Harmonic Suppression and Power Factor Correction of High Voltage Power Supply for High Power Broadcast Transmitter
High power broadcast transmitters require substantial electrical power, often tens to hundreds of kilowatts, to deliver their rated output power. The high voltage power supply that powers the transmitter final stage is a major load on the electrical grid. Without proper harmonic suppression and power factor correction, the power supply can degrade the power quality and incur penalties from the utility. Addressing these issues is essential for efficient and compliant transmitter operation.
Broadcast transmitters operate in amplitude modulation or frequency modulation modes, with the power consumption varying with the modulation. The final amplifier stage, whether using vacuum tubes or solid state devices, requires high voltage DC power. The power supply converts the AC grid power to the required DC voltage. The conversion process can introduce harmonics and reactive power that affect the grid.
Harmonics are frequencies that are integer multiples of the fundamental frequency. Nonlinear loads such as power converters draw current in pulses rather than sinusoidally, creating harmonic currents. These harmonic currents flow through the grid impedance, causing voltage distortion. The voltage distortion affects other loads connected to the same grid, potentially causing malfunction or overheating.
The total harmonic distortion quantifies the harmonic content. Standards such as IEEE five one nine limit the allowable harmonic current injection at the point of common coupling. The limits depend on the short circuit current at the point and the load current. Larger loads have stricter limits because they have greater impact on the grid voltage.
Harmonic suppression techniques include passive filters, active filters, and converter topologies with reduced harmonic generation. Passive filters use tuned LC circuits to provide low impedance paths for specific harmonics, diverting them from the grid. Multiple tuned filters can address several harmonic orders. Passive filters are simple and reliable but may interact with the grid impedance causing resonance.
Active filters inject harmonic currents that cancel the harmonics from the load. The active filter measures the load current, calculates the harmonic components, and generates compensating currents. Active filters can address multiple harmonic orders simultaneously and adapt to changing load conditions. However, they add complexity and cost.
Twelve pulse and higher pulse rectifier configurations reduce the harmonic generation at the source. A twelve pulse rectifier uses two six pulse bridges with their inputs phase shifted by thirty degrees. The harmonics from the two bridges cancel for certain orders, reducing the total harmonic current. Higher pulse configurations provide further reduction but require more complex transformer arrangements.
Power factor is the ratio of real power to apparent power. A low power factor means that the current is higher than necessary for the power being delivered. The excess current causes additional losses in the grid and reduces the capacity of transformers and cables. Utilities often charge penalties for low power factor to encourage correction.
The power factor has two components. Displacement power factor results from the phase shift between voltage and current fundamental components. Distortion power factor results from the harmonic content in the current. The total power factor is the product of these two components. Both must be addressed for good power factor.
Displacement power factor correction uses capacitors to supply reactive power. The capacitors are sized to compensate for the inductive reactive power drawn by the converter. The correction can be fixed, with a permanently connected capacitor bank, or switched, with stages that switch in and out to maintain the desired power factor as the load varies.
Active power factor correction circuits shape the input current to be sinusoidal and in phase with the voltage. Boost converters operating in continuous conduction mode can achieve this by controlling the current waveform. Active correction provides both displacement and distortion correction in a single circuit. It is commonly used in lower power applications and increasingly in higher power systems.
The interaction between harmonic filters and power factor correction capacitors must be considered. The capacitors can resonate with the grid inductance at harmonic frequencies, amplifying rather than reducing harmonics. Detuning reactors in series with the capacitors shift the resonance away from harmonic frequencies. Proper coordination of all power quality measures ensures effective and stable operation.
Monitoring of the power quality verifies compliance with standards and identifies any developing issues. Power quality meters measure the harmonic spectrum, power factor, and other parameters. The data can be logged for analysis and reporting. Trends in the data can indicate degradation of filter components or changes in the grid conditions.
