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

three-

phase 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

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 co-

generation 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 three-

phase

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 stand-

alone 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

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 co-

generation 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.

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

21

5

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.

&

WTT.

STS

I

Figure

1.

Block diagram of a

UPS

configuration

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

output voltage to the utility mains of fixed voltage

and frequency.

A

block diagram of the turbine-generator-

converter 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.

4C.AC CONVERTER

I

I

TURBINE

CINERATOR

m

pP

SUPERVISORY

CONIROL

Figure

2.

Block diagram

of

the standby power supply

studied

2.1

Operation

of the

system

The working fluid fed to the turbine through

valves SV and CV produces torque

in

the turbine

that is transferred to the generator connected to

it

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.

21

6

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.

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.

3

Power Factor Correction

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.

3.1 Compensating Reactive Power

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,

it

makes

it

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 turbine-

generator 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

QL,

to

obtain

unity

power factor,

a

reactive power

QL

has to be fed to

the mains. By sensing the reactive power needed, the

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.

4

Simulation and test results

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

400

I-----'

,

300

i

200

I

100

jlull

so

-qp

-200

-300

I

-400

J

I

I

ttmsl

I

i

3-phase

currents

40

20

90

-20

-40

Figure

3.

Test results. Upper traces line voltages, lower

traces converter output currents. Output power:

24

kVAr

21

7

Signal

I”

phase

Ursm 223.43

1

thd(U)

-~

17.84- 17.95

-1

7i

~

1-

Irsm

36.23 34.18 33.60

thd(1) 12.53 11.62 11.89

2’Id phase 3rd phase

Total

226.04 225.73

I

i

I

I

S(kVA)

I

8.10

I

7.73

I

8.26

I

24.08

I

-0. I2 -0.46

-0.03

-0.61

-7.7

I

-8.26 -24.07

-0.01 -0.06 -0.03

Table

1.

Laboratory test results

U1

(Magnitude)

100

-1

0

12

3

4

5

6

7

8 9

101112131415

Harmonics

I

,

I1

(Magnitude)

I

I

I

100

15

10

i

s

mil

1

5

0

I

0

1

2

3

4 5

6

7

8

9

101112131415

Harmonics

I

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].

21

8

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 &he measured 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 stand-

alone mode.

(A

linear load is assumed).

5.

Conclusion

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

Three-

Phase UPS Systems,

Proceedings

of

the 22'ld International

Power Conversion Conference, June 25-27,

1991,

Nuremberg, Germany

In the paper

a

special power systkm, th,at'

21

9