Implementation of capacitor banks

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Capacitor elements

Technology

The capacitors are dry-type units (i.e. are not impregnated by liquid dielectric) comprising metallized polypropylene self-healing film in the form of a two-film roll. They are protected by a high-quality system (overpressure disconnector used with a high breaking capacity fuse) which switches off the capacitor if an internal fault occurs.
The protection scheme operates as follows:

• A short-circuit through the dielectric will blow the fuse
• Current levels greater than normal, but insufficient to blow the fuse sometimes occur, e.g. due to a microscopic flow in the dielectric film. Such “faults” often re-seal due to local heating caused by the leakage current, i.e. the units are said to be “self-healing”
• If the leakage current persists, the defect may develop into a short-circuit, and the fuse will blow
• Gas produced by vaporizing of the metallisation at the faulty location will gradually build up a pressure within the plastic container, and will eventually operate a pressure-sensitive device to short-circuit the unit, thereby causing the fuse to blow

Capacitors are made of insulating material providing them with double insulation and avoiding the need for a ground connection (see Fig. L33).

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a)

b)

Electrical characteristics
Standard		IEC 60439-1, NFC 54-104, VDE 0560 CSA
Operating range	Rated voltage	400 V
	Rated frequency	50 Hz
Capacitance tolerance		- 5% to + 10%
Temperature range (up to 65 kvar)	Maximum temperature	55 °C
	Average temperature over 24 h	45 °C
	Average annual temperature	35 °C
	Minimum temperature	- 25 °C
Insulation level		50 Hz 1 min withstand voltage : 6 kV 1.2/50 μs impulse withstand voltage : 25 kV
Permissible current overload		Classic range(1)	Comfort range(1)
		30%	50%
Permissible voltage overload		10%	20%

Fig. L33: Capacitor element, (a) cross-section, (b) electrical characteristics

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Choice of protection, control devices and connecting cables

The choice of upstream cables and protection and control devices depends on the current loading.
For capacitors, the current is a function of:

• The applied voltage and its harmonics
• The capacitance value

The nominal current In of a 3-phase capacitor bank is equal to:
with:
- Q: kvar rating
- Un: Phase-to-phase voltage (kV)
The permitted range of applied voltage at fundamental frequency, plus harmonic components, together with manufacturing tolerances of actual capacitance (for a declared nominal value) can result in a 50% increase above the calculated value of current. Approximately 30% of this increase is due to the voltage increases, while a further 15% is due to the range of manufacturing tolerances, so that
1.3 x 1.15 = 1.5
All components carrying the capacitor current therefore, must be adequate to cover this “worst-case” condition, in an ambient temperature of 50 °C maximum. In the case where temperatures higher than 50 °C occur in enclosures, etc. derating of the components will be necessary.

Protection

The size of the circuit-breaker can be chosen in order to allow the setting of long time delay at:

• 1.36 x In for Classic range⁽¹⁾
• 1.50 x In for Comfort range⁽¹⁾
• 1.12 x In for Harmony range⁽¹⁾ (tuned at 2.7 f) ⁽²⁾
• 1.19 x In for Harmony range⁽¹⁾ (tuned at 3.8 f)
• 1.31 x In for Harmony range⁽¹⁾ (tuned at 4.3 f)

Short time delay setting (short-circuit protection) must be insensitive to inrush current. The setting will be 10 x In for Classic, Comfort and Harmony range⁽¹⁾ .

Example 1 
50 kvar – 400V – 50 Hz – Classic range

Long time delay setting: 1.36 x 72 = 98 A
Short time delay setting: 10 x In = 720 A

Example 2 
50 kvar – 400V – 50 Hz – Harmony range (tuned at 4.3 f)
In = 72 A
Long time delay setting: 1.31 x 72 = 94 A
Short time delay setting: 10 x In = 720 A

Upstream cables

Figure L34 gives the minimum cross section area of the upstream cable for Rectiphase capacitors.

Cables for control

The minimum cross section area of these cables will be 1.5 mm² for 230 V.
For the secondary side of the transformer, the recommended cross section area is u 2.5 mm².

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Bank power (kvar)		Copper cross- section (mm2)	Aluminium cross- section (mm2)
230 V	400 V
5	10	2.5	16
10	20	4	16
15	30	6	16
20	40	10	16
25	50	16	25
30	60	25	35
40	80	35	50
50	100	50	70
60	120	70	95
70	140	95	120
90-100	180	120	185
	200	150	240
120	240	185	2 x 95
150	250	240	2 x 120
	300	2 x 95	2 x 150
190-210	360	2 x 120	2 x 185
245	420	2 x 150	2 x 240
290	480	2 x 185	2 x 300
315	540	2 x 240	3 x 185
350	600	2 x 300	3 x 240
385	660	3 x 150	3 x 240
420	720	3 x 185	3 x 300

Fig L34: Cross-section of cables connecting medium and high power capacitor banks⁽¹⁾

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Voltage transients

High-frequency voltage and current transients occur when switching a capacitor bank into service. The maximum voltage peak does not exceed (in the absence of harmonics) twice the peak value of the rated voltage when switching uncharged capacitors.
In the case of a capacitor being already charged at the instant of switch closure, however, the voltage transient can reach a maximum value approaching 3 times the normal rated peak value.
This maximum condition occurs only if:

• The existing voltage at the capacitor is equal to the peak value of rated voltage, and
• The switch contacts close at the instant of peak supply voltage, and
• The polarity of the power-supply voltage is opposite to that of the charged capacitor

In such a situation, the current transient will be at its maximum possible value, viz: Twice that of its maximum when closing on to an initially uncharged capacitor, as previously noted.
For any other values of voltage and polarity on the pre-charged capacitor, the transient peaks of voltage and current will be less than those mentioned above.
In the particular case of peak rated voltage on the capacitor having the same polarity as that of the supply voltage, and closing the switch at the instant of supply-voltage peak, there would be no voltage or current transients.
Where automatic switching of stepped banks of capacitors is considered, therefore, care must be taken to ensure that a section of capacitors about to be energized is fully discharged.
The discharge delay time may be shortened, if necessary, by using discharge resistors of a lower resistance value.