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 L35 - top). The intense heat generated by this arc causes the metallization in the vicinity of the arc to vaporise (Figure L35 - middle).
Simultaneously it re-insulates the electrodes and maintains the operation and integrity of the capacitor (Figure L35 - bottom).
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 are cleared by self-healing.
- If the leakage current persists (and self-healing repeats), 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.
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 I_{N } |
Permissible voltage overload | 1.1 x U_{N }, 8 h every 24 h 1.15 x U_{N }, 30 min every 24 h |
Discharging unit | to 75 V in 3 min or less |
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 (%) | I_{MP}/I_{N} | ||||
---|---|---|---|---|---|---|---|
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 |
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 72 = 720 A
Upstream cables
Figure L39 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 mm^{2} for 230 V. For the secondary side of the current transformer, the recommended cross section area is ≥ 2.5 mm^{2}.
Bank power (kvar) | Copper cross- section | Aluminium cross- section | |
---|---|---|---|
230 V | 400 V | (mm^{2}) | (mm^{2}) |
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 | |
180 - 210 | 360 | 2 x 120 | 2 x 185 |
245 | 420 | 2 x 150 | 2 x 240 |
280 | 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 |
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.
Notes
- ^ Minimum cross-section not allowing for any correction factors (installation mode, temperature, etc.). The calculations were made for single-pole cables laid in open air at 30°C.