Abstract
The Eaton Powerware 9 Series IGBT transformer-free UPS is the most cost effective and reliable transformer-free UPS available for the supply of quality electrical power to critical electrical loads. It provides superior technology to older transformer based UPS systems, and results in cost-savings, smaller footprints, lower heat emissions and higher efficiencies.
This paper describes the following aspects of the Powerware 9 Series IGBT Technology, and its effect on transformer-free technology:
History of SCR’s (also known as Thyristors) and their use in UPS components and technology
IGBT Technology, and their incorporation into a “transformer-free” UPS
Key benefits and advantages of IGBT technology
History
As UPS systems were developed in the 1960’s and 1970’s the predominant technology used force commutated SCR inverters and phase angle fired SCR rectifiers. SCRs are rugged devices and, at the time, were one of the few power semiconductors readily available in useful voltage and current ratings.
SCRs do, however, present some practical limitations. An SCR can be easily turned on by applying a pulse to gate terminal, but it can’t be turned off this way. The only way to turn off an SCR is to reduce the current through a conducting SCR below the holding current for the SCR. This holding current is typically 2-3% of the rating of the SCR.
SCR Rectifiers and Static Switches
To turn off an SCR, the requirement to reduce current to 2-3% as mentioned above is easily achieved in SCR converters fed from an AC source as alternating voltage reverses in direction each half cycle. The voltage across a conducting SCR also reverses, thus causing the current through the device to attempt to reverse, which reduces the current level in the SCR below the holding current and causes the SCR to shut off.
SCR Inverters
The Inverter of a UPS is driven by a DC source. This creates a problem for the SCR which, in a Rectifier, relied on Alternating Current. A Direct Current source means that additional measures are required to turn off an SCR Inverter.
Additional circuits, called commutation circuits, are introduced to reverse current flow through the SCR. These circuits typically use auxiliary SCRs or high pulse current rated capacitors. In UPS applications these methods were used in the inverter functional block. Figure 3 shows a typical force-commutated SCR inverter.
Output Harmonic Filters
SCR Inverters have several inherent disadvantages. Due to limitations in SCR switching speeds they are unable to operate above a few kilohertz’s, which limits the range over which a pulse width modulation (PWM) control can be applied to the inverter. This in turn necessitates large output filter components to obtain an acceptable sinusoidal output waveform.
Due to its ability to operate at high frequency PWM, an inverter which uses IGBTs (Insulated gate bipolar transistors) re-creates a pure sinusoidal waveform and eliminates the need for bulky output filters.
“Isolation” Transformers
SCR technology required the use of double-wound transformers to be incorporated into UPS inverter and rectifier design. The term “isolation” is a misnomer, as will be discussed later.
Output Transformers
Commonly, SCR Inverters in older style UPS systems operated directly at the battery voltage, which can vary by +/-20% between boost charge and total battery discharge. This wide range of DC voltage feeding the SCR, the SCR’s limited PWM ability, and the necessity to operate the inverter at lower DC voltages due to component limitations meant that a double-wound output transformer had to be incorporated.
These output transformers converted the Inverter output voltage to the required load voltage, provided leakage reactance to be incorporated in the output filter circuit, and prevented the flow of DC current to the load in the event of a catastrophic inverter commutation circuit failure, as SCRs cannot be turned off when the commutation circuit malfunctions.
In UPS systems with transistor or IGBT inverters that are fed by SCR rectifiers, the wide DC voltage range is less of a problem, however this and the rectifier’s overall voltage level limit the inverter’s ability to convert to an appropriate AC output voltage without using an output transformer used to step up the output voltage.
In this configuration the transformer is not required for blocking of DC current as the transistors or IGBTs can easily be instantaneously switched off when DC is sensed in the output waveform. Figure 4 shows the incorporation of the transformer into the UPS, note the location of the static bypass switch after the transformer.
Input Transformers
On the input side of things, limitations in the voltage and current ratings of available components such as SCRs and commutation capacitors meant that older style UPS usually required a rectifier input transformer to feed the SCR the right voltage to obtain the desired DC Bus voltage on the other side.
This led to the situation where the input AC voltage is transformed to the voltage required by the input rectifier, converted to DC, reconverted to AC by the inverter and retransformed to the original input voltage by the inverter transformer.
For this reason older style UPS tend to be much larger, heavier, noisier, less efficient and more costly than current technology. In addition, even with input and output transformers, the static switch is a 3 pole device, which means that the UPS input and output neutral are not galvanically isolated from each other. Figure 4 illustrates. Note the straight through connection of the neutral
“In-built Galvanic Isolation” transformer misconception
It has been a common misconception that the in-built output and input transformers required in SCR and SCR/IGBT UPS automatically provide galvanic isolation to the load as an additional benefit.
This, in most cases, is not true. The main purpose of the output voltage transformer is for voltage conversion, not galvanic isolation of the neutral. As shown in Figure 4, typically the neutral of the UPS will be connected straight through, to simplify most site–requirements of an earth-bonded neutral, in which the MEN link occurs upstream of the UPS at the main feeder board. Therefore, any galvanic isolation of the load neutral will not occur.
In addition to that, galvanic isolation cannot be obtained as the static switch devices are placed after the transformer, as shown in Figure 4 (remembering the static switch is designed to switch between the UPS inverter/transformer output and the input bypass supply). ANY time the UPS transfers to static bypass means a bonding of the two electrical sources. In the case of an off-line or standby UPS, this occurs in Normal operation.
Galvanic isolation can be obtained by placing a double wound transformer on the output of the UPS. The purpose is to isolate both inverter output, and static switch output from the load. This can be often marketed as a “Bypass Isolation Transformer”, which is added on externally to the UPS. In fact, it highlights the fact that the bypass line is not isolated in normal internal operation.
IGBT Technology in UPS
Eaton’s latest UPS technology approaches the development of power-switching in a different manner. Our Powerware 9 Series UPS ranges now utilise IGBTs for both the rectifier and inverter. IGBTs are available in voltage and current ratings which allow the UPS designer to match the DC bus voltage to the required AC output voltage, allowing output transformers and bulky output filters to be eliminated.
SCR Static Switches
Eaton Static Bypass Switch use SCR’s, as a means of combining the best of both worlds, using the rugged nature of SCR’s to handle the overloads typically required in static bypass transfers, whilst still relying on quicker IGBT technology for inverter and rectifier components.
IGBT Rectifier & Inverter
Use of an IGBT rectifier means the rectifier can be made to draw near sinusoidal current, providing a high input power factor and a low harmonic current content (Eaton Powerware 9 Series UPS offer 0.99 input pf, and 3-5% THDi compared to 0.8pf and 30% THDi of a typical 6 pulse SCR UPS rectifier). The Rectifier includes a “Boost Converter” stage that converts DC Voltage to a high level which is sufficient for the inverter to convert to the required AC output voltage without the need for transformation. The same technology, indeed an identical power module, when used in the inverter allows a high output power factor rating and low output harmonic content.
In the design of the Powerware 9 Series the AC input and output are monitored for DC current and voltage component at PWM frequency. Should the DC component exceed acceptable limits the IGBTs will be shut off, the relevant contactor will be opened and diagnostic codes generated in the system log. In the case of a DC offset being detected in the inverter stage the load will be connected to the bypass supply to allow no break load operation.
The neutral is connected straight through the system, in exactly the same way as older designs using one or more low frequency transformers.
Even though two 50Hz transformers have been eliminated in the Powerware 9 Series UPS design we believe the system function and load protection is substantially improved in terms of protection against DC components, due to much faster protection. System performance is also substantially increased in the areas of AC to AC efficiency, DC to AC efficiency, acoustic noise level, size and mass.
CONCLUSION
In summary, IGBT Rectifier/Inverter UPS have the following advantages:
They have flexible voltage and current ratings, eliminating the need for input and output transformers
They can be driven on and off from the gate terminal, eliminating commutation circuits on the UPS Inverter.
They are able to turned on and off much faster, allowing much higher PWM frequencies, leading to a pure sinusoidal waveforms and much smaller filter components,
eliminating the need for bulky harmonic output filters
Their on state loss is comparable to SCRs, however the elimination of commutation circuits and low frequency transformers allow higher converter efficiencies to be
attained
As IGBTs respond much faster to gate signals they can be protected from overload at PWM frequency, meaning that overloaded devices can be shut off in 10-20
microseconds, as opposed to SCRs which cannot be shut off from the gate terminal at all, enhancing device failure protection.
These advantages have all been utilised in developing the latest technology Eaton Powerware UPS. Powerware pioneered the use of transformer-free UPS systems, releasing the first models in 1998, and in Australia alone have installed more than 1000 systems ranging in size from 7.5kVA to over 1000kVA.
Transformer-free inverters have also been used successfully for many more years in AC Variable Speed Drive applications controlling the speed of Squirrel Cage Induction motors where the presence of DC in the output is most undesirable as it causes a braking effect on the motor.
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