Improved Energy Efficiency Using Facts-Device Technique: a Case Study of Ogui-Enugu Power Distribution Network
Chapters One
OBJECTIVES/PURPOSE OF THE STUDY.
Electricity is modern society’s most convenient and useful form of energy. Without it, the present social and physical infrastructure would not all be feasible. The increasing per capita consumption of electricity throughout the world reflects a growing standard of living of the people [19]. The greater the per capita consumption of electrical energy in a country, the higher is the standard of living of its people. To reflect this global trend, the Electric Power Research Institute (EPRI) in the US launched the Flexible Alternating Current Transmission System (FACTS) initiative in the later 1980’s with two main objectives: To increase the power transfer capability of electric power system and to conveniently keep voltage constant over designated routes [20].
Without any doubt, the application of FACTS-DEVICE to Ogui-Enugu Power Distribution Network intends to achieve the following objectives:
- To determine if the voltage variations are undesirable and not within the prescribed regulatory limit of of declared voltage.
- To increase the power transfer capability of the power distribution network by controlling the flows in heavily loaded lines resulting in an increased loadability.
- To improve the stability of the network by using power electronic FACTS-DEVICE to keep the voltage magnitude constant at the consumers terminal. This would help to fulfill government policy of supplying consumers with of declared voltage.
CHAPTER TWO
LITERATURE REVIEW
GENERALITIES ON FACTS DEVICES
This chapter presents an overview of the most salient characteristics of the power electronic equipment currently used in the electricity supply industry for the purpose of voltage regulation, active and reactive power flow control and power quality enhancement. The emphasis has been on steady-state operation and a distinction is made between power electronic equipment, which uses conventional power semiconductor devices (i.e. thyristors) and the new generation of power system controllers, which use fully controllable semiconductor devices such as GTO’s and IGBT’s. The latter devices work well with fast switching control techniques, such as the sinusoidal pulse width modulation (PWM) control scheme and from the power system perspective, operate like voltage sources having an almost delay-free response.
Equipment based on thyristors have a slower speed of response, greater than one cycle of the fundamental frequency, and use phase control as opposed to PWM control. From the power system perspective, thyristors-based controllers behave like controllable reactances as opposed to voltage sources [25].
The TCR, SVC and TCSC belong to the category of thyristors-based equipment. The STATCOM, SSSC, UPFC and HVDC-VSC use the VSC as their building block [20]. It is known that all these power electronic controllers produce harmonic distortion, which is an undesirable side-effect as part of their normal operation [21].
The various means of harmonic cancellation open to the system engineers include switching control, multi-level configurations, three-phase connections and as a last resort, filtering equipment. The remit/crux of this thesis is not power system harmonics; hence, it is assumed that harmonic distortion is effectively contained at the source. The mathematical modeling conducted for the various power electronic controllers addressed in this chapter reflect this fact. The emphasis is on deriving flexible models in the form of nodal admittance matrices that use the frame of reference of the phases, which is a frame of reference closely associated with the physical structure of the actual power system/component. A major strength of this frame of reference is that all design and operational imbalances present in the power system are incorporated quite straightforwardly in the model. Nevertheless, it is acknowledged that very often it is desirable to reduce the comprehensiveness of the power system solution and to carry out the study in the frame of reference of the sequences rather than the phases. This has the advantage of speedier calculations, but key information could become unavailable since sequence domain modeling tacitly assumes that no imbalances are present in the plant component being modeled. When such an assumption is incorporated in the phase domain nodal admittance models, it yields simpler models expressed in the frame of reference of the sequences. Hence, in this thesis, the sequence domain nodal admittance model is used to develop the power flow equations of positive sequence power systems [26-28].
OPPORTUNITIES FOR FACTS DEVICES.
Power flow control has traditionally relied on generator control, voltage regulation by means of tap-changing and phase-shifting transformers and reactive power plant compensation. Phase-Shifting transformers have been used for the purpose of regulating active power in alternating current (A.C) transmission networks. In practice, some of them are permanently operated with fixed angles, but in most cases their variable tapping facilities are actually made use of. Series reactors are used to reduce power flow and short-circuit levels at designated locations of the network. Conversely, series capacitors are used to shorten the electrical length of lines, hence increasing the power flow. In general, series compensation is switched on and off according to load and voltage conditions. For instance, in longitudinal power system, series capacitive compensation is bypassed during minimum loading in order to avoid transmission line over-voltages due to excess capacitive effects in the system.
Conversely, series capacitive compensation is fully utilized during maximum loading, aiming at increasing the transfer of power without subjecting transmission lines to overloads. These aforementioned power systems, by and large are mechanically controlled. There is a widespread use of microelectronics, computers and high-speed communications for control and protection of present transmission/distribution systems; however, when operating signals are sent to the power circuits, where the final power control action is taken, the switching devices are mechanical and there is little high-speed control. Another problem with mechanical devices is that control cannot be initiated frequently, because these mechanical devices tend to wear out very quickly compared to static devices.
In effect, from the point of view of both dynamic and steady-state operation the system is really uncontrolled. Power system planners, operators and engineers have learnt to live with these limitations by using a variety of ingenious techniques to make the system work effectively, but at a price of providing greater operating margins and redundancies.
Furthermore, greater demands have been placed on the distribution/transmission network, and these demands will continue to increase because of the increasing number of generating companies and customers. Added to these is the problem that it is very difficult to acquire new rights of way, absence of long-term planning and the need to provide open access to generating companies and customers, all together have created tendencies toward less security and reduced quality of supply [29].
CHAPTER THREE
MATHEMATICAL MODELS OF CONVENTIONAL POWER FLOW AND POWER FLOW INCLUDING FACTS CONTROLLER (STATCOM).
METHODOLOGY
For the purpose of steady-state network assessment, power flow solutions are probably the most popular kind of computer-based calculations carried out by planning and operation engineers. The reliable solution of power flows in real-life transmission and distribution networks is not a trivial matter and over the years, owing to its very practical nature, many calculation methods have been put forward to solve this problem. Among them, Newton-Raphson type methods, with their strong convergence characteristics have proved the most successful and have been embraced by power industry [21]. Thorough grounding on conventional power flow theory with particular reference to the Newton-Raphson method is provided and adopted for this research work.
This technical research work is populated with efficient and elegant solutions for accommodating models of controllable equipment namely STATCOM FACTS controller into the Newton-Raphson power flow algorithms. The modeling approach used to represent controllable equipment can be broadly classified into two main categories, namely, sequential and simultaneous solution methods. The former approach is amenable to easier implementations in Newton-Raphson algorithms.
However, its major drawback is that the bus voltage magnitude and angles are the only state variables that are calculated in true Newton-Raphson, and a sub-problem is formulated for updating the state variables of the controllable devices at the end of each iteration thus such an approach yields no quadratic convergence.
Alternatively, the Unified approach combines the state variables describing controllable equipment with those describing the network in a single frame of reference for unified, iterative solutions using the Newton-Raphson algorithm [21]. The method retains Newton’s quadratic convergence characteristics. The unified approach blends the alternating current (AC) network and power system controller state variables in a single system of simultaneous equations viz:
CHAPTER FOUR
ANALYSIS OF OGUI-ENUGU POWER DISTRIBUTION NETWORK PARAMETERS
SIMULATION:
Depending on the nature of the actual physical system and the purpose of the simulation, the definitions of modeling and simulation tend to vary. Generally, simulation is a technique that entails setting up a model of a real situation and performing experiments on the model to predict the behaviour of the system from solutions or validify the properties of the mathematical model [58].
Different simulation packages are available each having its own unique features. These packages share the common merits of speed, improved accuracy and visualization over manual computation and presentation of results. In view of this, this thesis was simulated in a MATLAB 7.5 environment. MATLAB, developed by Math Works Inc., is a software package for high performance computation. It integrates visualization, Programming, Flexibility, Reliability and graphics in an easy to use manner where problems and solutions are expressed in familiar mathematical notation.
MATLAB is an acronym for Matrix Laboratory. Its basic data element is an array which allows the solution of many technical computing problems, especially those with matrix and vector formulation in a fraction of the time it would take to do so in a scalar non-interactive language such as C++ or FORTRAN [59].
CHAPTER FIVE
CONCLUSION
THESIS CONCLUSION
When the load demand on the supply system changes, the voltage at the consumer’s terminal also changes. The variations of voltage at the consumer’s terminals are undesirable and must be kept within prescribed limits of of the declared voltage.
However, it was observed from the analysis of Ogui-Enugu Radial Power Distribution network that the voltages at buses 11 to 13 are not within the acceptable voltage magnitude limits.
The drop in voltage magnitude beyond the acceptable limits experienced by consumers is usually caused by increased load demand on the network made by consumers, power theft and technical losses. In this thesis, voltage limit operating problem and power flow problem have been solved using STATCOM FACTS DEVICE to supply reactive power in order to maintain voltage magnitude constant in buses 11 to 13 and eventually got the bus voltages fall within the prescribed limits of of the declared voltage. Again the STATCOM FACTS DEVICE eminently increased the active power flow in buses 11 to 13 but conversely reduced the reactive power flow in those buses knowing full well that reactive power is neither consumed nor does any useful work in the network. It merely flows back and forth in both directions in the network. These improved voltage magnitudes and high-speed power flow control of Ogui-Enugu Radial power distribution network achieved in this research affirms the steady-state operational characteristics of STATCOM FACTS-DEVICE. The application of STATCOM FACTS-DEVICE solved the following problems being experienced by consumers’ of Ogui-Enugu Power distribution network.
- In case of lighting load, the lamp characteristics are very sensitive to changes of voltage.
For instance, when the supply voltage to an incandescent lamp decreases by of rated value, then illuminating power decreases by . On the other hand, when the supply voltage is above the rated value, the life of the lamp may be reduced by due to rapid deterioration of the filament.
- In case of power load consisting of induction motors, voltage variations cause erratic operation. When the supply voltage is above the normal, the motor may operate with a saturated magnetic circuit, with consequent large magnetizing current, heating and low power factor. On the other hand, when the voltage is too low, it reduces the starting torque of the motor considerably.
- Too wide variations of voltage cause excessive heating of distribution transformers. This reduces their ratings to a considerable extent.
- High speed power flow control is achieved since active (useful) power flow increased tremendously in the network understudy but conversely reduced reactive (un-useful) power flow. It is clear from the above postulations that voltage variations in a power system must be kept within acceptable limit and high-speed power flow control must be achieved in order to deliver good services to the consumers. With the trend towards larger and larger interconnected system, it has become necessary to employ STATCOM FACT-DEVICE in order to improve energy efficiency of power distribution networks.
SUGGESTIONS FOR FURTHER RESEARCH.
- In this thesis, the objective was to determine the steady-state operating condition of the Ogui-Enugu Radial distribution network. The steady-state was determined by finding out, for a given set of loading conditions, the flow of active and reactive powers throughout the network and the voltage magnitude and phase angles at all buses of the network. Buses where voltage magnitudes are outside acceptable limit, STATCOM was deployed to regulate such voltage magnitudes and increase the active power flow in the network.
However, with ever increase in the load sizes and operational complexities brought about by a widespread interconnection of power system, the operating philosophy had to be revised, and the concepts based on economic consideration like optimal power flow should be adopted. It is most likely that this thesis, although feasible, would not yield the most economic operating schedule. The OPF solution, in contrast, would optimize the power flow network equation subject to physical and operational constraints. Any solution point that satisfies all the constraints would then yield an economic feasible solution.
- The optimal power flow suggested above is usually solved by converting the constrained optimization problem into an un-constrained optimization problem using Newton’s method. This is achieved by constructing an augmented Lagrangian function. In view of the rigorous mathematical analysis involved in conventional optimization algorithms, combinatorial optimization problems are better solved using artificial intelligence. Sequel to this, it is suggested that expert systems such as Genetic algorithm and Fuzzy logic should be applied to simultaneously determine the best node/bus to connect the FACTS DEVICE, the type of FACTS DEVICE and the rating of the FACTS device in order to achieve the most economic feasible solution.
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