General Structure of Electrical Power System
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Introduction to Power generation
The high voltage electric transmission is the bulk transfer of electrical energy, from generating power plants to substations. This is from the local wiring between high voltage substations and customers are referred to as electricity distribution. Transmission lines, when interconnected with each other, become high voltage transmission networks. Transmission lines mostly use three phase alternating current (AC), although single phase AC is sometimes used in railway electrification systems. High-voltage direct current (HVDC) technology is used only for very long distances; undersea cables. Electricity is transmitted at high voltages to reduce the energy lost in long distance transmission. Power is usually transmitted through overhead power lines. Underground power transmission has a very high cost and greater operational limitations. The main problem in the Power distribution is that electrical energy cannot be stored so it is generated based on the necessity. A control system is required to ensure electric power generations match the demand. Power generating plant low voltage is produced. The generator terminal voltage is then stepped up by the power station transformer to a higher voltage for transmission over long distances.
Transmitting electricity at high voltage reduces the fraction of energy lost due to resistance. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. At extremely high voltages, conductor and ground, corona discharge losses are so large that they can offset the lower resistance loss in the line conductors. Transmission and distribution losses are generally below 10%. In general, losses are estimated from the discrepancy between energy produce and energy sold to end customers; the difference between what is produced and what is consumed constitute transmission and distribution losses. In an alternating current circuit, the inductance and capacitance of the phase conductors can be significant. The currents that flow in these components of the circuit impedance constitute reactive power, which transmits no energy to the load. Reactive current causes extra losses in the transmission circuit. The ratio of real power (transmitted to the load) to apparent power is the power factor. As reactive current increases, the reactive power increases and the power factor decreases. For systems with low power factors, losses are higher than for systems with high power factors. At the substations, transformers reduce the voltage to a lower level for distribution to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (33 kV to 132 kV) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to a low voltage.
Power Load balancing
The transmission system provides for base load and peak load capability, with safety and fault tolerance margins. The peak load times vary by region largely due to the industry. Power requirements vary by the season and the time of day. Distribution system designs always take the base load and the peak load into consideration. The transmission system usually does not have a large buffering capability to match the loads with the generation. Thus generation has to be kept matched to the load that prevents overloading failures of the generation equipment. Multiple sources and loads can be connected to the transmission system and they must be controlled to provide orderly transfer of power. In centralized power generation local control of generation involves synchronization of the generation units to prevent large transients and overload conditions.
In distributed power generation the generators are geographically distributed and the process to bring them online and offline must be carefully controlled. The load control signals can either be sent on separate lines or on the power lines. To load balance the voltage and frequency can be used as a signaling mechanism. In voltage signaling, the variation of voltage is used to increase generation. The power added by any system increases as the line voltage decreases. Voltage based regulation is complex to use in mesh networks, since the individual components and set points would need to be reconfigured every time a new generator is added to the mesh. In frequency signaling, the generating units match the frequency of the power transmission system. In droop speed control, if the frequency decreases, the power is increased. Wind turbines and other distributed storage and generation systems can be connected to the power grid, and interact with it to improve system operation.
5.2 Power generation and distribution through overhead lines with single line diagram
Electrical power system deals with the technology of generation, transmission and distribution of electrical energy. An electric power system consists of different subsystem are explained as follows
I. Generation subsystem
The conversion from one source to electrical energy through the process of electromagnetic conversion. This system consists of group of generation systems. Power system comes into existence with the growing demand of electrical energy. Power generations are classified as Hydraulic, nuclear power, fossil fuel and non-conventional power, solar power.
II. Transmission subsystem
The overhead transmission network transfers electrical energy from generating stations located at long distance to the distribution system. The Transformer and transmission line subsystems are designed in such way to transmit bulk power for consumption at the load line. The step-up transformers are used in various range of step-up voltage based on the requirements. A transmission voltage varies between 66 kV to 440KV in India.
III. Sub-transmission system
The sub-transmission network is the portion of transmission system connected to the high voltage substations using transformers.
IV. Distribution subsystem
By this process energy is connected to different distribution subsystem to a place a main transmission subsystem. A distribution subsystem consists of over headlines and underground cables. The distribution of power system is generally in two levels feeder or primary voltage at 11kV and secondary/consumer voltage at 415 Volt for three-phase and 230 Volt for single phase supply for house hold application. Each individual customer is connected to the secondary circuit through service leads and a meter. Distribution system is classified as Radial distribution system, loop distribution system and network distribution system. From the main switch electrical energy is distributed to the various points using distribution board system and tree system.
V. Control subsystem
This subsystem is formed by relays, switch and other control elements to protect other subsystems to protect faults and overloads to ensure efficient, reliable and economic operation of electric power system.
Figure 6.1 Block diagram of single line power system
Step-up and step down Transformers are used in all subsystems. At the sending end from generator step up Transformers are used and the receiving end step-down Transformers are used. Power and distribution Transformers are used in power line system depending upon the power handling capacity.
The earthling of electrical installation is undertaken for the following reasons.
- To avoid shocks to a living body.
- To ensure the potential with respect to the earth of any current carrying conductor does not rise above it's designed level.
- To provide safety to operating personal
- To avoid fire hazard due to leakage current.
Representation of the transmission line
A transmission line has series resistance, series inductive reactance, shunt capacitance and leakage resistance which are distributed evenly along its length. Except for long lines, the total resistance, inductance, capacitance and leakage resistance of the line can be concentrated to give a 'lumped-constant' circuit which simplifies calculation. The particular 'lumped-constant' circuit used depends on the length of the line and the required accuracy of the calculations. For the purpose of this introduction to power system calculations, we will consider a representation which is accurate for short transmission lines up to about 80 km in length. For this length of line, the shunt capacitance and leakage resistance can be ignored. It should be noted that this assumption is not valid for unloaded lines when the shunt capacitance dominates.
Power system analysis is required for a large number of different purposes
- System design and control to maintain consumer voltage at statuary levels as affected by conductor sizing and transformer tap charger position.
- Fault calculations to ensure that the maximum fault current can be interrupted by circuit breakers or fuses and that large fault currents cause the minimum of damage to the power system.
- Design of protection systems to ensure faulty circuits are switched off rapidly (<20 ms) to prevent damage and to ensure only the faulty circuit is switched off to minimize supply disruption.
- System design and control to maintain frequency within ±0.5 per cent.
- To ensure sufficient generation is available to meet the expected demand-load forecasting.
- To ensure that loads are supplied by the most efficient arrangement of generators -load scheduling.
5.3 Power distribution through underground cables
Instead of overhead power lines electric power can also be transmitted by underground power cables. The transmission of electrical power through underground cables is possible in the following areas
- Densely populated metropolitan areas.
- Areas where overhead planning is difficult.
- Sea, Rivers and other natural obstacles.
- Areas of historical importance.
Advantages of underground power cables are as follows
- Bad weather conditions such as lightning, wind and freezing cannot damage it.
- Reduced emission into the surrounding area of electromagnetic fields.
- Shielding provided by the earth surrounding underground cables restricts their range and power.
- Underground cables need a narrower surrounding strip of about to install, on the other hand an overhead line requires a wide surrounding strip to be kept permanently clear for safety and maintenance.
- Underground cables are significantly safer as they pose no shock hazard.
- Less illegal connections, sabotage, and damage from war.
Disadvantages of underground power cables are as follows
- Undergrounding is costly affair as the cost of laying cables at transmission voltages is higher than overhead power lines.
- Fault Finding and repairing overhead wire is easier than underground cable.
- Redundant cables are used to ensure uninterrupted supply of power during fault and hence increase the cost of the system.
- Due to their proximity to earth underground power cables cannot be maintained live but no problem overhead power cables to be live can be.
- Operations are more difficult since the high reactive power of underground cables produces large charging currents and so makes voltage control more difficult.
- The life-cycle cost of an underground power cable is approx two times the cost of an overhead power line.
Points to remember
The high voltage electric transmission is the bulk transfer of electrical energy, from generating power plants to substations.
Transmitting electricity at high voltage reduces the fraction of energy lost due to resistance.
Power generation and distribution through overhead lines with single line diagram
Underground cables are used in some special cases where overhead planning is not possible.
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