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Case Study:
Generator Compatibility with UPS Incorporating an IGBT-Based Front End


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.

The G9000 is a transformerless UPS, and is designed to go to bypass to provide fault‐clearing current once a fault is detected.

In the G9000, faults are sensed many ways:

  • Faults sensed on the AC output:
    1. AC Ground Fault on the output: Phase to Ground: in this case a monitoring circuit detects when the zero sequence voltage changes. In other words, the vector addition of all three phases should be zero; when that voltage (or leakage current) rises above a certain setpoint, the unit activates the ZERO PHASE OVERCURRENT alarm, and sends the unit to This function can be disabled.
    2. Phase to Phase: here the CT’s and voltage monitoring on the output sense a rapid rise in current and simultaneous drop in voltage, and gives the INVERTER OVERCURRENT alarm, and again goes to bypass. This function cannot be disabled.
  • Faults on the DC bus/internal to the UPS:
    1. DC ground fault: there is a separate circuit in the G9000 that detects DC ground faults. This measures the difference between the unit’s virtual neutral and the input/output phases, and will detect an alarm in the case of:
      1. Leakage of one of the components in the rectifier/inverter/charger‐chopper circuits
      2. An electrolyte trail from battery to ground
      3. An output AC Phase to Ground fault in systems with ungrounded or HRG (High Resistance Grounded) source: this circuit will detect a fault in HRG systems when the voltage in the UPS’s internal virtual neutral rises to phase voltage (277V).
      4. The result will be a DC GROUND FAULT alarm and the unit will be sent to bypass. This function can be disabled.
  1. DC Short circuit: A similar circuit measures the balance between the + and – legs of the DC bus, and is there to detect a hard fault in the battery This function cannot be disabled.
  2. Note On The G9000’s Bypass Path
    The bypass path has no fuses or breakers in it and therefore has a no interrupt rating, but rather the withstand rating, required by UL from the factory. However, if an interrupt rating is specified, or if fuse protection in this path is desired, there is a provision to add fuses to the bypass path, at 65 kAIC for 80‐225 kVA models, and 100 kAIC for 300‐750 kVA models.

Distributed Bypass systems add no additional cost as there is when a centralized bypass switch is required.  The integral bypass switch is built into each UPS, and so does expand the system footprint.  The Tie cabinet does not need any intelligence. It is simple, reliable, and vendor-independent.  In an N+1 configuration, the distributed bypass design provides redundancy in the event of a failed static switch.


The argument for centralized bypass is simply that simplicity = reliability. The contrary truth is made evident when one considers that a parallel UPS system is more complex than a single large UPS, though parallel UPS systems are commonly designed to provide redundancy and therefore reliability. Likewise, distributed bypass adds a very simple device to each static switch circuit (a contactor) indeed making the bypass path only marginally more complex, yet far more reliable as a result.

Other considerations are:

  • Increased cost: The centralized Static Switch would have to be purchased in addition to the UPS.
  • Increased footprint: The centralized SS would be in a dedicated stand-alone cabinet in addition to the rest of the system
  • Dependence on a single switch, breaker, and motor operator if the switch is momentary, introducing a single point of


In the G9000, the hybrid bypass switch is incorporated in each UPS module. This simple circuit incorporates a thyristor/contactor switch that adds minimum complexity in exchange for greatly increasing each units bypass MTBF to 3,000,000 hrs.


UPSs that operate in econo-mode (line-interactive topology) are touted as operating at higher efficiency
with commensurate reduced operating costs compared to online (double conversion topology) UPSs.
The truth is that, as with all engineering solutions, there are trade-offs. The table below lists common
adverse power events from power utilities, and compares the two topologies handling of these power events.

The philosophical difference between these two topologies is simply that a line-interactive UPS is always
in bypass mode and relies heavily on sensing and reacting to power events, whereas a double
conversion UPS is always on line and keeps supplying conditioned power regardless of input power
Finally, from a failure mode perspective, there are more ways for things to fail with a line-interactive
unit than with double conversion unit. More sensing means more potential for a sensor failure; more
switching means an increased probability of dropping the load.
When selecting an Uninterruptable Power System, look beyond raw efficiency and balance your power
quality requirements against the performance criteria of these two UPS topologies. Slightly lower
operating costs may come at the expense of power qualityand reliability.