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The Electric Power Engineering Handbook International Standard Book Number (eBook - PDF) . PART III Transmission System. The Electric Power Engineering Handbook, Second Edition, Leonard L. Grigsby 8 Concept of Energy Transmission and Distribution. George G. Karady. Electric Power Generation, Transmission, and Distribution,. Third Edition. Edited by Leonard L. Grigsby. Electric Power Transformer Engineering, Third Edition International Standard Book Number (eBook - PDF).
With smaller industrial reticulation the preservation of stability in the transient period is generally regarded as the most important case for investigation. The adaptation of the network in the dynamic interval is left largely up to the natural properties of the system and by automatic or operator control. Induction motors always run asynchronously and stability studies involve a consideration of the load characteristics before and after a system disturbance.
For a fault close to the induction motor the motor terminal voltage is considerably reduced. For a given terminal voltage the current drawn is a function only of speed. As the speed drops the current increases rapidly to several times normal full load value and the power factor drops from, say, 0. The torque of the motor is approximately proportional to the square of the terminal voltage. System Studies 29 Because of these characteristics substation induction motor loads are often characterized as: 1.
Square law loads such as centrifugal pumps will recover with greater ease than constant torque loads such as reciprocating compressors. These loads may be reconnected automatically after a delay. The system designer must, however, consider the possibility of voltage collapse upon reconnection as the starting of motors places a severe burden on generation reactive power supply capability.
The starting torque is only about 1. The motor VAr demand is, however, very large because of the poor starting power factor. The system voltage can be severely depressed before, for example, on-site generator AVR action comes into play.
Checks should be made to ensure that direct-on-line DOL starting of a large motor or group of motors does not exceed the VAr capability of local generation in industrial distribution systems. Such data collection can be time consuming and for older machines such data are not always available.
For example, a primary substation infeed from a large grid network with high fault level to an industrial plant can usually be represented as a simple generator with large inertia constant and a transient reactance equal to the short circuit reactance. Simple load representation to voltage variations falls into one of the following categories: 1. Constant impedance static loads 2. Constant kVA induction motors 3. System Studies 31 Induction motors remote from disturbance and represented as a static load : Fully loaded motors can be represented as constant kVA load.
Partially loaded motors can be represented as constant current loads. Unloaded motors can be represented as constant impedance loads.
System faults will depress voltages and restrict power transfers. Usually, generators will speed up during the fault and the load angle will increase. Some generators may experience an increased load during the fault and will slow down. For the same proportionate loss in load during the fault, generators with lower inertia constants will speed up more quickly. On-site generators may remain in step with each other but diverge from the apparently high inertia grid infeed.
Induction motor slips will increase during the fault. After the fault, stability will be indicated by a tendency for the load angle swings to be arrested, for voltages and frequency to return to prefault values and for induction motor slips to return to normal load values. If on-site generators remain in synchronism with each other but cannot match the on-site load requirements, a decline in frequency will occur.
Load shedding will then be necessary to arrest the decline. Practical examples of these principles are given in the case studies in Section 1. Type of fault three phase, single phase to earth, etc.
A three phase fault is normally more severe than a single phase fault since the former blocks 32 System Studies Figure 1. The single phase fault allows some power transfer over healthy phases. Duration of fault. If the fault persists beyond a certain length of time the generators will inevitably swing out of synchronism.
The maximum permissible fault duration therefore varies principally with the inertia constant of the generators, the type and location of the fault. System Studies 33 Determination of maximum fault clearing time is often the main topic of a transient stability study.
The limiting case will usually be a three phase fault close up to the generator busbars. Low inertia generators H : 1. Location of a fault. Extent of system lost by the fault. If a main transmission interconnector is lost, the generators may not be able to transmit total power and power imbalance can continue to accelerate rotors towards loss of synchronism. The loss of a faulted section may also lead to overloading of system parts remaining intact. A second loss of transmission due, say, to overload could have serious consequences to an already weakened system.
In order to improve transient stability, fault durations should be kept as short as possible by using high speed circuit breakers and protection systems, particularly to clear faults close to the generators. The incidence of three phase faults can be reduced by the use of metal clad switchgear, isolated phase bus ducting, single core cables, etc.
Impedance earthing further reduces the severity of single phase to earth faults. Appropriate system design can therefore reduce the extent of system outages by provision of more automatic sectionalizing points, segregation of generation blocks onto separate busbars, etc.
System transient reactances should be kept as low as possible in order to improve transient stability. Machines and associated generator transformers with low reactance values may be more expensive but may provide a practical solution in a critical case. These devices can supply leading or lagging VArs to a system and thereby maintain nearly constant voltages at the point of connection in the system.
The characteristics of such devices are shown in Fig. Normally the busbar coupler is open and grid infeed is via the non-priority busbar No. On-site generation and a major hp induction motor are connected to busbar No.
Other smaller motor loads are connected to busbars 3, 4, and 5. System Studies 35 Figure 1. The large induction motors connected to busbars 1 and 5 are represented in detail in order that slip and current variations during a disturbance may be studied. The two groups of smaller V motors connected to busbars 3 and 4 are not to be studied in detail and are represented as constant kVA loads.
On-site generator No. The results of the computer analysis associated with this system for case studies 1 to 4 have been replotted in Figs 1. Generator No.
A three phase fault is imagined to occur on the 6. The protection and circuit breaker are such that a total fault duration of 0. Clearance of the fault disconnects busbar 3 and its associated stepdown transformer from busbar 1 and all other loads are assumed to remain connected. In Fig. Shortly after fault clearance, a return towards the original operating load angle position is seen.
The generator terminal voltage is also seen to recover towards prefault value. The on-site generator No. System Studies 37 Figure 1. During the fault the slip increases. However, shortly after fault clearance the terminal voltage recovers and the slip reduces towards the prefault value.
Similar behaviour for motors 2 and 3 is shown in Fig. The main motor loads therefore seem to be able to operate under the fault condition; the smaller motor loads have not been studied.
This machine has a relatively low inertia constant compared to the existing on-site generator No. No changes are proposed to the existing protection or circuit breaker arrangements. Both site generators are supplying full load. The duration of the fault has caused generator 2 to lose synchronism with generator 1 and the System Studies 39 grid infeed. The ensuing power surging is not shown in Fig. Acting as a consultant engineer to the industrial plant owner what action do you recommend after having carried out this analysis?
These networks use components such as power lines, cables, circuit breakers , switches and transformers. The transmission network is usually administered on a regional basis by an entity such as a regional transmission organization or transmission system operator. Transmission efficiency is greatly improved by devices that increase the voltage and thereby proportionately reduce the current , in the line conductors, thus allowing power to be transmitted with acceptable losses.
The reduced current flowing through the line reduces the heating losses in the conductors. According to Joule's Law , energy losses are directly proportional to the square of the current.
Thus, reducing the current by a factor of two will lower the energy lost to conductor resistance by a factor of four for any given size of conductor. The optimum size of a conductor for a given voltage and current can be estimated by Kelvin's law for conductor size , which states that the size is at its optimum when the annual cost of energy wasted in the resistance is equal to the annual capital charges of providing the conductor.
At times of lower interest rates, Kelvin's law indicates that thicker wires are optimal; while, when metals are expensive, thinner conductors are indicated: however, power lines are designed for long-term use, so Kelvin's law has to be used in conjunction with long-term estimates of the price of copper and aluminum as well as interest rates for capital.
The increase in voltage is achieved in AC circuits by using a step-up transformer. HVDC systems require relatively costly conversion equipment which may be economically justified for particular projects such as submarine cables and longer distance high capacity point-to-point transmission. HVDC is necessary for the import and export of energy between grid systems that are not synchronized with each other.
A transmission grid is a network of power stations , transmission lines, and substations. Energy is usually transmitted within a grid with three-phase AC. Single-phase AC is used only for distribution to end users since it is not usable for large polyphase induction motors. Higher order phase systems require more than three wires, but deliver little or no benefit. The synchronous grids of the European Union The price of electric power station capacity is high, and electric demand is variable, so it is often cheaper to import some portion of the needed power than to generate it locally.
Because loads are often regionally correlated hot weather in the Southwest portion of the US might cause many people to use air conditioners , electric power often comes from distant sources. Because of the economic benefits of load sharing between regions, wide area transmission grids now span countries and even continents.
The web of interconnections between power producers and consumers should enable power to flow, even if some links are inoperative. The unvarying or slowly varying over many hours portion of the electric demand is known as the base load and is generally served by large facilities which are more efficient due to economies of scale with fixed costs for fuel and operation. Such facilities are nuclear, coal-fired or hydroelectric, while other energy sources such as concentrated solar thermal and geothermal power have the potential to provide base load power.
Renewable energy sources, such as solar photovoltaics, wind, wave, and tidal, are, due to their intermittency, not considered as supplying "base load" but will still add power to the grid. The remaining or 'peak' power demand, is supplied by peaking power plants , which are typically smaller, faster-responding, and higher cost sources, such as combined cycle or combustion turbine plants fueled by natural gas.
Hydro and wind sources cannot be moved closer to populous cities, and solar costs are lowest in remote areas where local power needs are minimal. Connection costs alone can determine whether any particular renewable alternative is economically sensible.
Costs can be prohibitive for transmission lines, but various proposals for massive infrastructure investment in high capacity, very long distance super grid transmission networks could be recovered with modest usage fees.
Grid input[ edit ] At the power stations , the power is produced at a relatively low voltage between about 2. The Losses[ edit ] Transmitting electricity at high voltage reduces the fraction of energy lost to resistance , which varies depending on the specific conductors, the current flowing, and the length of the transmission line.
Measures to reduce corona losses include conductors having larger diameters; often hollow to save weight,  or bundles of two or more conductors. Factors that affect the resistance, and thus loss, of conductors used in transmission and distribution lines include temperature, spiraling, and the skin effect.
The resistance of a conductor increases with its temperature. Temperature changes in electric power lines can have a significant effect on power losses in the line. Spiraling, which refers to the way stranded conductors spiral about the center, also contributes to increases in conductor resistance. The skin effect causes the effective resistance of a conductor to increase at higher alternating current frequencies. Corona and resistive losses can be estimated using a mathematical model.
As of , the longest cost-effective distance for direct-current transmission was determined to be 7, kilometres 4, miles. For alternating current it was 4, kilometres 2, miles , though all transmission lines in use today are substantially shorter than this.
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System Studies 1. Introduction 1. Load Flow 1. System Stability 1. Short Circuit Analysis Chapter 2. Drawings and Diagrams 2. Introduction 2.
Block Diagrams 2. Schematic Diagrams 2. Computer Aided Design 2. Case Study 2. Graphical Symbols Appendix A. Substation Layouts 3. Introduction 3. Substation Design Considerations 3. Alternative Layouts 3. Space Requirements Chapter 4. Substation Auxiliary Power Supplies 4. Introduction 4. DC Supplies 4. Batteries 4.
AC Supplies Chapter 5.
Current and Voltage Transformers 5. Introduction 5. Current Transformers 5. Voltage Transformers 5. Future Trends Chapter 6. Insulators 6. Introduction 6. Insulator Materials 6. Insulator Types 6.
Pollution Control 6. Insulator Specification 6. Tests Chapter 7. Substation Building Services 7. Introduction 7. Lighting 7. Heating, Ventilation and Air-Conditioning 7. Fire Detection and Suppression 7. Security Chapter 8. Earthing and Bonding 8. Introduction 8. Design Criteria 8. Substation Earthing Calculations 8. Computer Simulation 8.
Protective Multiple Earthing Chapter 9. Insulation Co-ordination 9. Introduction 9. System Voltages 9. Clearances 9. Procedures for Co-Ordination 9. Surge Protection Chapter Relay Protection Introduction System Configurations Power System Protection Principles Current Relays Differential Protection Schemes Distance Relays