Uninterruptible Power Supply (UPS) provides an interim energy source during an unexpected loss of power supply, before the backup generator set kicks in. There are two types of UPS – static and rotary – both are able to meet the basic requirements of providing a reliable supply of regulated power. Allow me to explain these types.
Static UPS have no moving parts throughout its power path. It uses battery farms / string of batteries. The rectifier inside of the static UPS system converts incoming utility AC current to DC, and the inverter converts DC back to clean sine-wave AC to supply the load. At some point in the power path, there will be a place where DC current interfaces with the energy storage medium (i.e. batteries), in which case it charges the batteries and receives power from the batteries when the utility source supply fails. Depending on the duration required, the battery farm could be significantly large and heavy.
Rotary UPS are sometimes also referred to Dynamic or Flywheel UPS because of its rotating component, e.g. motor-generator or flywheel. In a rotary UPS, the primary power source jump starts the flywheel spinning, which builds up a kinetic energy based on mass of the flywheel and speed it rotates. When the power source fails, it releases the built-up kinetic energy back to the data center, hence slowing down the spin of the flywheel. One of the key requirement of a rotary UPS is that the backup generator set must be able to kick start within 15 seconds or less. And the reason is that the flywheel’s spin speed will be reducing as it releases its kinetic energy out, and usually, depending on the speed of its spinning, it will take about 15 seconds or less to drop to an RPM where it is no longer able to support the required load.
Comparisons:
- Rotary UPS systems tend to be centralised architecture, while static UPS can have the flexibility to be deployed as distributed UPS.
- In terms of space, rotary UPS uses a lot less space than static UPS as they do not require a huge battery farm. e.g. a flywheel for a 675kW UPS module requires about 121 sqft area and weighs close to 4,200 kg, assuming a requirement of couple of feet side clearance and a 3.5′ front clearance. On the other hand, a wet cell storage battery on two-tier racks, for 10 minutes standby, for the same capacity UPS module requires more than 350 sqft of floor area, or almost 3 times more, and weighs 15,000 kg.
- In terms of maintenance, batteries tend to have a finite lifespan and need to be replaced every few years. Flywheel last longer and although you may need to replace some ball bearings, the total cost is probably lower in the long run.
- As mentioned, rotary UPS have a shorter runtime than static UPS, providing only seconds of backup. It relies on assurance of a quick generator activation. What this means is that within the 15 seconds or less, the UPS must learn or recognize the utility loss, it must wait for the utility automatic transfer switch (ATS) to try to restore utility power, and if not restored, it must kick start the generator and transfer the UPS to the generator when its voltage is adequate… all within 15 seconds or less.
- Support for Gaia: All commonly used storage batteries contain an acidic electrolyte and lead, hazardous materials. Not really environmentally friendly. Flywheels, on the other hand, doesn’t contain large amounts of these hazardous materials.
- Efficiency and energy loss. This is a debatable argument as different vendors would publish their own benchmarks.
Both static and rotary UPS systems can meet the basic requirements of providing a reliable service of regulated power. Selecting which to use is dependent on your environment, the type of load to be supplied, and your preferences.
Tags: UPS