Reactive power compensation by power electronic drive

Additionally the input converter CONI supplying the electric drive can produce reactive component Q2 [2].. This fact may be used to produce reactive power to the supply line by the ele

REACTIVE BOWER CO ENSATION BY POWER ELECTRONIC D W E

R Teofla%;, W Koczara

Institute of Control and Industrial Electronics

Faculty of Electric Drives Warsaw University of Technology Koszykowa 75, W a r s z a ~ 00-662

Poland

Abstrad - This paper presents a concept of reactive power

compensation by electric drive system The priority task is

to supply electric drive with a&e power The amount of

m & e pewer comgonen(, that can be generated to the

supply b e by the drive system, depends on electric drive

power demand and hansistors' converter power mtiny

This paper considers conditions ander which reactive

power compensation I s possible The comilderations are

supported by simulation and verifBed by laboratory results

Modem electric drive system consists of input converter,

dc link capacitor and an inverter with electric motor Diode

input converters are usecl vastly in many industrial

applications However, they pollute the supply line with

higher karmorucs as they produce non sinusoidal current

[ 3 ] Additionally diode converters make the input current

lag the voltage wave Thus the current taken from the

network is enlarged by the reactive component The supply

system has to transfer this current, causing additional

p w r losses in the netmrk In consequence it leads to

voltage distortion aggravating parameters of energy

supplied to customers Having in mind new IEC standards

transistor converters will play more and more important

role as they are p l d instead d &ode converben Control

dgorithmns of such converters create psibilities for many

applications One of these advantages is that they give an

opportunity to improve the shape of the current taken from

the supply line by an electric drive [I]

Fully controlled powx converter can operate

simultaneously as real power supplier and as a source of

reactive power It enables power factor correction in the

supply line and it reduces negative influence of the load on

the supply line and other users Figure 1 presents the

electric drive system EDS and a three phase load TPLL

powxed from the same network Each phase of the EDS

has input inductance & Three phase load TPLL is

assumed to be linear The load requires apparent powr S,

which consists of active component P, and reactive

component Q, Each component may change in time

according to load's actual demand The active power

required by the electric drive is described as P2

Additionally the input converter CONI supplying the

electric drive can produce reactive component Q2 [2]

These two components express apparent pmr S, Pm

means the maximum value Qf the reaf p o w r that must not

be exceeded The nominal value of real power required by

the electric dnve Pthi is always less then the maximum

value P2u.u" The safe operating powr range for the

electric drive can vary in a range P2 E <O, P2N >

I

Electric Drive 1

System I

1

I

i

1

EDS

I

Usually an input converter, supplying an electric drive, is designed to operate with bigher powr ratings Especially

an electric motor start-up has to ke taken into

consideration d e n designing control systems Therefore

&er making a start-up of an electric motor, input converter operates with power S2 below its n o d value

It also happens that an electric motor is not full loade4

operating with p m e r P2 which is below its nomind value P2, This fact may be used to produce reactive power to the

supply line by the electric drive system, reducing reactive

component o f t h e current taken from the supply line by

other loads that are connected in parallel Additionally the

CON1 input converter can be used to generate reactive

powr to the network even when the electric drive,

supplied through the CON1, is not operating

It was assumed that the three phase load is linear therefore it takes sinusoidal wave of current from the

network a h i s current is shifted according to its phase

voltage by the phase angle pl The phase angle is calculated from :

Qi

s

p, = ants(-)

Neglecting reactive powr compensation of the electric drive system, reactive component of the loads current Im(il) becomes reactive component of the supply line

current Im(i) In order to diminish reactive component of

the supply line current, the electric drive system has to

become ai source of reactive power

2 REA(PpIW5 POWERCOMPENSATION STWSTEGY

Figure 2 presents control system for the input converter

CONI The system prowdes sinusoids? shape of the

current taken from the supply Bine and additionally it

enables reactive p w r generation in amount that is

actually required by the EQPLD ahe referena: signal of

voltage U : i s compared with its a c t d d u e Uc m e result

i s convented to the reference amplitude of the :q$y line

line current i n The

-4

I e+ -+

I

"he transistor CONI Gonwrter makes the current 6, taken

from tbe supply line, sinusoidal and in phase with the

voltage wave The converter operates with unity power

factor The apparent power S drawn fiom the sulply line

becomes only active p o w r P

S = + jQ, +P2 +jQ, =e + P2 + j(0, +Q,)

Therefore the supply line does not transfer any reactive

component o f current Figure 3 shows results of a

simulation when she supply line current i consists only of

active component ( H ) This current i is in phase with its

voltage wave U Current i, i s the phase current o f tllmc three

phase LOAD and i2 expresses input current of the electric

drive system, T h e shape of the current i can be treated as

6A ( + ~~

sa; _ _ + _ ._ -.+ - ._.+ - :

sinuscidal, however it hcl

reason for this is, that the

switching In order lo limit

inductance E, is applied in each phase

represents current required by the LOAD I

voltage and the amount of lag is about 5r

reactive current mompensation, the aeactiw: aompnien

r current i2 has to

component of the LOAD'S mment i ~

h ( i , ,S + lm(i, 1 = 0

Figure 4 presents the case, when reactive c m p " n t s of both v'ectors ip and i2 are equal As a result supply line

current i consists only of active compnent Re(ii,) and

w i z >

In this case the reactive component of the converter current

Im(i,) is relatively high, causing high amplitude cif the

converter CON1 current i2 Mower, the CON1 current i, is

limited by transistor converter power ratings FOP this reason, the maximum permissible value of the CONI

current irZ,,,= must not be exceeded If the reactive powr

649

6A; -4 +

4 .+ ~ i A :

In this m e the supply line c u ~ e n t i can

phase with its voltage wave In the CO

neetwork m m t r a d e r the difference of this reactive part

maximum pmum1

i2 Figure.6a and a5 p a e n t

wnverter CONI The tramistor converter CONI a n

ent power SZm Et means that it may

ve power csmpewtor and as a r e d pawer

er7 if the electric slrive r e d gowr demand

q d s to Pw? then c o m ~ ~ ~ i ~ n operation is not

possible Any other &"A which is less than P2-

enables such a csmpenmtisn The question is how much

reactive powr might be generated by the converter t~ the

electric drive Fig.6 show that

current increases the CON1 wnverter cm not provide fill

reactive pwr mmyensation It can generate reactive

pwr in m o u n t ofQ2 The AQB has to be transferred

h a m e t e a k, will express how muck pwr is required by

the electric drive in comparison with m i m u m value

Pw %e active p w r value required by the electric

drive c Ise for instance P2 Judging from Fig 4 the

conclusion is that the m ~ ~ n t of reactive powr generated

to the supply line could be up to Q2 Basing on the following relations k m n apparent, active and reactive Pomr

~ 2 M A x =: s,,,

If the electric drive does not operate, the m o u n t of

reactive gowr ahat can be generated to the supply line is

n m m u m and then

65 1

Figure 11: Phase voltage Y and total phase current i taka f?om the network

while limited reactive power compnsat~on

Figure 111 show the total supply line current i in the

case when the CONI current iz exceeded maximum

permissible value izz,, The CON% provides the

maximum reactive p o w r it can generate And yet the

demand for reactive pomr by the linear load is greater

This difference has to be transferred through the netwrk

The current taken from the supply line i lags its voltage

wave v by 3Odeg Tlie reasons for limited reactive p o m r

generation by the converter are transistor power ratings

and actual value of real power required by the electric

drive,

4 CQNCLUSIBWS This paper presented reactive power compensation by

an electric drive system The electric drive system

consisted of transistor input converter, dc link and

transistor inverter supplying an electric motor The main

eask of such a system was to supply the electric motor with

active pomr and additionally to generate reactive p o w r to the supply line The amount of reactive power produced by the electric drive system was calculated basing on tRe actual active power demand of the drive Therefore generation of reactive power was limited according to input converter power ratings and the drive power demand The electric drive system can provide maximum

of reactive power only when the electric motor does not operate

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