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sqj.1935.0033

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STATIC
IMPROVEMENT
OF
POWER
FACTOR
By R. W. HAIGH, Graduate
1
article deals solely with the improvement of power factor by stai
condensers, but it must be mentioned that there is a number of other
methods of obtaining the same results which should be considered for each
practical application, as one or other of them may be found to be more
suitable or economic for the given conditions.
Before discussing the various methods of controlling static condensers,
it is desirable to glance at the many advantages to be obtained by im-
THIS
The steady advance in condenser technique is resulting
in the increasing application of
static methods of power-factor
improvement. This illustration
shows a 150-kVA 2,200-volt
3-phase condenser
proving the power factor of electric supply systems and to reflect on the
theory and effects of capacitances in a.c. circuits, as these considerations
will influence the selection of the positions of the condensers and their
control gear.
The advantages of power factor improvement are:
(1) Less fuel required for prime movers.
(2) Smaller generator currents and increased load capacity for the kVA
rating.
(3) Smaller excitation currents than required for lagging power factor,
which exercises a demagnetizing effect.
(4) Reduced loading of transformer, switchgear, cables, and trans1
London Section.
— 177 —
Haigh — Static Improvement of Power Factor
mission lines giving less heating, lower losses, and increased load
capacity.
(5) New plant may be designed for lower continuous and overload
ratings with a possibility of considerable saving on first cost.
(6) Better voltage regulation due to improved power factor and reduced
current.
(7) Greater overload capacity for motors and brighter illumination
from lamps due to a reduced voltage drop on load.
(8) Should the electric supply tariff include a kVA maximum demand
charge, it will be found generally that, with industrial loads consisting
mainly of induction motors, the cost of the condenser often can be recovered
out of savings on this charge alone in one to two years.
Circulating Currents
Inductive machinery takes a magnetizing current, which lags by
90 deg. behind the applied voltage and the useful power current; also the
capacitance current taken by a condenser leads the applied voltage by
90 deg. It follows that if these lagging and leading reactive currents are
equal in the same circuit, they will exactly compensate one another,
leaving only the in-phase or power component to be drawn from the
supply. The energy stored in the electro-static field of the condenser
during the charging period (i.e. when the voltage is on the rising portion
of either the positive or negative half-cycle) is released when the supply
voltage is on the falling portion of the cycle; this coincides with the time
when the magnetic field is absorbing energy. Thus, energy flows backwards and forwards between the condenser and the inductive circuit.
All cables between the condenser and motor will be required to carry the
wattless current representing this oscillating energy, added vectorially to
the useful load current, while cables, transmission and generating plant
on the supply side of the condenser will only have to carry the load
current, together with a small current corresponding to the difference, if
any, in value of the two fields, i.e. this will be zero if the power factor is
corrected to unity.
To obtain maximum benefit from power factor improvement, it is
obvious that the correcting apparatus should be placed as near as possible to the source of lagging reactive current. Static condensers should
be connected, therefore, across the. terminals of individual motors, unless
other considerations render it undesirable. This method of installation
has a number of advantages. The condensers are fully automatic in
operation, no control gear, switches, etc., are required, and on shutting
down the motor the condenser will discharge through the stator windings,
obviating the use of resistances. In the case of slip-ring motors, the condenser can be mounted on the wall or floor near to the machine and the
leads connected to the three stator terminals of the motor, forming a highly
reliable, efficient, and robust combination. The connections for squirrelcage motors with star/delta starters are not quite so simple at first sight,
but a little consideration will show that this also is quite a straightforward
matter. The standard l.t. delta-connected condenser, complete with discharge resistances, can be connected across the supply between the main
motor switch and the starter; or a specially designed condenser, in which
the three phases are kept separate and brought out to six terminals,, may
be solidly connected to the motor terminals, in which case the condenser
— 178 —
Students' Quarterly Journal
June 1935
would discharge through the motor windings when the motor is shut
down and resistances would not be required. It may be necessary, in
some cases due to the resulting reduction in current taken from the supply,
to reset the overload trip coils in the motor starter.
Build-up of Voltage
The direct connection of condensers to individual motors is not always
desirable in practice. In factories where there is a large number of small
machines it is obvious that, in addition to the increased total cost, due to
the splitting of the total required leading kVAr into many small units,
the cost of installation will also be high, while it may be difficult to find
sites in every case for condensers adjacent to their respective motors.
Further than this, there is a danger that very large motors of, say, above
600 h.p. running down under their own momentum after having been
switched off on light load, may have high voltages induced in the windings
that may cause a breakdown of the insulation; This possibility is
accentuated when the condenser is large enough to give a leading power
factor when the machine is on no-load. The explanation of this phenomenon is as follows:
The motor undor these circumstances becomes a generator. It is
known that a condenser load connected to a generator will cause the
armature reaction to increase the magnetization of the main field which
will raise the terminal voltage and, in turn, the charging current of the
condenser, which will again increase the excitation, and so on, until the
iron of the magnetic circuit becomes saturated. This danger can be
overcome by connecting the condenser on the supply side of the motor
starter through an oil circuit-breaker fitted with a no-volt trip coil connected
across one of the motor phases, so that the condenser is automatically
switched off when the motor is shut down. This arrangement retains
practically the full advantages of individual motor correction, and if the
starting switch and condenser circuit-breaker are placed close together there
will be little danger of the condenser being overlooked when the motor is
started.
Location of Units
When condensers are required to reduce the kVA maximum demand
purely for tariff purposes, a single unit is usually installed at the supply
point, where, if a bulk supply is taken, it sometimes may be connected
permanently, preferably through an isolator, to the l.t. side of the transformer. Generally the question of control gear has to be considered, bearing
in mind the special duties and characteristics of the condenser. Where the
l.t. condenser line currents are below about 50 amps, an airbreak metalclad switch and fuse-box may be used; but for larger currents it is desirable
to use an oil circuit-breaker fitted with overload trip and no-volt release
coils. The circuit-breaker should be rated liberally or it will be found
that the maintenance cost of the contacts is abnormally high due to the
heavy current surges which occur when switching in a large condenser. In
general, such condensers can be switched in and out by hand, but in
certain cases, where the kVAr rating is in excess of the desired kVA maximum demand, due to the original power factor being very low, special
care is required in arranging the control of the condenser, as obviously it
— 179 —
Haigh — Static Improvement of Power Factor
should not be in circuit at times of light load. This can be done automatically if the installation be large enough to warrant the expense—the
method employed being to use contactors operated by a relay actuated
either by the kW load or, preferably, by the reactive kVAr and set to
switch the condenser into and out of circuit at a load value which will not
endanger the new maximum kVA demand figure. Another way of overcoming the difficulty, where the plant is suitable, is to install as large a
condenser as possible at the supply point and a number of smaller ones
to make up the total required leading kVAr, the latter being placed across
the terminals of certain motors which are only used when the plant is
running near its full capacity.
A compromise between correction at individual motors and bulk cor-
200-hVAr
440-volt
3-phase oil-immersed
•paper condenser
rection usually gives the best results, especially in large factories where
there are long lengths of cable feeding a number of distribution points.
Condensers placed at these distribution points, or at selected load centres,
will result in a considerable saving in distribution losses and improvement
of voltage regulation. Firms of large size usually employ a qualified
electrical engineer, who would arrange for the switching of the condensers
at given times or according to the variations of the mean load. In
smaller firms where the electrical plant is cared for by a less experienced
man under the maintenance engineer, some simple form of automatic
control is often required. Under such circumstances reliable and satisfactory control of the condenser is obtained by means of a contactor, the
operating coil of which may be either electrically interlocked with a
" key " motor starter or energized by an electric time switch.
Furnace Loads
A special application of static condensers for power factor improvement is that of high-frequency induction furnaces, in which the load and
— 180 —
Students' Quarterly Journal
June 1935
power factor vary over a very wide range during short intervals of time.
The method usually employed is to have a bank, consisting of a number
of condensers, connected in parallel with the furnace circuit so arranged
that units can be switched in and out by an attendant as required, so as
to give approximately the same readings on two ammeters inserted
respectively in the condenser and furnace circuits.
The need for the automatic switching of a bank of condensers in a
number of steps, depending on varying load conditions, does not often
occur in practice, as condenser installations are designed usually for the
maximum load. The power factor at average or reduced load conditions
is of little importance, either in the case of a private generating plant or
when power is purchased under a maximum demand tariff, providing it
does not attain an excessively high leading value. There are some plants,
however, where the load may vary considerably between maximum
demand and no-load during working hours, and then possibly it may be
desirable to have a variable equipment.
In conclusion the author desires to express thanks to Messrs. Johnson
& Phillips, Ltd., for permission to publish the photographs.
A FOUNDRY
SCENE
by Eric Wilkinson
It is entitled
" I couldn't find a halfinch sieve so I brought
two quarter-inch ones "
— 181 —
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