CHAPTER 9 SYSTEMS: ELECTRICAL POWER SYSTEM—THE POWER BEHIND IT ALL 158 DC-to-DC Converter Units (DDCUs) actually make use of AC to change voltage levels. The ISS also uses DC-to-AC inverters to provide US standard 110 Volt AC power to user loads in an effort to reduce cost and make use of commercial off-the-shelf (COTS) equipment. As mentioned above, 28 Volt DC systems have become an aerospace standard, and this standard was used for the RS EPS. However, low-voltage DC systems require higher currents and therefore larger, heavier cables to meet user power demand. To reduce mass, especially in a distributed power system, the USOS was designed to use higher voltages, which would require lower currents and smaller cables. The USOS uses an approximately 160 Volt DC primary and an approximately 124 Volt DC secondary system. Although this accomplished the goal of allowing smaller cabling to reduce mass, it also resulted in a few design impacts. As a new standard, user equipment had to be designed to either use the higher voltage or require additional support hardware in the form of power converters and inverters. Additionally, the higher voltage poses safety risks to both the hardware and the crew. Higher voltage presents a greater chance of electrical arcs, which could potentially cause damage to connections that are being mated (connected) or demated (disconnected). One result of this damage could be the release of molten metal, which might penetrate a spacesuit or cause physical harm to the crew in microgravity. Additionally, the higher voltage increases the electrical shock risk to the crew member when working with electrical connections. Therefore, safety requirements call for multiple levels of inhibits (i.e., steps to prevent electrical current flow, such as adding an open switch) to prevent exposing the crew and hardware to this high-voltage potential during electrical connector operations. Operationally, this requires removing power at higher levels, or “upstream,” in the EPS architecture and therefore powering down multiple pieces of equipment to replace one device. To relate this to the average home, it would be comparable to requiring the resident to turn off the room circuit breaker as well as the light switch when replacing a light bulb. Although this many levels provide additional protection against arcing and shock, it can add risk by unpowering perfectly good devices and removing redundancy. Trip Coordination Another overall design aspect of the EPS is current trip coordination. This is called a “safing function” because it places the hardware in a safe (unpowered) configuration while also trying to preserve as much functionality when faced with a malfunctioning piece of equipment. Electrical current sensors throughout the EPS monitor and report the amount of current flowing between electrical devices. When one of these sensors detects a higher-than- expected current, automated actions called “trips” are taken to open switches or deactivate equipment to remove the potentially hazardous situation. Usually, the higher current is caused by an electrical short (e.g., electrical wires crossed or touching) in the hardware and could lead to an electrical fire if left uncorrected. This is identical to a fuse or circuit breaker in a house activating and removing power when too many devices are connected to one electrical socket or when one device has become damaged. Trips need to be fast—on the order of microseconds or milliseconds—to protect hardware and cabling from damage. If a lower- level device did not trip fast enough, the higher current draw would be seen by higher-level devices. If the high-level device trips, power would be removed from more equipment than necessary. Therefore, the trip times are fastest at the lowest levels and lengthen higher in the EPS architecture. However, some devices on the ISS (e.g., motors on larger pumps) have startup transients that require additional current for short durations. This additional current may be enough to trigger the EPS trip functions. Some EPS devices were designed to “current limit” prior to tripping to handle these situations, where necessary. Current limiting is a function where an EPS device can actually adjust both output voltage and current so that, on average, the level is safe in an attempt to manage total output power. Limiting the current prevents upstream EPS devices from seeing higher current draws, thus preventing trips, while allowing a short time for device startup transients. Usually, the current is limited for only hundredths of seconds. If the transient has not ended in that time, the trip function will remove power from “downstream” load(s). Remote Control A design goal of the ISS was to make as many systems as possible remote controllable. Although this created the need for a robust Command and Data Handling system (see Chapter 5), it allowed for mass savings (i.e., less physical switches) and overall control from the ground for times when the crew was not available. In turn, this allowed the crew to focus on scientific research instead of day-to-day ISS operations. Most USOS EPS devices have firmware