Grid Harmonic Suppression and Power Factor Correction of High Voltage Power Supply for High Current Proton Accelerator
High current proton accelerators require substantial electrical power, potentially megawatts, for the radio frequency systems that accelerate the beam and the auxiliary systems that support operation. The power supplies for these systems draw significant current from the electrical grid, and without proper design, can introduce harmonics and operate with poor power factor. Grid harmonic suppression and power factor correction are essential for maintaining power quality, meeting utility requirements, and enabling efficient operation of the accelerator facility.
The electrical loads in a proton accelerator facility include the radio frequency amplifiers that provide the accelerating fields, the magnet power supplies for beam steering and focusing, the vacuum systems, and various auxiliary systems. The radio frequency amplifiers often use high power klystrons or solid state devices that draw pulsed or varying current. Magnet power supplies may be constant current or ramping depending on the accelerator type. The combination of these loads creates a complex current draw from the grid.
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. Power electronic converters including rectifiers and switching supplies are common sources of harmonics. The harmonic currents flow through the grid impedance, creating voltage distortion that can affect other equipment connected to the same grid. Excessive harmonics can cause overheating in transformers and conductors, malfunction of sensitive equipment, and interference with communication systems.
Power factor is the ratio of real power to apparent power, where real power performs useful work and apparent power is the product of voltage and current magnitudes. A power factor less than unity indicates that the current is not fully utilized for real power transfer, with some current flowing as reactive power. Low power factor increases the current required for a given real power, requiring larger conductors and transformers and potentially incurring utility penalties.
Rectifier based power supplies, common in high power applications, draw current with significant harmonic content. The current waveform depends on the rectifier topology, the input inductance, and the conduction angle. Six pulse rectifiers produce harmonics at orders 5, 7, 11, 13 and higher. Twelve pulse and higher pulse number rectifiers cancel some lower order harmonics, reducing the total harmonic distortion. The choice of rectifier topology affects the harmonic generation.
Active harmonic filters provide dynamic compensation of harmonic currents. The filter measures the harmonic currents drawn by the load and injects compensating currents that cancel the harmonics at the point of common coupling. Active filters can address multiple harmonic orders simultaneously and can adapt to changing load conditions. The filter rating depends on the harmonic current magnitude to be compensated.
Passive harmonic filters use tuned LC circuits that provide low impedance at specific harmonic frequencies, allowing harmonic currents to flow into the filter rather than the grid. Single tuned filters target specific harmonics, while multiple tuned filters address several harmonics. Passive filters are simpler than active filters but are less flexible and can interact with the grid impedance in ways that may cause resonance issues.
Power factor correction improves the power factor by supplying reactive power locally, reducing the reactive current drawn from the grid. Capacitor banks provide leading reactive power that compensates the lagging reactive power from inductive loads such as transformers and motors. The capacitor sizing depends on the reactive power to be compensated. Automatic switching of capacitor stages maintains optimal power factor as the load varies.
Active power factor correction circuits in switching power supplies shape the input current to follow the input voltage, achieving near unity power factor. These circuits use boost converters operating in a mode that controls the input current waveform. Active power factor correction is effective for power supplies with moderate power levels but becomes more challenging at very high power levels where the converter ratings become large.
Grid connection studies for accelerator facilities analyze the impact of the facility loads on the grid and specify the required harmonic suppression and power factor correction. The studies model the facility loads, the grid impedance, and the mitigation equipment to predict the harmonic levels and power factor at the point of common coupling. The results guide the design of the electrical system and establish the requirements for harmonic filters and power factor correction equipment.
Compliance with utility standards and grid codes sets specific requirements for harmonic levels and power factor. Standards such as IEEE 519 specify limits on harmonic current and voltage distortion at the point of common coupling. Utility interconnection agreements may specify power factor requirements, often requiring power factor above 0.95. The harmonic suppression and power factor correction design must achieve these requirements under all operating conditions.
