Flywheel technology
May 1, 2007 12:00 PM, BY GARY RACKOW
As spring storms begin, stations should consider this technology as an alternative power system.
Operating costs
A UPS system's operating costs can vary significantly, depending on the energy storage chosen. Annual operating costs for a 900-kVA flywheel UPS system will amount to about $29,000, assuming an electricity cost of seven cents per kilowatt-hour. Battery-based UPS system operating costs vary between $51,000 in years when battery replacements are minimal, to about $105,000 in years when full battery system replacements are required.
In most cases, conventional UPS systems have not been able to meet the technical or financial criteria required to justify capital expenditures in broadcast transmitting environments. There are several reasons for this.
First, the total cost of the UPS system installation, including facility modifications to create a suitable environment for the batteries, has been too high, even when the cost of UPS equipment alone is well within required capital cost limits.
Second, UPS system maintenance costs, including those resulting from the ongoing costs of scheduled and unscheduled battery replacements, significantly increase annual operating expenditures. By comparison, most flywheels have a design life of 20 years or more, so replacement costs are nonrelevant. Utility costs increase due to electrical loss of the UPS equipment.
Finally, the possibility of transmission interruptions due to crowbar events offsets any upfront financial advantages held by traditional UPS systems. A crowbar event is the automatic shutdown method used in high-power transmitters as a safety circuit. The shutdown protects the transmitter amplifier tube or the inductive output tube (IOT) in the event of an arc-over inside the IOT. The function of the crowbar is to remove the high voltage from the amplifier as quickly as possible, typically within a few microseconds of the detected problem. The crowbar circuit shorts out the high-voltage DC power supply to the IOT for a brief period of time, often in the range of several milliseconds.
This function is typically performed with a device called a thyratron, which is a gas-filled tube that is similar in construction to a vacuum tube. The thyratron is connected directly across the high-voltage DC supply. It is a fast, high-voltage switch, comparable to a silicon-controlled rectifier but with much higher voltage ratings and speed. When a problem is detected in the amplifier, the crowbar acts quickly. Otherwise, the IOT would be destroyed, resulting in significant capital expense. The crowbar action produces a current of several thousand amps on the AC input of the high-voltage supply and, therefore, on the output of the UPS system supplying the transmitter.
The event is equivalent to a short-circuit applied directly to the output of the UPS system, which can draw up to 20 times rated current. Assuming the input power supply to the transmitter can supply the large current that is demanded, the action of the crowbar does no harm to the transmitting equipment and after a few seconds, the high-voltage supply returns to normal.
In the event of an overload from a crowbar event, an integrated flywheel-based UPS system switches to bypass in order to help supply the desired current from the lowest impedance source, and it does so without disturbing the operation of other transmitters on the same circuit.
Transmitter protection
The architecture of an integrated, line-interactive flywheel-based UPS system is designed to handle overloads and large step loads that could disrupt studio transmissions or crowbar events that are inherent to transmitter sites.
Other typical power events are easily taken care of with the flywheel UPS design. Two examples of these types of installations are given below — Telemundo and the Christian Television Network.
Telemundo
Telemundo's KVEA-TV in Los Angeles, CA, knew that the slightest power disturbance could cause interruptions in studio transmission of programming and possible loss of views in a competitive Spanish-speaking market. As the fastest growing network in the country, Telemundo Communications Group wanted an expandable power quality system that could provide maximum protection for the station's sensitive digital equipment while withstanding the existing environmental conditions in the switchgear control room. Legacy battery-based systems could not tolerate the environmental requirements due to the need for constant battery cooling.
The station satisfied its requirements with an integrated flywheel UPS system along with a generator set inside an outdoor enclosure that includes a base tank. KVEA is now assured critical load protection from all IEEE 587 power anomalies — from transients to long-term outages — without disruption to the studio's production.
After its first flywheel UPS system successfully protected the critical load from numerous utility disturbances, Telemundo placed additional flywheel UPS systems across the country. As a result, when the network needed uninterruptible power to protect multiple transmitters at the top of the Empire State Building, two flywheel UPS systems were the obvious choice, due to their efficiency, space savings and environmental benefits.
Christian Television Network
When evaluating a UPS system for broadcast applications, one factor to consider is programming loss. The costs of replacing transmitter tubes, operational efficiency and maintenance are important factors in operational efficiency and budgets.
When Christian Television Network's (CTN) chief engineer needed UPS systems for the network's transmitters, he recalled working with another station that had a large, battery-based UPS system for its transmitters. He remembered that batteries caused problems, so he steered his acquisition efforts toward flywheels.
All of CTN's transmitters are high-current devices with voltages in excess of 20kV. This was key in driving the network's capital expenditures for its UPS system. CTN's transmitters put out 60kW of power, 24 hours a day. A power interruption shuts off the transmitter and stops cooling water flow. This is a nonstandard shut down that dramatically shortens tube life. The various tubes that CTN uses to power its equipment cost anywhere from under $30,000 to more than $40,000, so the initial cost of a midpower flywheel UPS system can potentially pay for the costs incurred in just one outage, in addition to the constant savings due to low maintenance and efficient operations.
The network, which has a potential audience of more than 15 million viewers, has several flywheel UPS systems installed at different transmitter sites throughout the Southeastern United States. At a site near Tampa, crowbar testing was performed on the output of one of its flywheel UPS systems. This transmission site has a primary 60kW analog transmitter. A backup 30kW transmitter shares the output of the UPS system with a new 65kW digital transmitter equipped with a crowbar circuit.
The UPS system was tested several times in order to view the response of the system during a digital transmitter crowbar event. The current delivered by the UPS system during the crowbar test peaked at 3000 to 4000 amps, varying due to the position of the voltage sine wave when the crowbar fired.
In one of the crowbar tests, the UPS system only discharged for a short period and did not have to go to bypass. In five of the six crowbar tests, the system went to bypass via static bypass switch and displayed a warning message due to the severe step load.
The UPS system then returned to normal online operation within a few seconds. The UPS system is configured so that if this same warning message occurs more than once per hour, the system will remain in bypass mode and will require an operator to reset it. Additionally, an external contact signal from a delay relay may be used to automatically return the UPS system to online operation if multiple crowbar events are expected within one hour.
The other transmitter at CTN's site, which was connected to the output of the UPS system, stayed on the air without a glitch during all of the crowbar tests. This is partly because the UPS static bypass switch allows transfer to the bypass source without affecting downstream equipment; otherwise, the critical loads could be subject to power disturbance during the event. The crowbar test at CTN's transmitter site indicates that flywheel-based UPS systems perform exceptionally well under extreme conditions and deliver stable, uninterrupted power to sensitive loads — even during severe transient conditions.
The future of flywheel technology
These two examples illustrate some of the common power supply experiences of broadcast stations and the compelling benefits of flywheel-based UPS systems. There is little question that the provision of high-quality, uninterrupted power for broadcast facilities will be a continued source of concern for broadcast engineers for the foreseeable future.
In the absence of alternatives, batteries have been the prevailing energy storage solution for decades, but UPS systems based on flywheel technologies have been proven, are more widely adopted today and offer many advantages. These sysetms are inherently more reliable than battery-based systems, particularly in broadcast applications, where they handle crowbar events effectively.
They also cost less over the life of the system, due principally to the lower installation and maintenance costs and reduced utility costs that result from UPS system efficiency losses. They are an environmentally friendly solution, which makes them easier to manage and ideal for transmission sites.
In short, flywheel UPS systems help broadcasters stay on-air and maintain a competitive edge in a world of increasing power challenges and proliferating sources for news and entertainment.
Gary Rackow is vice president of sales for the Americas at Active Power.
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