Literature Review Of Load Shedding Methods

Literature Review Of Load withdraw MethodsIn chapter 1, a brief discussion about active dispersal networks was presented. The importance of effect of islanded dispersion networks was discussed. This chapter is intended to give the reader a better downstairsstanding of the turn on contriveding dusts lively applied and proposed over the years. However, it is assumed that the reader is known with basic power st scoregy engineering.In branch 2.2, the bea of probability of islanding and the need for shoot down throw off is discussed. To achieve this objective, existing reduce withdraw manners atomic number 18 reviewed to understand their working principle, requirements, advantages and limitations. The main categories identified ar the following (i) Manual / SCADA Load cast off (ii) Load Shedding utilize thresholds, (iii) Adaptive Load Shedding, (iv) smart as a whip Load Shedding and (v) Load Shedding Based on Static Optimisation which are expound and discussed i n sections 2.2 through 2.7 respectively. Finally a thick is presumption in section 2.8 from which a new turn on shake off method for an islanded distribution network able to address the limitations of existing methods leave alone be proposed.probability of islandingThere is by now a considerable derive of literature on weight shedding. That most of the literature however deals with large merged strategys. For smaller systems when a loss of mains / grid event occurs the islanded distribution network have different operating marks and restrictions that require different fill up shedding guidelines. These are due to the low inertia of the distributed generators, the limited spinning reserve and limited communication systems 0 0. Load shedding is a practice utilise power system and serves as a perish to try to arrest any relative relative absolute frequency or potential drop when a charge isolating part of the distribution network occurs.Faults in power systems are ine vitable, for various reasons such as adverse weather checks, ageing and failure of equipment, accident, and animal contact. In general, faults happen when an brachydactylic physical contact occurs between lines or on lines to earth that create a short-circuit itinerary. If the system is not well protected, the high fault current due to the short-circuit path usher out cause damage to the equipment in the system. Faults also affect the reliability and quality of the power supply, leading to power recess by frequency and potential difference dispel and potency sag events. Regardless of the interruption period, the losses are often enormous both to the customers and power public utility company companies.There are two types of fault, determined by the physical nature of the short-circuit path temporary or permanent. Common causes of temporary faults on crash lines are lightning strike resulting in a flashover of the insulator bird or animal contact and momentary contact due to wind or trees. Faults caused by these events exist for a very short period of duration. On the other hand, a permanent fault stay ons in the system until the short-circuit path is removed. Common causes of permanent faults in power system networks are cable insulation failure, objects falling on the overhead lines, dirt on insulators and lines falling to the ground.When faults occur, a protection device ope browses to isolate the faulty line from the rest of the system (loss of mains / grid). The generators designated to provide voltage and frequency control will answer to control the island voltage and frequency. In severalise to achieve smooth transition to island operation, the generators must initiatoryly ride through the fault or failure and secondly act to counterbalance the active and reactive power in the islanded network. With a carefully designed bill shedding method the operation of the islanded distribution network readiness be manageable. It is important howe ver that the design of the blame shedding method is designed on the understanding of the characteristics of the system involved, including system topology and kinetic characteristics of its generation and load. A poorly design method whitethorn be ineffective and eventually lead to total customer interruption.Over the years, however, utility experience and extensive studies on a come in of systems have resulted in different methods guidelines. In the following section, principles and guidelines for load shedding methods are reviewed.manual / SCada Load SheddingManual or operator initiated load shedding 0 is not a dependable method to be used to avoid frequency deviation. However it ordure be used by some utilities to manually shed load or open ties (interconnectors) with adjacent areas at a frequencies below impulsive underfrequency thresholds. This type of action great poweriness be necessary to prevent any further frequency deviation and to recover the frequency back to t he nominal value. This load shedding intrigue cannot be used for the islanded distribution network as it will be very retard as the frequency and voltage in the network will collapse within few seconds making it impossible for the operator to decide the correct defence action compulsory for safe operation.automatic Load Shedding utilise thresholdsAn automatic load shedding for infection system using different schemes such as underfrequency, undervoltage and combinations of the two can be utilise to avoid frequency or voltage collapse during a significant asymmetry between generation and load. These types of load shedding methods are very dependant on shoot line studies of the systems dynamic performance and only consider the greatest probable imbalance between generation and load. These methods have to be coordinated with the protections of the generating units, shunt capacitors and other automatic actions that occur in the system during frequency and voltage variations.Unde rfrequency load SheddingThe underfrequency load shedding scheme as explained in the following text file 0 0 uses relays detecting the systems frequency. These are designed to operate on the instantaneous frequency value where they trip when the frequency drops below the set point of the relay. The shedding is utter(a) in the systems distribution or transmission stations where major load bird feeders can be controlled by tripping of the circuit breakers (CB) automatically. Different desktops can be applied in these load shedding schemes.Multiple stages can be used in the scheme 0. The substation loads can prioritised and grouped harmonise to the importance of the load. The relays can be set to control one or more groups of loads and when there is a frequency drop these can be disconnected sequentially where the group with the highest probability being disconnected the last. Each group disconnected should contribute to the system rate of shift of frequency regrets. If the load to be disconnected is small compared to the overall imbalance then the contribution will be insignificant and would cause further problems to the systems frequency decline.Another setting usual for this type of scheme is the time hold water 0. The time may can be required and used usually to avoid any frequency transient dips that could arise in the system. The time delay also avoids surplus load shedding by allowing the load / frequency controls in the system to respond to the frequency deviation. However load shedding performed with long time delays should be set appropriately as it will pay off the system more undefendable to system perceptual constancy if eventually load shedding is required. This method will work adequate in a situation where the system frequency decline is slow.For example, as discussed in 0, in the UK as stated in the NationalGrids GridCode each transmission area has to disconnect a set apartd percentage of the peak shoot for away that each Network O perator whose system is connected to the GB Transmission ashes shall disconnect by low frequency relays at a figure of speech of frequencies. The defined frequencies and the add up of loads are given in dishearten 1 -1.Table 11 Load Shedding Scheme sedulous in the UKFrequency (Hz)% Demand disconnection for eachNetwork Operator in Transmission AreaNGETSPTSHETL48.8548.75548.71048.67.51048.57.51048.47.5101048.348.27.5101048.05101047.85Total % Demand604040The percentages in Table 1 -1 are cumulative such that, for example, should the frequency fall to 48.6 Hz in the NGET Transmission Area, 27.5% of the total Demand connected to the GB Transmission System in the NGET Transmission Area shall be disconnected by the action of low frequency relays.A significant drawback of this method is that the systems frequency must be already be low before the relay can operate which can delay the load shedding action and the frequency recovery of the system. additionally these types of schemes usu ally shed more than the required amount of load.Undervoltage load SheddingUndrevoltage load shedding method has been successfully deployed in transmission systems to protect them from voltage collapse 0 0. System studies are required to determine which systems are potential candidates for suitable the undervoltage load shedding method. This method is most useful in slow decaying systems where the undervoltage load shedding relay time relays can coordinated accordingly and operate to alleviate the system from overload conditions and low voltages.Voltage collapse can be studied using steady state simulations for the identified areas using a power flow analysis. System planning engineers conduct numerous studies using P-V and Q-V as well as other analytical methods to determine the amount of load required to be shed to preserve voltage stability under different disturbances.Dynamic simulations can then determine the speed of the collapse and load shedding settings.An example as discus sed in 0 in the US in the Puget Sound area, which is prone to voltage collapse has been studied. The voltage trip thresholds were determined from the results of steady state simulations of worst contingencies. The time delays for the relays were coordinated to address control actions of the automatic capacitor switching, generator limits, on load tap changing transformer using dynamic simulations.Table 12 Load Shedding Scheme employed in the USVoltage (pu)Time delay (s)% Demand disconnection forNetwork Operator in Transmission Area0.903.550.925.050.928.05When the monitored bus voltages fall to 0.90 pu or glower for a minimum of 3.5 s then 5% of the load is disconnected. Additionally other 5% of load disconnection should occur when the voltage falls to 0.92 pu or lower for 5.0 s.There limitation associated with proper application of undervoltage load shedding is the location of its application.to where the relaying may be appropriately applied. If it is placed on a distribution lin e the effects of auto tap changers mask a system overload condition from the relay, or alternatively a line switching operation or the startup of a large industrial plant on one feeder could fool the relay. The relay would not be appropriate at locations directly adjacent to generation powerful enough to control bus voltages even during terrible overloads. The relay is best applied to locations with fairly stiff voltages under all normal conditions, so a low voltage condition will reliably indicate a severe overload condition, as may be assumed to be the case at large substations associated with bulk power transmission lines and therefore this method cannot be effectively applied in islanded distribution networks where DG unit power and load guide varies.combination load SheddingIn order to increase the security of the above discussed methods for underfrequency load shedding the relay could be set up to supervise the voltage, the current or the rate of change of frequency. Accord ing to their combined settings, the relay could either be blocked or initiate tripping of the CB to avoid any misoperations.One combination load shedding scheme is to use an underfrequency load shedding relay with voltage supervision. Basically the operation procedure of load shedding is blocked from operating unless the voltage is below a given threshold. The underfrequency relay will be able to trip the CB as long as the bus voltage it is supervise is lower than a set point.Another combination is to use current supervision instead of the voltage. The purpose of the current supervision is to select which feeders to trip. This can achieved by monitoring which feeders are loaded above a legitimate point and then the relay will initiate the load shedding signal.An alternative is to use the rate of change of frequency for supervision 0 0. During a disturbance the supervision of the rate of change of frequency can block the tripping for very fast frequency changes but would allow for typical frequency decay rates. besides instead of touchstone the instantaneous rate of change of frequency supervision is to use the frequency change trend. In other words by monitoring the average rate of frequency change will provide a more steady-going decision for tripping during disturbances. The load shedding decision of the scheme is made by monitoring the frequency change over a specified amount of time usually few hundred milli seconds. Therefore making the operation of the relay slower than the ones employing the rate of change of frequency.automatic ADAPTIve Load SheddingAdaptive control involves updating the amount of load to shed used by the method to cope with the fact that the conditions such as the power imbalance between generation and load of the system are time-varying or uncertain. It is important in these circumstances to minimise consumer disruption through proper design of the load shedding arrangements. An adaptive load shedding, is based on the relays re acting to a disturbance either by being instructed the amount to shed or by having certain defined criteria based on the rate of change of frequency.Anderson and Mirteydar in 0 present an adaptive methodology for setting of underfrequency relays that is based on the initial rate of change of frequency at the relay. The frequency performance of the islanded is represented by a linear system frequency response as shown in Figure 1 -1 and presented in more detail in the literature in 0.Figure 11 Simplified frequency response with disturbance inputwhereH= inertia aeonian (s)FH= fraction of total power generated by HP turbineTR= reheat time constant (s)Km= mechanical power gain computeR= droop characteristic (pu)D= damping factorClearly the only observe quantity that gives any clue as to the size of the disturbance is the initial slope of frequency decline.The use of the initial slope to estimate the magnitude of the disturbance requires that every substation in the island will observ e slightly different slopes and will therefore shed load based on different estimates of the disturbance. However on average the system as a whole will shed approximately the correct amount of load.To set the line of reasonings for the relays as explained they are based on a simulation of the frequency response for the system. In the example given (H = 3.5 s, FH = 0.3, TR = 8.0 s, Km = 0.85, R = 0.06 and D = 1) the evaluation of the frequency and its slope against different amounts of disturbances are given in Table 1 -3.Table 13 Initial Slope and Maximum Deviation vs Upset (frequency nominal 60 Hz)Pstepdf/dfmaxfminpupu/sHz/sHzHz-0.2-0.0286-1.7143-1.643858.356-0.3648-0.0521-3.1260-3.000057.000-0.4-0.0571-3.4286-3.287656.712-0.6-0.0857-5.1429-4.931355.069-0.8-0.1143-6.8571-6.575153.425-1.0-0.1429-8.5714-8.218951.781The lowest frequency permitted in the system is 57 Hz from the nominal 60 Hz. Therefore when a magnitude greater than -0.0521 pu/s is observed load shedding must be trigg ered.This method relies on the fact that the amount of load shedding is a function of only the inertia constant and the observed slope. The inertia constant is the rotating kinetic energy of all units in the island divided by the total connected volt ampere rating of the units. This parameter has to be estimated. Therefore, the initial slope is the only unknown. The load shedding amount is computed in per unit, which makes it easy to apply to every load and to every load shedding relay.A positive is that communication is not required between relays and the boundaries of the island are not required to be known. However the drawbacks are that if it is applied for the islanding application of islanded networks this might not be possible as the method needs good estimates of the inertia of the system D, R, TR, Km and FH. This can significantly change with the varying DG units and loads in the distribution network.Another adaptive load shedding method presented by Terzija in 0 uses simil arly as the previous method a variation of the typical swing equation. Due to the dynamic responses of turbines, governors, other control actions, spinning reserve, loads are not taken in account in the calculation of the required amount of load to be shed as given in .Where H is the inertia constants and assumed to be known in advance to the disturbance. The adaptive barbel is based on real time estimation of fc (frequency of equivalent inertial centre) which is proposed to be calculated centrally by measuring the local frequencies at each generator.The proposed method assumes that the time constants in the power system are large and with modern communication this method would be possible for big power systems. However in distribution networks communication is believed not to change drastically in the near future making this application operose to implement. This is because the estimation and control information are evaluated after the disturbance occurred.Van Cutsem and Otomega proposed a method in 0 which relies on a set of load shedding controls distributed over the region susceptible to voltage instability. Each controller monitors the bus voltage and act on a set of loads located at that bus. Each controller acts when its monitored voltage falls below some threshold and trips at different time according the severity of the drop. The action can be repeated until the voltage is above the threshold voltage.The principle of operation of the controller is described as follows.The delay depends on the time evolution of V as follows.A block of load is shed at a time t0 + such thatwhere C is a constant to be adjusted. This control law yields an inverse-time characteristic the deeper the voltage drops, the less time it takes to reach the value C and, hence, the faster the shedding. The larger C, the more time it takes for the integral to reach this value and hence, the slower the action.Furthermore, the delay is lower boundedto prevent the controller from rea cting on a nearby fault. Indeed, in normal situations time must be left for the protections to clear the fault and the voltage to recover to normal values.Similarly, the amount Psh of power shed at time t0 + depends on the time evolution of V throughwhere K is another constant to be adjusted, and Vav is the average voltage drop over the t0, t0 + interval, i.e.,Moreover, the whole system will tend to shed first where voltages drop the most. This location changes with the disturbance. Hence, the proposed scheme automatically adjusts the shedding location to the disturbance it faces. Note that the above features are achieved without resorting to a dedicated communication network. The controllers do not exchange information, but are rather informed of their respective actions through the power system itself.The drawback for this method for distribution network is that the tuning which consists of choosing the best values for Vth, C and K. A C and K combination suitable can be identifie d by minimising the total load shedding over all disturbance scenarios. Clearly this method would shed more loads for some scenarios. An additional concern is that the dynamic performance of the DG units and loads is not taken in account when acting load shedding if applied to the islanded distribution network and by trying to shed in steps the frequency drop in the network might drop significantly.automatic Intelligent Load SheddingApplications of intelligent load shedding in power system engineering (e.g. familial algorithms, artificial flighty networks, MonteCarlo etc.) have been demonstrated in 0 0. The characteristics which are inherent to intelligent methods, such as the ability to learn and generalization make it feasible for applications such as load shedding.You et al. in 0 discuss of a method that uses the rate of change of frequency to load shed. The method uses the same approach to calculate the required amount of load as in 0 and at the same time, the conventional lo ad shedding method with undefrequency thresholds is incorporated to form a new two level load shedding method.The conventional load shedding method has longer time delays and lower frequency thresholds which can be used to prevent unnecessary load shedding in response to small disturbances. If the disturbance is large, the second layer will be activated and a block signal to the first layer is enabled. The second layer based on the rate of change of frequency load shedding will shed more load promptly at the early stage of the disturbance.Similarly as to paper 0, this method will have the same limitations when applied to the islanded distribution network. In the paper 0 which follows this study, the comment of the selection of the settings for the relays is discussed. Agent technology is to try to assure that the method will withstand all possible disturbances. Traditionally after a major disturbance, the system is revisited and settings of devices and control actions are changed so that the system will withstand the same disturbance in the future. This however due to analysis of the system significant time and cost will be required.For the autonomous and adaptive learning capableness for the operators, the reinforcement learning technique is used. Reinforcement learning is learning by interaction.The agent tries actions on its environment and then, the tendencies of taking particular actions are reinforced by receiving scalar evaluations of its actions. Thus determining the amount of load to be shed required to avoid collapse.The paper does not discuss whether the technique is applied online or offline through simulation. Clearly for the online this would not be ideal as it will take a lot of number of failures until the agents are properly set for that particular disturbance. For the offline simulation a concern is that for islanded distribution networks the topology, DG unit power and load demand will change thus making the decision of the action of the agents is difficult to train. Another concern is communication between agents. Fast communication would be required for coordinated decisions.Another approach to load shedding is the use of fuzzy expert system and is described in 0. In this paper Sallam and Khafaga described a method to control the voltage instability by load shedding using fuzzy technique as fuzzy controller.The operation of the method relies on the experts knowledge which is expressed by language containing ambiguous or fuzzy description. The aim of this study is to design and analyse a fuzzy controller for the study to control against load and voltage instability by calculating the optimum load shedding as output.Similarly in 0 the authors propose genetic algorithms for the optimum selection of load shedding. These techniques search and optimise the amount of load shedding using objectives and constrains required for a practical load shedding method.Also in 0 the authors realize another technique using the arti ficial neural networks is presented. To prepare the training data set for the artificial neural network, transient stability analysis of the power system is required and to find the minimum load shedding for various scenarios. By selecting the total power generation, total load demand and frequency decay rate as the input neurons for the method, the minimum of load shedding is determined to maintain the stability of the power system.In paper 0 Thalassinakis and Dialynas introduce a computational method using MonteCarlo simulation approach for the calculation of the settings of the underfrequency load shedding relays is discussed. The frequency performance as previously discussed in section 1.5 is used here as well. The strategy for the relay settings will be determined against amount of load to shed, time delay, rate of change of frequency and underfrequency level. A new strategy is developed by changing these settings. The MonteCarlo then computes the system through reliability ind ices of generating units, the system frequency and load shedding indices.load shedding based on still optimisationThe first theory of applying load shedding using an on line dynamic simulation of the power system network was introduced by La Scala et al. 0. Followed by an improvement of the method combing a control action to attend angle and voltage stability sweetening in 0. The first paper that introduced the same concept applied for large power systems to the smaller distribution network is described in 0 by Nelson and Aponte. A more recent study using similar technique is also presented in 0.The paper presented in 0 describes the philosophy and the implementation of a preventive load shedding control algorithm for the application in dynamic security assessment. The methodology is based on nonlinear programming techniques, for assessing control actions to guarantee the dynamic security of power systems. The basic idea is that the online dynamic preventive control can be seen a s a static optimisation problem with minimising function and comparability and inequality constrains. The equality constrains consist in the discretisation at each time step of the differential algebraic set of equations representing the power system. The inequality constrains define a domain where the system trajectories should be contained in order to satisfy the requirements for the system performance stability and steady state voltage dips.In 0 the reflexion includes corrective actions based on load shedding. The proposed method assumes that the analysis is performed to detect particular disturbances threatening the dynamic security of the system. The analysis is based suing the n-1 rule which is performed in advanced and applying the results immediately after the detected contingency. Each analysis has its associated strategies consisting with the corresponding amount of load to be shed at a fixed number of controlled nodes. The optimisation however is evaluated based on the steady state values of angle, voltage and active power (generator and load). Load shedding based on static optimisation performs load flow to calculate the initial P, V for all the nodes in the system. Then the method performs a transient simulation assessment to ensure the system is stable against angle and voltage. Followed by an approach to the minimisation of a function in presence of equality and inequality constrains consist in incorporating the inequalities in the cost function by adopting the penalty factor method and treating the whole problem as a minimisation in presence of the sole equality constrains by the use of Lagrange multipliers.This method has been used for synchronous generators in transmission systems. However in distribution networks because of the diversity of the generators and their ride through capability this approach could result in conditions where optimised solutions do not meet the requirements as shown in Figure 1 -2 0.Figure 12 Ride through capabili ty of Generating Unit, DC Converter or Power Park Modules.Explanation of graph required.Each Generating Unit, DC Converter or Power Park Module shall remain transiently stable and connected to the system without tripping. However for small generating units connected in the distribution network their transient behaviour could be as shown in Figure 1 -2 b and c where local protection and circuit breaker operation of generators or sensitive equipment will be disconnected after such a response. Similar to the voltage is for the frequency range.Therefore the load flow with corrective control for angle and voltage stability approach for the load shedding optimisation is not appropriate for distribution networks.In 0 and 0 describe of a method implemented in distribution networks where not only the amount of load shedding is optimised but also the time for the disconnection. The current trend is to apply the corrective measures as soon as possible or slow for the sake of event discriminat ion. The study and results however show that when the corrective action is applied at the optimal time increased damping and enhanced response are observed.summaryThe use of load shedding as a tool to keep the network stable has been constantly evolving, and different approaches have been formulated. Relaying schemes like underfrequency and ROCOF 0 0 are some examples of the mechanisms implemented to trigger a load shedding event. Typical load shedding schemes based on predefined threshold set points is quick, simple and reliable measure against system disturbance. When the frequency of the system reaches a specified threshold value, a time delay is inserted prior to the shedding action in order to avoid overshedding and assist the coordination of the next stage of load shedding action. This technique however when adopted for the islanded operation of small distribution networks would have several disadvantages. as well few frequency levels could lead to overshedding, but on the ot her hand, time delays between stages could add up and may not allow for enough load to be shed in time to re-establish nominal frequency.The implementation of ROCOF techniques mitigates some of these problems. The ROCOF value calculation is an immediate indicator of the power imbalance but for the distribution network the variation of the DG units operation would make this measurement unreliable. Also the average ROCOF calculation may take too long and eventually make the load shedding method slow in operation. Even if accurate measure of the islanded distribution network ROCOF valu