برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات.

دایره المعارف تاسیسات برق (اطلاعات عمومی برق)

من میخوام یک تابلوی بانک خازن طراحی کنم و تجربه کافی ندارم لطفا من را
راهنمایی کنید.


شما ابتدا باید برآورد صحیحی از ظرفیت بانک خازنی مورد نیاز داشته باشید. برای
اینکار لازم است میزان ضریب توان مطلوب که آن نیز از برآوردهای اقتصادی برای تعیین
ضریب توان اپتیمم بدست می آید، نتیجه شود. اگر میزان بار اکتیو شما P باشد، ظرفیت
بانک خازنی مطلوب برای رسیدن از زاویه توان φ1 به زوایه مطلوب φ2 نیازمند ظرفیت
جبران سازی معادل  Qc مطابق رابط زیر خواهیم بود.





این ظرفیت می توان به طور یکجا برای جبران سازی اثر یک ترانس یا موتور
الکتریکی به سیستم اعمال شود و یا اگر توان P شامل یک گروه از بارهای جزئی تر
مانند  موتورهای الکتریکی کوچکتر است، مقدار ظرفیت جبران سازی می تواند بصورت
پلکانی از  طریق رله تنظیم کننده مخصوصی که بطور اتوماتیک بانکهای خازنی جزئی را
وارد مدار می  کند، به سیستم اعمال شود.





به برخی ملاحظات فنی در خصوص تابلو و طراحی سیستم جبران ساز اتوماتیک
برگرفته از یکی از هندبوکهای تأسیسات الکتریکی توجه کنید.


Since compensating system devices is also associated with the
same  power system as a switchgear assembly, it should generally meet the same
specifications except for small variations in operating conditions and  test
requirements. Capacitors generate excessive heat when in service. Installations
employing large capacitor banks must therefore have a capacitor mounting
structure suitable to dissipate heat freely and permit circulation of fresh air
during normal operation. To achieve this, open-type enclosures are usually
preferred when it is possible to house the panel in a separate room or mount
the  units on a structure in an open switchyard. It may also be provided with an
expanded metal enclosure. Forced cooling within the enclosure or the room where
such banks are installed is common practice to dissipate the excessive heat.
For  more details refer to the following publications:
IEC 6083 1 - 1 and
6093 1 - 1 for LT, and IEC 6087 1 - 1 and
IEC 60143- 1 for HT systems
When
selecting the voltage of the capacitor units care must be taken that during
operation the voltage across the capacitor units does not fluctuate beyond  +/-
l0%. If this happens the voltage rating of the capacitor units must be chosen so
that the variation across the units under unfavorable operating conditions does
not exceed
+/- l0%.

Automatic PF correction of a
system


For an industrial or power plant application or an
installation with a number of inductive load points a group capacitor control
is  always more effective, simple and economical. Such an installation generally
has  a frequent variation in its load demand due to some feeders coming on the
bus  and some falling out at different times. There may be variation in the
individual feeder’s load demand, such as a tool room, where not all the
machines  will be working at a time, or a pulp mill and paper mill, where the
paper mill  has a continuous load, the pulp mill an intermittent one. A water
treatment  plant or a pump house are similar installations where all or some of
the loads  would be in operation at a time. For such installations, the total
capacitive  load demand at a required power factor level is worked out and the
total  capacitor banks are installed at a convenient point and suitably grouped
(banked) for the type of loading and system demand. Each bank is controlled
through a power contactor and a common power factor correction relay to
automatically monitor and control the power factor of the system to a
predetermined level, preset in the relay, by witching a few capacitor banks ON
or OFF, depending upon the load demand and the power factor measured by the
relay. The relay actuates the required number of capacitor feeders through
their  contactors.
Automatic correction is always recommended to eliminate
manual  dependence and to achieve better accuracy. It also eliminates the risk
of a  leading power factor by a human error that may cause an excessive voltage
at the  motor and the control gear terminals.
The following example
illustrates the  method of selecting the capacitors’ value, their grouping and
their control for  a system having a number of load points.



Switching
sequences

A switching sequence that can be employed for a
particular  load cycle may be one of the following:

First in last
out

This is the simplest type of switching. Capacitors are switched
ON in the sequence of 1-2-3 . . . n and switched
OFF in the reverse sequence
i.e. n . . . 3-2-1. In this switching the last switched capacitor is made to
switch again. This switching is therefore more stressful for the capacitor as
well as for the system, due to surge voltages.
During a switch ON therefore
some time delay must be introduced into the switching circuit for the capacitor
charge to decay to a safe level. In this sequence, the capacitors of each step
are normally the same. It is a primitive and unscientific switching sequence
and  therefore not in much use.

True ‘first in first out’
(FIFO)

Capacitors are switched ON in the sequence of 1-2-3 . . . . n
but an additional logic is used to switch OFF in the same sequence 1-2-3 . . .
n, i.e. the oldest OFF capacitor is switched ON first and the oldest ON
capacitor is switched OFF first. This apparently is the best switching
sequence,  giving enough time to an OFF capacitor to discharge before it is
switched again.  Each stage capacitor rating must be equal to avoid a wide
fluctuation in the  p.f. correction and hence undesirable subsequent switchings,
which may be  necessitated when the capacitors are not of equal
ratings.

Pseudo  (false) ‘first in first out’
In a
way this is more appropriate than  the FIFO, for it can have unequal stage
capacitors. Capacitors, however, are  arranged in ascending order such as 10,
20, 50 . . . kVAr etc., or large or  small capacitors. But, as mentioned above,
this may not provide accurate  correction in the first instance. Now the relay
may have to operate on the  theory of probability and resort to a lot of
sequencing to arrive at the right  correction. Such sequencing may also lead to
hunting and cause voltage  fluctuations, because of varying p.f. s and also
cause switching currents and  voltage surges.

1+2+2.
..

The first capacitor is half  the rating of the remaining ones,
which are all equal. The p.f. correction is  delayed, due to allowing discharge
time. The smallest unit is more  stressed.

Direct combinatorial
or auctioneering

Here  the relay assesses the kVAr  requirement of
the system and switches ON all the  required capacitors simultaneously.
Theoretically any size of capacitor units  can be used. But when a capacitor is
taken out for maintenance, this can create  confusion in the p.f. correction.
The relay will not be aware of this and act  accordingly. The bank size may also
be large and cause switching  surges.

Special
sequencing

These are sequencers and  can sequence the switching of
capacitors in any fixed pattern. Capacitors can be  automatically taken out of
the circuit and others introduced in their place by a  device known as ‘the load
rotator.
A good relay can be modified to perform  a particular switching
sequence during ON and OFF and both sequences need not be  same. The following
is a type of relay that can be modified to perform any  desired switching
pattern.


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