Auxiliary power systems are needed at HVDC converter stations to operate all to the equipment which supports the operation of the main circuit power transfer equipment including the control and protection systems, the cooling fans and oil circulation pumps of the converter transformers, pumps and fans of the cooling equipment for the semiconductors installed in the converter valves, heating, ventilation and air conditioning of the valve halls and control buildings, as well as all other building and station loads such as communications systems, lighting, heating, maintenance and site security systems. HVDC transmission systems are typically required to achieve between 98% and 99% energy throughput availability. In order to achieve such high HVDC system availability, the auxiliary power supplies need to be designed with virtually 100% availability and very high security (low forced outage rates) similar to the power supplies of facilities such as nuclear power plants. In addition, the auxiliary power systems need to be able to shield the converter station auxiliary equipment from disturbances on the power supply sources, to ride-through high or low voltage in steady state or during dynamic events, to ride-through high or low frequency events, and restart automatically following faults. High security of HVDC auxiliary power systems is essential because even momentary interruptions of critical protections or cooling functions can result in tripping or possibly damage to the converter equipment. Longer term outages of auxiliary supplies can result in other concerns such as deep discharge of batteries or potential damage due to environmental factors such as freezing damage to water-filled equipment and plumbing. The fundamental requirements and common solutions to satisfy these requirements are discussed in this chapter.