Voltage Stability and Control - Coursework Example

Chapter 2. a) Discuss briefly the importance of the following parameters in the design and application of High Voltage direct current supplies: Voltage stability and control Voltage stability can be described as the capacity of a system to steadily sustain the systems’ voltage at the desired levels without succumbing to a disturbance at any of its buses. The stability reached at a design is usually considerate of the conditions under which the power system is to be installed and subjected to. Disturbances are characterized by a considerable and steady drop in voltage, usually occasioned by changes in the conditions subjected to the power system or even a sudden rise in load demand. A disturbance or voltage instability can therefore be summarized as the manifestation of inability of the system to supply a certain power demand. Voltage stability at the design level on the other hand can be described as the systems deliberate measures to ensure that a steady voltage is maintained throughout the conditions as well as demand. Voltage control measures are the intervention strategies introduced for purposes ensuring that the power system remains stable throughout (Kundur, 1994, p17). Voltage control is aimed at ensuring minimum disturbances by maintaining voltage within the acceptable limits and must satisfy the following; voltage utility at the terminals is always within the required limits, system stability issues are minimized and that reactive flow is maintained at a very low magnitude to facilitate low RI2 and XI2. Voltage stability is maintained if for every system bus, V (voltage magnitude) increases by increases in the utilization at the other end of Q (reactive power injection). However, voltage instability occurs if as a minimum, one bus has V decreasing while Q increases. Otherwise put, voltage stability will be achieved if for all buses V-Q sensitivity is positive or instability will occur if negative for one bus as a minimum. Transient Stability Transient stability is the capacity of a power system to sustain synchronism is exposed to strong transient instabilities or disturbances. The most defining characteristic of the instability that the system is meant to overcome is a system failure that requires a very short critical clearing time. This type of instability is however serious since the generator rotors experience a rapid kinetic energy accumulation so huge that the first power swing is not enough to release it. Insulation System High voltage power cables require high quality insulation due to the nature of the high electric strength of the currents they transmit. To ensure that the insulation system meets the standard for high voltage involved, several attempts have been made since a long time ago. One of such insulation designs generated over time includes the oil-impregnated paper dielectric. Insulation of high voltage transmission systems must take certain factors into consideration, one of them being the type of cable used. High voltage cables to be used must have capacity to conduct heavy currents. The most efficient insulation systems developed for high voltage transmissions have been found to be dielectric systems which posses desirable features such as; high alternating current and impulse electric strength, low permeability, low power factor, physical stability, chemical stability, high thermal conductivity, flexibility, available and cheap. The oil-impregnated paper dielectric used in high voltage transmission systems possesses most of the desired features, better than many other systems. Output Current Output current is the current obtained after power generation has taken place and can be enhanced by the use of various control and regulation techniques. Generation issues must be overcome to facilitate an efficient output current recovery. Ripple and Regulation Ripple voltage constitutes the unwanted intermittent fluctuation in the output current (direct current) in a power system usually originating from another source that is of an alternating current (ac) in nature. Voltage ripple generally arises when current is delivered to a load and the expected output voltage considerably falls due to the generation of this unnecessary voltage. The voltage ripple is usually of the same frequency with the main current supplied to the load. To regulate voltage in order to eliminate the loss of output voltage at the load, capacitors are introduced. Ripple voltage (δV) causes a reduction in the expected voltage by the factor ΔV thereby reducing efficiency of the system. Regulations through capacitance are aimed at reducing the factor δV as much as possible. High capacitance capacitors are suitable in mitigating the impacts of ripple voltage. Component Rating Component rating is involved in the determination of the best insulation system to be adopted. The best material to be used in the power system must be chosen after considering a number of factors affecting the power system. Component rating enables selection of the best material. Size and Cost During the design of a power system, the availability issues of materials used in the system must consider the size as well as cost applicability. There is high quality and size responsive materials for use in a system design if enough research is done on the same. b) Continuous 100A, 6kV supply required for a small domestic air cleaner Suitable Circuit The most suitable circuit to be applied in the design of a small domestic air cleaner with a supply of continuous 100A, 6kV is probably a negative ion generator circuit. The reason why this circuit is appropriate is their silence and power conservation capacity. Regular replacement of physical filters is not necessary which makes this circuit better in a list of many others. It is cheap and manageable to house the small sized gadget within the domestic setting. Voltage regulation is easily done since just a small magnitude of current is involved hence the high voltage is not a major problem of the system. With efficient insulation during the design, there are few problems that are expected in the operation of the gadget. Chapter 3 Use of Voltage Dividers in High Voltage Measurement A voltage divider is a gadget designed to apply resistors or capacitors or both in the reduction of high voltage by diving it. Creation of a volume control circuit and generation of reference voltages are some of the major usages of voltage dividers. Using Resistors Voltage dividers apply the premise of Ohms Law where one volt if spread across two equal value resistors gets halved in the middle. The generated resistance by the resistors determines the amount of voltage. This implies that if two resistors of the different resistance values are used to divide voltage, the resultant output usually corresponds to the amount of resistance there is at each of the resistors. Below are illustrations of dividers (equal and different value resistors). Equation for calculating output current. According to the illustrations in the previous page, each divider is using equal and different value resistors dividing input volt at the middle to give the output according to the resistance at the bottom. Where more than one equal value resistors are involved in the divider, the input voltage is divided into four equal values as illustrated in each output voltage value. Voltage dividers apply the above voltage division in a similar manner when subjected to both alternating and direct currents. These two illustration assist in explaining how 10V and 5V can be divided down into 1V and 4V respectively using the two different value resistors. However, resistors are not just placed in the divider since the amount of heat energy dissipated must be considered in the output voltage. Considering the resistance value in determining the output voltage must ensure that the heat energy dissipated does not overcome the capacity of the resistor and the divider as a whole. Heat dissipated can actually burn down the system. Advantages of Resistors Resistors assist in reducing the amount of voltage in high voltage devices that could otherwise result in excessive heat energy which is destructive. With the right information from the manufacturer, voltage coefficient of resistance can be relied upon to determine the best type of resistor to apply in different high voltage systems. It is possible to apply various resistors of different resistance values in the same system to considerably reduce the amount of destructive voltage. Disadvantages of Resistor The resistor choice is dependent on several technical considerations to be made in the design stages of the system. Resistors generally have an error during the construction of dividers in that they present a frequency dependent error. There is a high frequency limit in generally all resistance dividers due to their operation technique. Using Capacitors Capacitors can be used in a similar manner as observed above with resistors to reduce amount of voltages in high voltage systems. There are two types of capacitor applications that can be used in the designing of a voltage divider namely; single standard capacitor and composite capacitor with a series of elementary capacitors. While the standard capacitor is used for purposes of measuring voltage in dividers, the capacitive divider with a series of capacitor components is used in other applications. A representation of a single standard capacitor is on the left of the diagram while a series capacitor is on the right. Advantages of capacitors Capacitors aid in control and regulation of voltage by facilitating the dividing technique to be implemented. Alternatively, they assist in the balancing of reactive power at the terminals of the high voltage system. Disadvantages of Capacitors Single standard capacitors have the inherent problem of load inductance. While series capacitors are capable of overcoming the load inductance challenge, it is their limitation that they are unsuitable due to their high pulses. This is occasioned by their internal travelling wave oscillations. For this reason, the series capacitors cannot adequately handle the demands of a good divider alone. Series capacitors have a limitation in dividers efficiency by virtue of causing a ration error which is constant. In nearly all cases of capacitive dividers, there is an inherent weakness of low frequency limit, which is undesirable in the efficiency of the dividers. Compensated Dividers Mixed dividers constitute of both resistors and capacitors, in an attempt to maximize the benefits of each in developing a good divider. In this illustration, both the capacitors and resistors have been incorporated in the divider design. In this arrangement, it is capable to overcome some of the limitations experienced in each of the distinct divider designs when set up separately. Chapter 5 Why is it so important to achieve uniform distribution of voltage in high voltage equipment? Maintaining a uniform distribution along high voltage transmission equipment is important in protecting the equipment since surge voltages are destructive. The capacity of high voltage equipment to achieve uniform voltage distribution without incurring surge voltages is dependent on the level of capacitance it possesses (Kamaraju and Naidu, 2009, p20). According to the author, it is important that transient voltages are considered in all regions since application of an impulse could be an opportune condition for surge voltage at times. On the same note, the author notes that the duration of the transient voltages should be considered in advance particularly for those that last for over one microsecond. To ensure that equipment is strong enough to withstand surge voltages, high voltage testing must be done. Improvements on voltage distributions are made by ensuring that the conductors are stress free, high strength dielectric insulation especially at various stress points as well as material use on permittivity rating. Voltage distribution is not always possible in high voltage systems since they are often faced by sudden rise to the crest value while the accompanying decline is usually slower than the rise. Compare the techniques used to achieve uniform distribution of voltage, discussing carefully the practices used in transformers (30 marks) For transformers whose windings lie in concentric cylinders, usually in two distinct designs that determine uniformity in voltage distribution. To achieve a uniform voltage distribution, electric stress is reduced in between windings in order to ensure an even voltage distribution. In both the H.V and L.V windings, the amount of stress is calculated as shown in the equation below. Reduction in stress must also be done in the fields in the HV windings as well as field from HV to the bushings. Impulse stresses are also reduced for voltage distribution uniformity by ensuring that the front is not too steep while the tail should be less flat. The capacitance elements of the generator are also targeted for improvements that ensure uniform distribution. Metal oxide surge absorbers are also used in the ensuring a uniform voltage distribution in transformer windings. According to Amarnath, Ansari and Gurumurthy (2009, p1457), providing metal oxide absorbing blocks at various sections of the transformer windings greatly improves the control of power distribution towards ensuring uniformity. According to the authors, voltage distribution is dependent on α (the square root of ground capacitance to winding capacitance). By applying this technique, the electrical stresses involved in the systems are transferred to certain areas of the winding where metal oxide absorbers are not provided. In Kundur (1994, p678), it is possible to ensure a smooth and uniform voltage distribution throughout the system if modifications can be made to form a tap changing transformer. Voltage transmissions and their controls are dictated by the nature of configuration as well as distribution design. A well coordinated incorporation of tap changers allows the control of voltage which enables achieving bus balancing and uniformity in distribution. The diagram below is an illustration of how tap changers are introduced in a system (p679). Compare the techniques used to achieve uniform distribution of voltage, discussing carefully the practices used in circuit breakers (30 marks) To facilitate a uniform voltage distribution across a circuit breaker and throughout the system is usually done in the following ways, to avoid restriking voltage or switching surges. There are three generally acceptable practices that can be used to ensure that a smooth transmission is achieved at circuit breakers without surges. These include; synchronization, using resistor insertions, one or multiple resistor closures. In the use of synchronization, unenergized lines or energized lines at the circuit breaker must have a voltage equal to zero (Chander and Ravindranath, 1977, p351). Depending on the type of reactor used, it could be difficult to predict the voltage, especially when shunt reactors are involved. Very high accuracy levels are expected to be met which makes the technique vulnerable to risks of errors. Resistor insertions are employed in the reduction of voltage surges particularly a series resistor. When making the insertion, it must be ensured that the immediately experienced voltage surge is allowed enough time to return to the circuit breaker. Resistance value chosen must not be too little, neither too high to avoid imbalances in voltage distribution. Very high resistance value causes overvoltage on shorting whereas too low causes overvoltage on resistor insertion. Application of two or more resistors along the transmission enables reduction of overvoltage through incorporation of sequential switching effect achieved by resistors. Insulation coordination can also be facilitated at certain flash over paths namely; firstly earthing through both internal as well as external surfaces and secondly by use of contacts (Chander and Ravindranath, 1977, p352). Compare the techniques used to achieve uniform distribution of voltage, discussing carefully the practices used in overhead line insulators (30 marks). Overhead line modification to ensure that the necessary insulation is achieved must take into consideration parameters such as size, performance under pollution, high mechanical loadings and reduce the high RFI levels resulting from corona. Overhead line insulators application is mainly for the insulation of pylons in high voltage live cables. The techniques used depend on certain factors, for instance height of the line. About six types of overhead insulation methods are available, namely; disc, longrod, pin, shackle, post, pot and Hewlett types. Disc insulation type involves usage of insulation discs or units put together through caps. The discs are locked together and insulated at the cap using porcelain. Longrod are applied in phase-to-phase insulation to protect the transmission system from galloping under heavy motion force of wind. Pin insulation has pin insulators on a shank usually fastened on the transmission pole or pylon and acts mainly as a jumper-line insulator system. Shackle insulators support transmission lines at a distribution junction. Post insulators use heavier insulation to achieve a Maximum Design Cantilever Load (MDCL) categorization. Pot types of insulators apply pins for mounting and are particularly applied in telephone lines. Hewlett insulation is a modification of disc type but have internal insulation and interlocks using metallic bolts in place of adhesive cement (Judin, 2008, p1) Chapter 6 What is meant by Insulation Coordination? Altering various components within a power distribution system to ensure that surges are maintained at specific values as well as containing possible flashovers to cause minimal damage in the event of their occurrence is termed as insulation coordination (Chander and Ravindranath, 1977, p352). According to Kamaraju and Naidu (2009, p360), the main purposes of insulation coordination is usually in the grading of insulation for equipment or overhead power lines. Explain, using diagrams, the principles and functions of Rod Gaps for the protection of electricity supply system equipment. Rod gaps are the cheapest way of protection against over voltage. Gap spacing is done to ensure that it facilitates the withstanding of high voltages as well as overvoltage without damage to equipment. They are applied to offer supplementary protection since voltage time and equipment shape always present a challenge to the intended function. Typical settings of rod gaps are illustrated below (elect.mart.ac, p173). Normal System voltage (kV) 66 132 275 400 Gap Setting (mm) 380 660 1240 1650 Alternative rod gaps are in the enhanced type that uses two rod gaps in series. They are referred to as duplex rod gaps. An example of how the gap setting is achieved is illustrated below. Nominal system voltage (kV) 11 33 Gap setting 2x31 2x63 The mechanism of operation of the rod gaps is that when a spark arises, the gap causes a collapse in the voltage. This ends up in a chopped wave that is less destructive to the electric equipment. Explain, using diagrams, the principles and functions of Surge Diverters for the protection of electricity supply system equipment. Many rod gaps and at least one non-linear resistor series are incorporated to make surge arrestors variously referred to as lightning arrestors. Application of zinc oxide in modeling of surge arrestors assist in conferring volt-ampere features to the apparatus. Non-linear arrestor (elect.mrt.ac, p175) How are protective devices chosen to provide the optimum insulation level in a power system? In order for insulation to attain guaranteed protection levels, certain factors are considered. The circuit parameters are defined An appropriate circuit protection component is then selected Establish the fault time using the time current curve Check on the operating conditions of the system Verify the apparatus power parameters for compatibility of voltage between input, output and ambient conditions. Perform independent tests and evaluate safety levels Having considered some of the above factors, it is possible to make the best choice of protection against high voltages using insulation. The insulation material applied will therefore pass the tests of; impulse strength, AC strength, dielectric losses. Considerations have to be made concerning other factors affecting the performance of the insulator namely; transient overvoltages, mechanical loads, radio-interference and polluted environment performance. Chapter 4: (a) Outline the principal ionisation processes in gases and explain the essential differences between spark, glow and arc discharges. Discuss briefly the various factors that influence breakdown in a gas. (50 marks) Principal Ionization Processes in Gases The ionization of a gas is a factor of electron impact. The ionization process starts when an electron is dislodged from the negative terminal inside a tube/envelope filled with gas and starts traveling towards the +ve terminal. The -ve charge is repelled from the negative electron and attracted to the +ve terminal The electron attains a speed that is proportional to the distance of travel before hitting a gas molecule and the voltage applied The molecule hit travels toward the +ve electrode and in the process hits other molecules on its way When energetic electrons hit molecules, they become positively charged more like those of liquids. These are attracted to the –ve terminal. At the negative terminal, they receive –ve charge and become neutral molecules again. If the electron fails to posses enough energy to ionize the gas, it bounces off and starts traveling again toward the +ve terminal If the pressure is extremely high, gas molecules become too dense for the electrons to gain adequate potential to cause ionization Arc Discharge Gases are excellent insulators at atmospheric pressure. When an air gap separates two electrodes and the electrodes connected to a variable power source, an arc is established between the two when the potential reaches a certain point. When the arc is formed, current begins to flow. This current is limited by the resistance (impedance) of the circuit as it has very low resistance. Once an arc is formed, the voltage that is required to maintain it is reduced. Current density over cathode rises to extremely high values (103-7 A/cm2) Noisy and luminous Associated with high temperatures and high electrons density and +ve ions Spark Discharge A spark is formed between two electrodes separated by an air gap with the two electrodes having a great potential difference between them. When there is no current between the two electrodes, the air in the gap is an insulator. The gas structure is changed as voltage increases between the electrodes. Once the voltage goes beyond the dielectric strength of the gas, the gas gets ionized. The ionized gas conducts and electrons are allowed to flow across the gap thus forming sparks. Glow Discharge A glow is a post breakdown discharge characterized by a luminous flow that is diffused. The colour of the glow depends on gas used and cathode material The glow discharge covers space between the electrodes and partly the cathode The anode has intermediate regions of brightness and darkness The voltage drop between anode and cathode mainly constant and depends on type of gas Factors affecting gas breakdown The ionization potential of the electrons is affected by two factors: Atmospheric pressure The air pressure impacts the voltage required for the attainment of ionization potential. If pressure is raised, the amount of potential needed to attain this potential is increased and vice versa The type of gas that surrounds the electrodes. Sodium vapour for example requires 5 volts while helium requires about 24.5 volts under similar conditions (b) The following table shows experimental results for pre-breakdown current measurements in a gas between parallel-plate electrodes, the electric field being maintained constant: Gap d(mm) 1 2 3 4 6 8 10 12 Current (A x 10-11) 1.4 1.8 2.45 3.1 5.2 8.6 20 50 Draw an accurate graph (on graph paper). Determine the value of Townsend’s first (  ) and second ( ) coefficients and the value of the electrode spacing at which a spark discharge will occur. (50 marks) Considering that Townsend’s equation is given by: Where I is the current flowing in the device, e (Euler’s Number) = 2.71828 αn is the first Townsend ionisation coefficient εi is Townsen’s second coefficient d is the distance between the plates I0 is the photoelectric current generated αp expressing the number of ion pairs First Coefficient: 10mm = 1cm Assuming I=I0, 1= 2.72 αn log 1 = αn log 2.72 1/αn = log (2.72 -1) αn = 4.2463 Second Coefficient 1= 2.724.2463/ (1- εi 3.2463) (1- εi 3.2463) = 2.724.2463 -3.2463 εi = -1+ 2.724.2463 εi = 1+ 2.724.2463/ (-3.2463) εi = xxxxxxxxxxxxx References Amarnath, J., Ansari, Z. A., & Gurumurthy, G. R., (2009) “Transfer Winding Sections Provided with Metal Oxide Surge Absorbers.” International Journal of applied Engineering Research, vol. 4 no. 8 pp.1457-1468 Chander, M & Ravindranath, B. (1977) Power system protection and switchgear. New Delhi, India: New Age International Insulation Coordination. [online] elect.mrt.ac. Available from: http://www.elect.mrt.ac.lk/HV_Chap10.pdf [Accessed 9 February 2011] Junid, A. (2008) Overhead Line Insulators: Types, Selection, Installation, Wearing and Maintenance [online]. Version 78. Knol. Available from: http://knol.google.com/k/adam-junid/overhead-line-insulators/22z3waum2dn6b/2. [Accessed 9 February 2011] Kamaraju, V. & Naidu, M. S. (2009) High voltage engineering. New Delhi, India: Tata McGraw-Hill., Kundur, P. (1994) Power system stability and control. London, UK: McGraw-Hill, Inc., Read More