Islanding Operation of Electrical Systems in Buildings

doi:10.4236/epe.2013.54B038

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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

Email: [email protected]

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)

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

48.5

49.5

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

500

1000

1500

-1 -0.500.51

Time (s)

Voltage (V)

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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-400

-300

-200

-100

100

200

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-1-0.5 0 0.51

Time (s)

Voltage (V)

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Figure 4. Voltage transient at re-connection. (a) Without

re-synchronisation. (b) With re-synchronisation.

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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-

search, Vol. 81, No. 2, February 2011, pp. 608-616.

doi:10.1016/j.epsr.2010.10.023