This slide presents about the basic and importance about load shedding in smart microgrid distribution systems. Later of the class i will discuss about in detail on the process of executing the load shedding.

1. Presented By:
Prof. (Dr.) Pravat Kumar Rout
Sheetal Chandak, Senior Research Fellow, CSIR, India
Department of Electrical and Electronics Engineering
Siksha ‘O’ Anusandhan University

2.  A power outage (also called a power cut, a power
out, a power blackout, power failure or
a blackout) is the loss of the electrical
power network supply to an end user.
 Power outages are categorized into three different
phenomena, relating to the duration and effect of the
outage:
➢ Faults.
➢ Brownouts.
➢ Blackouts.

3.  A Brownouts is an intentional or unintentional drop
in voltage in an electrical power supply system. Intentional
brownouts are used for load reduction in an emergency.
 The reduction lasts for minutes or hours, as opposed to short-
term voltage sag (or dip). The term brownout comes from the
dimming experienced by incandescent lighting when the
voltage sags.
 A voltage reduction may be an effect of disruption of
an electrical grid, or may occasionally be imposed in an effort
to reduce load and prevent a power outage, known as
a blackout.

4.  Blackout occurs when electricity demand exceed the
supply. In that case, an imbalance is said to exist between
generation and consumption.
 These cause load imbalance which can lead to frequency
decay, initial power deficiency, dangerous cascade effects, or
even shutdown of one or more generators.
▪ Blackouts affects utilities, ships,
refineries, mines, industrial
processes and almost every
power system in the world.

5.  Three main factors generally cause collapse incidents
worldwide.
✓First factor is loading limitation of transmission
line.
✓Second factor is load behaviour including on load
tap changer performance.
✓Third factor is the influence of protection and
control systems.

6.  The largest power outage in world history that
occurred in New Delhi in July 2012.
Factors for collapse included:
✓Weak interregional power transmission corridors
caused by multiple outages.
✓High loading on the 400 kV Bina–Gwalior–Agra
link, leading to the mis-operation of its protection
scheme.
✓Balancing excess generation caused by the load
shed.

7. Number of power outage in different parts of world [1]

8.  A selective power cut is a controlled shutdown
(Load Shedding) of the power supply in a given
area, so as to avoid a blackout.
 Various load shedding (LS) strategies adopted to
restore the generated and absorbed power balance.
 Load shedding is a way to distribute demand
for electrical power across multiple power sources.

9.  Load shedding is occurred by the deliberate electric power in
parts of a power distribution network to prevent the failure of
the entire system when demand exceeds supply.
 It is used to relieve stress on a primary energy source when
demand for electricity is greater than the primary power
source can supply.

10.  Stage 1 allows for up to 1000 MW of the
national load to be shed.
 Stage 2 allows for up to 2000 MW of the
national load to be shed.
 Stage 3 allows for up to 3000 MW of the national
load to be shed.
 Stage 4 allows for up to 4000 MW of the
national load to be shed.

11. Three main issues have to be considered for load
shedding:
 The amount of load for shedding,
 The timing to execute load shedding event,
 The appropriate location for load shedding.

12.  Tripping appropriate amount of load is a crucial deciding
factor in restoring the instability of a power system
network.
 Tripping fewer loads than necessary will not effectively
overcome voltage collapse.
 Tripping an excessive amount of load may raise a new
concern, which is the over frequency condition, because the
system will generate load in excess of the load demand.
 Load characteristics serve an important function in
determining the capability of an unstable power system to
regain its stability after the disturbance.

13.  Two types of load models that influence the amount of load
curtailment are static and dynamic loads.
 Static loads represent the active and reactive power consumed
by the load at a particular instant of time as a function of the
bus voltage and frequency at that instant.
 In steady state power analysis, system frequency is considered
constant, such that the power consumed is a function of the
bus voltage alone.
 By contrast, dynamic load models consider the time-varying
nature of the operating characteristics of power system
components, including induction motors, discharge-type
lamps, as well as thermostatically controlled loads, such as air
conditioners and under load tap changers.

14.  The load buses are usually categorized in the order of
the weakest bus to the strongest bus.
 Graphically obtained from the PQ curve, the weakest
bus tends to have the highest dV/dQ component and
tends to be more susceptible to voltage collapse.
 Consequently, the weakest bus is the most appropriate
candidate for load shedding.

15.  The timing of load shedding performance should consider the
amount of load shed and the location.
 Various system components attempt recovery at different time
frames.
 The minimum amount of time allowed before a LSS is activated is
the time taken from the commencement of system parameters
collapse.
 Moreover, the maximum amount of time allowed before a LSS is
triggered is the time taken for all the intervening system
components to attempt system recovery.
 Research undertaken to compute for the timing of load shedding
occurrence involves dynamic simulations for load shedding.

16. Load Shedding Strategies
(LSS)
Conventional
Load Shedding
Strategies
Adaptive Load
Shedding Strategies
Intelligent Load
Shedding Strategies
Under
Frequency
LSS
Under Voltage
LSS
Artificial
Neural
Network LSS
Fuzzy Logic
Control LSS
Adaptive Neuro
Fuzzy Inference
System
Optimisation
Algorithm

17.  The basic guideline in designing a UVLS scheme developed by the
Technical Studies Subcommittee of Under voltage Load Shedding Task
Force of Western Systems Coordinating Council (WSCC) :
i. Load shedding scheme should be designed to coordinate with protective
devices and control schemes for momentary voltage dips, sustained faults,
low voltages caused by stalled air conditioners, and so on.
ii. Time delay to initiate load dropping should be in seconds, not in cycles. A
typical time delay varies between 3 seconds and 10 seconds.
iii. UVLS relays must be on PTs that are connected above automatic LTCs.
iv. Voltage pick-up points for the tripping signal should be set reasonably
higher than the “nose point” of the critical P-V or Q-V curve.

18. v. Voltage pick-up points and the time delays of the local neighbouring
systems should be checked and coordinated.
vi. Redundancy and adequate intelligence should be built into the
scheme to ensure reliable operation and to prevent false tripping.
vii. Sufficient load should be shed to bring voltages to the minimum
operating voltage levels or higher.
 The VAR margin should be maintained according to the Voltage
Stability Criteria of the WSCC.
 UVLS has become the preferred strategy of power utilities because
it is a cost-effective solution to address voltage stability issues.

19.  The European Network of Transmission System Operators for
Electricity (ENTSOE) with a system of 50Hz has recommended
the following steps for under frequency load shedding:
i. The first stage of automatic load shedding should be initiated at 49
Hz.
ii. At 49 Hz, at least 5% of total consumption should be shed.
iii. A stepwise 50% of the nominal load should be disconnected by
using under frequency relays in the frequency range of 49.0–48.0
Hz.
iv. In each step of load shedding, a disconnection of no more than 10%
of the load is advised.
v. The maximum disconnection delay should be 350 ms including
breakers’ operating time.

20.  Recommendation for power plants to attain safe operation:
i. At 49.8 Hz, ‘quick-start’ plants should be connected to the grid.
ii. For a power system operating within 50 Hz (60 Hz) frequency, the
minimum allowable operating frequency usually specified by the
manufacturer according to the turbine type is 47.5 Hz (57.5 Hz) .
 This is necessary because
✓ The protection of the generator and its auxiliary equipment because
power plant auxiliary services begin to malfunction at a frequency
of 47.5 Hz; the situation becomes critical at about 44–46 Hz.
✓ Generator operation at 47.5 Hz or below could damage the turbine
blades and reduce its lifespan .

21.  Disadvantage :
✓ It does not estimate the actual
amount of the power imbalance.
 Thus result in either of the two:
✓ Over-shedding, which affects power
quality.
✓ Under-shedding, which leads to
tripping of electricity service.
Flowchart of the conventional LSS

22.  The strategy employs power swing
equation to shed the required amount of
loads.
 The power imbalance within the system
can be obtained by:
 Whenever system disturbance, there is a
variation in frequency as well as rate of
change of frequency(ROCOF).
 Using the equation, the power imbalance
is estimated, which helps to calculate the
required amount of load to be shed.
t
f
f
H
P


=
2
Flowchart of the adaptive LSS

23.  Advantages:
✓ This approach of load shedding can be applied to an
isolated power system with a single generation unit as well
as to an interconnected power system.
✓ Enhances the reliability of frequency and voltage stability.
✓ Enhanced transient behaviour when encountering severe
disturbance.
 Disadvantages:
✓ The values of ROCOF are different for similar amount of
load variations at base and peak capacity.
✓ Strategy suffers un-optimum estimation of power
imbalance due to the variations in ROCOF.

24.  The term ‘computational intelligence techniques (CIT)’
generally refers to a set of techniques that are applied to mimic
human intelligence.
 These techniques include:
✓ Artificial neural networks (ANN),
✓ Adaptive neuro-fuzzy inference system (ANFIS),
✓ Fuzzy logic control (FLC),
✓ Genetic algorithms (GA), and
✓ Particle swarm optimizations (PSO).
 These techniques can easily solve those nonlinear, multi-
objective problems in power systems that cannot be solved by
the conventional methods with the desired speed and accuracy

25.  ANN is a mathematical model based on human neural systems.
 The ANN training process:
✓ Three inputs – total generation, total load demand, and
frequency decay rate
✓ One output – minimum amount of load shedding.
 Advantage: The proposed ANN technique performed load
shedding more quickly as compared with the conventional
technique.
 Disadvantage: ANN can provide satisfactory results for known
(trained) cases only. ANN fails to predict accurate results for
unknown (untrained) or varying cases.

26.  FLC is a mathematical tool suitable for modelling a system
which is too complex and not well defined by mathematical
formulation.
 Some of these applications include load frequency control,
unified power flow controller (UPFC) application, flexible AC
transmission system (FACTS) application, and reactive
power/voltage control.
 The proposed technique was formulated on frequency (f),
rate of change of frequency (df/dt), and load prioritization.

27.  The ANFIS method is based on the combination of
artificial neural networks and fuzzy logic control.
 It combines the learning abilities of ANN with the fuzzy
interpretation of the FLC system.
 Drawbacks: It can only work with Sugeno-type systems.

28.  Genetic algorithms (GA) are the global optimization
technique for solving non-linear, multi-objective problems.
 The database for load shedding problems is obtained from a
power flow study and is successfully implemented on the
power system.
 The results indicate that the GA-based technique is feasible
and effective in providing optimal load shedding.
 Drawbacks:
✓ It has been observed that the computation time of GAs to
determine the amount of load shed is very large.
✓ This slow response limits their usage for online application.

29.  PSO has been proved as a robust and fast technique in solving
non-linear, multi-objective problems.
 PSO finds the global optimum solution more quickly as
compared to genetic algorithms.
 PSO ability of taking only minimal time may encourage its
implementation for real-time optimal load shedding in power
systems.
 Disadvantage: PSO is easily interrupted by partial
optimization.

30.  The GA and PSO techniques were used to solve generator
outage and line outage cases, and were validated on the
IEEE 30-bus system.
 The responses of GA and PSO in all the case studies,
results to show that: in terms of computation time ,PSO is
faster than GA; the minimum amount of load is shed by
GA

31.  Recent power blackouts that have occurred around the world make
the reliability of conventional UFLS techniques questionable.
 Conventional UFLS techniques are not suitable for today’s large
and complex power systems.
 Computational intelligence techniques have the ability to deal with
such modern power systems efficiently.
 Computational intelligence techniques in load shedding can reduce
the possibility of blackouts, and enhance the power system’s
reliability.
 Improvements are still needed to make these techniques compatible
with real-time applications.

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