Energy and Power Engineering, 2013, 5, 198-201
doi:10.4236/epe.2013.54B038 Published Online July 2013 (http://www.scirp.org/journal/epe)
Islanding Operation of Electrical Systems in Buildings
Control Methods for Voltage and Frequency Regulation and Re-synchronisation
David Johnston
School of Engineering, Northumbria University, Newcastle, United Kingdom
Received February, 2013
ABSTRACT
A control system was developed, which allows on-site generators within a building to operate in islanding mode, in the
event of loss of grid supply. The on-site generation included a photovoltaic array and a back-up induction generator.
Regulation include avoidance of sudden voltage and frequency changes at the transition to islanding mode, maintaining
voltage and frequency within limits during islanding, and phase matching when the grid supply is re-established. Possi-
ble modes of operation during islanding were photovoltaic array alone, generator alone, or both sources operating in
parallel. Control methods were considered for each of these, and the resulting voltage and frequency regulation was anal
sized. The results show that voltage and frequency could be kept within limits, except during the transitions to and from
islanding. At these times, the transients were minimised.
Keywords: On-site; Generation; Islanding; Photovltaic; Regulation; Re-synchronisation
1. Introduction
A number of electrical power sources are available on a
scale (kW range) that makes them suitable for on-site
generation in buildings. This includes renewable energy
sources, such as solar photovoltaic (PV) panels, which
are particularly suited to building integration, and small
scale wind turbines. In addition, combined heat and power
(CHP) systems may be installed in larger buildings. The
presence of on-site generation provides the potential for
continued electricity supply, in the event of loss of grid
voltage. This would be particularly advantageous in mul-
ti-storey buildings, where uninterrupted operation of lifts
and stairwell lighting would contribute to safety.
In order to allow islanding operation, a number of re-
quirements must be met in terms of electrical safety and
quality of supply [1].
1) A smooth transition from grid-connected to island-
ing mode, without sudden changes in voltage or fre-
quency.
2) Regulation of voltage and frequency during island-
ing operation [2].
3) Re-synchronisation (including phase matching) with
the grid supply, prior to reconnection, once the grid vol-
tage has been restored [3].
2. Electrical Systems and Regulation
A multi-storey building is modelled in this work. The
primary on-site generator is a PV array. A fuel driven
generator supplies back-up power during islanding op-
eration. (In the practical work, a D.C.-D.C. power supply
was used to emulate the PV array, and a D.C. motor with
a suitable control system was used to emulate the engine
driving the generator.) This supplys the PV array during
the day, and is the sole supply at night. Loads include
heating and/or air conditioning, lighting and one or more
lifts. During the day, if islanding occurs, it may be possi-
ble for the PV array and the generator to supply most or
all of the loads. However, at night the generator alone
would probably be able to supply only essential loads –
lift(s) and stairwell lighting.
Generators in the kW range are usually induction gen-
erators, as the less stringent speed control requirement
allows for a simpler control system. This type of genera-
tor is considered here. As one of the major loads is the
lift motor – which is an induction motor – capacitors will
be required to supply the reactive power necessary to
maintain voltage. If these are in the form of switched
parallel banks, they can be used for regulation during
islanding mode.
The lift motor is a large occasional load. It is supplied
from a battery bank via a variable voltage variable fre-
quency (VVVF) inverter. The battery in turn is recharged
from the PV array and/or generator running at the aver-
age load of the system. This allows a smaller generator to
be used, and the battery can respond to the sudden start-
ing and stopping of the lift more rapidly than the genera-
tor or the maximum power point tracker (MPPT) of the

D. JOHNSTON 199
PV inverter, resulting in better regulation.
Many of the larger loads are heating and/or air condi-
tioning systems. Due to the large heat capacity of the
building components – walls, floors, etc – and water
tanks, these loads can be operated for limited periods, if
necessary, independently of their normal thermostatic
controls, without adverse changes in temperature. This
feature allows such systems to be used as switchable
loads, for regulation of voltage and/or frequency during
islanding operation [4]. Other loads, such as room light-
ing and electronic appliances are controlled according to
user preferences. A simplified schematic circuit of the
power sources and loads is shown in Figure 1.
Depending on the available solar energy and the load-
ing conditions, three modes of operation are possible
during islanding.
 The output of the PV array is sufficient to supply
the essential services, the user-controlled loads and
some of the switchable loads. These switchable
loads are then used as the primary means of voltage
regulation.
 The output of the PV array is not sufficient to sup-
ply the essential services and the user-controlled
loads. The generator is then used to make up the
deficit. The associated capacitors provide an addi-
tional mechanism for regulation of voltage and/or
frequency.
 The generator is the only electrical power source
(islanding at night). The loads are reduced to the
essential services and possibly some of the higher
priority user-controlled loads. The capacitors again
provide an additional mechanism for regulation, in
this case in conjunction with the mechanical power
supplied to the generator.
The switchable loads can also be used to limit rapid
voltage and frequency deviations during the transition to
islanding mode. (As the generator is for back-up, it does
not switch on until shortly after this time, so reactive
power compensation – and hence capacitors – are not
present at the transition.)
During re-synchronisation, all currently operating ge-
nerators must be brought into phase with the grid voltage,
maintaining synchronisation with each other as
Photovoltaic
Array
Generator
Lift
Swit cha ble
Loads
User
Controlled
Loads
Grid
Connector
Grid
Suppl y
Figure 1. Simplified schematic circuit of on-site generation
and loads, together with grid connection, for a typical multi-
storey building.
they do so. Common practice for PV inverters is to use a
phase-locked loop (PLL) to synchronise its output with
the grid, after which the grid connection is closed. For
induction generators,the grid connection is re-established
first (generally via a soft start unit) and the generator
then automatically re-synchronises. In a system where
either or both of these generators may be operating, it is
easier to implement a universal control method if both
types re-synchronise in the same order. The capacitor
banks connected to the generator allow the frequency to
be controlled. A phase detector, using the grid voltage as
its reference, can be used with the generator/capacitors to
form a PLL. This can re-synchronise with the grid, after
which the grid connection is close – in the same order as
for the PV inverter.
3. Results – Regulation of Voltage and
Frequency
The results for voltage regulation for a PV array only are
shown in Figure 2. For a relatively large building, with a
high power array and a large number of switchable loads,
switching of a single load results in a small change in
voltage. Thus, the voltage can be kept within limits.
However, for a smaller building, with a lower powered
array and fewer switchable loads, switching a single load
results in a larger change in voltage. This may overshoot
or undershoot the voltage limits, in an unstable oscilla-
tion. The problem was corrected by implementing the
rectifier – connecting the battery to the building’s A.C.
cabling – as an inverter with bi-directional power flow,
allowing the battery to act as a load or source of electri-
cal power with a continuously variable input/output. The
resulting voltage is approximately constant and remains
within the limits.
200
210
220
230
240
250
260
0 102030405
Time (s)
Voltage (V)
0
Large bui ldi ng, many loadsSmall building, few loads
F ew loads + battery
Figure 2. Voltage regulation by switchable loads with PV
array as electrical source. Initial rise in voltage is due to
controlled increase in PV output power.

D. JOHNSTON
200
The capacitor banks (one on each phase) and the pow-
er applied to the generator were used to control the volt-
ages and frequency, in a two-input – two-output control
system, for maximum flexibility. Each capacitor bank
had capacitors of 1 μF, 2 μF, 4 μF, 8 μF and 16 μF, al-
lowing the total capacitance to be varied in 1 μF steps up
to a maximum of 31 μF. This allowed fine control of
both voltage and frequency, and was thus able to keep
both of these close to their nominal values.
It was observed that switching of the capacitors al-
lowed a more rapid response than changes in the genera-
tor power alone, resulting in better regulation. In addition,
the capacitor banks on each phase allowed individual
control of each phase voltage to compensate for unbal-
anced loading.
Large and rapid changes in voltage and frequency oc-
curred during the transition to islanding operation, de-
pending on the loading at the time relative to the level of
on-site generation. Where the load was larger than the
generation, as shown in Figure 3, there was a drop in
voltage, and to a lesser extent in frequency. By drawing
power from the battery, via the inverter, the deficit could
be largely compensated for, thus reducing both the depth
and duration of the voltage and frequency deviations.
This is also shown in Figure 3. As the graph shows, the
voltage deviation and frequency deviation both exceed
the limits (E.U. limits -6/+10 %) for approx. one second.
Figure 4 shows the effects of re-synchronising the
generator prior to reconnection to the grid. In Figure
4(a), re-synchronisation is not applied, and there is in
general a phase mismatch at the moment of reconnection.
This results in a large transient in both voltage and cur-
rent, until the torque on the generator brings it into phase
with the grid voltage. As can be seen in the graph, the
peak of the transient voltage can be over 1000 V, leading
to damage of the generator, cabling and other equipment.
Figure 4(b) shows the results when the capacitor banks
are used to control the phase and achieve re-synchroni-
sation. The transient is much lower – typically 50% lar-
ger than the nominal voltage. The will generally not be
sufficient to damage the generator or cabling, although
sensitive equipment, such as electronic appliances might
be affected.
When both the PV array and generator were operating
during re-synchronisation and reconnection, it was found
that they maintained better phase matching with each
other if the PV inverter tracked the phase of the generator,
rather than vice versa. The moment of inertia of the gen-
erator resulted in a slower time response than for the PV
inverter, making it difficult to maintain phase matching.
Conversely, the rapid response of the PV inverter al-
lowed it to easily track changes in the phase of the gen-
erator output.
150
170
190
210
230
250
012345678910
Time (s)
Voltage (V)
W ithout battery p owerWith batte ry powe r
(a)
47.5
48
48.5
49
49.5
50
50.5
012345678910
Tim e (s)
Freque ncy (Hz)
W ithou t battery powerWith battery power
(b)
Figure 3. (a) Voltage deviation during transition to island-
ing operation showing effect of battery power; (b) Fre-
quency deviation.
4. Conclusions and Further Work
By using the control systems and methods described, the
voltage and frequency could be kept within limits
throughout most of the duration of the islanding period.
However, transients remained at the start of islanding and
at reconnection to the grid, although the measures adopted
reduced the magnitude of these transients. Further work
may reduce them to the point where they no longer ex-
ceed operating limits.
The control methods described provided regulation in
each of the operating modes – PV array only, generator
only, and both systems operating. For a control system to
be fully universal, it would need to be able to detect
which power sources are operating, and adopt the appro-
priate control method. This could be implemented via a
micro-controller for maximum flexibility and will be one
of the main topics for future work.

D. JOHNSTON
201
(
-1500
-1000
-500
0
500
1000
1500
-1 -0.500.51
Time (s)
Voltage (V)
a)
bution network control system [7]. Future work could
include integrating these islanding operation methods
with smart grid technology.
5. Acknowledgements
The author would like to thank Prof. Ghanim Putrus, Dr.
Edward Bentley, Dr. Mahinsasa Narayana and Dr. Jaya-
raman Ramachandra for their collaboration on previous
projects on islanding operation of PV systems and induc-
tion generators, which have contributed to this work.
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Figure 4. Voltage transient at re-connection. (a) Without
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In the work described, islanding was initiated by a
controlled disconnection at the grid input. In a real situa-
tion, islanding would be due to a fault elsewhere on the
distribution network, resulting in a loss of grid voltage.
The building’s electrical control system would need to
detect this condition, and switch to islanding operation [5,
6]. This could be achieve by monitoring the grid condi-
tion – voltage, frequency, impedance, etc – or by an ac-
tive signal, indicating occurrence of the fault. In the latter
case, this would involve communication between the
building’s electrical control system and the local distri-
[7] A. Moeini, A. Darabi, S. M. R. Rafiei and M.
Karimi, “Intelligent Islanding Detection of a Syn-
chronous Distributed Generation Using Governor
Signal Clustering,” Electric Power Systems Re-
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