5.8 Power Management and Distribution

For the power generation and storage components to supply power to the components, the electrical power system must regulate the voltage and current to accommodate each component’s specifications. The spacecraft power generation technology has the option to directly distribute power to the spacecraft subsystems components or to store energy and distribute power from the energy storage technology. Either way, the power must go through power conversion, management, and distribution.

The power conversion, management, and distribution technology is the interfacing technology between the power source and the spacecraft subsystem components. These interfaces contribute to the EPS design requirements. The power conversion, management, and distribution technology usually reside on a centralized power distribution board that acts as this interface. On this board, common components include voltage regulators, current limiters, switches, shunts, and converters. Source control components include shunt regulators, series regulators, and shorting switch arrays. Power conditioning components include DC-DC converters, DC-AC inverters, and regulators. Energy storage control components include chargers and regulators. A schematic diagram of the electrical power system assists systems engineers and EPS specialists in graphically depicting the interfaces and detailing relevant information; a schematic of the International Space Station EPS architecture is shown in the figure above. You’ll see the end nodes are power sources and the components running along the power lines are power conversion, management, or distribution components with a collection of these components composing a printed circuit board. For large spacecraft, one must consider that DC voltage is typically supplied at 28V (heritage from aircraft) and long harnesses experience resistive power loss, which must be accounted for.

When designing the power conversion, management, and distribution (PDU in short) subsystem, you’re designing the interfaces between the power consumers, power generators, and energy storage components. The spacecraft PDU is likely going to be custom designed for the unique configuration of the payload and spacecraft bus components. The circuit design of the PDU will likely necessitate printed circuit board design software, like KiCAD or Eagle, schematic diagrams that display the interfaces, and circuit design domain knowledge. Here are some best practices when designing the PDU:

• DC switching – as a goal switches or relays should be in a positive line. The negative wire should be connected to the ground.
• ARC suppression – as near as possible to the source of ARC
• Modular construction desirable
• Grounding – ground cable preferable to ground via structure
• Maintain continuity between structural elements, thermal blankets, etc
• Maintain shield continuity, single shield ground point desirable
• Don’t overcomplicate

Integrated Power Systems

In the previous sections, we’ve discussed design drivers for power consumption, individual technologies for power generation and energy storage, and the design process for the PDU. This section will discuss the various configurations for integrated power systems and the design considerations for which configuration to use for characteristics missions. The various configurations are shown in increasing complexity.

As discussed briefly in the energy storage section, missions that rely solely on energy storage as the power source have finite lifetimes that count down once the power source is tapped into. This integrated power system, particularly the power management and distribution system, is typically simpler as the discharge power supplied to the spacecraft bus is predictable; more specifically, discharge voltage follows a known curve, and discharge current is constant. Discharge relies only on the energy storage system that is embedded in the spacecraft, which was readily available for testing on the ground prior to launch. Although RTGs were mentioned in the power generation section, RTGs are essentially nuclear batteries. Their power output and characteristics mimic the behavior of a battery, producing nearly constant power.

For an integrated power system that relies solely on a power generation source, the system must also incorporate a power conversion system, “such as converting between AC and DC; or changing the voltage or frequency; or some combination of these” [Wikipedia]. Solar cell arrays vary in power output depending on the visibility and intensity of the sun. This high power generation variability channeled directly to the power distribution and management system can overload the circuit and cause some catastrophic electrical failures, potentially damaging other hardware. The use of solar cell arrays solely to power spacecraft is very rare [KickSat] because if the spacecraft enters eclipse, the spacecraft is in danger of powering off and never turning back on. There must exist a power source during an eclipse to keep the current flowing through the electrical power system. These conditions also apply to the thermal power generation methods.

Suggested Reading

The most common integrated power system configuration includes both a power generation source and energy storage for sustained, reliable access to power. The power conversion, management, and distribution design will be more complex but this configuration allows for more capabilities. The regenerative power enables longer mission lifetimes, enabling end-of-mission extended goals. The energy storage provides robustness to eclipse and anomalies in power generation. The most common pair of technologies is the solar cell and secondary battery, which is the Artemis CubeSat kit’s configuration.