12/24/2018
Capacitor Banks In Power System (part four)
Electrical Lectures / Energy and Power
Capacitor Banks In Power System (part four)
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Capacitor Banks In Power System (part four)
Alternator
Alternator capability
capability curve
curve -- Green
Green area
area is
is normal
normal operating
operating range
range of
of aa typical
typical synchronous
synchronous
machine,
machine, yellow
yellow is
is abnormal
abnormal but
but not
not damaging
damaging and
and operating
operating in
in red
red regional
regional will
will cause
cause
damage
damage or
or misoperation.
misoperation.
Continued from technical article: Capacitor Banks In Power System
(part three)
PF correction for loads connected on
captive Diesel Generator (DG)
Let us consider that there is a captive diesel generator the rating of which
is specified as 1000kVA and PF 0.85. Rating in kVA specifies the
maximum current the alternator can deliver at the system voltage.
In the previous parts of this article we have seen that the role of power
capacitors in improving the power factor and reducing total cost of
electricity in an industrial installation is well established with regard to
supply of power from the Utilitys/utilities.
Hence it seems logical to extend the above application of power
capacitors when power is drawn from captive diesel generator to optimize
their performance.
It is however a common practice that DG set users generally switch
off capacitors or do not install capacitors at all when the DG set is in
use because of the following reasons:
1. Apprehension that the DG set may get over loaded due to the
fact that the kVA rating or current delivered by the DG set is
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Capacitor Banks In Power System (part four)
generally considered as the indicator of output of DG set. It is
well known that use of capacitors will reduce the kVAR
requirement from DG and hence kVA requirement will go down
which in turn will reduce the current drawn from the DG set and
could thus tempt the to add more loads on a given DG set.
.
2. The other reason for such an opinion is related to the risks
arising due to sustained leading power factor conditions that
would occur with the use of fixed capacitors in variable load
situations.
However with meticulous application of PF correction capacitor we can
improve the overall efficiency of DG set operation and result in
considerable economic benefits to the DG set user.
This article tries to analyze the same in the following paragraphs.
Diesel Generator Set Rating in kVA
As we have considered 1000kVA DG. This way of specifying the DG rating
is very logical because specifies the maximum current the alternator can
deliver at the system voltage.
DG set Rating in kVA at a particular PF
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The diesel generator which we had assumed was of 1000kVA at 0.85PF.
The relevance of PF in case of DG rating is as follows:
1. To find mechanical power rating of a diesel engine for a particular
diesel generator, first convert kVA to kW and thereafter kW to BHP.
This can only be done if we assume a certain average Power Factor
(PF) under which the DG set would operate.
.
2. The power factor so assumed should be in line with the average
power factor prevalent in the industry. A typical industrial load
comprises of induction motors (typical PF of 0.8 to 0.85), non-linear
loads (typical PF of 0.5 to 0.6) and combination of unity PF loads
(Resistive heating and incandescent lighting). Hence assuming an
average power factor of 0.85 for typical industrial loads is considered
acceptable by convention.
.
3. Consequently a power factor of 0.85 is used for calculating the kW,
which is then converted to the BHP rating of the prime mover. BHP
rating so obtained is the output of the prime mover. Considering
suitable engine losses it becomes possible to calculate the power
rating of the engine.
Now after understanding the DG set name plate rating parameter, let us
come back to the question should we connect the Capacitor Banks in
parallel to the loads conned to DG? Answer is YES, It is however,
important to ensure that under actual operating conditions the kW loading
and current loading should not be exceeded.
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Power Factor of loads supplied by DG sets can therefore be improved
closer to unity by use of suitable Reactive Power Compensation Systems
keeping in view the rated current loading is not exceeded.
Let us consider an example for the same:
** Any industry has a 1000 kVA DG set which is loaded at an average of
600 kW at 0.7 PF. In addition, there are 125 kW of other loads within the
same installation, which are not loaded on the DG set due to capacity
restrictions that arise during occurrence of short-term peak loads, such as
motor starting, and intermittent welding load. Due to this, productivity in
the Industry is lowered when the DG Set is in operation.
During the period when Utility supply is available all loads can be
operated. Is it possible to improve productivity when DG Set is in
operation?
** A well designed power factor correction capacitor bank panel can
improve the cost of electricity consumed from utility as well as improve
productivity when DG Set is in operation.
• DG rated capacity = 1000 kVA
• kW of load connected to DG = 600 kW
• Average load power factor in industry where DG is installed =
0.7
• kVA drawn at normal condition = 600 / 0.7 = 857 kVA
Hence percentage load on DG without Capacitor bank = 857
/1000 = 85.7%
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Now if we connect the suitably sized and designed (already discussed in
part1 to 3) capacitor bank in parallel to the loads connected to DG and
improve the average overall load power factor from 0.7 to 0.85 than for the
same percentage loading of 85.7% that is 857kVA the active power that
can be drawn is = 857 x 0.85 = 728.45 kW
Hence one can see the moment capacitor bank is connected in parallel to
the loads connected to the DG the additional requirement of 125kW is
comfortably met without exceeding the percentage loading on DG.
During the period when the Industry is using supply from the Utility the
Capacitor banks system can ensure consistently high PF, thereby
achieving demand savings and reduction in losses and elimination of any
PF penalty. Consequently, cost of electricity consumed from the EB will be
minimized.
The same Capacitor banks system can be also used when the Industry is
using supply from the DG set. The fast acting property of the Capacitor
banks system will reduce the peak load requirements that are to be met
from the DG set. This is achieved by providing instantaneous
compensation from the Capacitor banks system during conditions when
motors are started and / or welding machines are being operated. This will
enable the Industry to transfer the 125 kW of additional load on to the DG
set and ensure that productivity is improved when the DG set is in
operation.
Due to better loading, the DG set efficiency will improve as for
same 857 kVA; Active power now delivered is now 728.45 kW
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instead of 600 kW.
REACTIVE POWER COMPENSATION SYSTEMS by Capacitor Banks can
enable D.G set users to reconfigure their loads / D.G sets to achieve
better percentage loading and efficiency on the machines. As a result
reduction in cost / kWh can be attained.
Impact of leading kVAR on generators
Now since we have very well established that a suitably designed
Capacitor Banks can be connected in parallel to the loads connected to
DG. However what is the impact if one keeps on improving the power
factor and the power factor goes on leading side.
Some inherent characteristics of an alternator limit the amount of leading
kVAR that can be absorbed by a DG. We cannot go on switching ON the
Capacitor Banks as and when required, this can create over voltage
condition in DG and subsequently over fluxing.
There is a reverse kVAR limit of every generator.
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The ability of any generator to absorb the kVAR is termed as reverse
kVAR limit. This ability is defined as reactive capability curve. Below figure
shows typical generator reactive capability curve. X axis is the kVAR
produced or absorbed (positive to the right). Y axis indicates the kW
(positive going up). kVAR and kW are shown as per unit quantities based
on the rating of the alternator (not necessarily the generator set, which
may have a lower rating.
The normal operating range of a generator set is between zero and 100
percent of the kW rating of the alternator (positive) and between 0.8 and
1.0 power factor (green area on curve). The black lines on the curves
show the operating range of a specific alternator when operating outside
of normal range. Notice that as power factor drops, the machine must be
de-rated to prevent overheating. On the left quadrant, you can see that
near-normal output (yellow area) can be achieved with some leading
power factor load, in this case, down to about 0.97 power factor, leading.
At that point, the ability to absorb additional kVAR quickly drops to near
zero (red area), indicating that the AVR is “turning off” and any level of
reverse kVAR greater than the level shown will cause the machine to lose
control of voltage.
A good rule of thumb for generators is that it can absorb about 20% of its
rated kVAR output in reverse kVAR without losing control of voltage.
However, since this characteristic is not universal, it is advisable for a
system designer to specify the reverse kVAR limit used in his design, or
the magnitude of the reverse kVAR load that is expected.
Note that this is not specified as a leading power factor limit, but rather as
a maximum magnitude of reverse kVAR.
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Asif Eqbal
Bachelor of Engineering in Electrical & Electronics engineering, from
Manipal University, (Karnataka), India in 2006. Presently involved in the
design of EHV outdoor substation and coal fired thermal power plants for
more than seven years. Motto of joining EEP as a contributor is to share
my little engineering experience and help the budding engineers in
bridging the conspicuous gap between academics and Industrial practice.
“If you have knowledge, let others light their candles with it, so that people
who are genuinely interested in helping one another develop new
capacities for action; it is about creating timeless learning processes".
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