Inverter Topology and Transfer Time
The term Inverter comes from the physics principal of “Inversion” which is the conversion of direct current (DC) to alternating current (AC). Inverters have been deployed as a source of emergency power for many years, and while the physics has remained the same, different inverter topologies have been developed that are differentiated by the speed in which the equipment transfers from normal to emergency power. This “transfer speed,” or “transfer time,” measures the amount of time it takes the Inverter to begin supplying power to the emergency load from its battery supply once a power loss occurs.
Historically, transfer time was one of the most important considerations when designing an emergency lighting system with an Emergency Inverter. Some legacy lamp types, such as HID, require a constant arc be maintained to ensure proper operation after power is lost, thus requiring a short or immediate transfer to the inverter battery supply (typically less than 4ms). In modern LED lighting applications, transfer time becomes less of a concern. LED diodes will provide instant illumination when supplied by an emergency power source no matter how much time has passed since the AC utility power failed. Some emergency lighting designs, however, still specify that the transfer time be zero or near-zero based on yesterday’s plans. A common product solution to eliminate transfer time is utilizing an inverter with “Double Conversion” topology. This article will discuss the theory of operation behind Double Conversion and compare its usefulness in emergency lighting applications against the Line Interactive Fast Transfer topology utilized by IOTA IIS FT Inverters.
What is Double Conversion?
The Double Conversion topology eliminates transfer time by keeping the inverter circuitry continuously energized whether a power loss condition has occurred or not. This differs from a line interactive system where utility power is simply routed through the inverter to be supplied directly to the lighting load until a power loss is detected. In a double conversion system, the emergency lighting load never receives AC utility power directly and is instead always powered by way of the inverter circuitry. This means that there is never a measurable transfer time between AC utility and the battery supply feeding the inverter circuit.
The name “Double Conversion” comes from the fact that the incoming AC utility power is “converted” twice before reaching the emergency load (see Illustration 1). During normal power conditions, the incoming AC utility is first converted (or “rectified”) to DC power. This DC power travels through a DC link that routes the power to the Batteries for charging and to the inverter circuitry. The Inverter circuitry again converts, or in this case, “inverts” the DC power back to AC line voltage to supply the emergency lighting load. If a power loss occurs, the DC link ensures that battery voltage is immediately available to supply the emergency load without requiring time to transfer from AC utility to the battery supply.
In addition to eliminating transfer time, the double conversion process filters the output power to the connected load by de-constructing the AC utility and then re-constructing it to be a clean AC waveform. This provides protection for the load against power anomalies (such as surges or transients) which can be necessary in some applications with highly sensitive equipment (such as a data center). However, the voltage protection/filtering provided by double conversion is not critical in lighting applications. Emergency luminaires are designed to operate in typical AC utility power conditions and have a higher tolerance for power anomalies than more sensitive loads. This is evident through features such as universal input tolerances and healthy surge ratings. Considering that double conversion systems tend to be costlier than other alternatives, it is important to keep in mind a few limitations about double conversion that outweigh its value in lighting applications.
Limitations of Double Conversion in Lighting Applications
A major drawback of double conversion systems is lower power efficiency when compared to other Inverter topologies. Every time power is converted from AC to DC or vice versa, some energy is lost in the form of circuit inefficiencies or heat. Since double conversion systems are continuously converting the AC utility to DC and then back again, the energy loss can be significant. To compensate, double conversion systems must draw more power than other alternatives, increasing the energy cost to the building owner. Additionally, the heat generated by the continuous AC to DC to AC conversions requires forced-air cooling fans to run continuously. This produces extra noise, further increases power consumption and may add maintenance costs associated with replacing fans and/or filters.
Another limitation of double conversion systems is that the electronics are continuously energized. Most alternative system designs pass-through AC utility power during normal operation and only engage the inverter circuity during power loss conditions. Keeping the inverter circuitry energized in a double conversion system can reduce component lifespan and require building owners to more frequently replace the emergency lighting equipment. Additionally, operating the inverter circuitry at all times produces additional electrical noise on top of what is generated by forced-air cooling.
Lastly, where double conversion is most prominent is in applications that require an auxiliary power system listed to UL 1778. Conversely, The UL standard for Emergency Lighting Systems is UL 924 which involves a separate (and often more rigorous) set of equipment safety and reliability tests that must be passed to obtain a UL 924 listing. It is a safe assumption that many double conversion inverters are re-purposed UL 1778 systems that have been modified to meet the rigorous standards for life safety. This may affect the reliability of the equipment, especially when compared to an Emergency Lighting Inverter that was designed from the ground up to meet the UL 924 standard.
In emergency lighting applications, the disadvantages of Double Conversion often outweigh its value, especially when compared to the next alternative: Line Interactive Fast Transfer Systems.
Line Interactive Fast Transfer Systems
IIS FT (Fast Transfer) Inverters offer a more cost-effective solution for near-zero transfer time (2ms nominal). These systems were built specifically to the UL 924 standard and feature a line interactive design that rapidly engages the inverter circuitry when AC utility power is lost (see Illustration 2). During normal power conditions, IIS FT Inverters will pass-through AC utility power to the lighting load, while drawing power to charge its batteries as needed. This topology optimizes system efficiency and only adds the battery charging requirements to the bulk load rating of the system. If a power loss occurs, the inverter will disconnect the AC utility and rapidly engage the inverter circuit, allowing power from the batteries to energize the emergency lighting load.
In IIS FT systems, the transfer from AC utility to the battery supply occurs in only 2ms, which is adequate for any lighting load (including HID). Critically, this line interactive fast transfer design only converts power from AC to DC and then back when it is needed. This makes these systems more energy efficient than double conversion systems because the incoming power is not subjected to continuous energy loss from the double conversion process before it reaches the lighting load. Additionally, the inverter circuitry on IIS FT systems is only engaged during power loss conditions, causing less wear on the components than double conversion designs. IIS Fast transfer systems are ideal for today’s lighting loads and eliminate excess power loss, noise, and cost, while providing a reliable source of emergency power.
Conclusion: Are Double Conversion Systems Ideal for Emergency Lighting Applications?
Double Conversion systems do offer advantages with regards to protecting sensitive equipment against power anomalies, but these advantages are not realized in lighting applications. The limitations of double conversion in the form of reduced energy efficiency and reliability make it less attractive than alternative inverter designs. IIS Fast Transfer Emergency Lighting Inverters are a more ideal solution for a reliable and cost-efficient emergency lighting system.
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