165 SYSTEMS: ELECTRICAL POWER SYSTEM—THE POWER BEHIND IT ALL CHAPTER 9 on-the-fly materials (see Chapter 18). Due to these difficulties, no further plans have been made to retract the USOS solar arrays. However, some EPS maintenance requirements (i.e., Sequential Shunt Unit [SSU] replacement) originally called for solar array retraction to safe electrical connections. With no way to “turn off” the sun, and with the retraction option essentially off the table, the operations community had to develop plans to perform these maintenance tasks with time-critical steps during limited eclipse periods. This is another example of the designer’s intentions being changed through the lessons learned by operating the ISS. Sequential Shunt Units The SSU is the primary power regulation device that controls SAW output. The SSU maintains its Primary Power System voltage set point (typically 160 Volts DC) by balancing the system demand with the number of connected array strings. Each array string can be individually connected or disconnected from the primary power bus. Array strings that are disconnected from the power system are shunted (shorted or rerouted back to the array). The output from the SSU is therefore the sum of all connected strings at any given time. The SSU also contains multiple safing functions that cause it to automatically shunt all array current, including output overvoltage (indicating that the SSU is not functioning correctly) and output undervoltage (indicating a possible electrical short downstream). Batteries The actual storage devices of the Primary Power System batteries are nickel hydrogen battery assemblies— three per power channel. In early 2017, the team began replacing the nickel-hydrogen batteries with lithium-ion batteries. The new batteries provide more energy storage in a smaller box, with one lithium-ion battery replacing two nickel-hydrogen batteries. The batteries store power throughout the entire orbit. Array power is used to charge the batteries during insolation. A portion of the stored battery energy is discharged to supply the ISS loads during eclipse. Energy from the batteries may also be used to supplement the power-generation function during insolation. For example, if the load on a power channel is temporarily higher than the solar array can supply due to overloading, shadows cast by other space station structure, solar arrays purposely not tracking the sun, or a failure, then the batteries will discharge in parallel with the solar array output to maintain sufficient power to downstream loads. If all batteries are fully operational on a channel (i.e., not undergoing maintenance), the USOS EPS is designed to only discharge down to 65% SOC to supply the nominal ISS power needs during the period of orbital eclipse and can then be fully charged during a single period of insolation. The additional battery capacity would be used to support loads if solar array input power were to be lost. Battery Charge/Discharge Units The Battery Charge/Discharge Units (BCDUs) control the charging and discharging of the power channel batteries. During insolation periods, the BCDUs will charge, and then maintain the batteries at their maximum SOC. The discharge unit converter is a bidirectional power converter that can regulate the current level for charging the battery and regulate the voltage level produced when discharging the batteries. Typically, the BCDUs are set to regulate the output voltage level to 151 Volts DC. When the SSU is producing power (set point of 160 Volts DC), the BCDU will sense the output voltage above its set point and operate in a battery-charging or maintenance mode. When the SSU output voltage drops, the discharge unit will begin to reduce the battery- charging current when the voltage drops below the BCDU set point, the BCDU will begin discharging the batteries to maintain power to downstream loads. This transition is automatic and happens without crew or ground interaction. BCDUs also provide backup power to the Primary Power System components of the other power channel on the same PVM. This power enables only command and control of these components, and cannot be used to supply power to the downstream loads of that channel. Secondary Power System Direct-Current-to-Direct-Current Converter Units The DDCUs are the interface between the Primary Power System and the Secondary Power System. They convert the primary power range of 155 ± 22 Volts DC to the tightly regulated secondary voltage level of 124 ± 1.5 Volts DC. Numerous converter units are distributed throughout the USOS. In general, units are located in close proximity to the loads they power due to their operating at a lower voltage and higher current, which in turn requires larger cabling after