Power Factor Correction Concepts with respect to Fluorescent and High Intensity Discharge Lamps

1. POWER FACTOR:

All Discharge Lamps, such as Fluorescent Lamps, High Pressure Mercury Vapour Lamps, Sodium Lamps, Metal Halide Lamps, etc., require ballasts (chokes) or transformers for their operation. These devices are inductive in nature. When a Discharge Lamp is switched on, it draws Apparent Power from the mains. This Apparent Power (VA) has two components; one is the Active Power (W) actually being consumed by the lamp for illuminating it, and the other is the Reactive Power (VAr) feeding the electromagnetic circuit of the control gear.

Power factor is the ratio of Active Power (W) to the Apparent Power (VA) (Figure 1).

Power factor is also denoted by p.f. or cos Ø . Typical p.f. of ballasts is from 0.35 to 0.5.

2. POWER FACTOR CORRECTION:

As mentioned above, the inductive components, such as ballasts, draw Reactive Power (VAr) from the mains. It lags behind the Active Power (W) by 90^o (Figure 1). A capacitor, if connected across the mains, will also draw Reactive Power [VAr_(c)], but it leads the Active Power (W) by 90^o. The direction of the capacitive Reactive Power [VAr_(c)] is opposite to the direction of the inductive Reactive Power (VAr) (Figure 2).

If a capacitor is connected in parallel with an inductive load, it will draw capacitive Reactive Power [VAr_(c)]. The effective Reactive Power drawn by the circuit will reduce to the extent of the capacitive Reactive Power [VAr_(c)], resulting in reduction of Apparent Power from VA to VA₁. The phase angle between the Active Power and the new Apparent Power VA₁ will also reduce from Ø to Ø₁ (Figure 2). Thus the power factor will increase from cos Ø to cos Ø₁.

New p.f. = cos Ø₁ =

By selecting a capacitor of an appropriate value, the power factor can be corrected to 1. However, in practice, the power factor is improved to fall between 0.9 and 0.95.

3. ADVANTAGES OF POWER FACTOR CORRECTION:

The main advantages of the Power Factor Correction are:

1. The electrical load on the Utility is reduced, thereby allowing the Utility to supply the surplus power to other consumers, without increasing its generation capacity.

2. Most of the Utilities impose low power factor penalties. By correcting the power factor, this penalty can be avoided.

3. High power factor reduces the load currents. Therefore, a considerable saving is made in the hardware cost, such as cables, switchgear, substation transformers, etc.

4. CONNECTION OF CAPACITORS IN LUMINAIRES:

1. Parallel (Shunt) Connection:

This is the most popular method of connection. The capacitor is connected in parallel to the luminaire as shown in figures 3 and 4. The voltage rating of the capacitor is usually the same as ( or a little higher than ) the system voltage.

2. Series Connection:

In case of a double (twin) fluorescent luminaire, where two lamps are controlled by two ballasts, it is usual to over-compensate one ballast by connecting a capacitor in series with it, and to leave the other ballast uncompensated. The leading power factor on the first ballast, in conjunction with the lagging power factor of the second ballast, brings the total power factor to near unity. The scheme is shown in figure 5. The voltage rating of series connected capacitors is much higher than the supply voltage and must be correctly selected.

5. OPERATING INSTRUCTIONS:

1. The capacitors retain electrical charge, even when the power is switched off. It may be dangerous to touch the terminals of the capacitors, unless the capacitors are fully discharged. For safety requirements, all AMBER lighting capacitors are supplied with internal discharge resistors, so that the capacitor voltage drops to below 50 V within 1 minute of switch off. Even then, care should be exercised in handling the lighting fixtures.

Some times by oversight or by ignorance, capacitors meant for parallel operation are installed in series with the ballasts. Since the capacitors have sufficient safety margin in their insulation ratings, this error may pass unnoticed at the time of commissioning of the installation. However, with passage of time, the capacitor is overburdened and overheated, and may cause catastrophic losses. Therefore, care should be taken in selecting voltage rating of the capacitors, which should strictly be according to the application requirements.

2. As ballasts generate heat, capacitors with thermoplastic cases should be placed as much away from the ballasts as possible. Care should be taken that the capacitor case temperature does not exceed the rated temperature printed on the capacitor. If the heat dissipation within the luminaire is not proper, capacitors with aluminium case should be used.

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