What are harmonics and power factor on a CCR input?
A phase-controlled regulator draws late, chopped current from the supply. It distorts the voltage and raises the kVA the source must carry, most of all at low brightness steps.
On its input a CCR is not a simple resistive load. A phase-controlled (thyristor) regulator switches on part-way through each half-cycle, so it draws a chopped current that starts late. The lateness is a lagging displacement and the chopped shape is distortion: the current contains harmonics, whole-number multiples of the supply frequency such as the 3rd, 5th and 7th. Together they set the input power factor: true power factor is the displacement power factor multiplied by the distortion factor.
Both effects grow as the brightness step is lowered and conduction gets shorter. The harmonic currents flow through the source impedance and distort the bus voltage, and the low power factor raises the kVA that the cables, transformer and generator must carry for the same lamp power. A generator has a higher source impedance than the utility, so it shows the distortion more. Because many hours are spent on low and middle steps, this can be the normal operating condition rather than a rare one, and a measured, trended record is the way to know where a given site stands.
- Identify the CCR technology. Thyristor, ferroresonant or IGBT, from the nameplate and manual, because each loads the supply differently.
- Connect the analyser. At the CCR input or the main bus, through current and voltage sensing sized for the circuit, under the site’s isolation and permit-to-work procedure.
- Measure on every step. Record true-RMS current, total harmonic distortion, the main harmonic orders and power factor on each brightness step, at a realistic circuit load.
- Compare with the limits. Use the CCR manual, the utility agreement and the authority’s requirement. Harmonic limits are normally set at the agreed point of connection.
- Check the supply side. Look at the voltage distortion on the bus and, on a generator, at the kVA loading and not only the kW.
- Correct with care. Options include an input filter, a line reactor or a different CCR design. Do not add plain power-factor capacitors without a harmonic study, because they can resonate with the harmonics.
Chopped input current with harmonics, and a power factor that falls as the step is turned down. Filtering or a line reactor may be fitted to limit the effect.
A ferroresonant unit draws a near-sinusoidal current with a high power factor near full load, though it still falls at light load. An IGBT unit depends on its input stage, so read the data sheet.
Frequently Asked Questions
Q1What are harmonics on a CCR input?
Harmonics are currents, or voltages, at whole-number multiples of the supply frequency, for example 150 Hz and 250 Hz on a 50 Hz supply. A phase-controlled regulator switches on part-way through each half-cycle, so it draws a chopped, non-sinusoidal current that contains odd harmonics as well as the fundamental.
Q2Why is the power factor of a CCR lower at low brightness steps?
At low steps the regulator conducts for only a small part of each half-cycle, so the current starts late and is sharply chopped. The late current lags the voltage and the distorted shape widens the gap between real power and apparent power, so power factor falls even though the lamp load is small. True power factor is the displacement power factor multiplied by the distortion factor.
Q3Do ferroresonant and IGBT CCRs have the same problem?
Not to the same degree, and it depends on the design. A ferroresonant regulator draws a near-sinusoidal current with a high power factor close to full load, although power factor still falls at light load. An IGBT regulator depends on its input stage: one with active power-factor correction can draw a clean current, while a simple rectifier input does not. Check the manufacturer's data for each brightness step.
Q4What problems can harmonics and low power factor cause?
Harmonic currents flowing through the source impedance distort the bus voltage, heat cables, transformers and generators, and can upset other equipment, meters and protection. Low power factor raises the current for the same real power, so cables and the generator must be sized for the kVA rather than the kW. Power-factor-correction capacitors can resonate with harmonics, so they should not be added without a harmonic study.
Q5How are harmonics and power factor measured and limited?
A power quality analyser measures true-RMS current and voltage at the CCR input or the main bus and records total harmonic distortion, individual harmonic orders and power factor on each brightness step. IEC 61000-4-30 defines the measurement method, and IEEE 519 and EN 50160 are common references for limits. The limits that apply come from the utility agreement and the authority's requirement, and are normally set at the agreed point of connection.
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