Standby Power Supply Using Alternative Energy Through Cogeneration Technologies Telecommunications Special, The Third I

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STANDBY POWER SUPPLY USING ALTERNATIVE ENERGY
THROUGH COGENERATION TECHNOLOGIES
Dr. Jardan, R. K. *,Dranga, 0. (Ph.D. Student)*, Bereknyei, D. (Ph.D. Student)*

* BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS
H-1 1 1 1 Budapest, Budafoki,8. Hungary.
Tel: +361-463-2338, Fax: +361 4633163 e-mail: jrk@elektro.get.bme.hu

ABSTRACT
A special co-generation system, developed for producing electric and heat energy from low pressure saturated steam or
some other working medium has been developed. The system consists of a high speed turbine, coupled to a threephase permanent magnet synchronous generator, a three-phase ACIAC converter and a microprocessor supervisory
control unit. The main purpose of the system is to produce electric energy, that can be utilised directly to supply loads
in stand-alone mode or it can be fed back to existing utility mains. In the latter case the converter, connected in parallel
to the mains, can serve also as a source of reactive power and higher harmonic current components, i.e. the converter,
by changing its control method, can be turned into a system providing not only active power, but functioning as a
complete active filter or a.c. line conditioner at the same time. In the paper a further feature of the system, its ability to
serve also as a stand-by (SPS) or uninterruptible power supply (UPS) is presented in some detail.

1

INTRODUCTION

failures, it can serve as a standby power supply or UPS
for a selected
group of
loads including
telecommunication equipment.
A possible advantage of the system is that it
can use alternative energy sources that are suitable for
producing saturated or superheated steam with IOW or
medium pressure, thus it is applicable
in
telecommunication systems even in places where
electric energy is not directly available. In most cases
the electric energy can be produced within cogeneration or less frequently in direct generation
process. The alternative renewable energy sources
include solar energy (direct solar steam system or solar
energy with heat exchanger), geothermal energy (with
or without heat exchanger) and biomass energy. Various
waste energy sources can also be utilised by applying
the system. These include the energy extracted from the
procgs of pressure reduction in steam, gas and fluid
networks and the energy that can be recovered in the
process of seawater desalination.

The paper describes a special system ‘designed
to be used as an alternative power source for
telecommunication equipment or as an UPS-system.
The system is based on the application of cogeneration principle, that makes it possible to generate
electric and heat energy by utilising alternative
renewable and waste energy sources. The basis of the
solution is a system that consists of a high-speed turbine
coupled to a three-phase synchronous generator, a threephase ACIAC converter and a microprocessor
supervisory control unit. The function of the system is
to utilise the energy content of some working medium,
such as steam, gas or pressurised fluid, by a special
turbine. A more detailed description of the system can
be found in [l]. The turbine is developed for various
working media of low energy content. Genetally, the
system, beside heat energy produces electric energy,
that can be utilised directly to supply loads in standalone mode in isolated rural areas as general-purpose
power supply for telecommunication and other loads or
feeding it back to existing utility mains. The additional
feature of the system that makes it possible to use it as
an a.c. line conditioner is presented in [ 2 ] .
As the energy source of the system is different
from the mains, it offers the possibility to use it as an
SPS or UPS. In the parallel mode of operation, when
the system feeds energy back to the mains, during mains

2. Description of the system
Various configurations of uninterruptible power supply
systems are available to increase the reliability of the
energy supply and improve the overall performance.
Development of an up-to-date high performance system
requires careful attention to every detail of it. In a
previous paper [4] solution to problems of the inverter

215

.

output voltage to the utility mains of fixed voltage
and frequency.
A block diagram of the turbine-generatorconverter system presenting the main units is shown
in Figure 2. The working fluid is fed to the turbine
through safety valve SV and control valve CV.
Turbine T is directly coupled to generator G that
supplies the input of the three-phase AC/AC
converter, while the output of the converter is
connected to the utility mains. In the basic
arrangement of the system there are two parameters
to be controlled. One controlled parameter is the
outlet pressure of the turbine that is changed by the
inlet, quantity of the working fluid through the
control valve, i.e. a flow control actuator.
The other controlled parameter is the speed of
the turbine-generator set. The micro controller unit
controls the output quantities of the converter by
accepting signals from the supervisory controller and
the output power sensor S2.

stage was presented, here the questions of application of
renewable energy sources is discussed in some detail.
At the beginning of static UPS applications the
system was built of a controlled rectifier an inverter
(including the filter) and a battery connected to the DC
link. It soon became evident that this simple
arrangement had two basic shortcomings. First, if any
problem occurred in the inverter operation, the supply
of the load was lost, even if the mains was present. The
second problem is connected with the internal
impedance of the inverter and filter stage. In case of a
transient load increase, the output voltage had relatively
large drops, that frequently resulted malfunction of
sensitive loads.
Great improvement in system performance was
achieved by the introduction of fast-acting static
switches.
In the scheme shown in Figure 1 static switch
SW is closed in normal operation while static transfer
switch STS is open. In case of failure or excessive
voltage drop detection, SW is open and at the same time
STS is closed. This way a large current transient can be
supported by the mains. If the load current returns to
normal and output voltage of the inverter is within
tolerance, the load can be transferred back to the
inverter.

&

I I

TURBINE
CINERATOR

4C.AC CONVERTER

m

WTT.

pP SUPERVISORY CONIROL

Figure 2. Block diagram of the standby power supply
studied

STS

I
Figure 1. Block diagram of a UPS configuration

2.1 Operation of the system

A brief description of the system is given
for the application in steam networks, however, it
should be emphasised that the key elements and the
basic principles of the operation are the same in the
case of other energy sources and working media.
Applying the turbine-generator system, the
functional block diagram shown in Figure 1. has to
be modified, as the rectifier is supplied by the
generator instead of the mains. The generator, driven
directly by the turbine, is a machine of special
construction. Due to the high speed and the
requirement of high reliability, a permanent magnet
solution has been selected. The output voltage of the
generator is fed to the rectifier R that is followed by
the inverter INV. The resulting AC/AC converter
connects the generator of variable frequency and

The working fluid fed to the turbine through
valves SV and CV produces torque i n the turbine
that is transferred to the generator connected to i t by
a clutch. The electric power obtained at varying
voltage level and frequency is to be fed back to the
utility mains of 400 V and 50 Hz by the help of the
ACfAC converter.
The output power of the converter is
cont;olled according to the reference signal of the
control unit, given by the speed controller. A
pressure controller loop keeps the pressure of the
working fluid at the outlet constant corresponding to
a reference signal.
There are two basic modes of operation of the
system when functioning as an SPS or UPS.

216

2.1.1. Stand-alone or Island Mode of Operation
In this mode the loads are directly supplied by the
converter
of
the
turbine-generator
system.
Theoretically, in this mode the turbine-generator
system, without any additional measures, is
functioning like a UPS. Naturally, the group of the
loads have to be selected to match the output power
of the UPS. The control strategy of the system is
based on controlling the input power on the
mechanical side to ensure power equilibrium
between the mechanical and electrical side.

converter can be controlled in such a way that it will
prodfice the requested power components The. contro!
unit has two input accepting reference signals P and Q
that determines the active P and reactive Q power
independently. For this purpose the amplitude and the
phase of the internal voltage of the controller has to be
controlled.
A converter dimensioned for rated current can supply
reactive power only if the active component is less than
rated. To produce reactive power by the converter at
rated active power evidently needs over-dimensioning
some parts of the converter. A 50 o/o over-dimensioning
in current ensures that reactive power can be supplied,
in the range of 110 to 150 %, depending on the active
power needed.

2.1.2. Parallel mode.
In this mode both the turbine-generator system (UPS)
and a selected group of loads are connected to the
mains. In normal mode the turbine-generator system
feeds all the power back to the utility mains, available
on the mechanical side, that is automatically ensured by
the speed regulation of the machine set. In case of mains
failure the group of loads is disconnected from the
mains by opening the static transfer switch STS and
further on the power will be supplied by the turbine generator system. Apart from some switching transients
the power supply of the loads can be regarded
continuous. When the static transfer switch is opened,
the control strategy of the turbine-generator system has
to be changed to correspond to the stand-alone mode.
After the mains returns to normal, the system can be
restored to the original state.

Simulation and test results

4

The idea described here has been checked by
computer simulations and laboratory tests.
A turbine generator converter system with an
output power of 20 kW and 25 kVAr has been built and
tested. The system is installed in an industrial
environment where the Total Harmonic Distortion
(THD) of the mains voltage is unusually high due to
rectifier type loads. The average THD of the mains was
over 9 % and it offered good opportunity to demonstrate
the effect of the converter on the power quality.
1

3-phase voltages

3

Power Factor Correction
400

300
200

In an industrial environment a considerable portion
of the loads have a poor power factor resulting from low
cos cp and high harmonic content of the load current.
The principles of compensating the reactive power and
the higher harmonic content will be discussed next.

I
,
i

I

jlull

100

s o

-qp
-200

3.1 Compensating Reactive Power

I

-300
I

-400 J

Assuming that the mains current contains only
fundamental component but the power factor needs
correction, reactive power has to be supplied to the
mains. If the power source is suitable to change the
reactive power supplied, a closed-loop power factor
compensation can be built by sensing the power factor
at a point of the network where compensation and
controlling the source of reactive power is needed to
supply the required amount of reactive power. When the
power capability of the reactive power source is limited,
i t makes i t only possible to improve the power factor or
to compensate the power factor of a selected group of
loads. This is the case with the converter of the turbinegenerator system as the rated power of the unit is
generally considerably lower than the power level
required e.g. by a factory.
If the reactive power of the loads is Q L , to obtain
unity power factor, a reactive power Q L has to be fed to
the mains. By sensing the reactive power needed, the

ttmsl

I

I
i

3-phase currents

40

20
9

0

-20
-40

Figure 3. Test results. Upper traces line voltages, lower
traces converter output currents. Output power: 24
kVAr

217

1

I ” phase

Signal

Ursm

223.43

thd(U)

-~

17.84-

3rd phase

226.04

225.73

Irsm

36.23

34.18

12.53

11.62

I S(kVA)

I 8.10
-0.I2

-0.01

7 i

17.95

thd(1)
I

-1 1-

2’Idphase

Total

~

33.60

11.89

i

I

I 8.26

I 24.08

-0.46

-0.03

-0.61

-7.7 I

-8.26

-24.07

I 7.73
-0.06

I

-0.03

Table 1. Laboratory test results

U1 (Magnitude)

,

I1 (Magnitude)

100

15

1-

I
I

100

I

mili

10

s
5
0

0

I

1

2

3

4

5 6

7 8

9 101112131415

0

Harmonics

1

2

3

4

5

6

7

8

9 101112131415

Harmonics

Figure 4. Harmonic voltages and currents test

Figure 5. Simulation result. Transient of a mains falilure. Upper trace: load voltage, middle trace: mains current,
lower trace: inverter output current. Values are given in [pu].

218

1I
I

The results were recorded by using network analyser
equipment inserted between the output of the converter
and the mains. The equivalent circuit of the mains, from
measured values, was calculated as R,=0.3 R (ohmic
resistance) and L,= 508 pH (inductance). The converter
filter parameters are R,=0.05 R and L,=2.5 mH.
Figure 3. shows that the shape of the output
voltage and the current is different from the ideal
sinusoidal one due to its compensation affect on the
network. These results are displayed in Table 1.
During the above test the converter supplied
24 kVAr reactive power to the mains and the THD was
reduced from 9.4 % to 7.8 % corresponding well to the
calculated attenuation coefficient kp0.83.
Using &hemeasured harmonic compqnents of
the line voltage and an above parameters the system was
simulated taking accurately into account the operation
of the converter with a switching frequency of 10 kHz
and applying a unipolar PWM.
In Figure 5. the results of a simulation run are
shown. In a system operating in parallel mode, a mains
failure is assumed at the instant of 50 ms. The static
transfer switch is opened and further on the load current
is supported by the inverter. The output current of the
inverter is reduced as the load is assumed to be lower
than rated. It can be seen that due to the distorted
waveform of the mains, the output current of the
inverter contains high harmonic content in normal
operation, while it becomes sinusoidal in the standalone mode. (A linear load is assumed).

consists of a high-speed turbine coupled to a three-phase
synchronous generator, a three-phase AC/AC converter
and a microprocessor supervisory control unit, was
presented. The system is designed to operate using
alternative energy source that is suitable also to serve as
a standby or uninterruptible power supply. A control
strategy, easy to implement, ensures closed loop control
of the power factor and at the same time the power
quality is favourably affected.

References
Jgrdan, R.K. Generation of Eleciric Energy by
High-speed Turbine -Generator Sets. Proceedings of
Intelec'95 Conference October 29-November 1. The
Hague, The Netherlands, pp.664-670
Casadei, D.-Grandi, G.-Jardan, R.K.-Profumo, F.
Control Strategy of a Power Line Conditioner for
Cogeneration Plants. PESC'99, Power Electronics

Specialists Conference, Charleston, South Carolina,
US. June 27-July 1, 1999.
Jardan, R. K -Nagy, I.- Korondi, P. Masada, E - Nitta,T.Ohsaki, H. Power Generation Systeiii for Utilising
Alternative Renewable and Waste Energy

IPEC-Tokyo-2000, April 3-7.2000 Tokyo
Jardan, R. K. . Prohlerns of Static Switches in ThreePhase UPS Systems, Proceedings of the 22'ld International
Power Conversion Conference, June 25-27, 1991,

5. Conclusion

Nuremberg, Germany

In the paper a special power systkm, th,at'

219



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