Harmonic Suppression and Power Factor Correction of High Voltage Power Supply for High Power Broadcast Transmitter
High power broadcast transmitters require substantial electrical power, with transmitter powers ranging from tens of kilowatts to megawatts for long wave and medium wave broadcasting. The high voltage power supply for the final amplifier stage draws significant current from the utility grid, and without proper design, can introduce harmonic currents and operate with poor power factor. Harmonic suppression and power factor correction are essential for meeting utility requirements, maintaining power quality, and operating efficiently.
Broadcast transmitters use various final amplifier types including vacuum tube class C amplifiers for medium wave and solid state amplifiers for newer designs. The amplifier characteristics affect the power supply requirements. Vacuum tube amplifiers typically operate from high voltage DC, with the power supply providing plate voltage to the tube. The amplifier draws current that varies with the modulation, creating a dynamic load on the power supply.
Harmonic currents are currents at frequencies that are integer multiples of the fundamental grid frequency. Nonlinear loads that draw current in pulses or with distorted waveforms generate harmonics. The high voltage power supply, typically using rectifiers to convert AC to DC, is a source of harmonic currents. The harmonic currents flow through the grid impedance, causing voltage distortion that can affect other loads on the same grid.
Harmonic limits are specified by standards and utility requirements. IEEE 519 specifies recommended limits for harmonic current distortion at the point of common coupling. The limits depend on the short circuit current ratio and the voltage level. Meeting these limits requires harmonic mitigation in the power supply design. The transmitter installation must verify compliance through measurement.
Harmonic mitigation techniques include passive filters, active filters, and multi pulse rectifiers. Passive filters use tuned LC circuits to provide low impedance paths for specific harmonic frequencies, allowing harmonic currents to flow into the filter rather than the grid. Active filters generate compensating currents that cancel the harmonic currents. Multi pulse rectifiers use transformer connections that cancel specific harmonics through phase shifting.
Twelve pulse rectifiers use two six pulse bridges with 30 degree phase shift between their input voltages. The harmonics from the two bridges cancel for the 5th, 7th, 17th, 19th and other harmonics of order 6k plus or minus 1 where k is odd. The remaining harmonics are of order 12k plus or minus 1, reducing the total harmonic distortion. Higher pulse numbers provide additional harmonic cancellation.
Power factor is the ratio of real power to apparent power, where real power performs useful work and apparent power is the product of RMS voltage and RMS current. Low power factor indicates that the current is not fully utilized for real power transfer. Poor power factor increases the current required for a given real power, requiring larger conductors and transformers and potentially incurring utility penalties.
Power factor correction improves the power factor by supplying reactive power locally. Capacitor banks provide leading reactive power that compensates the lagging reactive power from inductive loads. The capacitor sizing depends on the reactive power to be compensated. Automatic power factor correction switches capacitor stages to maintain the target power factor as the load varies.
Displacement power factor refers to the phase shift between the fundamental frequency voltage and current. Distortion power factor refers to the effect of harmonic currents on the power factor. The total power factor is the product of displacement and distortion power factors. Correction of displacement power factor uses capacitors, while correction of distortion power factor requires harmonic mitigation.
The modulated nature of broadcast transmitter loads affects the power supply requirements. The amplifier current varies with the audio modulation, from a minimum at carrier only to a maximum at full modulation. The power supply must maintain performance across this range. The harmonic generation and power factor may vary with the modulation level.
Filter design for broadcast transmitters must avoid resonance with the grid impedance at frequencies that could be excited by the modulation. The filter resonant frequencies should be well separated from the audio frequencies present in the modulation. Detuned filters that avoid resonance at specific frequencies can be used when the grid characteristics are known.
