Introduction
There is a growing trend in the UPS industry to create a highly efficient, more lightweight and smaller UPS system. Data centers across the world are constantly searching for new ways to maximize reliable UPS power while minimizing the UPS and associated equipment footprint. In addition, operating costs for large data centers always will be a priority.
In applications that require a smaller capacity UPS (less than 200 kW), true on-line double conversion transformerless UPS systems have emerged as the topology of choice since the early 2000’s. In larger applications, most legacy UPS systems consisted of a UPS with a transformer, or multiple smaller UPS power modules paralleled together to achieve the required capacity. Most UPS manufacturers were finding it difficult to create a true on-line double conversion transformerless UPS system larger than 200 kW due to factors such as ground faults, high frequency noise and efficiency.
Transformerless UPS systems utilize Insulated Gate Bipolar Transistor (IGBT) for all power conversion processes (AC/DC converter, DC/DC chopper and the DC/AC inverter). IGBTs are much faster than the traditional thyristor and can be controlled by simply toggling an on/off gate signal using a digital signal processor and a field programmable gate array as opposed to waiting for a zero crossing. When the gate signal is removed, the IGBT turns off. The combination creates a series of pulses to re-shape existing voltages (conversion from AC to DC and from DC back to AC).
As with any switching power electronic, the device itself has power losses. For the IGBT, the two primary losses are the conduction losses and the switching losses. Since the IGBTs are being turned on and turned off much faster, the switching losses will increase creating a less efficient system. This is a challenge most manufacturers were unable to overcome in larger capacity UPS systems up until just a few years ago.
For this reason, many UPS manufacturers remained using thyristors as opposed to IGBTs in the converter section of the UPS (rectifi er section) for many years. Although there are many benefits in using IGBTs in the converter section, a decrease in efficiency prevented them from being a preferred option for most in larger UPS kW rated systems (above 200 kW) for quite some time. In the UPS inverter, the benefits of the IGBT switching speed have far outweighed the decrease in efficiency.
It is important to note that a transformer serves many purposes in the UPS system. Even if the UPS utilizes IGBTs for switching, a transformer would still have a purpose. Other technologies such as the IGBT switching controls and fault detection still need to be considered.
This paper will discuss the new technologies used in transformerless UPS systems and the advances in the Mitsubishi 9900 series UPS systems.
Background
Before the advances in UPS controls and the benefits of the IGBT in the converter and inverter. The diode bridge converter (figure 3) is similar to the 6 pulse SCR rectifier, except the diodes are natural commutation (natural on and natural off). The benefits of the diode bridge are better efficiencies and lower harmonics compared to a traditional SCR rectifier can start to be appreciated, the primary and secondary purposes of the transformer in traditional UPS systems must first be discussed.
Converter Section:
The main purpose of the converter section of the UPS is to convert the AC utility power to DC power. The most popular power electronics used for this process are the Diode (6 pulse), the thyristor (SCR 6 pulse and SCR 12 pulse) and the IGBT. Until recently, the 6 pulse and the 12 pulse SCR rectifier had been the most popular.
As shown in figure 2, the 12 pulse SCR rectifier uses an isolation transformer in combination with two 6 pulse SCR rectifiers (figure 1 shows a 6 pulse SCR rectifier). The 30 degree phase shift provided by the transformer is the main purpose of the isolation transformer. The two 6 pulse SCR rectifiers will alternate to create twice as many pulses. The 12 pulse SCR rectifier will produce fewer harmonics than the single 6 pulse SCR rectifier allowing for a smaller harmonic filter.
The diode bridge converter (figure 3) is similar to the 6 pulse SCR rectifier, except the diodes are natural commutation (natural on and natural off). The benefits of the diode bridge are better efficiencies and lower harmonics compared to a traditional SCR rectifier.
Every rectifier, regardless of the technology or power electronics used, will produce harmonics. For all technologies but the IGBT (figure 4), these harmonics are greater than desired for most electrical systems, including the backup generator. Therefore, an input filter is required to reduce the harmonics to less than 10% iTHD. As shown in Figure 5, the input filter is comprised of an inductor with a parallel capacitor. The transformer, in this case, helps add inductance to the line to further reduce the harmonics. Figure 6 shows reflected harmonic distortion on input current waveforms from their respective rectifier or converter technology.
Finally, the isolation transformer used in the converter section of the UPS system will provide isolation between the UPS module and the utility. This isolation helps protect against DC ground faults caused by the batteries and will minimize damage to the upstream distribution system due to a separation in the transformer input and output windings.
Although some manufacturers have started using IGBTs in the converter section, most vary in the switching speed of the IGBTs. The benefit of using a slower switching speed for the IGBT converter is a higher efficiency (less switching means less switching losses). In this case, the IGBT converter will still produce larger amounts of harmonics, which will require a lower frequency input harmonic filter (similar to the harmonic filter for the SCR rectifiers).
The problem with lower frequency input harmonic filters on the UPS converter is the leading power factor at small loads. When the UPS system is operating at a reduced load, the ratio of capacitance in the input filter to the load becomes very large and will produce a leading power factor from the UPS. This leading power factor can result in generator compatibility issues. To eliminate these issues, an active input filter (switching harmonic filter capacitors in and out of the circuit depending on the load) will need to be used or the input filter would need to be disconnected both resulting in an increased harmonic content.
