Transient Over Voltages Analysis In Power System Engineering Essay
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Published: Mon, 5 Dec 2016
Transient over-voltage is one of main causes for unscheduled interruption in power transmission and distribution systems including a smart grid. A surge over-voltage due to lightning and switching operation results in damages in an electrical power system and often leads to power outages. Predictive calculations of over-voltages generated by the lightning and the switching operation in the transmission and distribution systems are most essential for an economical insulation design and a reliable operation of the transmission and distribution systems. The transient over-voltage can be high frequency, medium frequency or low frequency. The transient over-voltage is dangerous to both lines as well as the connected equipment and may cause damage to the equipment. This project analyzes the transient at the load after switching. This project will discuss about analyzing a transient over-voltages which is the cause and the effect of the transient over-voltages. The method to solve this transient over-voltages also been discussed in this project. The PSCAD simulation is used for the transient analysis in this project to design the power system circuits.
Voltage transient in Electric Power System appear several disturbances, sometimes very dangerous for the electrical equipment life, for the environment and for the human life. Switching transient phenomena produce in Power Systems over-voltages, over-currents and electrical fields, which haven’t to neglect. Thus was modeling and simulated the switching transient phenomena, consequently the electric fields and the possible negative influence about electrical equipment, environment and human life.
Voltage transient in power system are cause by switching operation, lightning and faults in the system. The over-voltages can be dangerous to both the lines as well as the connected equipment and may cause damage to the equipment. Purpose of this project is to analyze the transient over-voltages at the load and to identify the method to reduce the effect of transient over-voltages.
1.2 Project Objective
The objective of this project is as follows:
Identify the effect and the cause of transient over-voltages and also the method to solving transient over-voltages.
Simulate the transient over-voltages at the load after switching and design the power system circuits by using PSCAD simulation.
Analyze the result after switching and after use pre-insertion resistor.
1.3 Project Scope
In order to achieve the objective of this project, there are several scopes had been outlined as follows:
Analysis on the distribution system.
Generating the transient over-voltages waveform by using PSCAD simulation.
The analysis is just focusing into transient over-voltages that occur because of the switching capacitor.
2.0 Literature Review
Voltage transients in power systems are caused by switching actions, lightning and faults in the system. Different phenomena create different types of transients. Oscillatory transients are caused mainly by switching phenomena in the network. The most common switching action is capacitor bank switching. The most severe transients are caused by capacitor energizing while capacitor de-energizing only causes a minor transient. Oscillatory transients are characterized by duration, magnitude and spectral content. There are subclasses of oscillatory transients depending on the dominant frequency. In this project it will more focus on switching devices.
2.2 Power System
Electric power system is a very important part of the infrastructure of modern society. The power system today is very complex interconnected network. Electric power system is the system that can transform and change the form of the energy into electrical energy and transmit it to consumer. Technology today still cannot store the electricity that has been produced. The electrical energy only produce when needed or it will use after it is produced. As the effect, the management on the electricity becomes hard and difficult. The power system may be subdivided into the four major subsystems which are Generation subsystem, Transmission subsystem, Distribution subsystem and utilize subsystem. Figure 2.1 shows the power system that divided into generation, transmission and distribution.
Figure 2.1: The Electric Power System
2.2.1 Generation Subsystem
There are two major components in the generation system which is generators and transformers.
For generators, an essential component of power systems is the three phase alternating current, ac, generator known as synchronous generator or alternator . The source of the mechanical power, commonly known as the prime mover, may be hydraulic turbines, steam turbines whose energy comes from the burning of coal, gas and nuclear fuel, gas turbines, or occasionally internal combustion engines burning oil . Some alternate sources used are solar power, geothermal power, wind power, tidal power and biomass.
The power transformer transfer power with very high efficiency from one level of voltage to another level . The transformer is been used to step up or step down the voltage. Insulation requirements and other practical design problems limit the generated voltage to low value, usually 30 kV. The step up is used for transmission of power. At the receiving end of the transmission lines, step down transformers are used to reduce the voltage to suitable values for distribution or utilization. The electricity in an electric power system may undergo four or five transformations between generator and consumers .
2.2.2 Transmission Subsystem
An overhead transmission network transfer electric power from generating units to the distribution system which ultimately supplies the load. It also interconnects neighboring utilities which allow the economic dispatch of power within regions during normal conditions, and the transfer of power between regions during emergencies. The network that interconnected between the utilities and load is called system grid . The transmission line can be categorized into two categories which are high voltage transmission line and sub transmission line system. The difference between these two systems is in the voltage where for the high voltage, the level for transmission line voltage can reach 500kV and for sub transmission are in between 69kV to 138kV. All the transmission will be terminated at the substation .
2.2.3 Distribution Subsystem
The distribution system connects the distribution substations to the consumer’s service-entrance equipment. The voltage for this type of system has been reduced by using step down transformer from 66 kV to 22 kV and below. The secondary distribution network reduces the voltages for utilization by commercial and residential consumers. Lines and cables not exceeding a few hundred feet in length then deliver power to the individual consumers. The secondary distribution serves most of the customers at levels of 240 V for single phase and 415 V for three phases. Distribution systems are both overhead and underground. The growth of underground distribution has been extremely rapid and as much as 70 percent of new residential construction is via underground systems .
2.2.4 Utilization Subsystem
The utilities system or power system loads are divided into three main categories which are industrial, commercial and residential. Industrial loads are composite loads and induction motors form a high proportion of these loads . These composite loads are functions of voltage and frequency and form a major part of the system load . On the other hand, commercial and residential loads consist largely of lighting, heating, air conditioning and cooking . These loads are independent of frequency and consume negligibly small reactive power . The load varies throughout the day and power must be available to consumers on demand. The daily-load curve of a utility is a composite of demand made by various classes of users. The greatest value of load during 24 hour period is called the peak or maximum demand .
2.3 Transient Over-voltages
Transient is a sudden increase in current or voltage in a circuit that can damage sensitive components and instruments. Transient overvoltages are a voltage peak with a maximum duration of less than one millisecond. It can be high, medium, or low frequency. Transient overvoltages on power system are due to various causes and can be classified into two main categories, external and internal overvoltages . Natural overvoltages on low voltage networks are caused by direct lightning strikes. Lightning is an external overvoltage. The high level of energy contained in a direct lightning strike on a lightning conductor or an overhead low voltage line leads to considerable damage of the installation. The overvoltages can be over 20 times the nominal voltage. Operating or switching overvoltages linked to a network’s equipment create overvoltages of a lower level 3 to 5 times the nominal voltage but occur much more frequently, thus causing premature ageing of the equipment. Switching overvoltages is an internal overvoltage. Transient overvoltages are generally oscillatory and take the form of a damped sinusoid. The frequency of these overvoltages may vary from a few hundred Hz to a few kHz and it is governed by the inherent capacitances and inductances of the circuit.
2.2 Switching Capacitor
Equipment containing electronic switching components is also likely to generate electrical disturbances comparable to over-voltages. The consequences of which on sensitive equipment, albeit not visible, are no less detrimental: premature ageing and unpredictable or fleeting breakdowns. Operating over-voltages are produced when reactive or capacitive equipment is switched on and off. Furthermore, interrupting factory production, lighting or transformers can generate over-voltages which will themselves cause greater damage to nearby electrical equipment. In general, these over-voltages are caused by transient phenomena which appear when the state of the network is changed by switching operation or fault condition. Example of these over-voltages is switching on and off equipment, such as switching of high voltage reactors and switching of a transformer at no load. The time duration of the switching over-voltages is longer than lightning. This overvoltage is most disastrous to the power system equipments because it happen many time than lightning. Closing, opening, disconnection and re-striking in a power system circuit result in over-voltages six times than the normal voltage.
Shunt capacitors banks are common devices used in power system for reactive power compensation, voltage regulation and power factor correction. These capacitors are implemented in the system in order to control system voltage, increase power transfer capability, reduce equipment loading, and reduce energy costs by improving power factor of the system.
However, energizing these shunt capacitors produces a transient oscillation in the power systems. Due to the fact that the operation of switching shunt capacitors happens frequently, shunt capacitor switching is regarded as the main source of generating transient voltages on many utility systems. These transients can cause damages on both utility systems and customer systems, depending on the system parameters such as switched shunt capacitor size, transformer size, and the type of customer loads connected to the system. Transient frequencies due to utility distribution capacitor switching usually fall in the range 300 Hz to 1000 Hz. Transient over-voltages which result are usually not of concern to the utility, since peak magnitudes are just below the level in which utility surge protection, such as arresters, begins to operate. However, because of the relatively low frequency, these transients will pass through step-down transformers to customer loads. Secondary over-voltages can cause voltage magnification or nuisance tripping of adjustable-speed drives. Figure 2.2 show the example of single line diagram of the power system using shunt capacitor. Figure 2.3 show the transient voltage at the switched shunt capacitor. This is the example of the transient in the voltage waveform. Figure 2.4 show the transient voltage at the low voltage capacitor that has been magnetized.
Figure 2.2: Example of Single Line Diagram of the Power System Using Shunt Capacitor
Figure 2.3: Transient Voltages at the Switched Shunt Capacitor
Figure 2.4: Magnified Transient Voltage at the Low Voltage Capacitor
2.3 Pre-Insertion Resistor
There are several techniques to mitigation the switching transient in the distribution circuit and one of the techniques that use in this project is pre-insertion resistor. A pre-insertion resistor provides a means for reducing the transient voltages associated with the energization of a shunt capacitor bank. The resistors were connected in series with the controlled capacitor bank to damp the transient inrush current. The resistor is bypassed shortly after the initial transient dissipates, thereby producing second transient event. An additional switch is use to bypass this resistor. The performance of pre-insertion resistor is evaluated using both the insertion and bypass transient magnitudes, as well as the capability to dissipate the energy associated with the event and repeat the event on a regular basis. The size for this resistor was calculated from equation 2.1 and 2.2. The optimum resistor value for controlling capacitor energizing transients depends primarily on the capacitor size and the source strength. The value of the resistor is approximately equal to the surge impedance, Zo, from equation 2.3.
R = (2.1)
Z = (2.2)
Roptimum ≈ Z (2.3)
This chapter describes about the step that needed in this project. Figure 3.1 show the block diagram of analysis and figure 3.2 show the flowchart of the process analysis. This block diagram shows the step from the first step of the project which is design the circuit to the final step which is result and analysis.
Placing Input and Output Devices
Result and Analysis
Figure 3.1: The Block Diagram of Analysis
In order to get the result from the Power System Computer Aided Design (PSCAD), the user should follow the step as follow in the Figure 3.2.
Figure 3.2: Flowchart of Process Analysis
3.2 Design Circuit
This is the process to build up the diagram of the circuit for the analysis. The user need to choose and selecting component from master library and put it in the main page where the user will construct the circuit. There are many components with a different type of setting. The user only need to double click on the component to edit or changes the setting and parameter. Project development consists of two parts electronic and software designs.
Figure 3.2 show the three phase source that use in this project. This source impedance is type R because resistor connected series with the source. The source in this project is being controlled through fixed parameter.
Figure 3.2: Three Phase Source
Figure 3.3 show the three phase transformer that use in this project. The type of the transformer is a three phase and two winding transformer. The transformer is connected in star delta connection. Other characteristic for the transformer in this project is the transformer use to step up the voltage.
Figure 3.3: Three Phase Transformer
3.3 Placing Input and Output Devices
This process is used to get the measurement, signal and waveform of the graph in selecting part or component. The output device must be placed at the point of measurement before plotting can be done on the drawing space. Without this device, the PSCAD cannot create the plotting and the result cannot be obtained. After the circuit had already been executed and there is no error, the output from the circuit or diagram will be obtained. Figure 3.4 shows some of the output and input devices that used in this project.
Figure 3.4: The input and output devices
3.4 Running Simulation
After complete all the circuit design and placing input and output device, the user need to run the circuit to get the result. To run the simulation the user only need to click on the run button in the main toolbar. Figure 3.5 show the location of run button at the main toolbar. The run toolbar have a green color. When this button is pressed, PSCAD will go through several stages of processing the circuit before starting the EMTDC simulation.
Figure 3.5: The run button at main toolbar
During the run time, the work will be compiling by the PSCAD. The result or output only will produce if there is no error in the setting of the circuit in the drawing or all connection is connected. If there is an error, the warning will appear at output space. Once the program is no error and running, the graph and also the measurement will be produce depending on the selected node. The user also able to pause and zoomed the graph.
3.5 Graph Calculation
The graph give the certain value to calculated the overshoot, resonant frequency and also the curve can be identified as an overdamped, underdamped or critical damped. Equation 3.1 is use to calculate the overshoot. Equation 3.2 is uses to calculate the resonant frequency, ωo.
% OS = -100 (3.1)
ωo = (3.2)
The curve is overdamped if α is bigger than ωo, underdamped if α is smaller than ωo and critically damped if α is equal to ωo. Equation 3.3 to 3.8 shows the step to get a value of α. Figure 3.6 shows the example of the series RLC circuit.
Figure 3.6: The Series RLC Circuit
Equation 3.3 is equation for series RLC circuit. Equation 3.4 and 3.5 is a root for quadratic equation for 3.3.
The root for equation 3.4 and 3.5 can be express to equation 3.6 and 3.7.
From equation 3.4 and 3.6, the value for damping factor, α, calculated as equation 3.8.
4.0 Expected Result
The transient over-voltages that occur at the load during capacitor switching will be discuss base on result at all buses in the circuit that will be design before doing the simulations. The discussion is about the voltage waveform when transient over-voltages occur and voltage waveform when the simulations using pre-insertion resistors to reduce transient over-voltages. The results of waveform at all buses that will be obtain from PSCAD simulation will be discuss in term of peak voltage, overshoot and weather that waveform is overdamped, underdamped or critically damped. Figure 4.1 show the voltage waveform that will get during capacitor switching at all buses. Figure 4.2 show the voltage waveform that will get at all buses after simulate it using pre-insertion resistor. The transient will be reduce after doing the simulation using pre-insertion resistor.
Figure 4.1: Transient Over-voltage Waveform at Buses.
Figure 4.2: Voltage Waveform at Buses after Pre-Insertion Resistor.
In PSM 1 all the literature review regarding this project are been studied so at the end of PSM 1 it can be summarized that the objectives of the project will be fulfilled in the next PSM 2, which is to design the circuit, running the PSCAD simulations and analyze the result that obtain from the PSCAD simulations in term of peak voltage, overshoot and weather that waveform is overdamped, underdamped or critically damped. This progress will need more commitment and efforts. In conjunction to achieve that, scope and objectives of this project will be the guideline.
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