Case Study:
Generator Compatibility with UPS Incorporating an IGBT-Based Front End
Scope:
Lack of generator compatibility with uninterruptible power supplies (UPS) has been an issue since continuous power solutions have been in use. Typical applications involve a double conversion UPS with an input composed of a six pulse rectifier and an input filter (See Figure 1). With this input, typical input current harmonics range from 10% current total harmonic distortion (THD) at 100% load to over 30-40% current THD at 25% load or less.
It should be noted that for generator applications a tuned L-C (inductor and capacitor) filter is added to the line side of the rectifier of the traditional UPS. It is tuned to work optimally at 100% load and is connected to the input by a contactor. In situations where the load drops below ~25% (NOTE: It is common for UPS systems to be loaded to only 15% to 50% of their full load rating since most people unknowingly oversize the UPS to support the nameplate rating of the downstream equipment.), the tuned filter must be removed from the circuit to prevent a leading power factor from being presented back to the input source which is of grave concern when operating on generator power. When the UPS load drops below 25%, the input filter is removed from the circuit and over 30 to 40% current THD is reflected back to the source. Therefore, one can expect to see the traditional UPS manufacturer’s published 8-10% input current THD specification at 100% load only. The tuned filter has fixed parameters and as the load decreases, the filter becomes less efficient in mitigating harmonics and thus, harmonic content will increase.
High harmonic current reflected from the UPS input to the generator causes generator compatibility issues including excessive heating in the generator windings and high voltage THD. A catastrophic situation may occur if the input filter of the UPS is not taken off-line at light loads. If the filter fails to disengage, the input power factor will be extremely leading, causing current to flow back to the genset and excite the generator windings. This causes the generator windings to see a much higher voltage and shutoff for self-protection, dropping the generator from the circuit. While this occurrence is rare, it is obviously a potential point of failure. Regardless, when the UPS performs normally and disengages the filter, input current distortion will always rise to over 30%-40%.
To lessen the occurrence of these problems, most generator manufacturers recommend over-sizing the diesel genset by a factor of two (2X) and over-sizing natural gas gensets by a factor of three (3X) when feeding a traditional 6-pulse rectifier-based UPS. This is a costly and non-ideal solution.
The Optimal Solution:
Traditional UPS designs have been greatly improved by putting an IGBT converter in the front end, in lieu of a six pulse rectifier with an input filter. The input IGBT’s switch at 22 kHz (converting AC power to DC power) and at this rate, the harmonics reflected back to the source are almost non-existent (See Figure 2):
The following harmonic graph for the Toshiba UPS with an IGBT-based front end shows that the current harmonic distortion is well below the published 3% specification (See Figure 3):
The Test:
Testing was done at the Onan facility in Fridley, Minnesota using a Dranetz PP4300. The Toshiba 15kVA 4200 Series UPS was loaded to 100% load with a standard three-phase load bank. The test simulated the elevation in source impedance and the interaction between the UPS and the Onan generator. The Onan test engineer concluded that the Toshiba UPS performed like a nearly perfect linear, power factor corrected, resistive load.
The values increase only slightly when the UPS is supplying a fully nonlinear load.
Additionally, performance was monitored at a field installation at a major software company in Minnesota. A 175kVA 7000 Series Toshiba UPS was applied to an available 160kW generator load. It has been in service for over three years with monthly generator exercising and outage events and has had none of the generator overheating or compatibility problems associated with the typical, commercial grade UPS.
In Conclusion:
Toshiba guarantees generator compatibility on all of its UPS systems from single phase to large three phase. The formula used is:
Generator kW Rating = Toshiba UPS Output kVA
An example of this would be that a 50kW standby diesel genset would be adequately sized to support a 50kVA Toshiba UPS. It should be noted that other loads such as air conditioners and lighting circuits are often required for a true continuous power system and that the generator should be sized to support these additional loads.
Toshiba is the recognized worldwide leader in IGBT design and has been supplying complete UPS systems from its Houston, TX factory since 1986. From small point-of-use single phase systems to large, multi-MVA three phase applications, Toshiba has a double conversion, front end IGBT-based to UPS to handle any critical application.
