Implementation of capacitor banks

Capacitor elements

Technology

Capacitors at low voltage are dry-type units (i.e. are not impregnated by liquid dielectric) comprising metallised polypropylene self-healing film in the form of a two-film roll.

Self-healing is a process by which the capacitor restores itself in the event of a fault in the dielectric which can happen during high overloads, voltage transients, etc.

When insulation breaks down, a short duration arc is formed (Figure L34 - top). The intense heat generated by this arc causes the metallization in the vicinity of the arc to vaporise (Fig. L34 - middle).

Simultaneously it re-insulates the electrodes and maintains the operation and integrity of the capacitor (Fig. L34 - bottom).

Fig. L34 : Illustration of self-healing phenomena

Protection scheme

Capacitors must be associated with overload protection devices (fuses, or circuit breaker, or overload relay + contactor), in order to limit the consequences of overcurrents. This may occur in case of overvoltage or high harmonic distortion.

In addition to external protection devices, capacitors are protected by a high-quality system (Pressure Sensitive Disconnector, also called ‘tear-off fuse’) which switches off the capacitors if an internal fault occurs. This enables safe disconnection and electrical isolation at the end of the life of the capacitor.

The protection system operates as follows:

• Current levels greater than normal, but insufficient to trigger the over-current protection sometimes occur, e.g. due to a microscopic flow in the dielectric film. Such “faults” often reseal due to local heating caused by the leakage current,
• If the leakage current persists, the defect may produce gas by vaporizing of the metallisation at the faulty location. This will gradually build up a pressure within the container. Pressure can only lead to vertical expansion by bending lid outwards. Connecting wires break at intended spots. Capacitor is disconnected irreversibly.

Fig. L35 : Cross-section view of a three-phase capacitor after Pressure Sensitive Device operated: bended lid and disconnected wires

Main electrical characteristics, according to IEC standard 60831-1/2: "Shunt power capacitors of the self-healing type for a.c. systems having a rated voltage up to and including 1000 V".

Electrical characteristics
Capacitance tolerance	–5 % to +10 % for units and banks up to 100 kvar –5 % to +5 % for units and banks above 100 kvar
Temperature range	Min: from -50 to +5°C Max: from +40 to +55°C
Permissible current overload	1.3 x IN
Permissible voltage overload	1.1 x UN , 8 h every 24 h 1.15 x UN , 30 min every 24 h 1.2 x UN , 5min 1.3 x UN , 1min 2.15 x UN for 10 s (type test)
Discharging unit	to 75 V in 3 min or less

Fig. L36 : Main characteristics of capacitors according to IEC 60831-1/2

Choice of protection, control devices and connecting cables

The choice of upstream cables, protection and control devices depends on the current loading.

For capacitors, the current is a function of:

• The system voltage (fundamental and harmonics),
• The power rating.

The rated current I_N of a 3-phase capacitor bank is equal to:

[math]\displaystyle{ I_N = \frac{Q}{\sqrt{3}. U} }[/math]

with:

• Q: power rating (kvar)
• U: phase-to-phase voltage (kV)

Overload protection devices have to be implemented and set according to the expected harmonic distortion. The following table summarizes the harmonic voltages to be considered in the different configurations, and the corresponding maximum overload factor I_MP/I_N. (I_MP = maximum permissible current)

Configuration	Harmonic order					THDu max (%)	IMP/IN
	3	5	7	11	13
Standard capacitors						5	1.5
Heavy Duty capacitors						7	1.8
Capacitors + 5.7% reactor	0.5	5	4	3.5	3	10	1.31
Capacitors + 7% reactor	0.5	6	4	3.5	3	8	1.19
Capacitors + 14% reactor	3	8	7	3.5	3	6	1.12

Fig. L37 : Typical permissible overload currents

Short time delay setting of circuit breakers (short-circuit protection) should be set at 10 x I_N in order to be insensitive to inrush current.

Example 1:

50 kvar – 400V – 50 Hz – Standard capacitors

[math]\displaystyle{ I_N = \frac{50}{\sqrt {3}\times 0.4} = 72A }[/math]

Long time delay setting: 1.5 x 72 = 108 A Short time delay setting: 10 x 72 = 720 A

Example 2:

50 kvar – 400V – 50 Hz – Capacitors + 5.7% detuned reactor

I_N = 72A

Long time delay setting: 1.31 x 72 = 94 A Short time delay setting: 10 x I_N = 720 A

Upstream cables

Figure L38 gives the minimum recommended cross section area of the upstream cable for capacitor banks.

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 current transformer, the recommended cross section area is ≥ 2.5 mm².

Bank power (kvar)		Copper cross- section	Aluminium cross- section
230 V	400 V	(mm2)	(mm2)
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	2x95
150	250	240	2x120
	300	2x95	2x150
180-210	360	2x120	2x185
245	420	2x150	2x240
280	480	2x185	2x300
315	540	2x240	3x185
350	600	2x300	3x240
385	660	3x150	3x240
420	720	3x185	3x300

Fig L38 : Cross-section of cables connecting medium and high power capacitor banks

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.

References

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