BACKGROUND

1. Technical Field

The present disclosure generally relates to uninterruptible power supplies. More particularly, the present disclosure relates to compact uninterruptible power supplies that use a multi-level two stage dc-dc converter and a multi-level inverter to supply power from an energy storage device.

2. Background of Related Art

There is large demand for data centers to store data due to the emergence of Web-2.0-enabled businesses in the financial, e-commerce, pharmaceutical, and multi-media industries. The digital storage market doubles every 18 months, which translates to an annual growth rate of approximately 150% for the next 5 years. Computer equipment manufacturers continue to expand their data collection and storage capabilities of their servers, which are widely used in data centers across the world. This expansion has led to an increase in the total power requirements of data centers both while connected to an electrical utility and during an interruption in power from the electrical utility. In particular, data centers now demand power in the megawatt range and voltage in the kilovolt range. As a result, data centers require uninterruptible power supplies (UPSs) that can meet these high power and high voltage requirements when there is an interruption in the power supplied from the electrical utility.

Over the past ten years, the cost of copper has increased approximately 400% (from about $0.77/lb to about $4/lb). By using medium voltage (6.6 kV or 13.8 kV) distribution, it is possible to reduce the size of the copper power supply cables, thereby reducing the cost of the power supply cables. It is also possible to reduce the critical power losses between the utility grid and the server computer rack by under 5% by using a transformerless medium voltage (MV) UPS and using a MV distribution system.

In boost mode, the DC-DC converter for the energy storage device of a UPS may use a single power semiconductor device to step up the voltage provided by the energy storage device, e.g., a battery, in the UPS. However, a single power semiconductor device is not available to step up the output voltage of the UPS so that it can connect across medium-voltage lines, for example, 6.6 kV or 13.8 kV AC lines. Therefore, the AC output of UPSs typically uses a step up transformer to step up a voltage of a battery. For example, the transformer may step up the voltage of a battery at 700 V DC or some other low voltage to the AC voltage of the power supplied by the utility supply, for example, 13.8 kV or some other medium voltage.

FIG.1shows a system100for supplying power to information technology (IT) and/or mechanical load155according to the prior art. The system100includes a utility/generator power supply system195and a UPS115that includes a step-up transformer140. Under normal load conditions, power is supplied to the load155entirely by the utility supply165. The utility supply165supplies an AC voltage ranging from about 3.3 kV to about 13.8 kV. The mechanical portion of the load155includes electrical power required to operate cooling equipment required to remove waste heat generated by the IT portion of the load155.

A surge protector180is used to limit voltage spikes in the power supplied by the utility supply165. A bypass line162allows maintenance tasks or other work to be performed on system171-173when ON/OFF switch of bypass line162(not shown) is closed and a static transfer switch (STS)175is opened. Line filters170are coupled to each AC line171,172, and173to reduce harmonics in the power supplied by the generator160or the utility supply165. The STS175supplies power to a step-down transformer150when the STS175is closed. The step-down transformer150can convert the medium voltage supplied by the utility supply165, e.g., 13.8 kV, to a low voltage, e.g., 400 V. The low voltage is then supplied to the load155having an appropriate current level.

When an interruption or disturbance in the power supplied by the utility supply165is detected, the STS175opens and the UPS system115starts supplying about 100% of the power to the load155via the UPS's transformer140. The UPS system115can supply power to the load155for a short period, e.g., approximately five minutes, but generally the generator160starts generating power if the interruption is more than a few seconds.

The UPS system115generates power from a low-voltage energy storage device105, e.g., one or more low density lead-acid batteries B. The low voltage VBof the energy storage device105can range from about 300 V to about 600 V. The low voltage is then converted to a high voltage, e.g., approximately 700 V, by a bidirectional DC-DC converter110. The bidirectional DC-DC converter110includes one stage for converting the low voltage DC to a high voltage DC. The high voltage DC is then converted to a low AC voltage, e.g., approximately 400 V, using a two-level inverter120.

The AC voltage output from the two-level inverter120passes through filter130, such as an inductor-capacitor (LC) filter, to a step-up transformer140. The step-up transformer140converts the low AC voltage to a medium AC voltage, e.g., about 13.8 kV. The medium AC voltage output from the step-up transformer140is then provided to the step-down transformer150, which converts the medium AC voltage to a low AC voltage, e.g., about 400 V, that is appropriate for the load155.

Once the generator160has reached its reference speed and stabilized, transfer switch190shifts the primary power source from the utility supply165to the generator160. During this shift, the output voltage of the UPS system115is synchronized to be in phase with the output voltage of the generator160. Once the STS175is closed, a soft transfer from the UPS system115to the generator160is executed until the load155is entirely powered by the generator160. The energy storage device105of the UPS system115is then recharged by the power generated by the generator160.

After the power interruption or disturbance ends, the load155is shifted from the generator160to the UPS system115because the utility supply165may be out of phase with the generator160and the STS175shifts the primary power source to the utility supply165. The output voltage of the UPS system115is then synchronized to be in phase with the output voltage of the utility supply165. Once the output voltage of the UPS system115and utility supply165are synchronized, the load155is quickly transferred from the UPS system115to the utility supply165. Then, the energy storage devices105, e.g., batteries B, of the UPS system115are recharged from the utility supply165so that the UPS system115is ready for future interruptions or disturbances in the utility supply165.

The step-up transformer140in the UPS system115meets the power requirements of the load155; however, the step-up transformer140is a large and bulky component of the UPS system115. As a result, the power density of the UPS system115is lower because the transformer140occupies a large amount of floor space, which, in some cities, can be quite expensive. The transformer140also introduces considerable losses (approximately 1 to 1.5% of the power) into the system thereby reducing the efficiency of the UPS system115. Also, when traditional sinusoidal pulse width modulation (PWM) technique is used to operate the inverters and an ON-OFF PWM technique for bi-directional single stage DC-DC converters110, current distortion increases. As a result, LC filters130, which are expensive and bulky, are placed at the output of the two-level inverters120to reduce the current distortion or harmonics as demanded by the IT and/or mechanical load155.

SUMMARY

The systems and methods of the present disclosure provide power to a load using a medium voltage uninterruptible power supply (UPS) without using an output transformer. The UPS includes a DC-DC converter and an inverter. The DC-DC converter may be a two-stage multi-level DC-DC converter that may be configured for unidirectional or bidirectional power flow. The DC-DC converter generates a high DC voltage from a low or medium voltage energy storage device such as a battery and/or ultra capacitor. The multi-level inverter converts the high DC voltage into a medium AC voltage (from about 3.3 kV to 35 kV, e.g., about 13.8 kV). The UPS may also include a small filter to remove any harmonics generated by the DC-DC converter and/or the multi-level inverter.

In one aspect, the present disclosure relates to a transformerless uninterruptible power supply (UPS) for an electrical load. The UPS includes an energy storage device, a two-stage DC-DC converter, and a multi-level inverter outputting a medium AC voltage. A negative terminal of the energy storage device, a negative terminal of the two-stage DC-DC converter, and a negative terminal of the multi-level inverter are electrically coupled to a common negative bus. The medium AC voltage may be between about 3.3 kV and about 35 kV.

The two-stage DC-DC converter may include a first stage that generates a first output DC voltage and a second stage that generates a second output DC voltage higher than the first output DC voltage. A positive terminal of the second stage of the DC-DC converter and a positive terminal of the multi-level inverter may be electrically coupled to a common positive bus. The first stage may include two levels and the second stage may include more than two levels. The second stage may include three levels or five levels.

The two-stage DC-DC converter may include a plurality of switches that form the levels of the first and second stages and a plurality of capacitors coupled together in a flying capacitor topology having a common negative bus. The medium AC output may be a three-phase AC output, the multi-level inverter may include three sets of switches, each of which corresponds to one of the three phases of the three-phase AC output, and each set of switches may be configured in a diode-clamped multi-level topology.

The multi-level inverter may convert the second output DC voltage into a third output voltage that is an AC voltage smaller than the second output DC voltage. The multi-level inverter may include more than two levels. The transformerless uninterruptible power supply may further include a filter electrically coupled to the AC output of the multi-level inverter to remove harmonics from the AC output of the multi-level inverter. The filter may be an inductor-capacitor-inductor filter.

The transformerless uninterruptible power supply may further include a DC-DC converter controller and a multi-level inverter controller. The DC-DC converter controller controls the first stage with pulse width modulation control signals and controls the second stage in flying mode configuration with fixed duty cycle control signals. The multi-level inverter controller controls the multi-level inverter using space vector PWM control signals so as to perform neutral point voltage balancing.

The two-stage DC-DC converter may be a bidirectional converter that allows the flow of power in a first direction from the energy storage device to the AC output of the multi-level inverter and in a second direction from the AC output of the multi-level inverter to the energy storage device. Alternatively, the two-stage DC-DC converter may be a unidirectional converter.

The energy storage device may be a low voltage energy storage device. The low voltage may be between about 700 V and about 1200 V. The energy storage device may be a battery, an ultra-capacitor, or a battery and an ultra-capacitor electrically coupled to one another.

In another aspect, the present disclosure features a transformerless uninterruptible power supply for an electrical load including an energy storage device, a single stage DC-DC converter, and a multi-level inverter having a medium AC voltage output. A negative terminal of the energy storage device, a negative terminal of the single stage DC-DC converter, and a negative terminal of the multi-level inverter are electrically coupled to a common negative bus.

The single stage DC-DC converter may include a plurality of switches that form the levels of the single stage DC-DC converter and a plurality of capacitors coupled together in a flying capacitor topology having a common negative bus.

The energy storage device may be a high voltage energy storage device. The high voltage may be between about 4 kV and about 7 kV. The energy storage device is a battery, an ultra-capacitor, or a battery and an ultra-capacitor electrically coupled to one another.

In yet another aspect, the present disclosure features a method for supplying power from a transformerless uninterruptible power supply to an electrical load when an interruption in power occurs. The method includes supplying a first DC voltage from an low voltage energy storage device to a DC-DC converter, converting the first DC voltage into a second DC voltage, providing the second DC voltage to a multi-level inverter, and generating an AC voltage from the second DC voltage. The AC voltage is a medium voltage less than the second DC voltage.

