Written by Robert Collins, lead Solarcraft Engineer
The operation of metering, communication, and emergency systems can be maintained by an uninterruptible power system (UPS). Generators fulfill this need in most circumstances, but not without some interruption. When truly uninterrupted power is required, battery-based UPS systems are used to bridge the interruption. These systems can also operate without a back-up generator, entirely from the reserve energy stored in batteries.
There are several types of battery-based UPS systems. Emerging technologies offer promise with increases in efficiency and reductions in equipment cost. However, problems with reliability and utility have slowed industry acceptance. With reliability paramount in back-up applications, focus on tried and true methods for two different applications: the double-conversion AC UPS and the DC UPS.
The double-conversion AC UPS has long been regarded as the gold standard for AC UPS design. These systems comprise three major parts:
The battery charger is an AC/DC converter that uses line power to charge the batteries with direct current. The storage battery maintains an energy reserve for powering the inverter during back-up events. The inverter is a DC/AC converter that uses direct current from the storage battery to create AC power for the end-use equipment. The system derives its name from the two conversion processes at work: AC to DC, and DC to AC. Power flows through the system from charger to battery to inverter. If the system storage battery is fully charged, the power from the charger essentially ‘skips over’ the battery and powers the inverter directly.
As mentioned before, these systems are often used to provide power on a temporary basis during the period of time required to start and warm up a generator. Automatic switchgear can then transfer the AC input of the battery charger from the power line to the generator. During the entire process, all power for end-use equipment is provided by the storage battery and inverter; there is absolutely no interruption in power to enduse equipment. This application is typical of facilities that require power at all times, such as hospitals, air traffic control, and defense installations. The length of the backup period is limited only by fuel storage for the generator.
Power for the enduse equipment is provided for a limited period of time from the storage battery and inverter alone. The inverter will simply run until it is manually shut down, or until the batteries have discharged to the low-voltage disconnect (LVD) point. The LVD function is employed to prevent permanent damage to the battery from excessive discharge. This application is typical of facilities that require short-term power in order to properly shut down processes and machinery. Some examples are server farms that require some time to close programs and databases, and water or chemical processing facilities that need to close or open vital valves.
The DC UPS is essentially a simplified version of the Double-Conversion AC UPS. The system has two major parts:
Like the AC UPS, the first two stages are the battery charger and the storage battery. The variation lies in the third component: the inverter is omitted and replaced by a switch employing the LVD function to protect the batteries. Rather than supplying AC power, DC power is drawn directly from the storage battery to power DC equipment. Power flows through the system in the same way as an AC UPS, the battery charger essentially powering the end-use equipment directly when the batteries are fully charged.
DC UPS systems offer distinct advantages. Because no inverter is employed, the system complexity and cost are reduced. In addition, the absence of an inverter boosts efficiency by avoiding the power losses associated with DC to AC conversion. This increase in efficiency is doubly rewarded because most DC powered equipment draws less power than its equivalent AC-powered counterpart. The AC-powered counterpart usually contains yet another AC to DC converter within, to provide power for internal use; virtually all electronic equipment uses DC power internally. Specifying DC-powered load equipment avoids these unnecessary conversion losses altogether.
Emergency lighting in commercial buildings is a good example of DC powered load applications. AC line power is used to charge a battery. In the event of a power failure, the reserve energy is used to directly power low voltage DC lighting.
The telephone system has long been powered by DC storage batteries, charged by the AC power line. Cellular and data communication networks have employed the same topology.
Double-conversion UPS system with end-use equipment connections. Identifying the components from top, three-phase DC to AC inverter, battery charger, controls for end-use equipment, and two 265Ah batteries
Dual AC and DC UPS configured system. Enclosure houses three-phase DC to AC inverter, DC UPS battery charger, and two 265Ah batteries
Battery charger selection and sizing is a potentially complex issue with many possible strategies. Reducing the size of the battery charger reduces systems costs. Chargers are available with AC input voltages ranging from 120 to 480VAC. DC output voltages can range from 12 to several hundred VDC. Voltage selection will generally be dictated by the inverter selection or, in the case of a DC UPS, by the desired voltage to supply the DC enduse equipment.
Power handling can span from as little as 50W to several hundred thousand watts. In all cases, the power rating of the charger must exceed the average power consumption of the end-use equipment. To do otherwise would deplete the battery. Additional power handling must also take into consideration power losses in the inverter and storage battery. In addition, some amount of additional power handling may be required to recharge the storage battery while at the same powering the end-use equipment.
In any application, the minimum charger capacity will be set by the specified recovery time established for the system battery. Beyond this, the key in considering alternatives lies in understanding the average power consumption of the end-use equipment.
For example, it is entirely possible to power a very large load intermittently– such as a valve actuator motor consuming several thousand watts – from a relatively small charger with a capacity of several hundred watts. Careful consideration must be made of the duration, frequency, and power consumption of back-up events. A hidden benefit is that this same motor could ultimately be powered by a much smaller AC power line than the actual power requirements of the motor itself would suggest it was connected directly.
Battery capacity required is simply the product of the time required for the support of end-use equipment and the average power consumption by the inverter or DC loads during the back-up event. This value of watt-hours will equate to an equivalent value of amp-hours once a battery voltage is chosen.
The value of amp-hours should then be adjusted upward for loss of capacity through aging and, more importantly, to limit the depth of discharge during a back-up event. These factors can be determined more precisely after environmental specifications are set and battery chemistries are chosen. Inverter selection and sizing are straightforward as well. Input and output voltages will be dictated by the application. Power handling will be set by the peak-power requirement of the end-use equipment. Consideration should be given to any load that presents high inrush or start-up currents. Motors are a prime example here: a motor requiring 1,000W to run at its rated horsepower may require an inverter with 4,000W of capacity in order to start properly.
An off-the-shelf UPS is an economical solution, and there are various options available. In many cases these systems satisfy the application requirements. However, applications with unique requirements might benefit from a custom designed UPS system. Custom systems are common; the components are modular in nature and the design process is fairly straightforward. A custom UPS system can be a more cost-effective approach, and in some cases the only solution for uninterruptible power.
Written by Robert Collins, lead Solarcraft Engineer
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