161 SYSTEMS: ELECTRICAL POWER SYSTEM—THE POWER BEHIND IT ALL CHAPTER 9 Contingency—Jumpers Whereas cross-tie functionality provides for the loss of a power channel, certain failures in the Secondary Power System or Thermal Control System would still remove power from critical ISS systems. As the ISS was assembled, the operations and engineering teams devised the use of physical connectors and cables to “jumper” around these failures, kind of like an electrical detour. Sometimes these jumpers make use of electrical connections that were originally intended for temporary use during ISS assembly (see Introduction). For example, the Lab-Truss Contingency Jumper provides secondary power from a DDCU inside the US Laboratory module to critical loads on the P1 or S1 truss segments. This capability originally existed to provide power to truss loads prior to the permanent ETCS activation, and can now be used if an ETCS loop fails. Other jumpers reroute power between DDCUs, potentially stealing power from payloads for critical systems (e.g., sacrificing science to maintain life support). Specific jumper cables and procedures have been designed for multiple failure scenarios. However, not all scenarios can be covered. The potential for a failure to cause the loss of a particular device that is needed for safety or mission success still exists. In these cases, the crew could use spare electrical wiring and a pin kit to build a new connection (see Chapter 16). A solution cannot be guaranteed, but the operations support officers and the engineering teams have often proven their ingenuity and creativity in the use of pin kits in the face of unexpected failures. The downside to any jumper is that the crew must take physical action to install it. This can pull the crew away from scientific research, create the need to wake the crew in the middle of the night, or, worst case, cause an extended duration of equipment loss if the crew is unavailable to install a jumper. When an external cooling pump failed in 2010 and again in 2013 (see Chapter 20), jumpers were used to provide power to redundant systems until a spacewalk could be performed. Contingency—Planning, Energy Balance, and Load Sheds The solar array of each power channel can produce approximately 30 kilowatts (kW) of power—or about three times the average household power consumption in the United States. However, once Primary Power System battery charging, housekeeping power (i.e., the power needed to operate the EPS devices themselves), and inefficiencies (i.e., energy lost, often in the form of heat, due to resistance in cables and during voltage conversions) are accounted for, each power channel can nominally supply about 12 kW of power to downstream loads. Higher loads, up to approximately 15 kW, can be supported for short durations at the risk of having an additional single-point failure cause an entire channel to trip off. Many factors can lower the power generated by a channel. The inability of solar arrays to track the sun, the ISS in an off- nominal attitude, or Primary Power System failures or maintenance can all lower the power available from a power channel. Power planning is one of the most work-intensive, ongoing operations for the Station Power, Articulation, Thermal, and Analysis (SPARTAN) flight controllers. Power planning includes determining the power availability and the load demand of each channel. Power availability is mainly driven by the solar array configurations and external environmental forces. During normal operations, the ISS solar array rotary joints are configured to track the sun as the vehicle moves along its orbit, thus maximizing the solar energy gathered for power production. However, due to multiple constraints, dynamic operations such as visiting vehicle arrival or departure and spacewalks may require the solar arrays to be fixed in specific positions called solar array “feathering.” Typically, at least one solar array feathering event happens weekly. When the solar arrays are not actively tracking the sun, power production can be greatly reduced to the point of not providing enough power to meet minimum power channel loads. Also, the natural occurrence of changing solar beta angle (see Chapter 7) affects the total power generation on the ISS. Low beta angles cause longer eclipse periods (i.e., where the arrays do not receive sunlight), which drove the design of the Primary Power System batteries. Eclipse periods are shorter at high beta angles at their maximum value, the ISS is in continuous daylight for multiple days in a row. Although more sunlight might result in more power, it also impacts operations (e.g., the equipment can get too warm). If the Primary Power System batteries are not discharged, they can overcharge, thereby causing damage to the batteries. Or, they can develop a memory, meaning the full depth of discharge (i.e., time