Although GUNNS is meant to be generic as possible, the GUNNS electrical aspect is focused on modeling the power systems of modern space vehicles. All space vehicle electrical systems have 3 main functions:

• Generation: the creation of electrical power. Most space vehicles use a combination of batteries and solar arrays. For applications where there isn’t enough sunlight or solar arrays are practical, they’ll replace or supplement solar arrays with power generated from on-board ‘fuel’ source like RTG’s or fuel cells. Even nuclear fission reactors have been used in the past, and likely will be in the future.
• Distribution: the circuit that routes and regulates power from its generation sources to the vehicle equipment, the ‘loads’. This will typically include redundant paths, with voltage and current regulation and fault isolation functions. Power converters and regulators, switches, diodes and fuses are commonplace.
• Loads: the vehicle equipment that consume the electrical power to make the vehicle operate. Almost every other system in a vehicle consumes some electrical power, and there is a vast range of equipment types. Heaters, radios, computers, lights, pumps, fans & valves, robotic joint motors, sensors, and electric thrusters are common.

The GUNNS model computes the various currents and voltages produced by the generation and routed through the circuit, in response to demands from the loads. In addition to nominal operation, GUNNS includes a lot of fault modeling for things like open circuits and short circuits, switches failing or stuck open/closed, regulation & power converter failures, battery cell failures & fires, etc., as well as the responses of the system to those faults and fault recovery.

The EPS typically has many current and voltage sensors to allow the vehicle flight software and operators to monitor the system and detect faults. The EPS is typically operated by sending commands to effectors: opening & closing switches and configuring power converters & regulators. Fault isolation is designed to isolate as far downstream as possible, to minimize impact on the rest of the vehicle. Isolation is typically reversible, by using switches that trip open to isolate a fault, but can be reset and closed again when the fault is fixed. Faults that need isolation most often are short-circuits, so a switch immediately upstream of the short is opened to avoid damaging the rest of the vehicle – similar to the circuit breakers in your house. A set of controller models will typically be used, as a signal aspect, to wrap the electrical aspect and drive the sensors and effectors as a go-between the electrical aspect and flight software.

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