The present invention relates to an UPS (Uninterruptible Power Supply) system operating in the economical mode (ECO-mode) for reduction of a capacitive current in an AC bypass circuit. During the ECO-mode operation, a load of the UPS system is supplied from a power source - by the AC bypass switch circuit.

A commonly known an UPS system, indicated as 1 in Fig.1 , is connected in between an AC power source 2 and a load 3 and composed of an AC/DC converter 4, DC/AC converter 5 and AC contactor 6 electrically connected in series. An energy storage circuit 7 is connected to the common connection point created between the AC/DC converter 4 and the DC/AC converter 5. The DC/AC converter 5 comprises an AC output filter 10. The AC output filter 10 is a low-pass filter and has standard LC topology presented in Fig. 2. Its purpose is to filter out an output voltage of the DC/AC converter 5 in order to achieve near AC sinusoidal voltage at the load 3. In UPS system 1 a bypass switch 8 create alternative connection path between AC power source 2 and load 3. A first terminal of the bypass switch 8 is connected to the input common connection point created between AC power source 2 and the AC/DC converter 4 while the second terminal of a bypass switch 8 is connected to the output common connection point created between AC contactor 6 and load 3. The operation mode of the UPS system 1 is selected by the UPS control circuit 9. In general there are two modes of operation of UPS system 1 in which power is transferred form the AC power source 2 to the load 3. The first one is called double conversion mode when power flows though AC/DC converter 4, DC/AC converter 5 and AC contactor 6. The second one is called economical mode or shortly called ECO-mode when power flows from the AC power source 2 to the load 3 through bypass switch 8. The most of time UPS system 1 is operating in ECO-mode that is utilized to increase overall UPS system efficiency and can be used when AC power source 2 is stable. During ECO-mode operation the AC bypass switch 8 is closed the AC contactor 6 remains closed the DC/AC converter circuit 5 is blocked and the AC power source 2 supplies load 3 through the bypass switch 8. During the double-conversion mode the AC bypass switch circuit 8 is open, Ac contactor 6 is closed and the load 3 is supplied by the AC source 2 through operating AC/DC converter 4, the DC/AC converter 5 and AC contactor 6. During the ECO-mode operation, when bypass switch 8 is closed, AC contactor 6 remains closed and DC/AC converter circuit- 5 is blocked by UPS control circuit 9, AC output filter 10 has substantially no function. AC contactor 6 remains closed because it is slow device and, in case of AC power source 2 fault, UPS system 1 would need to change power flow path from bypass switch 8 to the DC/AC converter 5 with minimum delay to not disturb normal operation of load 3. Therefore the AC output filter 10 is all the time connected to the same common connection point as the load 3. Due to this fact, during ECO-mode operation, current of closed bypass switch 8 is a sum of the current of load 3 and capacitive current of the AC output filter 10 of the DC/AC converter 5. Capacitive current of the AC output filter 10 is a source of several problems:

• It is a source of increased conduction losses in the bypass switch 8,

• It is a source of the reactive capacitive power, which may lead to reactive power overcompensation and power quality issues,

• It is a source of power quality problems due to generation of current oscillations in systems with distorted grid voltage,

• It can be a reason of spurious tripping of external protection devices, triggered by the inrush current of the AC output filter capacitors.

Presented above UPS system 1 could be found for example in the US patent US 6,295,215.

From US patent application US 2014/0368043 a method for reduction of a capacitive current of the AC output filter 10 is known. The method action is to keep the AC contactor 6 opened during ECO-mode operation. Thus the AC output filter 10 is not energized, and the capacitive current is not conducted by the bypass switch 8. This solution has several drawbacks. The AC contactor 6 has to be fast in order to reduce transition time from the ECO-mode to the double-conversion mode of the UPS system 1. Because of that, expensive semiconductor based technology for AC contactor 6 is preferred (SCR, IGBT, IGCT), which has to be rated to total load 3 current with 10 times overcurrent capability according to standard. It increases cost and losses of AC contactor 6.

From US patent application US2017/163088 a UPS system is known. An uninterruptible power source is connected between (i) each of an AC power source and a bypass AC power source and (ii) a load. The uninterruptible power source includes: an input terminal configured to receive AC power from the AC power source; a bypass terminal configured to receive AC power from the bypass AC power source; an output terminal configured to output AC power to the load; a converter configured to rectify the AC power provided to the input terminal into DC power; an inverter configured to invert DC power output by the converter or DC power output by a power storage device into AC power, and to output the AC power to the output terminal; a power conversion controller configured to control the power conversion in each of the converter and the inverter; a bypass switch having one terminal connected to the output terminal; a first switch connected between the bypass terminal and the other terminal of the bypass switch; a second switch connected between (i) a first node located between the input terminal and an AC terminal of the converter and (ii) the other terminal of the bypass switch; a third switch connected between the input terminal and the first node; and a switching controller configured to control on/off of the bypass switch and the first to three switches. The uninterruptible power source has (i) a first operation mode in which AC power is supplied from the inverter to the load and (ii) a second operation mode in which AC power is supplied from the AC power source or the bypass AC power source to the load via the bypass switch. The uninterruptible power source is configured to change over to the first operation mode when power failure occurs to stop supply of the AC power from the AC power source or the bypass AC power source while the second operation mode is selected. When turning off the bypass switch by the switching controller, the power conversion controller is configured to (i) control the converter to invert the DC power output by the power storage device into AC power and output the AC power to the first node, (ii) control the inverter to invert the DC power output by the power storage device into AC power and output the AC power to the output terminal, and (Hi) control the inversions in the converter and the inverter such that phase and magnitude of an AC voltage output to the first node become equal to phase and magnitude of an AC voltage output to the output terminal. This system in the first embodiment of the invention has the ECO-mode as an operation mode. When power failure of an AC power source occurs during an ECO-mode in which AC power is supplied via bypass switch, the UPS is configured to change over to inverter power supply from an inverter. When turning off the bypass switch during power failure, a power conversion controller is configured to (i) control the DC/AC converter to invert the DC power from the battery into AC power and output the AC power to a load, and (ii) control the inverter to invert the DC power output by the battery into AC power and output the AC power to an output terminal. The US2017/163088 patent describes system that is equipped with an LC output filter connected at the output of inverter and a mechanical switch between the LC output filter and common connection point of Load and bypass. This mechanical switch between the LC output filter and load is relatively slow. Therefore in eco- mode has to stay closed to ensure quick response of UPS inverter to AC power source disturbance. This situation has been presented in Fig. 2 of above-mentioned patent. The main disadvantage of this solution is fact that capacitor of LC output filter remains connected to the grid and causes flow of high reactive capacitive power that increase excessive losses in bypass and lead to the overcompensation of reactive capacitive power delivered to the grid. The skilled person can understand that capacitor causing problem has to be disconnected from the system during eco-mode. It is not obvious that capacitance of capacitor could be changed to minimize capacitive current. Even if it could be considered option, it is not obvious if parallel or series connection should be more beneficial. The skilled person could, for example, minimize capacitance by minimizing number of parallel connected capacitors that is different than solution proposed in this invention

From US patent application US2008/0278005 UPS (Uninterruptible power supply) is known. The UPS comprising: at least one first input, at power supply output, a first converter which is shut down when operating in economic mode, a reversible second converter, a branching and/or selection circuit to connect an input directly to the power supply output when operating in economic mode and control means. The skilled person knows that reversible second converter consists output filter in order to filter the higher harmonics of the output voltage of the converter. The commonly practice is usage of LC output filter for this purpose.

Main drawback of system described in patent application US2008/0278005 is that during an ECO-mode operation, the LC output filter drain capacitive current by the bypass switch and thus increased conduction losses occurs in the bypass switch. Moreover AC filter is a source of capacitive reactive power which may lead to reactive power overcompensation and power quality issues like increase of the voltage RMS value at the point of common coupling and generation of higher harmonic currents in the distorted grid voltage. Above- mentioned patent does not solve excessive capacitive current flow in its permanent connection state.

There is known from US application US2005/0162219 an active filter that can reduce capacitance of a feedback section of an operational amplifier in a semiconductor integrated circuit device. The active filter includes an operational amplifier in which a plurality of capacitive elements are connected between an output terminal and an inverting input terminal or an input terminal. This class of active filter devices is dedicated to signal conditioning and doesn't belong to the category of AC power filters for power electronics devices like voltage converters. It uses small power unipolar signal MOSFET switch that cannot be used to solve problem with excessive capacitive current that requires bipolar switch operating with full voltage of 50Hz (or 60Hz) grid. Therefore, the person skilled in active filter devices described in patent application US2005/0162219 cannot use his knowledge to solve problem with capacitive current existing in UPS operating in eco-mode. None of the abovementioned patents solved the problem of excessive capacitive current flow by limiting its value or interrupting its flow in a manner that permits on quick transition from ECO-mode to double conversion mode of UPS system with minimum delay to not disturb normal operation of load.

The essence of the inventive UPS system which comprises an AC/DC converter circuit and a DC/AC converter circuit connected in series between a power source and a load wherein the AC/DC converter circuit and DC/AC converter circuit are bypassed by a AC bypass switch circuit, where the DC/AC converter circuit is equipped with an AC output filter being an LC circuit, is that the DC/AC converter circuit is equipped with an AC output filter in which the LC circuit is formed by at least two capacitors or banks of capacitors C1 and C2 connected in series having a different values of capacitances. One of these capacitors C1 or C2 is bypassed by a bidirectional switch connected in parallel to the capacitors or bank of capacitors which has lower value of the capacitance. When the bidirectional switch is closed and the AC bypass switch circuit is open then the UPS system is enable for working in a double-conversion operating mode for which the AC/DC converter circuit and the DC/AC converter circuit are operated with nominal LC filter parameters. When the bidirectional switch is open and the AC bypass switch circuit is closed then the UPS system is enable for working in an ECO-mode operation for which the DC/AC converter circuit is not operating and parameters of LC filter are modified by bidirectional switch to reduce capacitive current.

Preferably the bidirectional switch is a mechanical switch, a solid-state switch or a hybrid switch.

Preferably the solid-state switch or a hybrid switch comprises a at least one of

semiconductor valves: SCR, MOSFET, IGBT, IGCT or their combination.

Preferably the DC/AC converter circuit is connected to the load continuously both in the ECO-mode operation for which the DC/AC converter circuit is not operating and in the double-conversion mode for which the AC/DC converter circuit and the DC/AC converter circuit are operated and an optional contractor is closed. The advantage of the invention is a very simple construction which allows for reduction of the capacitive current in the UPS system. The bidirectional switch in the output AC filter is rated only to current of the capacitor C1 or C2, which has higher value of capacitance, what reduces cost of the bidirectional switch. Moreover the DC/AC converter can be continuously connected to the load when the AC contactor is omitted or closed during the ECO-mode operation and double-conversion mode. As a result the faster transition of the UPS system from ECO-mode operation to the double-conversion mode operation can be achieved. In the UPS system, according to the present invention, the AC contactor can be slow mechanical switch, which can be closed both during ECO-mode and double conversion mode.

The subject of the invention is presented as an embodiment in the drawing where fig.1 shows a known from the prior art UPS system, fig.2 shows a detail from fig.1. Fig.3 shows the UPS system according to the present invention and fig. 4 shows a detail from fig.3.

In comparison to the known prior art a novel UPS system comprises a new output AC filter 11 (fig.4) instead of the know AC filter 10 (fig.2). The AC output filter 11 of the DC/AC converter 5 has LC topology. Total capacitance of the AC output filter 11 is composed of two capacitors, C1 and C2. Capacitors C1 and C2 are connected in series. Preferably, capacitance of C1 and C2 are different. Capacitor C1 having the lower capacitance is bypassed by the bidirectional switch 12. The switch 12 may be a mechanical switch, solid- state switch as SCR, MOSFET, IGBT, IGCT or hybrid switch. In the UPS system 1 , according to the present invention the AC contactor 6 can be omitted.

When the UPS system 1 is operating in the double-conversion mode then the optional AC contactor 6 is closed, the bipolar switch 12 of the AC output filter 11 is closed and bypass switch 8 is closed. The value of capacitance Cdoubie_conversion of LC output filter 11 connected in parallel to the load 3 is equal: double conversion ~ ^2 (1)

When the UPS system 1 is operating in the ECO-mode then the optional AC contactor 6 is closed, the bipolar switch 12 of the AC output filter 11 is open and bypass switch 8 is open. The value of capacitance CEco-mode of LC output filter 11 connected in parallel to the load 3 is equal: C1 C2

^ECO-mode ~ Q^ ₊ 2 (2)

The resultant value of Csco-mode is smaller than the value of Caouue -conversion, because of series connection of capacitors C1 and C2. Thus, capacitive current in the bypass switch 8 of the UPS system 1 is reduced. The capacitance of the capacitor C1 can be selected in respect to the value of the capacitor C2 in that manner to achieve required reduction of capacitive current or to achieve new filtering functionality together with inductance of supplying grid. The value of capacitor C2 is selected by the designer of the LC output filter 11 in respect to the DC/AC converter 5. Both capacitors C1 and C2 could be replaced by a series or parallel connected banks of capacitors what is not presented in the drawing.

The process of the reduction of capacitive current according to the present invention is realized in the following way:

When the UPS system 1 is working in the double-conversion mode (what is decided in the control circuit 9) then the bipolar switch 12 of the LC output filter 11 is closed.

When UPS system 1 is working in the ECO-mode (what is decided in the control circuit 9), then the switch 12 of the LC output filter 11 is opened.