Current wave of a "switch mode" power supply
ENERGY EFFICIENT SYSTEMS ROLE OF CAPACITORS.
Mr. S. Prabhu. , Mr. Sachin. V. Shelar.
MADHAV CAPACITORS PVT. LTD.
Now a days there is increasing awareness about saving energy, utilising the available energy efficiently by employing energy efficient devices. Thus, there is a trend to convert conventional systems to energy efficient systems.
Capacitor, which improves the power factor of the system, forms an integral part of the energy efficient system.
Industry is rapidly changing over to energy efficient drives, devices these are listed as below.
etc.
Though these devices facilitate the use of energy efficiently they reduce the power factor of the system, inject harmonics in the system. These problems further gives rise to another problems and ultimately there is reduction in the overall efficiency of the system.
Devices and problems caused by them :
1. DC drives :
DC drives are used as they give better efficiency than that of AC drives also the speed control of DC drive is easy.
AC supply is converted into DC supply using converters. Converters reduces power factor introduces harmonics.
AC------> Converter------> DC
2. Frequency converters :
These are used to control the speed of AC drives. First AC supply is converted into DC supply, which is again converted into AC with different frequency. This also reduces the power factor and introduces harmonics.
AC(50 Hz) ------> DC(Using Converter) ------> AC (Using Inverter) (Variable frequency)
3. Electronic items :
For accurate and automatic control there is increase in use of electronic controls, also use of computers has been increased. All these electronic items require switched mode power supply, which draws current over a part of each half cycle as shown. This reduces the power factor.
Current wave of a "switch mode" power supply
These lamps draw an almost instantaneous current over a short portion of the voltage wave. This reduces the power factor.
Current Wave of a C.F. Lamp.
All these devices form what is called as nonlinear load, which not only causes the problem of low power factor but also problem of harmonics.
In an electrical distribution system , low power factor, harmonics can not be tolerated as it reduces the overall efficiency of the system and it also affects the working of other devices.
To improve the power factor use of capacitor is essential, where harmonic level is high only capacitor does not serve the purpose and use of power factor correction capacitors along with the harmonic filter becomes essential.
Reactive power compensation is very important as it not only improves the efficiency of the system but also reduces the penalty for low power factor
Benefits of high power factor :
Capacitor itself is an energy efficient product as it has low power loss, its efficiency as high as 99.9%. In addition to this, capacitors has low initial cost, flexibility in choosing rating, compact size, easy installation, less maintenance etc.
To install capacitors in a system we should study the system i.e. we should have certain data such as load in KW, existing power factor etc. Now we should decide the new required power factor, then we can calculate the required KVAR to improve the power factor very easily with the help of following table.
Capacitor KVAR for Power Factor Correction.
Ratio of Capacitor KVAR/Existing load in KWS to raise the power factor to
|
New Power Factor
|
||||||||
|
Existing Power Factor |
0.85 |
0.90 |
0.95 |
0.96 |
0.97 |
0.98 |
0.99 |
1.00 |
|
0.40 |
1.668 |
1.805 |
1.959 |
1.998 |
2.037 |
2.085 |
2.146 |
2.288 |
|
0.50 |
1.112 |
1.248 |
1.403 |
1.441 |
1.481 |
1.528 |
1.590 |
1.732 |
|
0.60 |
0.714 |
0.849 |
1.005 |
1.043 |
1.083 |
1.131 |
1.192 |
1.334 |
|
0.65 |
0.549 |
0.685 |
0.840 |
0.878 |
0.918 |
0.966 |
1.027 |
1.169 |
|
0.67 |
0.488 |
0.624 |
0.779 |
0.817 |
0.857 |
0.905 |
0.966 |
1.108 |
|
0.68 |
0.459 |
0.595 |
0.750 |
0.788 |
0.828 |
0.876 |
0.937 |
1.079 |
|
0.69 |
0.429 |
0.565 |
0.720 |
0.758 |
0.798 |
0.840 |
0.907 |
1.049 |
|
0.70 |
0.400 |
0.536 |
0.691 |
0.729 |
0.769 |
0.811 |
0.878 |
1.020 |
|
0.71 |
0.372 |
0.508 |
0.663 |
0.701 |
0.741 |
0.783 |
0.850 |
0.992 |
|
0.72 |
0.343 |
0.479 |
0.634 |
0.672 |
0.712 |
0.754 |
0.821 |
0.963 |
|
0.73 |
0.316 |
0.452 |
0.607 |
0.645 |
0.685 |
0.727 |
0.794 |
0.936 |
|
0.74 |
0.289 |
0.425 |
0.580 |
0.618 |
0.658 |
0.700 |
0.740 |
0.909 |
|
0.75 |
0.262 |
0.398 |
0.553 |
0.591 |
0.631 |
0.673 |
0.713 |
0.882 |
|
0.76 |
0.235 |
0.371 |
0.526 |
0.564 |
0.604 |
0.652 |
0.687 |
0.855 |
|
0.77 |
0.209 |
0.345 |
0.500 |
0.538 |
0.578 |
0.620 |
0.661 |
0.829 |
|
0.78 |
0.183 |
0.319 |
0.474 |
0.512 |
0.552 |
0.594 |
0.633 |
0.808 |
|
0.79 |
0.156 |
0.292 |
0.447 |
0.485 |
0.525 |
0.567 |
0.608 |
0.776 |
|
0.80 |
0.130 |
0.266 |
0.421 |
0.459 |
0.499 |
0.541 |
0.582 |
0.750 |
|
0.81 |
0.104 |
0.240 |
0.395 |
0.433 |
0.473 |
0.515 |
0.556 |
0.724 |
|
0.82 |
0.078 |
0.214 |
0.369 |
0.407 |
0.447 |
0.489 |
0.530 |
0.698 |
|
0.83 |
0.052 |
0.118 |
0.343 |
0.381 |
0.421 |
0.463 |
0.504 |
0.672 |
|
0.84 |
0.026 |
0.162 |
0.317 |
0.355 |
0.395 |
0.437 |
0.478 |
0.645 |
Illustrative Example. :
Then Capacitor KVAR/Load Kws ratio from the above table is 0.685. Therefore Capacitors required are 131 x 0.685 = 89.735 KVAR.
The Capacitors are rated at 440 volts to allow for high voltages prevailing on the systems. But they operate let us say at an average of 400 volts. Therefore an upward revision by 1.21 times is called for. The customer will require 1.21 x 89.74 = 108.6 KVAR. Choose 110 KVAR, which comes within standard sizes.
A desirable combination would be 50 : 25 x 2 and 10 KVAR.
General layout of system, only power factor correction.
We should not install more KVAR also we should not install less KVAR than the required KVAR as both give disadvantages, secondly we should decide optimum location of capacitors, it is preferable to install the capacitors as near to the load as possible. Further if there is wide variation in the load pattern we should go for automatic power factor control method in which capacitors are switched ON/OFF according to the load.
Energy efficient devices brings along with it problems of harmonics. So it is better to carry out harmonic survey and eliminate them by installing harmonic filters.
Harmonics have many disadvantages such as,
A normal power factor correction bank is divided into two parts. One is normal power factor correction bank, only for power factor correction purpose and second is harmonic filter bank, which besides reducing the harmonics also improves the power factor.
General layout of system along with harmonic filter.
A harmonic filter consists of a tuning reactor in series with the capacitor, it is connected in parallel with the normal untuned capacitor bank. KVAR rating of reactor and capacitor is higher as it has to carry normal power frequency current along with the harmonic current, also the voltages across them is high.
Now a days there is increasing awareness about the harmonics and its control. Although, presently there is no legislation to control the level of harmonics it will be introduced in near future and the consumer which is generating harmonics beyond specified limits may have to pay penalty.
There are standards like IEEE 519-1992, which recommends limits on the voltage and current distortion. CBIP has also taken steps in this direction and has published a guide for limiting voltage harmonics while guide for limiting current harmonics is in pipe line. Also current distortions limits are gaining increasing importance over the voltage distortion limits.
Table 1 Current distortion limits for general distribution systems.
(120 volts through 69000 volts)
Maximum harmonic current distortion in percent of IL
Individual harmonic order (odd harmonics)
|
ISC / IL |
<11 |
11<h<17 |
17<h<23 |
23<h<35 |
35< h |
TDD |
|
<20 |
4.0 |
2.0 |
1.5 |
0.6 |
0.3 |
5.0 |
|
20<50 |
7.0 |
3.5 |
2.5 |
1.0 |
0.5 |
8.0 |
|
50<100 |
10.0 |
4.5 |
4.0 |
1.5 |
0.7 |
12.0 |
|
100<1000 |
12.0 |
5.5 |
5.0 |
2.0 |
1.0 |
15.0 |
|
>1000 |
15.0 |
7.0 |
6.0 |
2.5 |
1.4 |
20.0 |
Even harmonics are limited 25% of odd harmonic limits above.
(When ISC / IL is say 1000, then a maximum limit of 12 % is allowed for the 11th harmonics. In other words, presence of 11th harmonics below 12% value does not strictly call for a filter.)
The IEEE standard 519 specifies voltage distortion limits as per the table below.
|
Bus voltage at PCC |
Individual voltage distortion % |
Total voltage distortion THD % |
|
69 kV and below |
3.0 |
5.0 |
|
69 kV - 161 kV |
1.5 |
2.5 |
Following example gives general idea and values for harmonic filter.
Data :
By calculation :
KVAR required = 1158 KVAR.
Out of this 30 % are used for harmonic filter.
Thus,
Untuned power factor bank = 808 KVAR.
Tuned harmonic filter = 350 KVAR.
KVAR rating of reactor = 399 KVAR.
KVAR rating of capacitor = 1203 KVAR.
(It can be seen that individual rating of reactor and capacitor is quite high.)
The cost of harmonic filter is many times the normal capacitor bank but the advantages gained are worth.
Capacitors play very important role in making a system energy efficient. A energy efficient system has low losses, high efficiency and capacitor helps in reducing losses and gaining high efficiency. Cost of the capacitor is low, also the payback period of capacitors is less. Life of capacitors is fairly high approximately 10 years, it has less maintenance.
In near future all conventional drives will be replaced by energy efficient drives , which brings along with it the problems of low power factor and harmonics. These problems can be easily solved by using capacitors and total system can be made more energy efficient.
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