Incoming electric power disturbances can easily disable a transmitter, resulting in dead air and, potentially, a corresponding loss in commercial revenue. A momentary power outage can cause some transmitters to experience a hard shutdown, which can lead to transient voltages, sudden loss of cooling or other control malfunctions that can damage costly transmitter components.
Unfortunately, the special needs of transmitters place extraordinary demands on UPS systems. In particular, a feature called a crowbar circuit protects TV transmitters with sensitive inductive output tubes (IOTs). One of the most common problems in these transmitters is damage that may result from an improperly handled crowbar event.
The function of the crowbar is to remove the high voltage from the tube, typically within a few microseconds of the detected problem, by placing a short-circuit directly across the high-voltage power supply of the transmitter. This function is usually performed by a thyratron. It serves as a fast, high-voltage switch, comparable to a silicon-controlled rectifier (SCR) but with much higher voltage ratings and speed. When a problem is detected in the amplifier, the crowbar acts quickly; otherwise the IOT could be destroyed.
One problem with a crowbar action is that it draws several thousand amps from the AC mains. This peak current can equal 20 times the rated transmitter input current. It is equivalent to a momentary short-circuit applied directly to the output of the UPS. As long as the UPS can supply the high current that is demanded, the crowbar protects the IOT and no harm occurs to the transmitter. After a few seconds, the high-voltage supply restarts and the transmitter returns to normal operation. If the crowbar action does not work properly, the IOT can be damaged. Further, several IOTs may be connected to a single UPS. To avoid dead air, the UPS must maintain good voltage to the others when one crowbars.
Figure 1. A power supply like the CleanSource flywheel UPS bridges the gap in the event of a power outage, carrying critical equipment until an automatic-transfer switch transfers the load to an engine generator. Click here to see an enlarged diagram.
At a typical transmitter site, the UPS is installed in conjunction with a backup engine-generator set and an automatic-transfer switch to provide backup power for extended outages caused by storms, floods, earthquakes, fires or other natural disasters, or by utility infrastructure failures. Frequently the engine generator is located in an outdoor enclosure. The transfer switch and UPS can also be located in an outdoor enclosure if space inside the broadcast facility is unavailable or inconveniently configured. As illustrated in Figure 1, the input of the UPS is switched between utility and the generator set using an automatic-transfer switch. This switch senses out-of-tolerance utility conditions, starts the engine generator and switches to generator power as soon as normal voltage and frequency are present. The UPS system continuously conditions and regulates power coming from either source and, most importantly, provides bridging power during the few seconds needed to start the engine generator and transfer the load to it.
While UPS systems can provide power conditioning and voltage regulation to ensure constant quality power to transmitters, and the necessary ride-through power until a standby generator takes over, not every UPS is equipped to handle crowbar events. Caterpillar’s Cat UPS uses Active Power’s CleanSource flywheel technology to offer a solution that features battery-free operation and high efficiency.
The architecture of the UPS handles overloads and step loads better than traditional, battery-based UPS systems, in part because it is designed to efficiently manage crowbar events. The flywheel-based UPS can handle overloads as great as 1000 percent for 10ms. Even when the crowbar event causes a large overload on the UPS, the fast-switching power electronics enable a smooth transition to bypass, supplying the desired current from the lowest impedance source. And it does so without disturbing the operation of any other amplifiers connected to the same UPS.
Combining a solid-steel flywheel with power electronics, the system stores kinetic energy in the constantly spinning, low-friction, 600-pound disk and uses this energy to provide protection from momentary power outages, while the power electronics eliminate voltage irregularity and harmonic distortion. Flywheel UPS systems are available that can support loads up to 3600kVA in a fraction of the space of battery-based systems. Battery-free operation eliminates facility design and operational problems arising from the less-desirable characteristics of the large valve-regulated battery systems commonly used in UPS systems. Also, the footprint required by flywheel systems is typically one-quarter to one-third of batte ry-based UPS systems. This allows the UPS to be more easily integrated into cramped broadcast facilities.
Flywheels are insensitive to temperature variations. Battery systems require temperature to be maintained between 72° F and 83° F, while flywheels operate within a range of 32° F to 104° F. Therefore, flywheels have no stringent HVAC requirements and, in many locations, can be cooled without air conditioning. In UPS applications, valve-regulated batteries are a consumable rather than a permanent piece of equipment. They are typically replaced every three to five years, and battery life can be further shortened by frequent use. Flywheels are designed to last for more than 20 years, and the number of power outages does not affect product life. Flywheels also require less maintenance than batteries. Finally, UPS battery systems are configured as long strings of battery cells, which reduce the overall mean time between failure (MTBF) of a valve-regulated lead acid battery system to less than a year, on average, during its life. The MTBF of flywheels is orders of magnitude greater, which greatly improves the overall reliability of the UPS system, particularly when the UPS is subject to frequent utility disruptions.
Flywheel technology offers an alternative to batteries for power protection in critical broadcasting environments and a proven solution for handling crowbar events. It is particularly beneficial in situations where high reliability is required, space is at a premium, and/or environmental conditions make the installation of battery systems inconvenient or expensive.
Bryan Plater is director of North American sales for Active Power.
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