Nitz & Stair, Jr.

Current Division Between;*

UNIVERSITY OF ILLINOISLIBRARY

Class Book Volume

My 08-15M

Digitized by the Internet Archive

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CURRENT DIVISIONBETWEEN

ALTERNATORS IN PARALLEL

Ingo Charles Nitz

Jacob Leander Stair, Jr.

THESIS FOR THE DEGREE OF BACHELOR OF SCIENCE

IN ELECTRICAL ENGINEERING

IN THE

COLLEGE OF ENGINEERING

OF THE

UNIVERSITY OF ILLINOIS

PRESENTED JUNE, 1908

YD OS

UNIVERSITY OF ILLINOIS

Juno 1, 190 8

THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY

imo CHARLES NITZ and JACOB LBANDER STAIR, JR.

ENTITLED CURREM* DIVISION BETWEEN ALTERNATORS IN PARALLEL

IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE

degree of Bachelor of Science in Electrical Englnaaring

Approved:

HEAD OF DEPARTMENT OF

114900

CONTENTS.

page.Introduction, 1.

Obj ect , 1

.

Description of Apparatus, 2.

Theory and Method, 4.

Arrangement of Apparatus, 10.

Operation and Discussion of Tests, 10.

Conclusion, 39.

1.

INTRODUCTION.

According to some preliminary investigations carried

on by Professor Morgan Brooks and M. K. Akers, it v/aa at first

thought that "by the method herein described, the load on two or

more alternators operating in parallel could he divided equally

or in such proportion as desired. It has been concluded, how-

ever, that nothing but the driving forces could effect the divi-

sion of load between alternators in parallel, and that if the

driving force of one machine was less than that of another, the

former could not carry as large a load as the latter. It was

thought that the same theory and method would apply to the divi-

sion of current irrespective of the power load division.

OBJECT.

The object of these tests was to investigate the effect

of transformers, connected in series with the machines, as shown

on page 12 , upon the division of the current output between two

alternators operating in parallel under various conditions.

These conditions were to include the following:

1. Alternators running under increasing load with driv-

ing forces equal and (a) with excitation of fields kept constant,

and (b) with terminal voltage constant. ~~

2. Alternators running under constant' load with the

driving forces equal and with the field excitation of one machine

varying.

3. Alternators running under constant load with con-

stant field excitation and with the driving force of one machine

2

varying.

Similar tests were also to "be macie without the trans-

formers in the alternator circuits, and the results compared with

those obtained when using the transformers. Lastly a 3 to 2

current, ratio test was to be performed with one transformer.

ial design which were built by the Wagner Electric Co., for use

in the University power plant. Each was of 4 kilowatt capacity

with a primary potential of 45 volts and a secondary potential

of 30 volts. The coils were of a few turns of very heavy wire,

making the resistance very low, the primary resistance being

.0058 ohm1 and the secondary resistance, .0024 ohm.

current generator No. 95584, manufactured by the General Elec-

tric Company. This machine, 7.5 K.W. in capacity, has a six pole

revolving field and therefore had to be run at a speed of 1200

R. P. M. to produce a frequency of 60 C2 rcles per second.

DESCRIPTION OF APPARATUS

.

The transformers used in these tests were two of spec-

Machine No. 1 was the 220 volt 2 or 3 alternating

The armature has six coils

a

10

6

5

9

7 The tests were run at 110 volts

apart as shown in figure 1.

tained from one coil, three

obtain a greater output from

tromotive force vectors are 30°

single phase, and in order to

so arranged that their elec-

the machine, than could be ob-

coils were connected as for

three phase delta (Fig. 2),

thus giving two branches in

which to generate the current.

This decreased the armature

impedance and thereby decreased

the IZ drop in the armature.

Alternator No. 2 was a 5

pole inductor alternator of 7.5

Pig. 2. K.W. capacitj' built by Durfee

and Drew, '06. This machine has two separate armature windings

which could be connected in parallel or in series for generating

110 or 220 volts. Its speed w?s kept at 720 R. P. M. to produce

a frequency of 60 cycles per second.

Alternator No. 1 was driven by a 15 H.P., 4 pole, 220

volt Bullock motor. The 'mot or was shunt wound and had a normal

speed of 110 R . P. M.

Alternator No. 2 was driven by a 15 H. P., variable

speed, 4 pole, 220 volt Westinghouse motor which was also shunt

wound

.

A reactance coil with an air core was used to aid in

holding the machines in step, the primary being stationary and

the short circuited secondary being so arranged that it could be

slipped in or out of theprimary to change the impedance as de-

sired. The coil had a resistance of .527 ohm and an impedance

of 4.23 ohms making the angle between current and pressure equal

to 82050'. This current would therefore be nearlv wattless.

4.

THEORY AND METHOD.

Before taking up the method for the adjustment of load

current "between alternators, it may be well to review some of

the generally accepted ideas, as given "by Dr. Steinmetz and

others, of the conditions that affect parallel operation.

There are a number of conditions that determine the

successful operation of alternators in parallel. Among them

are the following, differences in wave form and the effect of

the higher harmonies, differences in the characteristics of the

two machines, inequalities in driving force, variations in ex-

citation^ and the react ions due to armature current.

The effect of differences in wave form between two

machines operating in parallel is to cause a circulating current.

Even when the machines are similar the fundamentals may be close-

ly in phase; but if the similar harmonies are not in phase a

current is caused to pass between the machines.

Owing to differences in the regulation of two machines

there may be a circulating current at certain loads, while at

other leads the machines operate with no cross current what-

ever. The difference in the characteristics causes inequaliti es

in the voltage of the machines.

The driving force is probably the most important fact-

or to be considered. Inequalities in this respect, tend to give

rise to inequalities in frequency. The result is that circulat-

ing currents are caused which transfer energy from the machine

whose driving power tends to increase its speed to the other

machine, thereby causing the load on the first machine to in-

crease. At a light load or no load the cross currents cause one

alternator to drive the other as a synchr onous motor, while for

the condition of load the machines do not share the load in pro-

portion to their capacities.

The prime mover does not maintain a constant speed

for all loads but ?ieverthel ess , the two machines that are operat

ing in parallel must have the same frequency. The loads on the

generators will so adjust themselves as to give a speed that

corresponds to the same frequency. It is then seen that the

division of load "between alternators not rigidly connected me-

chanically depends almost exclusive^ upon the speed regulation

of the prime movers. This speed regulation must be the same for

the prime movers of each generator to divide the load in propor-

tion to their capacities. Kence, if two machines are operating

together and one of the engines governs more closely than the

other, the alternator v/ith the better governed prime mover will

assume more than its share of the load at full load and less

than its share at light load.

With, customary centrifugal type of governor the speed

varies inversely with power. V/ith some form of electrical gov-

erning>power might be increased without decrease of speed.

It is sometimes supposed that the division of load

between alternators can be determined by adjusting the relative

field strengths of the two machines. As an example, take two

similar alternators direct connected to two steam engines.

These machines will, when operating in parallel, make with mathe

matical exactness the same number of revolutions per minute. If

the load is 500 kilowatts, and the governor of one engine admits

sufficient steam to generate 100 kilowatts at the same speed

that the governor of the other admits sufficient steam for 400

kilowatts, the load will divide itself in this ratio, no matter

what may be done with the generator fields, either "before or

after synchroni sm . It is evident that a change in the relative

field strengths of the two alternators can in no way affect the

governors of the engines. Thus the division of load does not

depend on the relative strength of the current in the alternator

fields. If the load could "be divided toy increasing the strength

of one field and decreasing that of the other, the lav; of the

conservation of energy would he violated, since no change has

resulted in the position of the engine cut-off. As long as the

indicated horse-power of the prime movers remains the same, the

output cf one cannot he materially raised or that of the other

lowered. When there is a difference in the exciting currents

in the fields, the machines adjust themselves to practically the

same field strength. This is done "by an exchange of wattless

cross currents that null down the voltage of the machine with

the stronger field, and "boosts" the voltage of the machine with

the weaker field until the two approach equality. The generally

accepted idea, then, is that the adjustment of load division de-

pends not upon the relative field strength either before or af-

ter the machines are in parallel, but upon the load-speed char-

acteristics of the prime movers.

Armature reactions also play a part in the successful

operation of alternators in parallel. The lower the armature

reaction, hence the closer the regulation of the machines, the

7

more sensitive they are to variations in field excitation. In

cases of low armature reaction the syrichroni zing power may he

so large as to require careful adjustment "before the machines

are thrown together, in order to prevent excessive circulating

currents. On the other hand, for machines of high armature reac-

tion the synchronizing power may not he sufficient to keep the

alternators in step when heavy overloads occur. Hence, it is

not desirable to have too low or even too great an armature reac-

tion when machines are to he operated in parallel.

The conditions reviewed above that affect the load

division will also have much to do with the division of load

current between the machines. It is the latter with which we

are concerned in this investigation.

The device used for effecting the division of load

currents between two or more alternators running in parallel

consists essentially of transformers Tiand T2 connected as shown

in the figure on page 12. The primaries of the transformers

are connected in circuit with one line terminal of the machine,

while the secondaries are connected in series forming a closed

circuit. All the transformers have the same number of turns in

the primary, which is designed to carry a current that is in di-

rect proportion to the full load current of the machine in whose

circuit it is connected. ,?he secondaries have the number of

turns that is proportional to the share of the current that its

corresponding primary is to carry. It is evident, since the sec-

ondaries are all connected in series, that the same current will

flow %n all.

0.

This method for load current division also employes

a synchroni a4ng coil that will hold the machines in step inde-

pendent of thevalue of the transformer inductance. The synchro-

nizing coil must he placed across the terminals of the machines.

The coil has such an impedance as will allow enough synchroniz-

ing current to flow to hold the machines in step. A sufficient

cross current would not flew were the transformers "between the

machine and the reactance coil.

The operation of the device for the division of load

current is as follows:- When currents proportional to the capaci-

ties of the machines are flowing through the primaries, the trans

formers will he non-inductive, since the ampere-turns of the

primary and secondary are equal. Therefore, for these conditions

practically no electromotive force exists at the terminals of the

transformers. Just as soon, however, as there is any change in

the value of the current flowing in either primary, without a

corresponding change in the other, the transformers are no long-

er non-inductive, and the current flowing from the machines is

opposed by the reaction of its primary. When the current in the

primary coil changes It causes a corresponding change in the cur-

rent from all the generators, through the short-circuited sec-

ondaries of the transformers. The proper proportion of load

current is thereby restored, and each machine gives out a current

that is proportional to its capacity. The effect is the same

when any change occurs in the driving force or excitation of

eith er machine.

The fundamental principle, therefore, of the load cur-

rent division method is to have transformers so connected as to

be approximately non-inductive so long as the desire^ division

of load current exists, but automatically inductive when the pro-

per division is disturbed, whereby a satisfactory balance of the

load currents is restored.

10.

ARRANGEMENT OF APPARATUS.

The diagram of connections on page 12 shows the leads

from the alternators to the load consisting of lamps. Nearest

to the machines is placed the synchronizing "bus "bar with the

synchronizing lamps and the reactance coil. The transformer

primaries were connected in corresponding leads of the two al-

ternators, and their secondaries were short circuited thru each

other in such a manner that the pressures induced in them would

aid one another in causing a secondary current to circulate.

Ssc Ssc were switches for short circuiting the transformers, and

Sco Sco were switches for cutting the transformers out of cir-

cuit. Thus when the former were open and the latter closed, the

transformer primaries were in circu.it, and when the former were

closed and the latter open, the transformers were cut out of the

alternator circuits. Sm Sm were main switches connecting the

machines to the main "bus bars thru the indicating ammeters and

wattmeters. This method of connecting the transformers, which

were or equal number of turns, should give an equal division of

the current.

OPERATION AND DISCUSSION OP TESTS.

In each case the machines were brought up to synchro-

nous speed and excited to generate 110 volts before they were

connected in parallel by closing the synchronizing switch Sg .

The main switches ^ Sm were then closed, connecting the machines

to the main busses.

The object of test No. 1 was to determine the division

of current between the two alternators under various loads from

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to 40 amperes, with the driving forces on the two machines

equal and the field excitation constant. The data on page 13

show the power and current output and the pressure supulled

each alternator. With the transformers in the circuit it was

easy to adjust one motor field and Veep the driving forces equal,

because any difference in driving forces was immediately shovm

by current circulating thru the reactance coil circuit. Without

the transformers this was not possible because the circulating

current would take the path of least impedance which was through

the bus bars. Ths slight inequality of driving forces with the

increase of load, was due to the difference in the speed load

characteristics of the motors. The curves on plate I show that

the division of current was nearer to the theoretical ratio when

using transformers than when they were not in use. The advantage

of having a better adjustment of driving forces may have caused

this result. The transformers, on the other hand, caused a larg-

er drop in the pressure supplied as the load was increased from

to about 40 amperes. This greater drop is due to the impedance

of the transformers which as soon as the primary currents were

slightly different, became of such values as to produce an appre-

ciable IZ drop that would have been more noticeable had it been

in phase with the current.

In part (b) of test I, it was desireable to keep the

terminal pressure at 110 volts to give a condition more nearly

that of actual' power plant operation. To do this and keep the

two alternators excited in equal proportion, that is, to force

each one to generate 110 volts at all loads, the data on page 16

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showing ratio between field current and amperes output for the

alternators operating at the proper speed was taken, and ^'rven

plotted as shown on plate II and ITT. Starting the test with

driving forces equal, at no load, the current division was de-

termined for all loads both with and without transformers: page

19 . In both cases the driving forces were allowed to change

according to the motor load-speed characteristics. Again the use

of transformers Showed more exactness in dividing the current,

and better still was the division with transformers, the driv-

ing forces having been, accurately adjusted thruout the run: page

21.

In test No. 2, pages 24 and 25, the number of lamps

was kept constant to give about 30 amperes output for each al-

ternator. Starting with driving forces equal, the current divi-

sion was determined for values of field current of alternator

No. 2 varying from .88 to 1.60 amperes, while the excitation on

alternator No. 1 was kept constant. Normal excitation on alter-

nator No. 2 was 1.32 amperes for this load. Calculations of the

current output of alternator No. 1 in per cent of the total cur-

rent, and the resulting curves, plate VI, show a decidedly better

division of the load current when transformers were used than

when they were not used. The average voltage, however, dropped

from 115 volts to 90 volts with a change of field excitation

from 1.60 to .88 amperes when transformers were used, showing

a much poorer regulation than the drop to 97.5 volts without

transformers for -a corresponding change in excitation.

Test No. 3, page 28, was run with constant load and

constant field excitation, and. with an increasing driving force

24.

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27.

on alternator No. 1, that on alternator No. 2 being constant.

In order to obtain readings both with and without transformers

for exactly the same condition of driving forces, the readings

with transformers were secured first, and then the transformer

primaries were short circuited and cut out of circuit by means

of the switches, 83c and S C o to secure the other readings. In

this case the curves on nlate VII show that with a great differ-

ence in the driving forces, the current division changed, the

larger load going to the machine of greater driving force. This

was more true with transformers than without them. Plate VIII

shows a slight rise in voltage as the driving force on alternat-

or.No. 1 was increased when running without transformers, and a

most decided drop when transformers were used, showing that as

the difference in current "became greater, the impedance of the

transformers increased..

To test a 2 to 3 ratio of current division, the primary

coil of one transformer was placed in the lead of alternator No.

1, and the secondary coil in the corresponding lead of alternat-

or No. 2, Alternator No. 2 should therefore have assumed three

fifths of the current load, hut the data and curves, pages 31

and. 32 show a nearly equal division of current load. Having

started with driving forces equal at no load, they remained near-

ly so thruout the test, because the load "speed characteristics

of the motors are quite similar. This equality of driving forc-

es would not allow the division of current in another ratio.

In all- tests, alternator No. 1 assumed a larger current

load than alternator No. 2. To determine if this was due to a

difference in the transformers, a primary- secondary current ratio

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33.

test wis made on the traxiflfoners . This was acoonpi i shed by run-

ning the alternators separately and short circuiting the trans-

former secondaries thru ammeters. For any Riven primary current,

the secondary current of transformer Ho. ?. was 3lightly less than

that of transformer l!o. 1. In order to have the secondary cur-

rents equal, the primary current of transformer No. 1 should have

been less than the primary current of transformer No. 2,, and

therefore alternator No. 2 should have carried the greatest part

of the current.

The characteristic curves, plate XI, show that the

pressure of alternator No. 2 decreases faster as the load increas-

es, than that of alternator No.. 1. This makes the operation of

the two machines in parallel less successful.

The electromotive force waves traced "by means of the

oscillograph are shown to "be so greatly different in form that

they also add difficulties to the successful parallel operation

of the two alternators.

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— lM.ach.in-e No. I—

39.

CONCLUSION.

It is 3een that in every test except with varying driv-

inf forces, a "better division of current was secured when the

transformers were use ^d than without them. This is due to the

fact that a more exact regulation of the driving forces could bo

secured when the transformers were used because a slight dif-

ference in the driving forces would immediately he shown by the

current flowing thru the reactance coil circuit. A still bet-

ter division could have been attained if the transformers had

been of smaller capacitj" and of more turns. As it was, the trans-

formers never had full load current applied and the iron losses

were entirely too large at the loads used in the tests. A much

larger drop in pressure was caused by the transformers than with-

out them, and this would be an objectionable feature in practical

operation of alternators in parallel. The current division is

almost entirely dependent upon the prime movers. It is known

that the power load division is determined by the driving forces.

Assuming equal voltage, when the machines are operating with un-

equal driving forces and consequently with unequal power load di-

vision an equal current division may be secured only by having

unequal power factors. But as the load on each alternator was

non-inductive, and as equal current will give similar conditions

in each transformer, unequal power factors can not he secured. It

follows therefore,, that the current division will not be equal

unless the load division isequal, and therefore not unless the

driving forces are equal.

1I

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