System and method for overvoltage protection

In one aspect, the present subject matter discloses a method for overvoltage protection of an electrical system. The method may generally include detecting an overvoltage condition on an electrical system; and switching on, in response to the detected overvoltage condition, an impedance connected to the electrical system, wherein the impedance clamps voltage on the electrical system.

FIELD OF THE INVENTION

The present subject matter relates generally to electrical generation and, more particularly, to a system and method for limiting overvoltage events during islanding of one or more sources of electrical generation.

BACKGROUND OF THE INVENTION

In some instances, sources of electrical generation may be located in remote areas far from the loads they serve. This is particularly true for renewable energy sources such as wind turbine generators, solar/photovoltaic generation, hydroelectric generators, and the like. Typically, these sources of generation are connected to the electrical grid through an electrical system such as long transmission lines. These transmission lines are connected to the grid using one or more breakers. Sudden tripping of the transmission line breaker at the grid side while the source of generation is under heavy load may result in an overvoltage on the transmission line that can lead to damage to the source of generation or equipment associated with the source of generation such as converters and inverters.

Accordingly, an improved system and/or method that provides for sufficient voltage limitation to prevent damaging the sources of generation and equipment associated with the sources of generation would be welcomed in the technology.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of embodiments of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.

In one aspect, the present subject matter discloses a method for overvoltage protection of an electrical system. The method may generally include detecting an overvoltage condition on an electrical system; and switching on, in response to the detected overvoltage condition, an impedance connected to the electrical system, wherein the impedance clamps voltage on the electrical system.

In another aspect, the present subject matter discloses a method for overvoltage protection for a grid-islanding event of an electrical system. The method may generally include detecting a grid islanding event on a poly-phase electrical system, wherein the grid islanding event is caused by disconnecting of one or more sources of electrical generation from an electrical grid; switching on, in response to the detected grid islanding event, an impedance connected between each phase of the poly-phase electrical system, wherein a overvoltage caused by the grid islanding event is limited by the impedance clamping voltage on the poly-phase electrical system; and switching off the impedance connected between each phase of the poly-phase electrical system when the overvoltage event drops below a threshold voltage value.

In another aspect, the present subject matter discloses a system for overvoltage protection of an electrical system. The system may be comprised of one or more impedance elements; one or more switches in series with the one or more impedance elements; and a controller, wherein the controller is configured to: receive an indication of a detection of an overvoltage condition on an electrical system; and cause the one or more switches to connect the one or more impedance elements to the electrical system in response to receiving the indication of the detected overvoltage condition, wherein the overvoltage condition is limited by the one or more impedance elements clamping voltage on the electrical system.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present subject matter is directed to a system and methods for limiting voltage on an electrical system. In particular, aspects of the invention are directed at limiting voltage on an electrical system caused by the islanding of one or more sources of electrical generation. Islanding occurs when one or more sources of electrical generation such as a wind park comprised of one or more wind turbine generators abruptly and unexpectedly is disconnected with the electrical grid. For example, islanding can occur when a breaker on the grid side of an electrical system such as a transmission line opens thereby disconnecting the one or more sources of electrical generation from the grid. This can result in high voltages on the electrical system. If the one or more sources of electrical generation utilize an AC-DC converter and/or a DC to AC inverter, then high voltages can occur on the DC link that connects the converter and inverter (if used) and can damage converter and/or inverter components. This can be more readily seen with reference toFIG. 1.

FIG. 1is a schematic diagram of an exemplary power generation system100that includes at least one power generation unit102. Power generation unit102includes a wind turbine, a solar panel or array, a fuel cell, a geothermal generator, a hydropower generator, and/or any other device that generates electrical power. More specifically, in the exemplary embodiment, power generation unit102can be a device that generates direct current (DC) electrical power from at least one renewable energy source. Alternatively, power generation unit102is a gas turbine, a steam turbine, and/or any other device that generates DC or alternating current (AC) power from a renewable or non-renewable energy source.

In the exemplary embodiment, power generation unit102is coupled to a power converter system104, or a power converter104. DC power generated by power generation unit102is transmitted to power converter system104, and power converter system104converts the DC power to AC power. The AC power is transmitted to an electrical transmission and distribution network106, or “grid.” Power converter system104, in the exemplary embodiment, adjusts an amplitude of the voltage and/or current of the converted AC power to an amplitude suitable for electrical transmission and distribution network106, and provides AC power at a frequency and a phase that are substantially equal to the frequency and phase of electrical transmission and distribution network106. Moreover, in the exemplary embodiment, power converter system104provides three phase AC power to electrical transmission and distribution network106. Alternatively, power converter system104provides single phase AC power or any other number of phases of AC power to electrical transmission and distribution network106.

In the exemplary embodiment, power converter system104includes a DC to DC, or “boost,” converter108and an inverter110coupled together by a DC bus112. Alternatively, power converter system104may include an AC to DC converter108for use in converting AC power received from power generation unit102to DC power, and/or any other converter108that enables power converter system104to function as described herein. In one embodiment, power converter system104does not include converter108, and inverter110is coupled to power generation unit102by DC bus112and/or by any other device or conductor. In the exemplary embodiment, inverter110is a DC to AC inverter110that converts DC power received from converter108into AC power for transmission to electrical transmission and distribution network106. Moreover, in the exemplary embodiment, DC bus112includes at least one capacitor114. Alternatively, DC bus112includes a plurality of capacitors114and/or any other electrical power storage devices that enable power converter system104to function as described herein. As current is transmitted through power converter system104, a voltage is generated across DC bus112and energy is stored within capacitors114.

Power converter system104includes a control system116coupled to converter108and/or to inverter110. In the exemplary embodiment control system116includes and/or is implemented by at least one processor. As used herein, the processor includes any suitable programmable circuit such as, without limitation, one or more systems and microcontrollers, microprocessors, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), programmable logic circuits (PLC), field programmable gate arrays (FPGA), and/or any other circuit capable of executing the functions described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term “processor.”

In the exemplary embodiment, control system116controls and/or operates converter108to adjust or maximize the power received from power generation unit102. Moreover, in the exemplary embodiment, control system116controls and/or operates inverter110to regulate the voltage across DC bus112and/or to adjust the voltage, current, phase, frequency, and/or any other characteristic of the power output from inverter110to substantially match the characteristics of electrical transmission and distribution network106.

During an islanding event, power generation unit102becomes disconnected from the grid106. This can result in an overvoltage on the electrical system that connects the generation unit102with the grid106. An overvoltage can be a short-term or longer duration increase in the measured voltage of the electrical system over its nominal rating. For example, the overvoltage may be 1%, 5% 10%, 50% or greater, and any values therebetween, of the measured voltage over the nominal voltage. This overvoltage on the AC side of the inverter108causes energy to be pumped into capacitors114, thereby increasing the voltage on the DC link112. The higher voltage on the DC link112can damage one or more electronic switches such as a gate turn-off (GTO) thyristor, gate-commutated thyristor (GCT), insulated gate bipolar transistor (IGBT), MOSFET, combinations thereof, and the like located within the inverter110and/or converter108.

FIG. 2Ais a simplified single-line diagram of an electrical power system network200. In this exemplary arrangement, the electrical generation unit102is connected to the grid106through a transformer202that steps up or steps down the AC power created by the generation unit102alone or in cooperation with a convertor110and/or inverter108. AC power is routed through a generator-side breaker204, a transmission line206, a grid side breaker208and to the grid106. Components of the network200can be comprised of AC single phase, AC poly-phase or DC electrical apparatus, as needed. For example, the transmission line206may be a three-phase (AC) transmission line. As shown, both the generator-side breaker204and the grid-side breaker208are closed.

FIG. 2Billustrates the electrical power system network200ofFIG. 2A, where the grid-side breaker208has opened. This can be the result of a fault on the grid106, a fault on the transmission line206, a malfunction of the grid-side breaker208, an accidental opening of the breaker208, and the like. The transmission line206may also be opened by an open circuit fault such as that caused by cutting or breaking the transmission line206. Such an open circuit condition, whatever the cause, can create an overvoltage on the affected phases of the transmission line206if the electrical generation unit102is under load and producing power at the time of the open circuit event. In some instances, the controller may not recognize the open circuit or islanding event, or may react too slowly to the event, and damage may occur to components that comprise the electrical system. In particular, electronic switches used in a convertor110and/or inverter108may be damaged during the overvoltage.

FIG. 3Aillustrates a simplified form of a three-line diagram of an embodiment of an electrical network200further comprising an embodiment of a system for overvoltage protection. In this embodiment, the overvoltage system comprises one or more impedance elements302connected between the phases304of the electrical system206. Though the electrical system206illustrates three phases, it is to be appreciated that embodiments of the present invention can be configured to adapt to any single-phase or poly-phase electrical system. The shown embodiment further comprises one or more switches306in series with the one or more impedance elements302. In one aspect, the switches306can be one or more electronic or mechanical switches or combinations thereof. For example, the switches can be mechanical switches that are controlled by motors, springs, and the like. In another aspect, the switches306can be electronic switches such as silicon controller rectifiers (SCRs) that are controlled by a gate, as are known in the art. Other electronic switches that may be suitable include integrated gate-commutated thyristors (IGCTs), insulated gate bipolar transistors (IGBTs), and the like.FIG. 3Billustrates a simplified form of a three-line diagram of an embodiment of an electrical network200further comprising an embodiment of a system for overvoltage protection, wherein the one or more switches306are comprised of three SCRs308.

Returning toFIG. 3A, the impedance elements302can be comprised of resistance elements, inductive elements, or combinations thereof. Generally, the impedance elements302can be connected between each phase304of a poly-phase electrical system206. The impedance elements302can be sized based at least in part on a voltage of the electrical system206, impedance of the electrical system310to the point where the impedance elements302are connected to the electrical system206, current supplied by any sources of electrical generation102connected to the electrical system206, and a desired clamping level of the overvoltage condition. Though not shown inFIG. 3A(orFIGS. 3B, 3C and 3D), switches306can be controlled (i.e., placed in a conducting or non-conducting state) by commands from a controller. In one aspect, the controller can be configured to receive an indication of a detection of an overvoltage condition on an electrical system206; and cause the one or more switches306to connect the one or more impedance elements302to the electrical system206in response to receiving the indication of the detected overvoltage condition, wherein the overvoltage condition is limited by the one or more impedance elements302clamping voltage on the electrical system. In this way, the impedance elements302act as a voltage divider for the phases of the electrical system206(in conjunction with the line impedance310), and thus the desired clamping level can be set by sizing the impedance elements302to the desired level. In one aspect, the overvoltage condition on the electrical system206can be caused by islanding of one or more sources of electrical generation102from an electrical grid106. In various aspects, the one or more sources of electrical generation102can comprise one or more wind turbine generators, one or more sources of solar/photovoltaic generation, one or more hydroelectric generators, one or more gas turbine generators, one or more steam turbine generators, combinations thereof, and the like.FIGS. 3A and 3Bgenerally illustrate the impedance elements302connected in a wye configuration. This configuration has the advantages of lower voltage stress on the SCR308or switch306because L-N voltage is lower than L-L voltage.

FIGS. 3C and 3Dillustrate alternate embodiments of simplified forms of three-line diagrams of embodiments of an electrical network200further comprising embodiments of systems for overvoltage protection. The configuration illustrated inFIGS. 3C and 3Dallow a single impedance element302to be connected between the phases304of the electrical system206. This can be compared toFIGS. 3A and 3Bwhere two impedance elements302are connected in series between two phases when the switches306are in a conducting state. InFIG. 3D, the overvoltage system comprises two SCRs308per impedance element.FIGS. 3C and 3Dillustrate the impedance elements302connected in a delta configuration. This configuration has the advantages of redundancy, such that any one of the SCRs308or switches306can fail, and the remaining two of the three switches will still provide a short circuit across all three phases (if the component values are designed/selected to take advantage of this redundancy). Similarly, if any one of the impedance elements302or the wiring in any phase fails open, the remaining phases can still provide a short circuit to limit the voltage across all the phases. To do this, the value of impedance element302is lower to limit the total voltage to a desired level. Also the current rating of the switches306must be higher to allow them to carry the total current in two switches, which would have been divided between three switches when all three are working properly.

Advantages of embodiments of this invention in general include being less expensive than adding dynamic braking to every wind turbine in the wind farm. It allows for a single control to decide when to operate it, which avoids any problems of individual turbines acting independently. In other words, avoids any possibility of the sources of electrical generation102fighting each other, some turning on and off at different times. This also offers the advantage of higher reliability and higher availability because it requires fewer components than adding dynamic braking to every sources of electrical generation102such as every wind turbine in a wind farm.

Referring now toFIG. 4, as noted above, some embodiments of systems for overvoltage protection can include a control system or controller36. In general, the controller36may comprise a computer or other suitable processing unit. Thus, in several embodiments, the controller36may include suitable computer-readable instructions that, when implemented, configure the controller36to perform various different functions, such as receiving, transmitting and/or executing control signals. As such, the controller36may generally be configured to control the various operating modes (e.g., conducting or non-conducting states) of the one or more switches306and/or components of embodiments of the overvoltage protection system. For example, the controller36may be configured to implement methods of operating embodiments of the overvoltage protection system.

FIG. 4illustrates a block diagram of one embodiment of suitable components that may be included within an embodiment of a controller36, or any other controller that receives signals indicating overvoltage and/or islanding conditions in accordance with aspects of the present subject matter. In various aspects, such signals can be received from one or more sensors58,60, or may be received from other computing devices (not shown) such as a supervisory control and data acquisition (SCADA) system, a turbine protection system, and the like. As shown, the controller36may include one or more processor(s)62and associated memory device(s)64configured to perform a variety of computer-implemented functions (e.g., performing the methods, steps, calculations and the like disclosed herein). As used herein, the term “processor” refers not only to integrated circuits referred to in the art as being included in a computer, but also refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits. Additionally, the memory device(s)64may generally comprise memory element(s) including, but not limited to, computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD) and/or other suitable memory elements. Such memory device(s)64may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s)62, configure the controller36to perform various functions including, but not limited to, directly or indirectly transmitting suitable control signals to one or more switches306, monitoring overvoltage and/or islanding conditions of the electrical system206, and various other suitable computer-implemented functions.

Additionally, the controller36may also include a communications module66to facilitate communications between the controller36and the various components of the electrical system206and/or the one or more sources of electrical generation102. For instance, the communications module66may serve as an interface to permit the controller36to transmit control signals to the one or more switches306to change to a conducting or non-conducting state. Moreover, the communications module66may include a sensor interface68(e.g., one or more analog-to-digital converters) to permit signals transmitted from the sensors (e.g.,58,60) to be converted into signals that can be understood and processed by the processors62. Alternatively, the controller36may be provided with suitable computer readable instructions that, when implemented by its processor(s)62, configure the controller36to calculate and/or estimate whether a detected overvoltage condition of the electrical system206is the result of islanding of the one or more sources of electrical generation based on information stored within its memory64and/or based on other inputs received by the controller36.

Referring now toFIG. 5, there is illustrated one embodiment of a method for overvoltage protection of an electrical system. This embodiment may be implemented by the controller36or other computing device. As shown, the method generally includes step502, detecting an overvoltage on an electrical system. Generally, this is accomplished by the controller36receiving one or more signals that indicate the presence of an overvoltage condition. In one aspect, the controller36can determine that the overvoltage is caused by an islanding of one or more sources of electrical generation102. In one aspect, detecting the overvoltage condition on the electrical system comprises detecting when a voltage on the electrical system meets or exceeds a voltage threshold value. For example, the voltage threshold value can be adjustable and can be set at 1%, 5%, 10%, 15% or any other value over the nominal voltage of the electrical system. In one aspect, a delay in overvoltage protection may be implemented in order to avoid acting on signals that may be mere noise rather than an actual overvoltage condition. Alternatively, at step502, a grid islanding event may be detected (not shown inFIG. 5). This event may be detected by overvoltage or by other signals not associated with overvoltage such as, for example, reverse power flow, sharp swings in phase angle, and the like. Embodiments of the present invention include current methods and systems to detect grid islanding as well as those that may be later developed. At step504, an impedance connected to the electrical system is switched on in response to the detected overvoltage, wherein the impedance clamps voltage on the electrical system. In one aspect, switching on, in response to the detected overvoltage condition, an impedance connected to the electrical system, wherein the impedance clamps voltage on the electrical system comprises switching on the impedance using one or more electronic or mechanical switches or combinations thereof. In one non-limiting example, the electronic switches can comprise one or more silicon controlled rectifiers (SCRs). In one aspect, switching on, in response to the detected overvoltage condition, an impedance connected to the electrical system, wherein the impedance clamps voltage on the electrical system comprises switching on an impedance connected between each phase of a poly-phase electrical system. In one aspect, the impedance connected between each phase of a poly-phase electrical system is sized based at least in part on a voltage of the electrical system, a grid impedance of the electrical system, current supplied by any sources of electrical generation connected to the electrical system, and a desired clamping level of the overvoltage condition. In one aspect, switching on, in response to the detected overvoltage condition, an impedance connected to the electrical system, wherein the impedance clamps voltage on the electrical system comprises switching on an impedance comprised at least in part of one or more inductors.

FIG. 6illustrates an embodiment of a method for overvoltage protection for a grid-islanding event of an electrical system. This embodiment may also be implemented by the controller36or other computing device. At step602, a grid islanding event is detected on an electrical system, wherein the grid islanding comprises disconnecting one or more sources of electrical generation from an electrical grid. As noted above, detecting a grid islanding event can be performed by systems and methods now known in the art or those later developed. At step604, an impedance connected to the electrical system is switched on in response to the detected grid islanding event. In one aspect, the impedance is connected between each phase of a poly-phase electrical system, wherein an overvoltage event caused by the grid islanding is limited by the impedance clamping voltage on the poly-phase electrical system. At step606, the impedance connected to the electrical system is switched off when the overvoltage event drops below a threshold voltage value.

As described above and as will be appreciated by one skilled in the art, embodiments of the present invention may be configured as a system, method, or a computer program product. Accordingly, embodiments of the present invention may be comprised of various means including entirely of hardware, entirely of software, or any combination of software and hardware. Furthermore, embodiments of the present invention may take the form of a computer program product on a computer-readable storage medium having computer-readable program instructions (e.g., computer software) embodied in the storage medium. Any suitable non-transitory computer-readable storage medium may be utilized including hard disks, CD-ROMs, optical storage devices, or magnetic storage devices.

Throughout this application, various publications may be referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which the methods and systems pertain.