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• Muhammad Rooshan

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• Sistema de energía eléctrica
• Red eléctrica
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• Electricidad
• Energia electrica

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What is Load Shedding For the sake simplicity, a balanced power system is one in which Demand And Supply of power are completely matched. To predict Demand and to Supply power to achieve balance power system is impractical and impossible in today world because of uncertainity and unstability of load and transmission system. Different methods are used for loadforecasting by utilities e.g utility measure power over 15 minutes intervals and predict the load for next 15 minutes. “Loadshedding is a emergency control action taken to maintain the stabilty of power system to shutdown load to achieve desired system frequency and voltage”. In the peak hours, sharp rise in load occurs and system frequency and voltage declines at fast rate ,to prevent power system spinning reserves plays important role to supply excess power.But if load demand is higher than the availible power the situation is intense and emergency load shedding is necessary to save the system.Electrical load consist of both Active and Reactive load,excess in Active load causes system “Frequency” to decline and excess of Reactive load hurts the “Voltage”.LS(Loadshedding) continues to shut load until equilibrium is achieved in preset time else system will collapse causing Blackout.

Load Shedding Methods Most of load shedding existing techniques used Voltage and Frequency via UFLS(Under Frequency Load Shedding),UVLS(Under Voltage Load Shedding) methodologies.UFLS and UVLS work without any co-ordination in power system.

i)Under frequency Load shedding In this technique the under frequency relay is used which continuosly monitors the system frequency.As long as frequency above the threshold pick-up value the relay works normally.But is frequency falls below pick-up value the relay trips the breakers that feeds the load.The techniques involve following steps. 1: Amount of load need to shut in each step 2: Delay between steps 3: Location of shed load

ii)Under voltage Load shedding This technique works in similar manner as UFLS but the difference is that it do so on the basis of Voltage rather than frequency.

There are various schemes for UFLS/UVLS namely 1:Static Load Shedding 2:Dynamic Load Shedding 3:Traditional 4:Half Adaptive 5:Adaptive

Static or Fixed Load Shedding Static load shedding curtails constant block of load at every stage. The calculation of time to shed the load is acquired from the system overload and rate of change of frequency. In static load shedding scheme, it is better to have more stages, but with smaller load in each stage, to minimize overshedding. Moreover, tripping a big block of load at one timewill have a large impact on an already weakened system.However the drawback is that it takes longer time to stabilize the frequency.

Dynamic Load Shedding Dynarmc load shedding on the other hand curtails dynamic amount of load at each stage by taking into account the magnihlde of disturbance, voltage and frequencycharacteristic of the system at each stage. This allows for the shedding of larger load for larger system imbalance,andsmaller load for smaller system imbalance.

Traditional LS In the traditional approach, when the frequency is lower than the first setting value, the first level of load shedding will be implemented. If the frequency continues to decline, it is clear that the first load shed amount is insufficient. When the frequency is lower than the second setting value, the second stage of load shedding will be implemented. If the frequency continues to decline, the further load shed stages will be activated until the normal frequency value is restored. However, the load amount to be shed is determined in the case of serious events. Therefore, when the frequency declines to its first shed level, the load that is shed is often excessive for the particular event.

Half Adaptive The half adaptive approach is similar to the traditional approach to some extent. However, the specific amount of load to be shed is determined in terms of the measuring value of the rate of change of frequency (ROCOF).

Adaptive

The adaptive approach is an improved and yet more reasonable algorithm. The frequency response model is built on the basis of the frequency differential equation and the rotor motion equation in this approach. Based on the variations of frequency,the amount of load to be shed by the adaptive approach can be determined more accurately.

Requirments For Load Shedding Schemes An optimal scheme requires thurough understanding of power system,process requirment,dynamic and static behavior of components,transient stabiltiy and magnitude of disturbance in system.A brief list is given below 1:Pre-Fault Condition a) Total system demand b) Amount of power exported to grid c) Generation of each generating unit d) Configuration and topology of generating unit e) Spinning reserve of each generating unit f) System configuration i-e tie-lie numbers,tie-line status and power flow,transformer and feeder status load etc) 2:Post-Fault Condition a) New Load Demand b) Remaining generation from on-site generation c) Spinning reserve remaining for each unit d) Time to bring up spinning reserve e) New system configuration 3:Type and duration for distubance and fault a) Electrical or mechanical fault b) Load addition impact c) Complete or partial loss generation on-site or grid d) Location of disturbance e) Subsequent network disturbances 4:System transient response to disturbance a) System frequency decline and rate of change b) System voltage response to disturbance c) Operation of protective device 5:Dynamic behavior of system after the fault 6:A communication and control network for continuos monitoring of power flow,analysis and resoration of system after the fault.

Conclusion Load shedding serves as ultimate guard to protect system from induced disturbance by using UFLS/UVLS schemes.Most of techniques used in LS schemes are broadly classified as Static and Dynamic (adaptive) LS.Each

techinique have their unique Pros and Cons.Like Dynamic is more fast,accurate than static one ,whereelse it static costs less than dynamic one.LS techniques based on adaptive and time optimal algorithm are more realiable than traditional UFLS,UVLS and PLC load shedding.Main factors which make a LS scheme appropriates includes minimized load curtailment,less time delay,less under frequency and over frequency conditions and above all identifying the Zone creating the Disturbance with in quick time enough to stop Frequency Decline to prevent system from collapsing.Load shedding scheme based on both Voltage and Frequency is more close to practical system thus achieve better results.

Reference 1: Intelligent Load Shedding Need for a Fast and Optimal Solution By Shervin Shokooh,Tanuj Khandelwal, Dr. Farrokh Shokooh, Jacques Tastet ,Dr. JJ Dai 2: Static and Dynamic Under-frequency Load Shedding: A Comparison A. A. Mohd Zin, Senior Member, IEEE, H. Mohd Hafcz, Member, IEEE and W. K. Wong 3: Modified Adaptive load Shedding H.A.RAKHSHANI, A.R.HEIDARI, M.M.KAYKHA Islamic Azad University Zabol Branch, Zabol, IRAN, [email protected] Engineering Department University of Zabol, Zabol, IRAN, [email protected] 4:Power Load Shedding Simulation And Rudnick,Armengol Blanco,Celzo Gonzalez

Optimization

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