Asecure and sufficient electricity supply is crucial for industry driven economic growth. To exemplify with a fast developing nation, India has plans to increase its installed capacity by 50 per cent between 2007 and 2012. During the same period, the interregional transmission capacity between the Indian regions is likely to be doubled.

Such a rapid development of power systems requires good planning and practices of providing effective Reactive power compensation devices like capacitors.

Capacitor is one of the most efficient and very high power density devices. However, being a low impedance member in an electrical system, all polluting elements like harmonics tend to flow towards it and makes its application more complicated.

This paper gives major application of capacitors, benefits, associated problems and possible solutions.

APPLICATION
One of the most important benefits of providing capacitors is to reduce losses of the power system. T&D losses in the Indian context, even at a conservative estimate, are around 40 per cent. This includes commercial and technical losses. Some of the major factors to the high technical losses are uses of lower conductor sizes, low voltages; poor power factors that are lack of reactive power in the system. In India major portion of losses is accounted for in the low voltage sections. Low voltages and poor power factors of induction motors driving agriculture pumps compound this problem. The reflection of the problem is even felt up to high voltage side. For example a low voltage of 320kV was reported on 400kV line indicating severe shortage of reactive power. Because of acute shortage of MVAR, the substantial portion of generation capacity is getting wasted.

Table 1: Cost Comparison
Equipment of 11kV capacitor	Cost in multiple of single-step cost
	(`100/kVAr)
	Capacitors	Capacitors	Capacitors
	in 1-step	in 2-step	in 3-step
Total 2000kVAr 11kV capacitors	1.00	1.10	1.20
6% Series reactors	1.12	1.80	2.22
RVT	0.15	0.30	0.45
Outdoor breaker	1.50	3.00	4.50
Isolator	0.20	0.40	0.60
Structures	0.50	1.00	1.50
Control panel	0.60	0.90	1.20
Interconnection	0.05	0.10	0.15
Total cost of supply	5.12	8.60	11.82
Cost in terms of 1-step	1.00	1.68	2.31

The ideal way of attacking the problem is to provide KVAR at the point where it is required that is at motor terminal of agriculture pumps. State utilities with finances from Rural Electrification Corporation (REC) is very well aware of the problem and has done number of experiments like making installation of capacitor compulsory for providing connection, free distribution of capacitors, providing capacitors on LT transformer feeding supply to distribution net work including agriculture pumps, automatically switched capacitors etc. But so for success rate is not satisfactory. Reasons can be attributed to: no incentives to farmers providing capacitors, poor quality of capacitors, malpractices etc.

The next convenient point is on 11kV side. This is midpoint of EHV lines and consumers to improve voltages and reducing losses. Pole mounted capacitors with auto switches were provided in large numbers. Similarly substation capacitors both at 11kV and 33kV were provided but this also failed. First apparent reason was quality of capacitors and second was over compensation during lean load conditions. Manual capacitors used to be switched off during nighttime and they continue to remain off during peak load hours. Present scenario remains injecting flexible power that is automatic switched Capacitor system (APFCR) either directly on 11kV side or on 44VAC side at 11kV substation by providing one 11kV/440VAC transformer the new trend is providing automatically stage switched capacitors at 11kV side.

CAPACITOR WITH AUTOMATIC POWER FACTOR CONTROLLERS
Automatic PF correction panel are very common in use in 415/440V system. These panels ensure correction of the PF to desired level automatically which saves the botheration of maintenance engineer to constantly monitor the PF. It is the normal practice to provide HT capacitors as fixed type to achieve major correction in PF and then fine-tune the PF with LT APFC panels. Nowadays customers are also going in for automatic PF correction in high voltage like 3.3kV, 6.6kV and even 11kV. Due to the higher cost of breakers, contactors and repetition of other equipments like reactors, RVTs, isolators etc, the cost of providing automatic analysis in terms of capacitor cost for 1, 2 & 3 steps automatic correction is high.

 It can be seen from Table 1 that for 2-step cost is more than 1.5 times the cost of single step correction. Three steps scheme can be more than two times the cost. If you add erection and civil work then the cost of multiple steps will further escalate. One should therefore make a prudent choice before going in for multi step PF correction scheme in high voltage capacitors. It is to be remembered that the purpose of providing capacitors is to save on electricity cost. If the cost of capacitor system is very high the pay back becomes very long.

Considering all above, the best option in the present scenario remains injecting flexible power that is automatic switched capacitor system (APFCR) either directly on 11kV side or on 44VAC side at 11kV substation by providing one 11kV/440VAC transformer.

Table 2: Voltage distortion limits
Bust voltage at PCC	Individual Voltage Distortion (%)	Total Voltage Distortion THD (%)
69kV and below	3.0	5.0
69.001kV up to 16.1kV	1.5	2.5
161kV and above	1.0	1.5

HT Capacitor bank at 66kV,132kV and 220kV has also been installed. The loads average out here and fluctuation s between maximum and minimum are also not very high. This takes care of minimum base load. This place has been found satisfactory but major benefits are felt at Generators and transmitters, and gives marginal improvement in the overall system. There are other advantages of capacitors some of which are:

• Improves voltage level of the system where they are connected
  
• Reduction in load current reduces resistive losses, which in turn reduces unit consumption.
  
• Releases capacity of transformer, which can then take more loads
  
• Helps in reducing unnecessary tripping of motors due to voltage dips
  
• Capacitors, with tuned reactors, filter out unwanted harmonics and reduce THD levels to within limits of IEEE-519.
  

CAPACITORS AS HARMONIC FILTERS
Increasing use of nonlinearly behaving power electronic converters in industries and buildings has resulted in generation of harmonic voltages and currents in considerable levels. These harmonics often create problems for the stable and economic operation of power systems. Efforts are being made to reduce the level of harmonics through the introduction of guidelines, recommended practices and standards. A commonly adopted solution for this problem is to install harmonic filters at suitable location. Harmonic filter consists of properly designed Capacitor- Reactor combination.

This serves the dual purpose of harmonic filter and improvement of power factor. Software has been developed to check the effect of various Filters, their effect on Total Harmonic distortion levels. Based on optimum harmonic filter systems are designed. In India large number of such effective Filters are in operation at 440VAC, 11kV and 33kV sides. IEEE-519 has specified limits of THD (Total Harmonic Distortion), which are given in Table 2.

Capacitors offer lower impedance to higher harmonics. Hence harmonics flow to capacitors causing overloading and possible premature failure. There is also a risk of resonance with any of the harmonic present in the system. In such a case it can damage not only capacitor but also other equipment in the line. Even if there is no resonance there is magnification of harmonic currents causing higher THD, which may exceed limits specified in IEEE-519.

It is very important to provide properly designed harmonic filter after carrying out and analysis of proper harmonic with different load conditions, eliminating the possibility of resonance, checking transformer fault level, impedances etc.

SIZING OF CAPACITOR BANK AND SERIES REACTOR
When capacitor bank voltage is increased its out put increases in square proportion. For example1 MVAr capacitor if required at 11kV and the rated voltage is chosen as 12.1kV, then the corrected rating at 12.1kV will be:

Rating of Capacitor in MVAr
= 1* (12.1*12.1/11*11)
= 1.21

When capacitor bank is provided with series reactor, say 6 per cent, the rating of capacitor needs to be corrected for loss in series reactor? For the same requirement of 1 MVAr at 11 KV, the rated voltage is chosen as:

Rated Voltage =11*1.1/(1- 0.06) kV
                       = 12.87 kV

Capacitor impedance at 11kV bus,
Xc = 11*11 / 1
      = 121 ohms

Corrected capacitor impedance
Xc' = Xc/(1 - 0.06) ohm
      = 121/0.94 ohms
      = 128.72 ohms

Hence
Cap. Output = KV*KV / Xc'
                    = 12.87*12.87/128.72
                    = 1.28 MVAr
S R imp.       = 0.06* Xc' = 7.73 ohms

Hence 1.28 MVAr, 12.87kV capacitor bank with 6 per cent series reactor will give 1 MVAr at 11 KV bus.

(P.K. Bhandari is Senior Vice President—Capacitor, Universal Cables Ltd, Satna, Madhya Pradesh)
 

Table 3: Maximum harmonic current distortion in % of IL
(Current distortion limits for general distribution systems: 120V to 69,000V)
Individual harmonic order (odd harmonics)
Isc/IL	h <11	11h<17	17h<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
Note:
Even harmonics are limited to 25 per cent of the odd harmonic limits above
Current distortion that results in a direct current offset (e.g. half wave converters not allowed)
*All power generation equipmetn is limited to these values of current distortion, regardless of actual Isc/IL
Isc=Maximum short circuit current at point-of-common-pooling
IL=Maximum demnd load current (fundamental frequency) at point of common coupling
H=harmonic number

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