. Depending on individual experience and viewpoint, there is no doubt various explanations for the increased concern with stability. The underlying cause, however, would appear to be very extensive interconnection of power systems with greater dependence on firm power flow over ties.
This magnifies the undesirable consequences of instability and complicates the analytical process through which acceptable system behaviour is assured. The possible consequences of instability are loss of generation, loss of transmission facilities and loss of load.
Stability therefore denotes a condition in which the various synchronous machines of a power system remain in synchronism or “in step” with each other. Conversely instability denotes a condition involving loss of synchronism, or “out of step”. To understand the different aspects and characteristics of power system stability, the following issues need to be considered [12, 13]:
- Beside the highly non-linear nature of a power system, this system is continuously subjected to changing operating conditions (e.g. loads, generation, etc.), hence, the stability of the system depends on the initial operating conditions.
- Power systems are usually subjected to a wide range of disturbances. These are classified as small disturbances (e.g. load changes) or large disturbances (e.g. fault conditions). For example, short circuits and transmission line outages can lead to structural changes from the reaction of the protection devices to isolate the faulty elements.
Based on the previous discussion, power system stability is categorized based on the following considerations:
- The nature of the resulting instability mode indicated by the observed instability on certain system variables.
- The size of the disturbance which consequently influenced the tool used to assess the system stability.
- The time margin needed to assess system stability.
CHAPTER THREE
POWER SYSTEM MODELLING
Philosophy of Engineering Modelling
Electrical power network is concerned with complex system, which either already exists or is being planned for the future. Because of the size of these systems, it is therefore necessary to resort to investigation based on models. A model is a representation of the more important properties of a system that can be used to predict the behaviour of the system over a range of operating conditions in its operating environment [74]. Models may be physical or mathematical. The physical models are the laboratory models like a switching and transmission systems, which are used to evaluate their performance and to connect design faults before systems are put into manufacture. However, mathematical models range from simple tables and graphs to complicated sets of equations and logarithms for performing computer simulation. However, diagrams and maps are also models and are extensively used in studies on electric power networks.
CHAPTER FOUR
METHOD OF ANALYSIS OF VOLTAGE STABILITY
Introduction
It is important to have an analytical method to investigate voltage stability in the power system, particularly with a complex and large interconnected network. The modal analysis or eigenvalue analysis can be used effectively as a powerful analytical tool to verify both proximity and mechanism of voltage instability. It involves the calculation of a small number of eigenvalues and related eigenvectors of a reduced Jacobian matrix. In the past, the electric utility industry has largely depended on conventional power-flow programs for static analysis of voltage stability. Stability is determined by computing the V-P and V-Q curves at selected load buses. Generally, such curves are generated by executing a large number of power flows using conventional models. While such procedure can be automated, they are time –consuming and do not readily provide information useful in gaining insight into causes of stability problems. In addition, these procedures focuses on individual buses, that is, the stability characteristics are established by stressing each bus independently. This may unrealistically distort the stability condition of the system. Also, the buses selected for V-Q and V-P analysis must be chosen carefully, and a large number of such curves may be required to obtain complete information. In fact, it may not be possible to generate the V-Q curves completely due to power-flow divergence caused by problems elsewhere in the system.
CHAPTER FIVE
SAMPLE SYSTEMS MODELLING, SIMULATION AND RESULTS ANALYSIS
Introduction
The Modal analysis method has been successfully applied to two different power systems. A power flow program based on Matlab is developed to:
- Calculate the power flow solution
- Analyze the voltage stability based on modal analysis
- Generate the Q-V sensitivities and Participation factors
- Generate Q-V Curves
Simulation Modelling Of the IEEE 14 Bus System
The Modal analysis technique is applied to the IEEE 14 Bus System shown in Figure 5.1. The data required for the simulation are as follows: Line data represented in Table 5.1; while Table 5.2 represents load distribution, all indicated as appendix A and B respectively. The voltage profile of the buses is presented from the load flow simulation as shown in Figure 5.2. It can be seen that all the bus voltages are within the acceptable level (± 5%). The lowest voltage compared to the other buses can be noticed in bus number 3.
CHAPTER SIX
ACHIEVEMENTS, CONTRIBUTIONS, RECOMMENDATIONS AND CONCLUSION
Achievements of the Thesis
- This thesis successfully modelled an interconnected large electric power system network that continually track system changes and assess the voltage stability. The models are included in Figure 5.1 and Figure 5.7 of pages 84 and 90 respectively.
- A matlab program was developed for the simulation of the voltage stability of an interconnected electric power system network. The program has the capability of running the system load flow, determining the system eigenvalues, the Q-V sensitivity and the participation factors.
- The possible causes of voltage stability problems were identified in the thesis to include long transmission lines, over loading of the network, generation and transmission line limitations and inadequate reactive power support.
- The sensitivities and the major contributing factors that provide insight into the system characteristics that will assist in developing remedial actions as identified in the thesis are Q-V Sensitivity, Generator, Bus and line participation factors these were determined in chapter five of the work.
- The above achievements were in line with the set out objectives as stated in purpose of the study of Chapter one.
Contributions of the Thesis to Knowledge
The contribution of this work to knowledge can be summarized thus:
- Development of Modal evaluation for voltage stability.
- Development of a simulation model that included compensators in the interconnected power system network.
- Development of Simulation model for large scale electric power system using software package operated in a Matlab environment.
- Confirmation of results obtained from Modal technique using Q-V curves generated on the suspected weak bus.
- Computation of the system margin based on reactive power demand. This would help the system operators to plan and compute the capacitive reactance of equipment required to improve the system.
- The power system reactive power margin computed with these techniques can be used to design an expert system for the protection and control of electric power system.
- This thesis demonstrated that the voltage stability computation with respect to system parameters yields practical information useful for the control, planning and operation of an interconnected electric power system network.
- The results can be used to determine placement locations for reactive support and Flexible Alternating Current Transmission System (FACTS) controllers in the interconnected power system network.
Recommendation For Future Work
There are a number of issues that still need to be addressed in this subject area.
- Simulation modelling of other interconnected power system componets such as generators, transmission lines and flexible alternating current transmission system (FACTS) controllers such as Static Synchronous Compensators (STATCOM), Static Synchronous Series Compensator (SSSC), Thyristor Controlled Series Capacitor (TCSC), Thyristor Controlled Phase Shifter (TCPS), Static VAr Compensator (SVC) to determine their sensitivities and participation factors for overall system stability.
- Incorporation of the method in this thesis for accounting for the effects of power system controllers and stability on power dispatch and electricity market prices.
The effect of deregulation in the reliability of electric power system in Nigeria has generated a lot of concern to the citizens of the country. The power utility company is in need of a reliability indicator that can be tracked over time and used to quantitatively assess system reliability. This role will be played by security margins. The first limit violation of the loading margin of a bus could be used to compare different buses relative security at different time.
Conclusion
In this dissertation, voltage stability of an interconnected power system network is investigated and assessed. Two interconnected power system networks were used as test systems they are: The IEEE 14 Bus system and the Nigerian 330kV, 30Bus interconnected power system. The systems were modelled using NEPLAN software operated in Matlab 7.5.0 environment. Power system analysis tools (PAST) is used in the modelling of the systems. The systems were modelled elements by elements with the parameters of each element included. The model for the Nigerian system incorporated compensators identified in the various locations of the system grid. The inclusion of these compensators in the interconnected power system network resulted in a great improvement in the voltage stability of the system.
The modelled network is simulated for investigation and assessment of voltage stability.
The following can be concluded:
1 The modal analysis technique is applied to investigate the stability of IEEE 14 bus system and the uncompensated and compensated Nigerian interconnected 330kV, 30Bus power system network. The method computes the smallest eigenvalue of the reduced Jacobian matrix using the steady state system model. The magnitude of the smallest eigenvalue gives the measure of how close the system is to voltage collapse. The Q-V sensitivity is used to identify the bus that has the highest sensitivity which is the bus that will have highest contribution to voltage collapse .The Q-V curve is used to confirm the result.Then the participation factors is used to identify the weakest node or bus, branch and generator in the system associated with the minimum eigenvalue.
2 The loadflow is calculated using the software, the nodes’ results showed that all the buses’ voltage ranges were within the acceptable (±5%) for the IEEE 14 Bus bus system and the compensated NIPS, while the uncompensated system has acceptable voltages except Maiduguri, Gombe and Jos with 0.9106, 0.9175 and 0.9359 respectively.
3 The Q-V sensitivity is used to determine the buses with highest sensitivity and the identified buses are Maiduguri with sensitivity figures of 0.1474 followed by Gombe, Kano and Jos with 0.1312, 0.0823 and 0.0370 respectively for the uncompensated Nigerian system while the compensated system has Maiduguri with sensitivity figures of 0.1398 followed by Gombe, Kano and Jos with 0.1247, 0.0807 and 0.0351 respectively.The results show that the compensated Nigerian system has reduced Q-V sensitivity values which is an indication of improved stability of the system.
4 The buses with highest participation factors at the least eigenvalue of 3.1549 are Maidugri and Gombe with figures of 0.4960 and 0.4413 respectively. The branches with highest participation factors at the least eigenvalue of 3.1549 are L469-Jos- Kaduna B19-B17, L517- Ikeja West – Benin B14 to B21, L453- Aba-Enugu B24-B26 L421- Jebba TS – Shiroro T.S B8-B9, L445- Shiroro T.S – Kaduna B9 – B17, L597 – Afam – Aba B5 – B24 with the figures of 1.0000, 0.4706, 0.2584, 0.2263, 0.2232 and 0.2170 respectively. The generators with highest participation factor at the least eigenvalue of 3.4951 are SM 347-Shiroro and SM 823 with figures of 1.0000 and 0.7939 respectively.
5 The use of compensators in the interconnected power system network resulted in a great improvement in the voltage stability of the system.
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