Emergency power systems
October 01, 2003
Appeared in Broadcast Engineering
Just about every news or entertainment broadcast facility requires a backup generator system. Today, such a system must be highly reliable to meet demanding uptime needs. It must also work seamlessly with the facility’s mechanical and electrical infrastructure, especially its UPS system. Selecting a generator with the appropriate characteristics and interfacing it with other facility systems poses a number of key challenges.
To meet these challenges, first consider generator voltage and power capacity. The generator’s voltage depends on the facility’s power-system configuration. Its capacity depends on the type of loads it must handle and how it handles them. Generator manufacturers offer products in a range of voltages and capacities, with a variety of options and features.
Voltage and configuration
The first step in choosing a generator is understanding the facility’s operating voltage and the voltage of the equipment the generator will support. For example, to use a 480V generator in a building with 208V, three-phase service, you’d have to add infrastructure to match the two. Fortunately, generators are available in a variety of voltages in both single and three phase, so there should be no problem finding a generator that matches the facility’s operating requirements. Most UPS systems and mechanical equipment are 480V, three-phase systems, and the generator should follow suit. Some facilities are configured as a campus, with one main building distributing power to several other buildings. In such cases, the main building often supplies power at a medium voltage (4160V, for example) to reduce feeder sizes. With the generators also located in the main building, the emergency distribution would follow suit and supply power at 4160V. Such a distribution system would require 4160V generators.
Generators are most commonly used for safety applications, such as emergency lights, fire pumps and fire-alarm systems. But if a building’s generator is serving broadcast equipment as well as safety equipment, it could present a problem. Modern safety codes require that a separate automatic-transfer switch (ATS) serve safety and emergency equipment. In facilities where the generator serves both broadcast and safety equipment, the ATS system is set up per code, so life safety has priority. Should the generator have trouble supporting the load, it can shed non-life safety load, including broadcast equipment. Generator testing can prevent such an incident and, in extreme reliability cases, a separate generator is employed.
To determine the generator capacity your facility requires, begin by tallying the known loads within each category of equipment. These categories include HVAC, UPS (the actual loading on the UPS, not just its capacity), general technical equipment that is not on UPS (i.e., general receptacles and equipment with built-in battery or UPS backup) and non-code-required lighting.
Sometimes, a device’s normal operating load rating can be deceiving. For example, a generator serving a UPS cannot be sized to the UPS’ full-load kVA rating because the UPS’ battery-recharge load and magnetization can make it “look” 60 percent larger to the generator. Similarly, you might have to inflate mechanical loads by up to 50 percent to handle start-up in-rush currents, depending upon how the equipment is started and controlled. In smaller installations, it is not unheard of to have a generator loaded to its maximum capability but rated at twice the kW rating of the UPS and HVAC loads. By contrast, lighting and general-receptacle systems don’t present large start-up in-rush currents, so you don’t need to inflate these loads.
Based on the load, and considering inflation factors, you can determine an approximate generator load. Be sure to include all individual loads within each category, all of which must run off the generator to ensure continued operation during a power failure. For the mechanical system, look at all the components – not just those within, say, master control. For example, for HVAC systems, include the pumps, air-handling equipment, compressors, cooling towers, etc. Don’t forget to include the control system because, without control power, the entire mechanical system will not operate.
In a base building environment, where a larger building cooling system feeds the technical rooms, it is important to look at where the heat rejection is taking place and make sure those systems are backed up. For example, if a studio is located in a large high-rise, it is impractical to use the massive building chiller and/or cooling tower system to handle the relatively smaller studio loads. Instead, it would be necessary to set up separate smaller HVAC systems that can run off a generator. Broadcast engineers often make mistakes when planning generator-powered emergency cooling systems. They sometimes assume that all the heat can be rejected into a non-technical space during a power outage, or that a fan coil running off a larger base-building chiller system (that will be offline in a power outage) will have adequate chilled-water capacity to run continuously during a prolonged power outage. Many times these systems will not work. They typically offer perhaps an hour or two of cooling capacity, but cannot handle a prolonged outage. A qualified HVAC engineer can verify such scenarios.
During the schematic-design stages, if you don’t know the loads of the systems, you can use certain rules of thumb to estimate sizes. You can estimate 2kVA per ton of refrigeration, 100W per square foot for data-center space, 75W per square foot for studio space, including studio lighting, and 25W per square foot for control rooms, including lighting. But, since these are only estimates, it is essential to use actual loads, when available, in the design of the system. Every facility is unique. Besides, all cities will require actual load calculations for their approval before they issue building permits.
The final step is to refine the generator capacity. When attempting to pedal uphill on a bicycle from a dead standstill, it is much easier to start on a gradual slope than a steep one. The same goes for a generator attempting to power several systems. Depending on the generator, it may be possible to step the loads (i.e., have the loads start up in a specifically timed sequence instead of all at once) to reduce the required generator size. Allowing 15 seconds between the addition of UPS, HVAC and other building loads gives the generator a chance to “breathe” and catch up with each successive load. This process can reduce the overall capacity requirements of the generator.
Next, identify the facility’s most critical path and work with the generator manufacturer to identify and select the appropriate alternator. At a certain engine size and kVA range, there is usually a specific range of alternators you can consider. For example, one alternator may be able to handle more loads on a steady-state basis, while another may better handle high in-rush currents.
Redundancy, reliability and testing
Reliability and redundancy go hand in hand. In a perfect world, every system would have a backup. In reality, broadcast systems are often redundant, but many times the power feeding them has several single points of failure. For any mission-critical facility, such as a live-news broadcast facility, relying on a single piece of equipment for backup is asking for trouble. Just as a facility would not rely on a single piece of critical broadcast equipment, the generator system must be designed with backups and alternate routes. For example, if the system will rely on a single ATS, it should have a bypass isolation option. This option allows the power to be routed around the ATS during periods of maintenance or in case of a failure.
To combat critical failure points, other strategies employ parallel generators, each of which is capable of handling the critical entertainment-equipment loads. Or, a mission-critical facility might have feeds from two separate grids with a medium-voltage transfer switch, reducing the risk of downtime.
Increasingly, facilities are using dual-cord technology to provide two power inputs into each piece of equipment, with an internal power supply that controls the source from which it draws power. With such a system, power supplied to the equipment can potentially be completely redundant from the generators down to the plug. This is the ultimate in the elimination of single points of failure on the electrical system. But if you power each system from the same point, you’ve gained nothing. It sounds obvious, but it’s a common mistake.
The last step in any generator design and construction is to coordinate and schedule testing. Failure to perform commissioning and regular testing of the generator system is a hidden danger in the operation of generator systems. It is surprising how few systems really work the first or second time they are tested. The process of commissioning (in-depth testing) will identify and solve possible power failures in different load scenarios before project completion and prior to operations relying on the system. Even a simple case of miswiring, in which a piece of equipment is accidentally wired to utility power, can bring the whole facility down. Once you’ve successfully commissioned the system, run it weekly and perform regular (at least annual) tests simulating a power failure to verify reliable operation.
Bridging the gap
It takes 10 seconds for a generator to achieve sufficient RPM to carry its load, but it only takes a fraction of a second of power loss for a computer system to go down. To bridge this critical gap, UPS systems provide battery storage or flywheel momentum. Battery UPS systems are more reliable, offer more flexibility, and provide a longer backup time than newer flywheel systems. Flywheel systems are only good for 15 to 30 seconds. Moreover, if a second power failure occurs two or three minutes later, the flywheel will be worthless because it may require a couple of hours to regain momentum. In contrast, UPS batteries can handle multiple outages of 10 minutes or longer, depending on the quantity of batteries provided.
Unfortunately, battery-based UPS systems are inherently undesirable loads on a generator. This may be counterintuitive, but the UPS creates third and fifth harmonics and adds them to the fundamental power waveform. The generator’s governor, which monitors the governor’s output frequency and voltage, can interpret this as a problem. If not properly set up, the governor will see the distorted waveforms, assume the generator is causing the problem, and either “hunt” for the proper level or shut down. This is more prevalent in older generator and UPS systems. Make sure the generator is equipped with the latest generator regulator software and devices. Filtering on the UPS system can also help prevent this problem in older generators.
The bottom line
Downtime is money lost. Generator systems are a critically important component in maintaining reliable operations at news and entertainment broadcast facilities. Sizing, designing and integrating these systems present many challenges. But a well-conceived system design, coupled with proper maintenance and regular testing, will provide years of reliable operation.
Bryan Gauger, PE, is a senior associate in the Los Angeles office of Syska
Hennessy Group, a consulting engineering technology and construction firm specializing
in broadcast, entertainment and arena facilities.