Patent Description:
DC power systems offer improved compatibility between loads, generators and storage systems internally operating at DC or requiring DC along the power conversion chain from generation to consumption. Examples of DC generators, loads and storage systems include solar photovoltaic systems, electric vehicles, fuel cells, variable-frequency drives, battery energy storage systems, LED lighting fixtures, etc. Improved compatibility enables to eliminate or simplify AC/DC and DC/DC power conversion steps between the devices that are interconnected, offering a number of related advantages. It increases the energy efficiency of the power system, the system's dependability and ultimately lowers the costs.

Therefore, DC power systems are favored over traditional AC power system architectures in a number of applications, for example in datacenters, building-level nanogrids and district-level microgrids for interconnecting electric vehicle charging infrastructure and high-power devices (heating, ventilation, air-conditioning and elevator drives), industrial manufacturing facilities to facilitate the integration of on-site storage and improve the power quality, electric vehicle charging stations, street lighting poles, cellular network masts, traffic signalization, and on-board applications, including aircraft and shipboard electrical power systems.

The majority of current DC power systems adopt a two-wire, unipolar DC architecture comprising a positive and negative pole. However, to transfer more power per unit conductor cross-section and to cover larger distances, the bipolar, three-wire DC architecture can be used. Considered voltage levels are for example +/-380V (positive pole: 380V, neutral: 0V, negative pole: -380V), +/-190V and +/-350V. The bipolar DC architecture provides two voltage levels, namely the positive-to-negative pole-to-pole voltage (e.g. 760V) and the pole-to-neutral voltage (e.g. 380V), for connecting devices at the most suitable voltage level. However, continuously stabilizing two voltage levels in bipolar DC power systems renders voltage control more complex than in unipolar systems, especially with the presence of devices connected alternately between the positive pole and the neutral and the negative pole and the neutral. If the current in the positive pole conductor differs from the current in the negative pole conductor, the neutral conductor can carry current, leading to a difference in the positive and negative pole-to-neutral voltages. Eventually, this difference may cause over- and undervoltage in the system, threatening the system's dependability.

Thus, bipolar DC power systems require active control of the DC pole-to-neutral voltages measured between the positive pole and the neutral conductor and the negative pole and the neutral conductor. Even in the presence of unbalanced loading conditions, in which case the power off-take and infeed on the positive pole and the negative pole differs, the pole-to-neutral voltage levels need to remain stable. Stacking of two AC-DC or DC-DC converters that separately control the positive pole-to-neutral voltage and the negative pole-to-neutral voltage has been described in Korean patent application <CIT>. This system relies upon its main control unit for voltage control. Alternatively, in <CIT>, an AC-DC converter connected to the positive and the negative conductor of the bipolar DC system has been disclosed which allows regulation of the positive-to-negative pole-to-pole voltage, and a voltage balancer is introduced for controlling the neutral terminal voltage. The voltage balancer is a voltage converter that transfers power from the positive pole to the negative pole in order to equalize the pole-to-neutral voltages. The system will fail if the voltage balancer or bidirectional converter fail. Several voltage balancer topologies exist. Examples are described by <NPL>, in Korean patent application <CIT> and in <NPL>. Another alternative is a three-port DC-DC converter with a positive, neutral and a negative output terminal for interfacing with bipolar DC systems and controlling the positive and negative pole-to-neutral voltages. Both isolated DC-DC converters, as described in Chinese patent application <CIT>, and nonisolated DC-DC converters, as described in <NPL> and in <NPL>, have that capability.

In three-wire bipolar DC systems, two-terminal AC-DC and DC-DC voltage converters connected to the positive pole and the neutral can be equipped with droop control, as well as voltage converters connected to the negative pole and the neutral. In that case, the bipolar DC system is controlled as two unipolar subsystems. Another method allows parallelizing multiple two-level half-bridge voltage balancers by a variant of the voltage droop control method for unipolar DC systems, as described in <NPL>. That method controls the voltage difference between the positive pole-to-neutral voltage and the negative pole-to-neutral voltage and the voltage difference setpoint reduces in proportion to the current injected in the neutral terminal. However, bipolar DC systems require decentralized controllers and a voltage control method for parallelizing multiple aforementioned AC-DC and DC-DC, two-port and three-port voltage converters with the ability to regulate the pole-to-pole and pole-to-neutral voltages. Such controllers and control method would increase the system dependability and enable to modularly scale-up to higher power levels. Such controllers and control methods are not currently available.

It is an object to provide good systems for controlling single bipolar DC systems, electric appliances comprising such control systems and good methods for controlling single bipolar DC systems.

Embodiments of the present invention provide a system with multiple voltage converters to control a single bipolar DC system, such that there is redundancy and the system does not encounter a single point of failure. Scalability is also enabled, as by combining multiple voltage converters in a modular manner, the total power level increases proportionally.

However, parallelizing multiple voltage converters in a bipolar DC power system requires a voltage control system, which ensures that the magnitudes of the positive pole-to-neutral voltage, measured between the positive conductor and the neutral conductor, and the negative pole-to-neutral voltage, measured between the neutral conductor and the negative conductor, remain within acceptable operating limits, but avoids counteraction between voltage converters. Therefore, the voltage control system according to embodiments of the present invention allows coordinating the setpoints sent to all voltage converters. These voltage setpoints eventually allow determining how much power each voltage converter contributes to control the pole-to-neutral voltages. Suitable selection of the voltage setpoints avoids undesired interactions between the voltage converters. Thus, controlling a voltage comprises controlling the voltage as a function of a respective output current or power by determining per converter a setpoint voltage in dependence upon the respective output current or power value, and by maintaining per converter the respective voltage at the setpoint value of the converter.

More in particular embodiments of the invention disclose a system for controlling a bipolar DC power system, wherein the bipolar DC power system comprises a positive conductor, a neutral conductor and negative conductor, wherein a positive pole-to-neutral voltage is a voltage between the positive conductor and the neutral conductor and a negative pole-to-neutral voltage is a voltage between the negative conductor and the neutral conductor. The system comprises control means for controlling the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, the control means comprising a first voltage converter configured to control a sum or difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage; and a second voltage converter, wherein, if the first voltage converter is configured to control the sum of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, then the second voltage converter is configured to control the positive pole-to-neutral voltage, the negative pole-to-neutral voltage, or the difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, and wherein if the first voltage converter is configured to control the difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, then the second voltage converter is configured to control the positive pole-to-neutral voltage, the negative pole-to-neutral voltage, or the sum of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, wherein controlling a voltage comprises controlling the voltage as a function of the respective output current or power. One or more of the voltage converters of the control means may be configured for controlling the positive pole-to-neutral voltage and the negative pole-to-neutral voltage.

In one aspect, the present invention also relates to a method of controlling a voltage in a bipolar DC power system comprising a positive conductor, a neutral conductor and negative conductor, wherein a positive pole-to-neutral voltage is a voltage between the positive conductor and the neutral conductor and a negative pole-to-neutral voltage is a voltage between the negative conductor and the neutral conductor, using a system for controlling the bipolar DC power system, whereby the system for controlling comprises a first voltage converter configured to control a sum or difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage; and a second voltage converter, wherein, if the first voltage converter is configured to control the sum of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, then the second voltage converter is configured to control the positive pole-to-neutral voltage, the negative pole-to-neutral voltage, or the difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, and wherein if the first voltage converter is configured to control the difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, then the second voltage converter is configured to control the positive pole-to-neutral voltage, the negative pole-to-neutral voltage, or the sum of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, wherein controlling a voltage comprises controlling the voltage as a function of the respective output current or power. The method comprises per converter receiving a voltage value in the system for controlling, receiving in the system for controlling an output current value, determining a setpoint voltage in dependence upon the output current or power value, and maintaining the voltage value at the setpoint value.

Further features of the present invention will become apparent from the examples and figures, wherein:.

Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated. The term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. In the drawings, like reference numerals indicate like features; and, a reference numeral appearing in more than one figure refers to the same element.

Where in embodiments of the present invention reference is made to a control means, reference also may be made to a controller.

Where in embodiments of the present invention reference is made to a control means for controlling the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, reference may be made to a control means configured for controlling the positive pole-to-neutral voltage and the negative pole-to-neural voltage. Furthermore, in embodiments, reference may be made to one or more of the voltage converters of the control means being configured for controlling the positive pole-to-neutral voltage and the negative pole-to-neutral voltage.

Where in embodiments of the present invention reference is made to a voltage converter, reference also may be made to a voltage source converter.

Referring to <FIG>, a system <NUM> for controlling a bipolar DC power system <NUM> according to embodiments of the present invention is shown. The power system <NUM> may be a power transmission system or a power distribution system. The bipolar DC system <NUM> comprises a positive conductor <NUM>, a neutral conductor <NUM> and a negative conductor <NUM>. A positive pole-to-neutral voltage is a voltage between the positive conductor and the neutral conductor and a negative pole-to-neutral voltage is a voltage between the negative conductor and the neutral conductor.

The system <NUM> comprises control means for controlling the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, the control means comprising a first voltage converter <NUM> and a second voltage converter <NUM> which together can control the pole-to-neutral voltages of the bipolar DC system <NUM>. As used herein, a voltage converter refers to a power electronic converter that has the ability to control either the positive pole-to-neutral voltage, the negative pole-to-neutral voltage or a mathematical function of the negative and positive pole-to-neutral voltages, for example by maintaining the voltages at a setpoint value.

In some embodiments, the first voltage converter <NUM> and the second voltage converter <NUM> are located within the same box or enclosure (not shown). In some embodiments, the first voltage converter <NUM> and the second voltage converter <NUM> are not located within the same box or enclosure and can be moved independently of each other. For example, the first and second converters may be connectable to a bipolar DC system spanning multiple buildings and the first converter may be connectable to the bipolar DC system in a first building whereas the second converter is connectable to the bipolar DC system in a second, different building. In another embodiment, the first converter may be connectable in the basement of a building and the second converter may be connectable at the rooftop of a building, for example connectable to a photovoltaic installation. The system <NUM> may comprise one or more further, non-voltage regulating converters (not shown) between the first and second converters, for example for supplying loads or generators. The system may comprise one or more electronic components or mechanical contacts connected in series with the positive, neutral or negative conductor, between the first and second converters as protective devices.

Hereinafter, voltage converters are categorized into four categories:.

The four categories are not mutually exclusive.

In a system <NUM> according to embodiments of the present invention, the first voltage converter <NUM> is a balanced or an unbalanced voltage converter. If the first voltage converter <NUM> is a balanced voltage converter, then the second voltage converter <NUM> is an unbalanced voltage converter, a positive voltage converter, or a negative voltage converter. If the first voltage converter <NUM> is an unbalanced voltage converter, then the second voltage converter <NUM> is a balanced voltage converter, a positive voltage converter, or a negative voltage converter.

The voltage converter types are schematically shown in <FIG>. Referring to <FIG>, a balanced voltage converter <NUM> comprises two front-end terminals configured to connect to the positive <NUM> and the negative <NUM> conductor respectively and an optional front-end terminal configured to connect to the neutral conductor <NUM>. Referring to <FIG>, an unbalanced voltage converter <NUM> comprises three front-end terminals configured to connect to the positive <NUM>, the neutral <NUM> and negative <NUM> conductor respectively. Referring to <FIG>, a positive voltage converter <NUM> comprises two front-end terminals configured to connect to the positive <NUM> and neutral <NUM> conductor respectively. Referring to <FIG>, a negative voltage converter <NUM> comprises two front-end terminals configured to connect to the neutral <NUM> and negative <NUM> conductor respectively.

Each of the voltage converters <NUM>, <NUM>, <NUM> and <NUM> comprises a power conversion stage, a control module and two or three DC front-end connection terminals. Optionally, voltage converters are able to interface by means of two or more electrical conductors with input devices <NUM>, <NUM>, <NUM>, <NUM> such as generators, loads, storage systems, the AC grid, or another DC system from which they receive a voltage to be converted.

Referring to <FIG>, a balanced voltage converter <NUM> comprises a positive terminal <NUM> and a negative terminal <NUM>, an optional neutral terminal <NUM>, a processor <NUM>, a control module <NUM>, and a power conversion stage <NUM>. The balanced converter <NUM> is configured to interface with an input device <NUM> by means of two or more electrical conductors <NUM>. The positive terminal <NUM>, the optional neutral terminal <NUM> and the negative terminal <NUM> are referred to as the front-end of the balanced voltage converter. A balanced voltage converter is connectable via the positive <NUM> and negative <NUM> terminal to the positive <NUM> and negative <NUM> conductor of a bipolar DC system respectively. If provided, the neutral terminal <NUM> is connectable to the neutral conductor <NUM> of a bipolar DC system. The power conversion stage <NUM> contains power semiconductor devices, gate drive circuits and input and output filters for DC/AC or DC/DC conversion. The input device <NUM> can be an AC grid, another DC grid, an AC end-use device or a DC end-use device. The input device <NUM> is connectable to the power conversion stage <NUM> via two or more electrical conductors <NUM>.

The power conversion stage <NUM> is configured to transfer power from the input device <NUM> to the front-end of the balanced voltage converter. The direction of power transfer can be bidirectional.

The processor <NUM> is configured to determine a balanced voltage signal vb2=(vp2+vn2)/<NUM> (with vp2 and vn2 the positive and the negative pole-to-neutral voltage, respectively - see <FIG>), a balanced current signal ib2=(ip2+in2)/<NUM> and a balanced voltage setpoint signal vb2,set, which is a function of the balanced current signal ib2. Voltage signal vb2 is thus related to the sum of vp2 and vn2. For example, <FIG> depict example functional relationships between the balanced voltage setpoint signal vb2,set, determined by the processor, and the balanced current ib2. The functional relationships are based on some or all of the following parameters: the nominal balanced voltage vb,nom, the maximum balanced voltage deviation Δvb<NUM>, the minimum balanced voltage deviation Δvb<NUM>, the balanced current deadband Δidbb2, the balanced voltage deadband Δvdbb2, the maximum balanced current ib<NUM> and the minimum balanced current ib2. The maximum and minimum balanced voltage deviation are determined from the voltage tolerances defined for the bipolar DC system. The maximum and minimum balanced current is determined by the current limits of the balanced voltage converter. The balanced current deadband may vary between zero and ib<NUM> + ib<NUM>. The balanced voltage deadband can vary between zero and vb<NUM> + vb<NUM>. The voltage-current relationship is preferably monotonically decreasing, for example described by a piecewise linear function or a polynomial. In the preceding description, balanced power can be used instead of balanced current equivalently, in which case the balanced power pb2 is defined as pb2=(vp2ip2+vn2in2)/<NUM>.

The control module <NUM> is configured to receive the balanced voltage setpoint signal vb2,set <NUM> and to send pulse-width modulation signals <NUM> to the power conversion stage <NUM>, such that the balanced voltage signal vb2 tracks the balanced voltage setpoint signal vb2,set. The processor <NUM> and the control module <NUM> can be embedded in a single computational unit (not shown).

Referring to <FIG>, an unbalanced voltage converter <NUM> comprises a positive terminal <NUM>, a neutral terminal <NUM> and a negative terminal <NUM>, a processor <NUM>, a control module <NUM> and a power conversion stage <NUM>. The positive terminal <NUM>, the neutral terminal <NUM> and the negative terminal <NUM> are referred to as the front-end of the unbalanced voltage converter. The unbalanced voltage converter is connectable to an input device <NUM>, interfaced by means of two or more electrical conductors <NUM>. The unbalanced voltage converter is connectable via the positive terminal <NUM>, neutral terminal <NUM> and negative terminal <NUM> to the positive conductor <NUM>, neutral conductor <NUM> and negative conductor <NUM> of a bipolar DC system respectively. The power conversion stage <NUM> contains power semiconductor devices, gate drive circuits and input and output filters for DC/AC or DC/DC conversion. The input device <NUM> can be an AC grid, another DC grid, an AC end-use device or a DC end-use device. The input device <NUM> is connectable to the power conversion stage <NUM> via two or more electrical conductors <NUM>.

The power conversion stage <NUM> is configured to transfer power from the positive pole to the negative pole, or from the input device <NUM> to the positive pole or from the input device <NUM> to the negative pole of the bipolar DC system. The direction of power transfer can be bidirectional, but not necessarily. The processor <NUM> is configured to establish an unbalanced voltage signal vu3=(vp3-vn3)/<NUM> (with vp3 and vn3 the positive and the negative pole-to-neutral voltage, respectively - see <FIG>), an unbalanced current signal iu3=(ip3-in3)/<NUM> and an unbalanced voltage setpoint signal vu3,set, which is a function of the unbalanced current signal iu3. Voltage signal vu3 is thus related to the difference of vp3 and vn3. For example, <FIG> depict example functional relationships between the unbalanced voltage setpoint signal vu3,set, determined by the processor, and the unbalanced current iu3. The functional relationships are based on some or all of the following parameters: the nominal unbalanced voltage vu,nom, the maximum unbalanced voltage deviation Δvu<NUM>, the minimum unbalanced voltage deviation Δvu<NUM>, the unbalanced current deadband Δidbu3, the unbalanced voltage deadband Δvdbu3, the maximum unbalanced current iu<NUM> and the minimum unbalanced current iu3. The maximum and minimum unbalanced voltage deviation are determined from the voltage tolerances defined for the bipolar DC system. The maximum and minimum unbalanced current is determined by the current limits of the unbalanced voltage converter. The unbalanced current deadband may vary between zero and iu<NUM> + iu3. The unbalanced voltage deadband can vary between zero and vu<NUM> + vu<NUM>. The voltage-current relationship is preferably monotonically decreasing, for example described by a piecewise linear function or a polynomial. In the preceding description, unbalanced power can be used instead of unbalanced current equivalently, in which case the unbalanced power pu3 is defined as pu2=(vp3ip3-vn3in3)/<NUM>.

The control module <NUM> is configured to receive the unbalanced voltage setpoint signal vu3,set <NUM> and to send pulse-width modulation signals <NUM> to the power conversion stage <NUM>, such that the unbalanced voltage signal vu3 tracks the unbalanced voltage setpoint signal vu3,set. The processor <NUM> and the control module <NUM> can be embedded in a single computational unit (not shown).

Referring to <FIG>, a positive voltage converter <NUM> comprises a positive terminal <NUM> and a neutral terminal <NUM>, a processor <NUM>, a control module <NUM>, and a power conversion stage <NUM>. The positive terminal <NUM> and the neutral terminal <NUM> are referred to as the front-end of the positive voltage converter. The positive voltage converter is connectable to an input device <NUM>, interfaced by means of two or more electrical conductors <NUM>. The positive voltage converter is connectable via the positive terminal <NUM> and neutral terminal <NUM> to the positive conductor <NUM> and neutral conductor <NUM> of a bipolar DC system respectively. The power conversion stage <NUM> contains power semiconductor devices, gate drive circuits and input and output filters for DC/AC or DC/DC conversion. The input device <NUM> can be an AC grid, another DC grid, an AC end-use device or a DC end-use device. The input device <NUM> is connectable to the power conversion stage <NUM> via two or more electrical conductors <NUM>.

The power conversion stage <NUM> is configured to transfer power from the input device <NUM> to the front-end of the positive voltage converter. The direction of power transfer can be bidirectional, but not necessarily. The processor <NUM> is configured to establish a positive voltage signal vp4, a current signal ip4 and a positive voltage setpoint signal vp4,set, which is a function of the current signal ip4. For example, <FIG> depict example functional relationships between the positive voltage setpoint signal vp4,set, determined by the processor, and the positive current ip4. The functional relationships are based on some or all of the following parameters: the nominal positive voltage vp,nom, the maximum positive voltage deviation Δvp<NUM>, the minimum positive voltage deviation Δvp<NUM>, the positive current deadband Δidbp4, the positive voltage deadband Δvdbp4, the maximum positive current ip<NUM> and the minimum positive current ip4. The maximum and minimum positive voltage deviation are determined from the voltage tolerances defined for the bipolar DC system. The maximum and minimum positive current is determined by the current limits of the positive voltage converter. The positive current deadband may vary between zero and ip<NUM> + ip4. The positive voltage deadband can vary between zero and vp<NUM> + vp<NUM>. The voltage-current relationship is preferably monotonically decreasing, for example described by a piecewise linear function or a polynomial. In the preceding description, positive power can be used instead of positive current equivalently, in which case the positive power pp4 is defined as pp4-vp4ip4.

The control module <NUM> is configured to receive the positive voltage setpoint signal vp4,set <NUM> and to send pulse-width modulation signals <NUM> to the power conversion stage <NUM>, such that the voltage signal vp4 tracks the voltage setpoint signal vp4,set. The processor <NUM> and the control module <NUM> can be embedded in a single computational unit (not shown).

Referring to <FIG>, a negative voltage converter <NUM> comprises a neutral terminal <NUM> and a negative terminal <NUM>, a processor <NUM>, a control module <NUM>, and a power conversion stage <NUM>. The neutral terminal <NUM> and the negative terminal <NUM> are referred to as the front-end of the negative voltage converter. The negative voltage converter is connectable to an input device <NUM>, interfaced by means of two or more electrical conductors <NUM>. The negative voltage converter is connectable via the neutral <NUM> and negative <NUM> terminal to the neutral <NUM> and negative <NUM> conductor of a bipolar DC system respectively. The power conversion stage <NUM> contains power semiconductor devices, gate drive circuits and input and output filters for DC/AC or DC/DC conversion. The input device <NUM> can be an AC grid, another DC grid, an AC end-use device or a DC end-use device. The input device <NUM> is connectable to the power conversion stage <NUM> via two or more electrical conductors <NUM>.

The power conversion stage <NUM> is configured to transfer power from the input device <NUM> to the front-end of the negative voltage converter. The direction of power transfer can be bidirectional, but not necessarily.

The processor <NUM> is configured to establish a voltage signal vn5, a current signal in5 and a voltage setpoint signal vn5,set, which is a function of the current signal in5. For example, <FIG> depict example functional relationships between the negative voltage setpoint signal vn5,set, determined by the processor, and the negative current in5. The functional relationships are based on some or all of the following parameters: the nominal negative voltage vn,nom, the maximum negative voltage deviation Δvn5, the minimum negative voltage deviation Δvn<NUM>, the negative current deadband Δidbn5, the negative voltage deadband Δvdbn5, the maximum negative current in<NUM> and the minimum negative current in5. The maximum and minimum negative voltage deviation are determined from the voltage tolerances defined for the bipolar DC system. The maximum and minimum negative current is determined by the current limits of the negative voltage converter. The negative current deadband may vary between zero and in<NUM> + in5. The negative voltage deadband can vary between zero and vn<NUM> + vn<NUM>. The voltage-current relationship is preferably monotonically decreasing, for example described by a piecewise linear function or a polynomial. In the preceding description, negative power can be used instead of negative current equivalently, in which case the negative power pn5 is defined as pn5=vn5in5.

The control module <NUM> is configured to receive the voltage setpoint signal vn5,set <NUM> and to send pulse-width modulation signals <NUM> to the power conversion stage <NUM>, such that the voltage signal vp5 tracks the voltage setpoint signal vn5,set. The processor <NUM> and the control module <NUM> can be embedded in a single computational unit (not shown).

Referring to <FIG>, various configurations of the first and second voltage converters are possible within the scope of the present invention.

Referring to <FIG>, in a system <NUM> according to embodiments of the present invention, the first voltage converter <NUM> is a balanced voltage converter and the second voltage converter <NUM> is a negative voltage converter. Hence, the first voltage converter controls the sum of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage.

Referring to <FIG>, in a system <NUM> according to embodiments of the present invention, the first voltage converter <NUM> is a balanced voltage converter and the second voltage converter <NUM> is a positive voltage converter. Again, the first voltage converter controls the sum of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage.

Referring to <FIG>, in a system <NUM> according to embodiments of the present invention, the first voltage converter <NUM> is a balanced voltage converter and the second voltage converter <NUM> is an unbalanced voltage converter. Again, the first voltage converter controls the sum of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage.

Referring to <FIG>, in a system <NUM> according to embodiments of the present invention, the first voltage converter <NUM> is an unbalanced voltage converter and the second voltage converter <NUM> is a negative voltage converter. Hence, the first voltage converter here controls the difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage.

Referring to <FIG>, in a system <NUM> according to embodiments of the present invention, the first voltage converter <NUM> is an unbalanced voltage converter and the second voltage converter <NUM> is a positive voltage converter. Again, the first voltage converter controls the difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage.

Referring to <FIG>, in a system <NUM> according to embodiments of the present invention, the first voltage converter <NUM> is a balanced voltage converter and the second voltage converter <NUM> is an unbalanced voltage converter. The first voltage converter controls the sum of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage and the second voltage converter controls the difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage. The system <NUM> further comprises a positive voltage converter <NUM> and a negative voltage converter <NUM>. A system according to embodiments of the present invention may comprise more than two voltage converters, provided that the system comprises at least a first voltage converter which is a balanced or an unbalanced voltage converter, and a second voltage converter, wherein if the first voltage converter is a balanced voltage converter then the second voltage converter is an unbalanced converter, a positive converter, or a negative converter, and if the first voltage converter is an unbalanced voltage converter then the second voltage converter is a balanced converter, a positive converter, or a negative converter. In a system according to embodiments of the present invention, converters in addition to the first and second voltage converters may be any of balanced, unbalanced, positive, or negative converters.

In some embodiments, the first converter and the second converter may be the same converter. For example, a single converter may contain a single processor, which is configured to determine the balanced and unbalanced voltages and the balanced and unbalanced currents. In this example, the first converter and the second converter are comprised in a single converter.

Referring to <FIG>, a flow chart of a method of controlling the voltage of a system according to embodiments of the present invention is shown. The method comprises the following steps, in a system according to embodiments of the present invention:.

The method optionally comprises step S4 in which the setpoint voltage is provided to the control module of a voltage converter.

The method may be performed by the processors comprised in the respective voltage converters of a system according to embodiments of the present invention.

In embodiments of the present invention, a processor comprised in a voltage converter of a system according to embodiments of the present invention is configured to perform a voltage control method according to embodiments of the present invention.

In embodiments of the present invention, a single-point-of-failure is avoided, for example by equipping DC/AC AC load with the ability to control vp as a function of the positive current or power. Or alternatively, for example, by extending the system with a second voltage balancer and embedding a controller, controlling vp+vn as a function of ip+in, into the DC/DC BESS. The presented invention enables both to operate in parallel with the original system.

In embodiments of the present invention, an overlay control scheme relying upon communication is not essential.

An embodiment of the presented invention has been experimentally validated in the set-up depicted in <FIG>. The experimental set-up comprises a bipolar DC system comprising three DC-DC voltage converters, referenced to as DCDC1, DCDC2 and DCDC3. DCDC1 is connected to the power supply unit PSU <NUM> and the positive, neutral and negative conductor of the bipolar DC system. DCDC2 is connected to the power supply unit PSU <NUM> with the positive and the neutral conductor of the bipolar DC system. DCDC3 is connected to the power supply unit PSU <NUM> with the neutral and the negative conductor of the bipolar DC system. The test set-up furthermore comprises a positive constant power load Pp, connected between the positive conductor and the neutral conductor, and a negative constant power load Pn, connected between the neutral conductor and the negative conductor.

DCDC1, DCDC2 and DCDC3 each contain a control device. The processor of DCDC1 determines the positive pole-to-neutral voltage Vp1 and the negative pole-to-neutral voltage Vn1. The controller of DCDC1 controls the balanced voltage Vb1=(Vp1+Vn1)/<NUM> to a setpoint Vb1,set as a function of the balanced current Ib1=(Ip1+In1)/<NUM> according to the droop profile shown in <FIG> and the unbalanced voltage Vu1=(Vp1-Vn1)/<NUM> to a setpoint Vu1,set as a function of the unbalanced current Iu1=(Ip1-In1)/<NUM> according to the droop profile shown in <FIG>. The processor of DCDC2 determines the positive pole-to-neutral voltage Vp2. The controller of DCDC2 controls the positive pole-to-neutral voltage Vp2 to a setpoint Vp2,set as a function of the positive current Ip2. The processor of DCDC3 measures the negative pole-to-neutral voltage Vn3. The controller of DCDC3 controls the negative pole-to-neutral voltage Vn3 to a setpoint Vn3,set as a function of the negative current In3. DCDC1 is thus a balanced voltage converter and unbalanced voltage converter, DCDC2 is a positive voltage converter and DCDC3 is a negative voltage converter. They conjointly control the voltages of the bipolar DC system for varying loading conditions determined by the constant power loads Pp and Pn.

The test results are depicted in <FIG> for balanced and unbalanced loading conditions. The first graph, Fig 19a, shows the power injected in the positive and the negative pole contributed by the three voltage converters. DCDC2 solely injects power in the positive pole and DCDC3 solely injects power in the negative pole. DCDC1 is able to inject power in both the positive and the negative pole. The second graph, Fig 19b, depicts the balanced Pbx=(Ppx+Pnx)/<NUM> (x=<NUM>,<NUM>,<NUM>) and the unbalanced Pux=(Ppx-Pnx)/<NUM> (x=<NUM>,<NUM>,<NUM>) power for the three voltage converters. The third graph, Fig 19c, depicts the unbalanced power ratio Pu/Pb and the fourth graph, wherein Pb=(Pp+Pn)/<NUM> and Pu=(Pp-Pn)/<NUM>, Fig 19d, depicts the positive pole-to-neutral voltage Vp and the negative pole-to-neutral voltage Vn at the constant power load terminals.

In the first three operating conditions, the total power remains constant, but unbalance is introduced as more power is gradually injected in the positive pole relative to the negative pole. The power in the positive pole increases with a particular amount and the power in the negative pole decreases with that same amount so that the total power remains constant. In operating condition <NUM> to <NUM>, the unbalance shifts to the negative pole and more power is gradually withdrawn from the negative pole relative to the positive pole. In all operating scenarios, the processors of the voltage converters calculate voltage setpoints as a function of their respective current.

Claim 1:
A system (<NUM>) for controlling a bipolar DC power system (<NUM>), the bipolar DC power system (<NUM>) comprising a positive conductor (<NUM>), a neutral conductor (<NUM>) and negative conductor (<NUM>),
wherein a positive pole-to-neutral voltage is a voltage between the positive conductor (<NUM>) and the neutral conductor (<NUM>) and a negative pole-to-neutral voltage is a voltage between the negative conductor (<NUM>) and the neutral conductor (<NUM>),
wherein the system (<NUM>) comprises control means for controlling the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, wherein the control means comprises :
a first voltage converter (<NUM>) configured to control a sum or difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage; and
a second voltage converter (<NUM>), wherein, if the first voltage converter (<NUM>) is configured to control the sum of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, then the second voltage converter (<NUM>) is configured to control the positive pole-to-neutral voltage, the negative pole-to-neutral voltage, or the difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, and wherein if the first voltage converter (<NUM>) is configured to control the difference of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage, then the second voltage converter (<NUM>) is configured to control the positive pole-to-neutral voltage, the negative pole-to-neutral voltage, or the sum of the positive pole-to-neutral voltage and the negative pole-to-neutral voltage,
and characterized in that the first and second voltage converter are configured for controlling a voltage by controlling the voltage as a function of a respective output current or power by determining per converter a setpoint voltage in dependence upon the respective output current or power value, and by maintaining per converter the respective voltage at the setpoint value of the converter.