PresenterRaja Masood Larik

PE 133053

Faculty of Electrical Engineering

OPTIMAL DESIGN OF AN UNDER VOLTAGE LOAD SHEDDING SCHEME BY USING

HYBRID META-HEURISTIC TECHNIQUES

SupervisorProf. Dr. Mohd Wazir Bin Mustafa

CONTENTSIntroductionBackgroundLiterature Review Problem StatementObjectives of the projectVoltage StabilityMethodologyConclusionReferences

INTRODUCTION• What is Load Shedding?Load shedding isWhen there is not enough electricity available to meet the demand from all Eskom customers, it could be necessary to interrupt supply to certain areas. This is called load shedding. It is different from a power outage that could occur for several other reasons.

It is a last resort to balance electricity supply and demand. We will only apply load-shedding when all other options have been exhausted.

It is an effective way to avoid total collapse of the electricity supply grid (a national black-out) which will have disastrous outcomes for South Africa. If unbalances on the power is not managed this could lead to the risk of collapse of the entire power network. If this occurs, it could take more than a week to restore power to the entire country. By rotating and shedding the load in a planned and controlled manner, the system remains stable.

INTRODUCTION

• Before load-shedding is applied, Eskom makes use of:– Gas and hydro options– Contracted and voluntary options with certain

large customers to reduce their demand.– If all these measures have been exhausted and

demand still cannot be met, Eskom will proceed with load-shedding.

When Load shedding is applied?

INTRODUCTION

• Developing a UVLS program requires coordination betweenprotection engineers and system planners, who together can determine the amount of load and time delay required in the shedding program but system planning engineers conduct numerous studies using P-V (nose curves) as well as other analytical methods to determine the amount of load that needs to be shed to retain voltage stability under credible contingencies.

• Voltage collapse is most probable under heavyload conditions where large amounts of power are to be transported from remote generation sites and thebulk of the system load consists of motors.

Who Decides Under Voltage Load Shedding (UVLS)

BACKGROUND Power system stability has been recognized as a very

important issue for secure interconnected power system operation In a power system.

Frequency is a measure of the balance of MW generation and MW load. When MW generation and MW load are exactly in balance, the frequency is at the normal level of 50 Hz.

Voltage is a measure of the balance of MVAr load and MVAr capability within a power system. If that reactive support is not available, the voltage goes down. When load exceeds generation, the frequency and voltage goes down. Most of the blackouts are caused by the instability of power system.

LITERATURE REVIEW

S.No Author Technique Strength Weaknesses Journals/Yrs of Publish

1 Abbas Ketabi [20]

A sensitivity-based method for UFLS

Hybrid and Multi area Power Systems

Voltage stability is ignored

IEEE Trans.on smart grid 2015

2 Ahmad Ahmadi [24]

New Integer-value modeling 

Minimizing total amount of load shed and the amount of interruption cost

Hybrid Power system not considered

Journal ELSEVIER2014

3 M.M Hosseini[23]

A techno economic multi-objective optimization

Consider social welfare and smart market

Hybrid Power system not considered

Journal ELSEVIER2013

4 Alireza Saffarian[22]

Designing the 3-D combinational Load shedding method

adaptive combinational LS methods are proposed

Traditional LS schemes are not capable of dealing with combined instabilities

IEEE Trans. On power systems, 2011

5 Urban Rudez [21]

Frequency of center of inertia (COI) in adaptive LS schemes

Only under frequency load shedding considered

 

under Voltage load shedding not considered

 

IEEE Trans. On power systems, 2011

Summary of Related Work

LITERATURE REVIEW S.No Authors Techniques Strength Weaknesses Journals/

Yrs of published

6 M.H.A Hamid 

UVLS based on voltage stability index

Dynamic simulation DG’s were not considered

IEEE innovative Smart grid technologies Proceedings 2014

7 Arief ardiaty  

Trajectory sensitivity factor (TSF)

Reduce the amount of LS by 15MW by using DFIG

Fail to reduce asynchronous power generate from DFIG

Journal ELSEVIER2012

8 Tamree Ranjbar Mozafari [14]

Modal predictive control

Can be implemented on entire Power System

Sensitivity of the modal were ignore

Journal ELSEVIER2011

 

9 Y.Wang I.R Pordanjani

Modal analysis method

non-linear problem into aseries of linear programming problems

Only N-1 Contingency has considered

IET Generation Transmission and distribution 2010

10 K.Uma Rao 

Round Robin Technique

Novel grading scheme for loads to minimize the impact of load shedding by taking revenue loss, social factors into consideration factors into consideration

Dynamics were ignored.

IEEE ISGT Asia 2013 proceedings

LITERATURE REVIEW S.No Authors Techniques Strength Weaknesses Journals/

Yrs of published

11 Md.Qamrul Ahsan  

Traditional, semi-adaptive and adaptive

Auto load shedding and islanding scheme developed in a well manner

Voltage stability problem ignored

IEEE Trans. On power systems, 2012

12 Zhichao Zhang 

new adaptive load shedding algorithm based on WAMS

Consider frequency and voltage both at the same time

Load Dynamics were ignored

International Journal of control and automation2014

13 Junjie Tang  

Power flow tracing algorithm

He consider both frequency and voltage stability

Transmission line outage is not considered

IEEE Trans. On power systems, 2013

Problem Statement UVLS techniques are implemented to protect the power

system from voltage collapse. A look at major power blackouts that have occurred around the world show that most were caused by voltage instability problems [4].

Voltage instability generally occurs due to either forced outage of the generator or the line, or overloading. When this happens, the reactive power demand in transmission lines varies severely and may cause a blackout if not recovered quickly.

Objectives

To propose a novel hybrid optimization technique that considers voltage stability. By combining Genetic Algorithm and Particle swarm Optimization

To develop a load shedding scheme for minimizing the amount of load to be shed.

To prevent the post load shedding problems and over loading

Voltage Stabilization in Power Systems

Voltage stabilization refers to the ability of a power system to maintain steady voltages at all buses in the system after being subjected to a disturbance from a given initial operating condition. It depends on the ability to maintain/restore equilibrium between load demand and load supply from the power system. [1]

Classification of Power System Stability

Power system can be classified according to different categories

Rotor angle stability refers to the ability of synchronous machines of an interconnected power system to remain in synchronism after being subjected to a disturbance. It depends on the ability to maintain/restore equilibrium between electromagnetic torque and mechanical torque of each synchronous machine in the system.Frequency stability refers to the ability of a power system to maintain steady frequency following a severe system upset resulting in a significant imbalance between generation and load. It depends on the ability to maintain/restore equilibrium between system generation and load, with minimum unintentional loss of load. Instability that may result occurs in the form of sustained frequency swings leading to tripping of generating units and/or loads. Generally, frequency stability problems are associated with inadequacies in equipment responses, poor coordination of control and protection equipment, or insufficient generation reserve.

Voltage Stabilization in Power Systems

What is the voltage stabilization in power system?

Power system stability may be broadly defined according to different operating conditions, an important problem which is frequently considered is the problem of voltage stabilization.

This important issue of power system control is to maintain steady acceptable voltage under normal operating and disturbed conditions, which is referred as the problem of voltage stabilization

Voltage Stabilization in Power Systems

Why we need voltage stabilization? Instability that may result occurs in the form of a progressive

fall or rise of voltages of some buses. A possible outcome of voltage instability is loss of load in an

area, or tripping of transmission lines and other elements by their protective systems leading to cascading outages. Loss of synchronism of some generators may result from these outages or from operating conditions that violate field current limit. [1]

Voltage Stabilization in Power Systems

A major factor contributing to voltage instability is the voltage drop that occurs when active and reactive power flow through of the transmission network; this limits the capability of the transmission network for power transfer and voltage support.

Voltage stability is threatened when a disturbance increases the reactive power demand beyond the sustainable capacity of the available reactive power resources.

Voltage Stabilization in Power Systems

The type of Dynamic / Nonlinear load can also cause the voltage instability.

While the most common form of voltage instability is the power frequency variation, the progressive drop of bus voltages, and the overvoltage instability.

Overvoltage can be caused by a capacitive behavior of the network as well as by under excitation limiters preventing generators and synchronous compensators from absorbing the excess reactive power. [1]

Large-disturbance voltage stabilization:

• Large-disturbance voltage stabilization refers to the system’s ability to maintain steady voltages following large disturbances such as system faults, loss of generation, or circuit contingencies.

• Determination of large-disturbance voltage stabilization requires the examination of the nonlinear response of the power system over a period of time sufficient to capture the performance and interactions of such devices as motors, underload transformer tap changers, and generator field-current limiters.

The time frame of interest for voltage stabilization problems may vary from a few seconds to tens of minutes.

Therefore, voltage stabilization may be either a short-term or a long-term phenomenon. [2]

Short-term voltage stabilization involves dynamics of fast acting load components such as induction motors, electronically controlled loads. (several seconds)

Long-term voltage stabilization involves slower acting equipment such as tap-changing transformers, generator current limiters. (several mins)

Small-disturbance voltage stabilization:

Small-disturbance voltage stability refers to the system’s ability to maintain steady voltages when subjected to small perturbations such as incremental changes in system load.

This form of stability is influenced by the characteristics of loads, continuous controls, and discrete controls at a given instant of time.

This concept is useful in determining, at any instant, how the system voltages will respond to small system changes.

Principles of UVLS

Amount of Load CurtailmentAppropriate location for load

curtailment Timing to execute load curtailment

event

Meta-Heuristic techniques for UVLS

By applying such techniques we may easily handle large and complicated Power Systems

Less Power failuresHighly Stable Enhanced ReliabilitySelf healingReduce the amount of load shedReduce the possibility of Blackouts

BLACKOUTS• A blackout in a power system refers to the unavailability of

Electric power in a area for a short or long duration. These power blackouts occurs due to natural and Technical reasons.

• Natural reasons i. Animal contact with live conductor.ii. Vehicular accident resulting damaged transmission poles.iii. Tress falling on transmission line due to stormy weather.• Technical Reasonsi. Human errorii. Overloaded transmission lines iii. Faults (L-G,L-L-G L-L-L-G)iv. Stability issuesv. Faulty equipment

Consequences of Blackouts

Duration of power outages occurred in different parts of world.

Limitation of conventional Load Shedding

• Conventional load shedding techniques are limited by theirinability to provide optimum load shedding. They simply follow a preset rule in which a fixed amount of load is shed when frequency /voltage deviates from the nominal value. The main disadvantage of this method is that it does not estimate the actual amount of the power imbalance.

• The result is either over-shedding, which affects power quality, social and economical loss or under-shedding, which leads to tripping of electricity service and over frequency problem.

Flow chart of conventional load shedding techniques.

Computational intelligence Techniques

• Comparison features of Conventional and computational intelligence techniques

No Feature Conventional Technique Computational Technique

1 Optimum load shedding

Do not provide optimum load shedding

Have a ability to provide optimum load shedding

2 Complex Power System

Cannot deal efficiently with modern and complex Power System

Can deal efficiently with modern and complex Power System

Advantages and Drawbacks of Computational intelligence techniques

No Technique Advantages Drawbacks

1 Artificial Neural Networks ANN has a ability to ensure an optimum amount of load shedding

ANN can provide satisfactory reults for known cases only and may fail to predict accurate results for unknown or varying cases

2 Fuzzy Logic Controller (FLC) FLC can be used for load shedding application on Power System of any size

The membership parameters of FLC require prior system knowledge .Otherwise it may fail to provide optimum load shedding.

No Technique Advantages Drawbacks

3 Adaptive neuro-fuzzy interface system (ANFIS

FLC parameters are optimized by using ANN which may lead to accurate load shedding

It can work with Sugeno-type systems

4 Genetic Algorithm (GA)

GA is global optimum technique for solving non-linear ,multi-objective problems GA ensures a minimum amount of load shed

GA takes a long time to determine the load shedding amount .This relative slowness limits their usage for on line application

5 Particle Swarm Optimization

PSO computation is simple and has the ability to find the optimum value

PSO is easily interrupted by partial optimization

Real Power (MW) vs. Voltage (P-V) Curve -- Nose Curve

Power-Voltage (P-V) Curve

Power – Voltage (P-V) Curve

Voltage Stabilization Techniques in Power Systems.

Most sensitive loads which cause voltage instability

i. Induction Motors

ii. Discharge type Lamps

iii. Thermostatic Controlled Loads

iv. Load behind Under Load Tap Changers (ULTC)

The driving force for voltage instability is usually the loads. In response to a disturbance, power consumed by the loads should be restored.

A situation causing voltage instability occurs when load dynamics attempt to restore power consumption beyond the capability of the transmission network and the connected generation.[1]

What cause voltage instability?

Main factors generally responsible for voltage collapse incidents worldwide.

• Transmission system limitation• Load behavior, including on load Tap changer

performance.• Influence of protection and control system.

1. Load shedding scheme should be designed in coordination with protective devices and control schemes for momentary voltage dips, sustained faults, low voltages caused by stalled air conditioners, etc.2. Time delay to initiate load dropping should be in seconds, not in cycles. A typical time delay varies between 3 to 10 seconds.3. UVLS relays must be on PTs that are connected above the automatic LTCs.4. Voltage pick-up points for the tripping signals should be set reasonably higher than the “nose point” of the critical P-V or Q-V curve.5. Voltage pick-up points and the time delays of the local neighboring systems should be checked and coordinated.6. Redundancy and enough intelligence should be built into the scheme to ensure reliable operation and to prevent false tripping.7. Enough loads should be shed to bring voltages to minimum operating voltage levels or higher while Maintaining VAR margins according to WSCC’s Voltage Stability Criteria.

North American Electric Reliability Corporation (NERC) Western Electricity Coordinating Council (WECC)Design criteria for UVLS

Methodology

Literature Review Data collection Modeling of Fuel cell,PV and Wind turbine Simulation Studies Analysis and Evaluation Proposed a Load shedding scheme Thesis writing and Publications MATLAB/Simulink and PSCAD cont…..

Methodology• In order to achieve optimum system capacity, an

optimal under voltage load shedding to provide voltage stability is proposed and solved using hybrid genetic algorithm and particle swarm optimization technique

• Use IEEE 14 bus and 57 bus test system with two evolutionary methods: HGAPSO and PSO. The 14 bus test system is small power system and suitable for primary assessment and 57 bust test system is a relatively medium scale power system and suitable for computational efficiency and optimally of load shedding.

Base Model

•

Conclusion• An optimal under voltage load shedding to provide voltage stability is

proposed and solved using hybrid genetic algorithm and particle swarm optimization technique. The main object of proposed load shedding is improvement transmission line performance in contingency conditions by alleviating transmission line over loadings, maximization of its voltage stability and minimization active power loss.

• Also, the economical object is considered to purpose of minimization of customer interruption cost that conflicted by the other objects. Despite of operational objects, economical object tries to minimize load curtailment in power system. It is shown that the hybrid genetic algorithm and particle swarm optimization can identify a global optimum solution in compare of traditional PSO.

References

[1] C. P. Steinmetz, “Power control and stability of electric generating stations,” AIEE Trans., vol. XXXIX, Part II, pp. 1215–1287, July 1920.[2] AIEE Subcommittee on Interconnections and Stability Factors, “First report of power system stability,” AIEE Trans., pp. 51–80, 1926.[3] P. Kundur, J. Paserba, V. Ajjarapu, G. Andersson, A. Bose, C. Canizares, N. Hatziargyriou, D. Hill, A. Stankovic, C. Taylor, T. Van Cutsem, and V. Vittal, "Definition and Classification of Power System Stability IEEE/CIGRE Joint Task Force on Stability Terms and Definitions," IEEE Transactions on Power Systems, vol. 19, pp. 1387- 1401, 2004[4] El-Sadek MZ. Preventive measures for voltage collapses and voltage failures in the Egyptian power system. Electr Power Syst Res 1998;44:203–11 .[5] Amraee T, Mozafari B, Ranjbar AM. An improved model for optimal under voltage load shedding: particle swarm approach. IEEE Power India Conf 2006:6.[6] J.A. Laghari, H. Mokhlis, A.H.A. Bakar, H. Mohamad., “Application of computational intelligence techniques for load shedding in power systems: A review”, Energy Conversion and Management, 2013, 75, pp. 130-140.[7] C. J. Mozina, Power Plant Protection and Control Strategies for Blackout Avoidance, Georgia Tech Protective Relay Conference, April 2005.

References

[8] North American Electric Reliability Council (NERC), 1987 System Disturbance Report, p19, July 1998.[9] IEEE Power System Relaying Committee Report, Summary of System Protection and Voltage Stability, Transactions on Power Delivery, Vol. 10. No. 2, April 1995.[10] R. Verayiah, A. Mohamed, H.Shareef, I. Z. Abidin, “Review of Under-voltage Load Shedding Schemes in Power System Operation” PRZEGLĄD ELEKTROTECHNICZNY, 2014, 90, pp.99 -103[11] Sadati N, Amraee T, Ranjbar AM, “A global particle swarmbased-simulated annealing optimization technique for undervoltage load shedding problem”. Appl Soft Comput 2009;9:652–7.[12 ] J. Kennedy, R. Eberhart, “Particle Swarm Optimization”, IEEE International Conference on Neural Networks, Piscataway, NJ, pp. 1942-1948, 1995[13] Amraee T, Ranjbar AM, Mozafari B, Sadati N. An enhance under voltage load shedding scheme to provide voltage stability. Electr Power Syst Res 2007;77(8):1038–46.[14] Amraee T, Ranjbar AM, Feuillet R. Adaptive under-voltage load shedding scheme using model predictive control. Int J Electr Power Syst Res 2011;81(7):1507–13.

References

[15] S. A. Pourmousavi and M. H. Nehrir, “Real-time central demand response for primary frequency regulation in microgrids,” IEEE Trans. Smart Grid, vol. 3, no. 4, pp. 1988–1996, Dec. 2012.[16] F. Katiraei, M. R. Iravani, and P. W. Lehn, “Micro-grid autonomous operation during and subsequent to islanding process,” IEEE Trans. Power Del., vol. 20, no. 1, pp. 248–257, Jan. 2005.[17] IEEE Recommended Practice for Industrial and Commercial Power System Analysis, IEEE Standard 399-1997, Aug. 1998.[18] F. Echavarren, E. Lobato, R. Rouco, M. Navarrete, R. Casanova, and G. López, "A load shedding algorithm for improvement of load margin to voltage collapse," in Power Tech Conference Proceedings, 2003 IEEE Bologna, 2003, p. 6 pp. Vol. 1.[19] B. Otomega and T. Van Cutsem, "Undervoltage load shedding using distributed controllers," Power Systems, IEEE Transactions on, vol. 22, pp. 1898-1907, 2007.[20] A. Ketabi and M. H. Fini, “An underfrequency load shedding scheme for islanded microgrids,” Int. J. Elect. Power Energy Syst., vol. 62, pp. 599–607, Nov. 2014.[21] U. Rudez and R. Mihalic, “Monitoring the first frequency derivative to improve adaptive underfrequency load-shedding schemes,” IEEE Trans. Power Syst., vol. 26, no. 2, pp. 839–846, May 2011.

References[22] A. Saffarian and M. Sanaye-Pasand, “Enhancement of power system stability using adaptive combinational load shedding methods,” IEEE Trans. Power Syst., vol. 26, no. 3, pp. 1010–1020, Aug. 2011.[23] Hosseini-Bioki MM, Rashidinejad M, Abdollahi A. An implementation of particle swarm optimization to evaluate optimal under voltage load shedding in competitive electricity markets. J Power Sour 2013;242:122–31.[24] Ahmad Ahmadi ,Yousuf Alinejad-Beromi A new integer-vale modeling of optimal load shedding to prevent voltage stability. Electrical Power and Energy Systems 65(2015)210-219.

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