Energy and Power Engineering, 2013, 5, 437-445 Published Online September 2013 (
Load Shedding Application within a Microgrid to Assure
Its Dynamic Performance during Its Transition to the
Islanded Mode of Operation
Dorel Soares Ramos1, Tesoro Elena Del Carpio-Huayllas1*, Ricardo Leon Vasquez-Arnez2
1Department of Electric Power and Automation Engineering, University of São Paulo, São Paulo, Brazil
2FDTE (Foundation for the Technological Development of the Engineering Sciences), São Paulo, Brazil
Email:, *,
Received July 14, 2013; revised August 14, 2013; accepted August 21, 2013
Copyright © 2013 Dorel Soares Ramos et al. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
This article presents the simulation results and analysis related to the response of the generators within a microgrid to-
wards an accidental overload condition that will require some load shedding action. A microgrid overload can occur due
to various reasons ranging from poor load schedule, inadequate switching of circuits within the microgrid, outage of one
or more generators inside the microgrid, illegal load connections by some low voltage consumers, etc. It was observed
that among the main factors that determine the survival of the microgrid during its transition from the grid connected
mode to the islanded mode of operation are the size and type of the load connected (passive or dynamic load) as well as
the length of time during which the unexpected load is connected. Models of a speed and voltage regulators of a diesel
generator, and important for coping with the overload conditions are provided in the paper. The novelty of the work lies
in the load shedding simulation and analysis of the specific generators studied herein, regarding that in many countries
the microgrid technology is seen as an important alternative towards the ever increasing load demand and also to assist
the system during periods of blackout.
Keywords: Distributed Generation; Islanded Operation Mode; Load Shedding; Microgrid Systems
1. Introduction
At present, the establishment of microgrid systems is
regarded by the power industry as one of the alternatives
to not only keep running critical loads but also provide
electricity to some regular consumers during periods of
prolonged interruptions (e.g. blackouts). Microgrid sys-
tems are the compelled choice in remote areas where it is
difficult to provide power through the power system. The
microgrid concept is not new; actually, in the early days
of the power industry this was the kind of system estab-
lished in the urban and industrial areas. The interconnec-
tion of systems to strengthen and form the network came
years later. This was done to offer the system a high level
of safety regarding possible faults; thus, cope with stabil-
ity issues aside of allowing the surplus generation capac-
ity in one area to be used elsewhere in the system.
Ironically, this kind of system networking may also
lead to some major interruptions (due to cascading effect)
like those that affected central, south and southeastern
Brazil and all Paraguay in 2009, the northeastern part of
the USA in 2003, and all Italy in 2003. Also, nearly 100
million people in Indonesia were affected by a huge
blackout in 2005. The last major blackout occurred in
India in 2012 leaving virtually inoperative in the North-
ern, Eastern and Northeast part of the country. In the
aftermath of these undesired events nearly all the af-
fected systems set up independently a common action, to
seek for some alternatives capable to diminish their im-
pact among which the establishment of microgrid sys-
tems was also pointed out.
One of the key features of a microgrid is its ability to
separate itself from the distribution utility (e.g. during
unscheduled periods of interruption) in order to continue
feeding its own islanded portion. This is not a simple task
though, especially when taking into account the com-
pelled operational procedures and protocols to be fol-
Another outstanding characteristic of a microgrid is
that, provided an agreement with the grid, it can supply
its surplus generation to the utility, for example, during
*Corresponding author.
opyright © 2013 SciRes. EPE
peak periods of demand or whenever the microgrid has
excess capacity.
Microgrid systems, when properly designed, have the
ability to separate from the distribution utility, for exam-
ple, during disturbance or blackout periods experienced
by the utility. This gives them the possibility to continue
feeding their own islanded portion. Additionally, pro-
vided an agreement with the distribution utility, they can
supply their surplus generation whenever they have ex-
cess capacity. This is particularly attractive for periods
when the utility faces peak periods of demand. Nonethe-
less, it can occur that during the transition from the grid
connected mode to the islanded mode of operation, an
excessive load (larger than the microgrid can uphold)
could be connected to the microgrid.
This overloading condition can occur due to various
reasons, namely: poor load schedule, inadequate switch-
ing of circuits within the microgrid, an upstream tripping
of the utility circuit breaker that leaves part of its load
connected to the microgrid, illegal load connections by
some low voltage consumers, etc. Under this condition,
the most common way to save the microgrid from a com-
plete collapse is to shed part of the load connected. This
action can help the fading generator to become stable
again ensuring its safe operation.
Some critics say that the application of load shedding
at specific times during the 2003 major blackout in North
America could have prevented the cascading outages that
came after the initial tripping event.
Several interesting references addressing the load
shedding issue in large systems were found. There may
also be some other references having the same merit;
however, due to space restrictions of this article it will
not be possible to include them all.
Reference [1], for example, provides a comprehensive
coverage on the load shedding issue, load restoration and
generator protection schemes using underfrequency re-
lays during abnormal frequency conditions. In [2], an
underfrequency protection program developed for a cer-
tain region in North America is presented. The program
reportedly optimizes the system wide load shed schedule,
also checking up its coordination with a steam tur-
bine-generator underfrequency protection scheme. In [3],
a method for determining the maximum probable rates at
which a power system frequency will decay, following a
disturbance, is presented. Such an analysis is chiefly di-
rected to provide underfrequency protection for nuclear
power plants. Reference [4], presents a summary on Sys-
tem Protection and Voltage Stability prepared by the
IEEE Power System Relaying Committee. It describes
the risk and mechanism of voltage collapse as well as
suggests some operation, system upgrades and protection
solutions to avoid such a condition.
In [5] the effect of the reduced frequency upon the ca-
pacity of a power plant, with special regard to cases with
deficiency of generation, is presented. It is stated that on
systems with a high percentage of motor load (such as
pumping), a combination of frequency and voltage re-
duction may secure maximum load relief during an
emergency. In [6-9], some methods dealing with under-
frequency load shedding, so as to avoid voltage instabil-
ity and its further collapse, are presented.
In [10] an optimal load shedding algorithm based on
an economic criterion is developed. In [11], a strategy to
shed an optimal number of loads in an islanded distribu-
tion system, using factors like the rate of change of fre-
quency (RoCoF) and the customers’ willingness to pay
during periods of outage to stabilize its frequency, is
Finally, in [12] a scheme that combines frequency and
voltage changes to shed loads is proposed. The premise
behind this scheme is that in the last years, power sys-
tems have changed and yet no corresponding modifica-
tion of the underfrequency load shedding schemes was
made; thus, the load shedding procedure would still be
based on the disconnection of pre-selected loads.
As can be seen, most of the above reviewed load
shedding methods and strategies are directed to large
systems, hence the need to develop a study on the exces-
sive overloading effects on microgrid generators. The
article presents the main control components of a gen set
and the various situations the microgrid generators can
face during the transition from the grid connected to the
islanded mode of operation.
The microgrid current status and some of the chal-
lenges presently encountered by the microgrid technol-
ogy, are presented in [13]. In [14,15] a microgrid island-
ing condition following a fault and its respective stability
behaviour are investigated. The load shedding alternative
is applied once the system frequency starts to decay from
its nominal operative value (50 or 60 Hz).
According to [16,17], the two load shedding methods
widely used are:
1) Traditional frequency drops with load percentage
shed. Typically, the load shedding scheme can be done in
3 (and up to 6) steps [16]. For example, in a three-step
method, the percentage of load shed would be:
Step f (Hz) (%) of load
1 59.3 10
2 58.9 15
3 58.5
as required to avoid going
below 58.2 Hz
2) Use of the rate of change of frequency (RoCoF) and
load percentage sheds. It evaluates the speed at which the
frequency (df/dt) is declining. This enables characterizing
the kind of contingency occurring in the system at vari-
Copyright © 2013 SciRes. EPE
Copyright © 2013 SciRes. EPE
nance, energy costs, etc. During such a transition the
power control of the microgrid generators must act
quickly as they start controlling the frequency of the
islanded section.
ous instants, thus, provides the system a most adequate
load shedding scheme [17]. For example, regarding the
above frequency drop of 59.3 Hz, the df/dt could be set
Whenever power in the network is lost, the microgrid
generators assigned to provide power to the intentionally
islanded portion should be able to pick up and feed the
load of the islanded system after the switch at the Point
of Common Coupling (PCC) has opened. The generation
sources herein referred can be any of the sources cited by
the IEEE std. 1547 [18]: PV arrays, wind turbines, fuel
cells, microturbines, conventional diesel, gas-fired tur-
bines, and energy storage technologies.
59.3 Hz ….. df/dt = 0.4 Hz/s …. 10% of total load.
59.3 Hz ….. df/dt = 1.0 Hz/s …. 25% of total load.
59.3 Hz ….. df/dt = 2.0 Hz/s …. 35% of total load.
The above under-frequency control methods are com-
monly used by many distribution utilities. In the context
of this paper, the entire load exceeding the normal power
demand of the microgrid internal generation will be shed.
This is done considering the simulation response of the
generators in returning to the pre-overload condition.
Also, the inherent differences existing between the grid
and a microgrid should be considered. In the latter case,
for example, the inertia of the generation sources is much
The microgrid system to be analyzed is connected to
the utility through a circuit-breaker (CB), in series with a
6 MVA, 13.8/2.4 kV transformer, as depicted in Figure 1.
The energy sources in the considered microgrid are: a
synchronous generator driven by a diesel engine (con-
nected to PCC through CB-1); a wind turbine driving a
synchronous generator (connected to PCC through CB-3)
and another small synchronous machine which could
represent a small hydro-generator (SHG) connected to
the PCC through CB-2. No PV array sources (solar pan-
els) or sources requiring energy storage elements were
considered because the primary focus of this research is
to analyze the dynamic behaviour of the considered
sources. Also, the machine equations are not presented
here as they can be found in the available literature deal-
ing with electric machines.
Generally, gen-sets like the one considered here have
no overloading capability. Wind generators and small
hydro generators may admit little overload (up to 10%).
Nevertheless, a brief analysis on what would be the per-
centage of load to be shed, if the above methods were
also used, will be included whenever appropriate.
2. Microgrid Load Shedding Application
The transition from a grid-connected to an islanded mode
of operation of a microgrid can occur due to reasons like
the presence of faults in the system, or due to some
pre-planned conditions like the system (grid) maint-
13.8/2.4 kV
Syn. Gen.
2.0 MVA
Load 1
2.0 - j0.6
Load 1A
Asyn. Motor
1367 HP
Load 3
Async. Motor
0.223 MVA
Syn. Gen.
0.29 MVA
2.4/0.38 kV
300 kVA
X = 8%
13.8 kV
Load 2
0.75 - j0.3
PCC2.4 kV
engine Wind
turbin e
Load 2A
Asyn. Motor
1.0 MVA
Syn. Gen.
1.0 MVA
CB-1 CB-2 CB-3
CB-5 CB-6
Load 3A
Asyn. Motor
0.3 MVA
Figure 1. Microgrid system used in the simulations.
The wind generator is connected to the microgrid
through a 2.4/0.38 kV transformer. In practice, due to
technical and economic reasons, most of the sources used
in wind power schemes are asynchronous generators.
One of the main reasons has to do with its ability to op-
erate at speeds different from the synchronous speed.
However, in this study, a synchronous generator was
chosen due to its less dependency from an external
source for providing reactive power. A Doubly Fed In-
duction Generator (DFIG) was also placed instead of the
wind (synchronous) generator; its response was compa-
rable to the synchronous generator used herein.
The loads fed by each generator, as well as the extra
loads (equivalent asynchronous motors) that simulate the
overloading condition (Load 1A, Load 2A and Load 3A)
are also shown in Figure 1. These extra loads are con-
nected to their respective generator through CB-4, CB-5
and CB-6. Load 1 and Load 2 were specified as constant
power loads. The normal load connected to the wind
generator (Load 3) is a dynamic load (equivalent asyn-
chronous motor). The complete system was implemented
in the EMTDC/PSCAD® program [19]. Some compo-
nents, like the electrical generators and circuit breakers
were taken from the software library, while others, like
the diesel generator speed and voltage regulators, etc.,
were independently built and defined.
The sequence of the load shedding procedure is as
follows: initially, all loads are primarily being fed by the
utility when, due to any reason (e.g. a three-phase fault),
the circuit-breaker (CB) is open. At this moment all three
generators start running and taking up their respective
loads. It is assumed that along with CB the other circuit
breakers (i.e. CB-1, CB-2 and CB-3) also trigger with the
fault. This way, it will be avoided the condition of any of
the generators from being carried away by another gen-
erator. This condition is critical in microgrids containing
several sources; otherwise, the whole microgrid system
can be jeopardized by the loss of synchronism among
them. Synchronization of fully-loaded generators is
commonly unachievable, hence the need to open CB-1,
CB-2 and CB-3. But that is an issue that for now is put
2.1. Diesel Generator Overloading
Among the control systems implemented on this genera-
tor are: a basic speed regulator, a constant mechanical
power regulator (Figure 2(a)) and a voltage regulator
(Figure 2(b)). As it will be shown later, a reasonably
robust voltage and speed regulator may be useful in
helping the machine cope with faults on the system. Most
of the synchronous generator parameters like the direct
and quadrature reactances as well as the transient and
subtransient time constants (Xq, T′′do, X′′q, T′′qo, etc.) were
estimated according to [20]. The machine starts as an
Figure 2. (a) Synchronous (diesel) generator model; (b)
Voltage regulator.
ideal source (t = 0.0 s) until it reaches its steady-state
condition. At t = 0.5 s the model enables the machine to
pass from an ideal source to a non ideal machine, simul-
taneously the voltage regulator control is inserted in to
the generator. No cylinder misfiring condition was simu-
At t = 2.0 s, once the initialization transient reaches a
stable condition, the dynamic model of the machine is
enabled. From this moment on, the machine electrome-
chanical equations start driving the generator, enabling
the variation of the speed and mechanical torque. Initially,
the diesel gen-set is feeding a linear load (L1 = 2.0 + j0.6
MVA) when at t = 3.0 s an equivalent induction motor
representing 50% of excess load (L1A = 1367 Hp) is con-
nected. The extra load (Load 1A) is switched off after
500 ms. It can be seen that at first the generator intends
to take up this extra load (see P_Dsl in Figure 3(a) and
also the stator current IL1 in Figure 3(b)) though failing
in its attempt. The instant the load shed occurs (opening
of CB-4 at t = 3.5 s) relieves the machine which quickly
returns to its previous loading condition. The P-I (pro-
portional-integral) constants define how quickly the ma-
chine will return to the pre-overloading condition.
The terminal voltage (V_Dsl_rms) shown in Figure 4(a)
Copyright © 2013 SciRes. EPE
D. S. RAMOS ET AL. 441
drops instantly, upon which, following a few oscillations
the voltage regulator carries it back to its previous opera-
tive value (1.03 pu). The gen-set frequency (F_Dsl) shown
in Figure 4(b) reaches a minimum value of 58.6 Hz. If
the conventional load shedding strategies were to be ap-
plied (see Section 1), the amount of load to be removed
would be the highest specified.
For example, according to [16], the percentage of load
to be shed, regarding the minimum value of 58.6 Hz,
2.5 2.75 33.253.5 3.7544.25 4.5
Ds l
2.5 2.7533.253.5 3.7544.25 4.5
( s)
( kA )
Figure 3. (a) Diesel generator power; (b) Current at the
1.5 22.5 33.5 44.555.5
( s )
V Ds l rms (kV)
1.5 22.5 33.5 44.5 55.5
( s )
Ds l
Figure 4. (a) Diesel generator terminal voltage; (b) Fre-
would be above 15% (i.e. as required to avoid falling
further the frequency). Now, according to [17], the per-
centage of load to be shed for this same case would be
35%. This is because the approximate RoCoF observed
in Figure 3(b) is about 12 Hz/s. Nonetheless, shedding
the entire extra load helped the frequency to get restored
in about 1.0 s. From the tests conducted, a maximum of
0.8% overload could only be upheld by the gen-set. The
time taken by the frequency to get restored was about 1.5
s. The main parameters of this and the other machines
used are presented in the Appendix section.
2.2. Wind Generator Load Shed
The wind generator shown in Figure 5 uses a simple PSS
(Power System Stabilizer). The wind turbine model is
available in the library of the program used [19]. The
main link between the generator and the wind turbine is
the mechanical torque (Tm) which is set to operate at a
constant value. This is because the wind speed and the
pitch angle of the wind turbine are also set to be constant
at WSP = 8 m/s and 11.5˚, respectively.
At t = 5.0 s both loads (L3 = 223.6 kVA and L3A = 300
kVA) are simultaneously connected (Figure 6(a)). At
this instant, the generator frequency (f_wnd) drops to about
58 Hz (Figure 6(b)). Notice that the overloading condi-
tion (set at t = 5.0 s) and its respective load shedding (t =
6.0 s) result in fast frequency oscillations which are ef-
fectively coped by the wind generator speed governor.
The rms voltage measured at the busbar where the wind
generator is connected to, showed a fair dip during the
overload and a slight overvoltage after the disconnection
of Load 3A.
Again, if the frequency drop and RoCoF methods were
applied, the percentage of load to be shed would be that
Figure 5. Synchronous wind generator model used.
Copyright © 2013 SciRes. EPE
exceeding the generator capacity. During this overload-
ing period, the speed regulator damps also these oscilla-
tions. Notice that the machine power (PL3), and the load
current (IL3), start dropping steadily (Figure 6(a)). At t =
6.0 s the equivalent extra load (Load 3A) is shed causing
the frequency to rise up to about 60.75 Hz, though, be-
coming damped and stable in approximately 0.5 s. The
electric torque (Te) has an opposite behaviour compared
to that of the frequency (Figure 6(c)). Due to the condi-
tions specified in the model, the mechanical torque (Tm)
remains constant at all times.
Wind turbines, especially small ones, are usually pre-
vented from operating under overload conditions. To
avoid this condition they are equipped with rigorous pro-
tection schemes like the stall and pitch angle control and
other protection systems.
2.3. Small Hydro-Generator (SHG) Load Shed
The machine dynamics previously described in Section
2.1 (i.e. the diesel generator model) also apply here. The
passive load and the unexpected load (equivalent motor),
is shown in Figure 7. In this case, two consequences of
the load shed procedure were considered.
2.3.1. Successful Disconnection of the Extra Load
It can be observed (Figure 8(a)) that at t = 2.5 s the gen-
erator is running at its nominal frequency (F_hydr) with an
44.5 55.5 66.5 7
(kA), P
44.5 55.5 66.5 7
44.5 55.5 66.5 7
( s )
T e, Tm
_____ Te
_____ Tm
Figure 6. Wind generator: (a) Total load connected; (b)
Frequency; (c) Electric and mechanical torques.
initial load (L4 =0.75 + j0.3 MVA). Then, at t = 3.0 s, the
extra load (i.e. L4A = 1.0 MVA) is connected. The output
power in the generator (PL4) rises up immediately,
though, failing to take up this load until at t = 3.5 s the
extra load is shed. The effect of the voltage regulator
(V_rms) in restoring the terminal voltage can be seen in
Figure 8(b).
Notice also how after the disconnection of the extra
load the machine frequency (F_hydr) returns to its normal
value not before facing some low frequency oscillations
Figure 7. Small hydro-generator model used.
2.5 2.7533.253.5 3.7544.25 4.5
( s )
2.5 2.7533.253.5 3.7544.25 4.5
( s )
Figure 8. (a) Output power of the SHG; (b) Terminal volt-
Copyright © 2013 SciRes. EPE
D. S. RAMOS ET AL. 443
(Figure 9(a)). The electrical and mechanical torque os-
cillation (T_elt and Tmech) and their subsequent damping
are shown in Figure 9(b).
2.3.2. Failure to Disconnect the Excessive (Extra)
Failure in disconnecting the extra load will inevitably
lead to a steady collapse of the SHG generator. In this
case, the switch CB-5 that connects the equivalent extra
asynchronous motor is not opened. Similarly to the pre-
vious case, the generator initially intends to take up the
extra load (Pout in Figure 10(a)); however, due to its lim-
ited capacity it quickly drops to zero. Notice that despite
the machine has a speed regulator, in situations like this,
there cannot be any speed regulator (or power/frequency
control) able to cope with such a condition.
A similar falling pattern after the failure to disconnect
the extra load can be observed in the case of the SHG
terminal voltage (V_rms in Figure 10(b)). Note that noth-
ing was mentioned about the overcurrent protection sys-
tem which would trip before the current reaches a certain
specified threshold.
The steady drop of the SHG frequency (Figure 11(a))
towards the excess load is more evident. Finally, both
electrical and mechanical torques (T_elt and Tmech in Fig-
ure 11(b)) start up a continuous rise in an effort to fulfill
the unexpected load demand.
3. Conclusion
The load shedding simulation results of some specific
generators within a microgrid are presented in this article.
A quick load shed applied to such a microgrid systems,
upon which large unforeseen loads are connected, is vital
01020 3040 50
010 20 30 40 50
( s )
T elt, T
mec (pu)
____ T mec
_____ T elt
Figure 9. (a) Frequency and (b) electric and mechanical
torques of the SHG.
2.4 2.6 2.833.2 3.4 3.6
( s )
2.42.6 2.8 33.23.4 3.6
( s )
Figure 10. (a) Output power and (b) terminal voltage of the
05 10 15 20
( s )
05 10 15 20
( s )
T elt , T me c (pu)
_____ T mec
_____ T elt
Figure 11. (a) Frequency and (b) electric and mechanical
torques of the SHG.
for keeping the generators running normally. Provided
their respective voltage and speed regulators all three
generators are considered resuming their operation and
frequency stabilization after sheeding the extra load. On
this regard, the article presents the main control compo-
nents of a gen-set and the various situations which the
microgrid generators can face during the transition from
the grid connected to the islanded mode of operation.
The size and length of time taken to shed the extra load
Copyright © 2013 SciRes. EPE
Copyright © 2013 SciRes. EPE
connected are important factors that can lead to the col-
lapse of the generator and the microgrid itself. Particu-
larly, the size of the extra load accidentally connected
will determine the frequency drop and the extent thus the
speed regulator will respond.
4. Acknowledgements
The authors would like to acknowledge the help of M. T.
Bassini for his aid with some of the machine control
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D. S. RAMOS ET AL. 445
The following are the parameters and constants used in
the simulations:
Z1 = (0.397 + j0.089) Ω/km, L1 = 1.0 km
Z2 = Z3 = (0.460 + j0.104) Ω/km, L2 = L3 = 0.8 km
Z4 = Z5 = (0.55 + j0.130) Ω/km, L4 = L5 = 2.0 km
Diesel generator:
Rotor inertia constant (H) : 0.55 s
Proportional (P)* : 5.0
Integral (I)* : 40.0 s
Gain (G) : 1.0
Time constant (T) : 0.005 s
Lead time constant (T1) : 1.0 s
Lag time constant (T2) : 1.0 s
*Offer the gen set a stabilizing and damping torque char-
Wind generator:
Rotor radius : 20 m
Air density : 1.229 Kg/m3
Gear box efficiency : 0.97
Angular mech. Speed : 16.667 Hz
Wind power coefficient (Cp) : 0.28
Small Hydro Generator:
Rotor inertia constant : 3.5 s
Mechanical friction & windage : 0.0 pu
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