Power System Contingency Analysis电力系统静态安全分析

Power System Contingency Analysis: A Study of Nigeria’s 330KVTransmission GridNnonyelu, Chibuzo Joseph Department of Electrical Engineering University of Nigeria, Nsukkachibuzo.nnonyelu@.ngProf. Theophilus C. MaduemeDepartment of Electrical EngineeringUniversity of Nigeria, Nsukka AbstractsAs new sources of power are added to the Nigeria’s power system, an over-riding factor in the operation of the power system is the desire to maintain security and expectable reliability level in all sectors –generation, transmission, and distribution. System security can be assessed using contingency analysis. In this paper, contingency analysis and reliability evaluation of Nigeria power system will be performed using the load flow method. The result of this analysis will be used to determine the security level of the Nigeria power system and suggestions will also be made on the level of protection to be applied on the Nigeria power system with aim of improving system security.Keywords: Contingency Analysis, Contingency, Power System Security, Overload Index1.INTRODUCTIONPower system protection is an important factor of consideration in all sectors of a power system during both planning and operation stages. This is because any loss of component leads to transient instability of the system and can be checked immediately by the help of protective devices put in place. As we propose and source new sources of power in order to meet up the Nigeria energy demand, it is important to access the security level of the existing grid in order to devise a more defensive approach of operation.Currently, the Transmission Company of Nigeria (TCM), projected to have the capacity to deliver about 12,500 MW in 2013, has the capacity of delivering 4800 MW of electricity. Nigeria has a generating capacity of 5,228 MW but with peak production of 4500 MW against a peak demand forecast of 10,200MW. This shows that if the generation sector is to run at full production, the transmission grid will not have the capacity to handle the produced power reliably [7]. This goes a long way to tell that the 330 KV transmission system is not running effectively as expected. Therefore to maintain and ensure a secure operation of this delicate system, the need for contingency analysis cannot be over emphasized.Contingencies are defined as potentially harmful disturbances that occur during the steady state operation of a power system [1] Contingencies can lead to some abnormalities such as over voltage at some buses, over loading on the lines, which if are unchecked, can lead to total system collapse.Power system engineers use contingency analysis to predict the effect of any component failure. Periodically, maintenance operation are carried out on generating units or transmission lines. During this, a unit is taken offline for servicing. The effect of this forced outage on other parts of the system can be observed using contingency analysis.As demand for power increases, more generating units are installed in Nigeria with no corresponding increase in transmission capacity. This makes the transmission lines run at their maximum power capacity which is very dangerous as there is too much power in the system at any moment. This power will be shifted to any available portion of the transmission system in case of any contingency thereby overloading the available portion. This effect can be analysed by the calculation of Line Outage Distribution Factor (LODF). Also, the overloading index of the remaining lines will can be obtained equally.2.POWER SYSTEM SECURITYOne of the most important factors in the operation of any power system is the desire to maintain system availability and reliability. This ensures a secure operation of the system and improved economic operation. Power system security is the ability of the system to withstand one or more component outages with the minimal disruption of service or its quality. System security involves practices designed to keep the system operating in emergency state when components fail and to restore it to its preventive state. For instance, a generating unit may break down or have to be taken off-line for maintenance purposes. This leads to frequency and voltage instability as the available generating unit experiences more loads than usual, hence frequency drops and bus voltages lowers. If this is not foreseen and defensively prevented by use of protective devices such as relays for load shading, it can lead to the collapse of the concerned system. Therefore the control objective in the emergency state is to relieve system stress by appropriate actions while economic consideration becomes of secondary.2.1CONTINGENCY ANALYSISContingency analysis is the study of the outage of elements such as transmission lines, transformers and generators, and investigation of the resulting effects on line power flows and bus voltages of the remaining system. It represents an important tool to study the effect of elements outages in power system security during operation and planning. Contingencies referring to disturbances such as transmission element outages or generator outages may cause sudden and large changes in both the configuration and the state of the system. Contingencies may result in severe violations of the operating constraints. Consequently, planning for contingencies forms an important aspect of secure operation [2].There are various methods of contingency analysis which include the following:a.AC Load flow methodb.DC Load flow methodc.Z-Matrix methodd.Performance Index methodOf all the above listed methods, methods based on AC power flow calculations are considered to be deterministic methods which are accurate compared to DC power flow methods. In deterministic methods line outages are simulated by actual removal of lines instead of modelling. AC power flow methods are accurate but they are computationally expensive and excessively demanding of computational time. Because contingency analysis is the only toolfor detecting possible overloading conditions requiring the study by the power system planner computational speed and ease of detection are paramount considerations. [1]Results of contingency analysis are usually ranked based on some indices calculated during the analysis. The choice of performance index to calculate depends on the engineer. In a deregulated power system, a discernibly purely profit driven, economic operation is of utmost importance hence Available Transfer Capability (ATC) is normally calculated to ascertain the available transfer capability of the system in case of any contingency. Other Indices include Voltage Stability Index (VSI), Active Power Loading Performance Index (APLPI), Line Outage Distribution Factor (LODF), Line loadability, etc.Generally, once the current working state of a system is known, contingency analysis can be broken down into the following steps:a.Contingency definitionb.Contingency selectionc.Contingency evaluationContingency definition involves preparing a list of probable contingencies. This typically includes line outages and generator outages.Contingency selection process consists of selecting the set of most probable contingencies; they need to be evaluated in terms of potential risk to the system. Usually, fast power flow solution techniques such as DC power flow are used to quickly evaluate the risks associated with each contingency. But in this work, the Newton-Raphson load flow method will be used to ensure higher accuracy.Finally, the selected contingencies are ranked in order of their security, till no violation of operating limits is observed.The algorithm for a typical contingency analysis is shown in Figure 1.Figure 1: Algorithm of a typical contingency analysis2.2 LINE LOADABILITYLine Loadability can be defined as Transmission-line voltages decrease when heavily loaded and increase when lightly loaded. When voltages on EHV lines are maintained within ±5% of rated voltage, corresponding to about 10% voltage regulation, unusual operating problems are not encountered. Ten percent voltage regulation for lower voltage lines including transformer-voltage drops is also considered good operating practice.In addition to voltage regulation, line loadability is an important issue. Three major line-loading limits are:a.the thermal limit,b.the voltage-drop limit, andc.the steady-state stability limit.The maximum temperature of a conductor determines its thermal limit. Conductor temperature affects the conductor sag between towers and the loss of conductor tensile strength due to annealing. If the temperature is too high, prescribed conductor-to-ground clearances may not be met, or the elastic limit of the conductor may be exceeded such that it cannot shrink to its original length when cooled.Conductor temperature depends on the current magnitude and its time duration, as well as on ambient temperature, wind velocity, and conductor surface conditions.The loadability of short transmission lines (less than 80 km in length) is usually determined by the conductor thermal limit or by ratings of line terminal equipment such as circuit breakers.For longer line lengths (up to 300 km), line loadability is often determined by the voltage-drop limit. Although more severe voltage drops may be tolerated in some cases, a heavily loaded line with V R/V S ≥ 0.95 is usually considered safe operating practice. For line lengths over 300 km, steady-state stability becomes a limiting factor [4].3.METHODOLOGYIn this paper, the AC load flow method of contingency analysis was adopted. The Newton-Raphson load flow algorithm, an algorithm under the AC load flow method, was used to solve the power flow problems during the analysis using MATLAB. This is because the NRLF method has more accuracy than other AC Load flow methods and converges faster. Newton-Raphon’s Load flow method is discussed more in [3, 4].3.1 Calculating System Line Overload Index (SLOI)To obtain the overall system overload index, a new performance index was proposed and calculated based on the Line Loadability discussed in Section 2.2. As stated, for safe operation, the ratio of the receiving end voltage and the sending end voltage must be greater than 0.95. This newly proposed index relies on this to calculate the system line overload index. It helps tell the system designer at a glance, the lines that should be given utmost attention in terms of protection. SLOI is computed by equation (1):SLOI=1− [min(V RV S )]k(1)whereV R, V S are the receiving end and sending end voltages respectively, andk the number of lines whose V R/V S < 0.95The Nigerian transmission grid is shown in Figure 2 with the single line diagram shown in Figure 3.Figure 2: the Nigerian power system. Blue lines indicate the 330-KV lines(Source: Nigeria System Operator)Figure 3: one-line diagram of the Nigeria 330-KV transmission gridThe network parameters – generator data, load data, and line & transformer data, of the Nigeria power system as used in this work were collated from [8, 9, 10] and are shown in tables 1, 2, and 3 respectively.Table 1: Generator data3.1 Simulation of Line OutageSimulation of transmission line outage is carried out by the formulation of the corresponding admittance matrix [5]. For instance, after outage of a line connecting bus ‘a’and ‘b’, the components of the Y bus that will be affected are Y aa, Y bb, Y ab, and Y ba. For a ‘π-modelled’transmission line, the admittance values after this outage is obtained by subtracting the admittance of the line a-b and the shunt susceptance jb ab/2 and jb ba/2 from Y aa and Y bb.Line outages was simulated by simply removing the line information from the line data matrix. This is similar to the line not existing initially as the information no longer exists.3.2 Simulation of Generator OutageThis simulates mainly outage of one unit (or more) in a power station. Let the total generation for the station at bus ‘m’ be P gm, and assume that there exist identical (g) units, then [6]:)(2) P gm′=P gm−n(P gmgwhereP’gm: Active power generated at bus m after the outageP gm: Active power generated at bus m before the outagen: Number of outage generation units in the stationP gm/g: Active power generated at bus m per generator unitIn this work, generator outages were not simulated as only the effect of line outages were desired.4.Results and DiscussionThe results of the analysis (the SLOI) is shown in Table 4 ordered by the SLOI from the most critical to the least critical.The result as shown in Table 4 contains the SLOI values of the different lines for line outages. It has been organised in the order in descending order.This shows that the outage of line 11 to 14 (Oshogbo to Aiyede) will have the most critical effect on the system followed by 11 – 15, 16 – 20, 20 – 24, 25 – 16. These lines have been shown to pose serious danger on the system stability if they fail, and therefore should be secured defensively to avoid the level of system instability caused by the outage of any of the lines.5.ConclusionFrom this study, it is has been shown with values, the importance of operating the transmission system defensively to avoid system collapse due to overloading. Also, the writer suggests that the Transmission Company of Nigeria (TCN) should adopts the (Flexible AC Transmission), FACT devices as they can improve the lines active power capability in any contingency event as have faster switching than the traditional compensation devices. Also additional lines should be used to connect Oshogbo to Aiyede through different routes to create more links for power to be transmitted through to Lagos area in order to reduce the SLOI value of Oshogbo to Aiyede line.References[1] Chary, D. M., “Contingency Analysis in Power Systems, Transfer CapabilityComputation and Enhancement Using Facts Devices in Deregulated Power System.”Ph.D. diss., Jawaharlal Nehru Technological University, 2011[2] Wood, A. J.; Wallenberg, B. F., “Power Generation, Operation and Control”. 2nd ed.,New York/USA: John Wiley& Sons, 1996, pp. 410-432.][3] Saadat, H., Power System Analysis, New Delhi: McGraw Hill, 2002, pp 189 – 256.[4] Glover, J. D., Sarma, M. S., Overbye, T. J., Power System Analysis and Design, 5h ed.Stamford: Cengage Learning, 2012.[5] Nara, K.,Tanaka,K., Kodama, H., Shoults, R. R., Chen, M. S., Olinda, P. 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