Solid-state protection for direct current networks

Systems, methods, techniques and apparatuses of fault protection. One exemplary embodiment is a protection system including a solid-state switching device, a galvanic isolation switching device, and a controller. The solid-state switching device is coupled between a switch arrangement of a power converter and a direct current (DC) link capacitor of the power converter. The galvanic isolation switching device is coupled between the DC link capacitor and a DC network. The controller is structured to determine a fault is occurring within the DC network, open the solid-state switching device in response to determining the fault is occurring, receive a measurement corresponding to an electrical characteristic of a fault current flowing through the galvanic isolation switching device while the solid-state switching device is open, and determine a location of the fault based on the received measurement.

BACKGROUND

The present disclosure relates generally to fault protection. Direct current (DC) distribution systems that include power converters interconnected by DC distribution lines also include protection systems to detect and isolate faults, such as short circuit faults. These protection systems may be placed near each power converter and on the DC distribution lines throughout the DC distribution system. In response to a detected fault on a DC distribution line, a protection system located next to a power converter will isolate the power converter from the DC distribution lines until the fault is isolated. Existing protection systems for DC distribution systems suffer from a number of shortcomings and disadvantages. There remain unmet needs including reducing protection system hardware requirements, reducing fault current interruption stress, and increasing fault location accuracy. For instance, conventional protection systems must interrupt the high magnitude fault current in order to isolate the detected fault. Furthermore, conventional protection systems only use a small set of measurements taken between the beginning of the fault and the fault isolation to determine a fault location. In view of these and other shortcomings in the art, there is a significant need for the apparatuses, methods, systems and techniques disclosed herein.

DISCLOSURE OF ILLUSTRATIVE EMBODIMENTS

For the purposes of clearly, concisely and exactly describing non-limiting exemplary embodiments of the disclosure, the manner and process of making and using the same, and to enable the practice, making and use of the same, reference will now be made to certain exemplary embodiments, including those illustrated in the figures, and specific language will be used to describe the same. It shall nevertheless be understood that no limitation of the scope of the present disclosure is thereby created, and that the present disclosure includes and protects such alterations, modifications, and further applications of the exemplary embodiments as would occur to one skilled in the art with the benefit of the present disclosure.

SUMMARY OF THE DISCLOSURE

Exemplary embodiments of the disclosure include systems, methods, techniques and apparatuses for fault protection systems. Further embodiments, forms, objects, features, advantages, aspects and benefits of the disclosure shall become apparent from the following description and drawings.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

With reference toFIG. 1, there is illustrated an exemplary direct current (DC) distribution system100. It shall be appreciated that system100may be implemented in a variety of applications, including utility grids, vehicular power systems, marine power systems, multi-drive power systems, DC charging systems, high voltage power systems, medium voltage power systems, and low voltage power systems, to name but a few examples.

DC distribution system100includes a bidirectional power converter110structured to convert power transferred between an alternating current (AC) network101and a DC network130. Power converter110includes a switch arrangement111and DC link capacitor114.

Switch arrangement111includes a plurality of semiconductor devices112a-farranged on three legs coupled across a DC bus113. A first leg includes semiconductor devices112aand112bcoupled in series at a midpoint connection. A second leg includes semiconductor devices112cand112dcoupled in series at a midpoint connection. A third leg includes semiconductor devices112eand112fcoupled in series at a midpoint connection. In the illustrated embodiment, semiconductor devices112a-fare insulated gate bipolar transistors (IGBTs) coupled in an anti-parallel formation with a freewheeling diode. In other embodiments, switch arrangement111includes bipolar junction transistors (BJTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), gate turn-off thyristors (GTOs), MOS-controlled thyristors (MCTs), integrated gate-commutated thyristors (IGCTs), silicon carbide (SiC) switching devices, gallium nitride (GaN) switching devices, or any other type of switching device structured to selectively control the flow of electric current.

In the illustrated embodiment, AC network101includes three phases, each phase being coupled to one midpoint connection of the three legs of switch arrangement111. In other embodiments, switch arrangement111may be adapted to be coupled to an AC network having a different number of phases, or may be adapted to be coupled to a second DC network. In certain embodiments, AC network101is a power generation device, such as a wind turbine or a natural gas generator, to name but a few examples.

Switch arrangement111may be controlled by controller127or by a separate controller. Switch arrangement111may be structured to transmit power unidirectionally or bidirectionally. For example, switch arrangement111may be structured to receive AC power from AC network101, convert the received power to DC power, and output the DC power to DC network130. Switch arrangement111may also be structured to receive DC power from DC network130, convert the received power to AC power, and output the AC power to AC network101. In other embodiments, switch arrangement111may be organized into another topology such as a multi-level converter, a DC/DC buck converter, a DC/DC boost converter, or a topology having more or fewer phase legs, to name but a few examples. It shall be appreciated that switch arrangement111may be of any topology and have components structured to receive DC power from or provide DC power to DC network130.

DC link capacitor114is structured to reduce transients, or smooth, DC power transmitted between switch arrangement111and DC network130. DC link capacitor114is also structured to store energy during the operation of DC distribution system100. During operation of system100, there is a capacitor voltage VCacross DC link capacitor114and a current ICflowing through DC link capacitor114. In certain embodiments, DC link capacitor114is structured to be an aluminum electrolytic capacitor, a film capacitor, or a combination thereof, to name but a few examples. In certain embodiments, DC link capacitor114is a plurality of capacitors.

DC network130includes DC distribution lines131and133. In the illustrated embodiment, a short circuit fault135is occurring on DC network130across DC distribution lines131and133. The portion of DC distribution lines131and133between fault135and protection system120has a line inductance137.

It is important to note that during a DC fault, fault current flows through a current path103including capacitor114, galvanic isolation switching device123, and portions of lines131and133having a line inductance137, such that line current ILmay be equal to capacitor current IC. With solid-state switching device121being opened to isolate power converter110from fault135, fault current135does not commutate from current path103to a second current path105including the diodes of switch arrangement111after capacitor114is fully discharged.

Protection system120is structured to isolate converter110from a fault135on DC network130. In certain embodiments, protection system120or a portion thereof is incorporated into a housing of converter110. In certain embodiments, protection system120or a portion thereof may be a retrofit kit structured to be coupled to converter110.

Protection system120includes a solid-state switching device121, a galvanic isolation switching device123, a measuring device125, and a controller127including a communication port129. Solid-state switching device121is coupled to DC bus113between switch arrangement111and DC link capacitor114. Solid-state switching device121is structured to selectively block the flow of current between switch arrangement111and DC link capacitor114. Solid-state switching device121may include any type of semiconductor switch. It shall be appreciated that the illustrated solid-state switching device is not coupled in parallel with an energy dispersion device such as a metal-oxide varistor (MOV). Since protection system120is arranged such that solid-state switching device121only opens while not conducting the fault current and only opens while conducting low current, such as a current having a magnitude less than the fault current, a current having a magnitude less than two times the nominal current magnitude, or no current at all, solid-state switching device121does not require an MOV to be coupled in parallel.

Galvanic isolation switching device123is coupled between DC link capacitor114and DC network130. Galvanic isolation switching device123is structured to selectively block the flow of current between power converter110and DC network130. Galvanic isolation switching device123may be a mechanical switching device, or any other type of switching device structured to galvanically isolate power converter110from DC network130. For example, galvanic isolation switching device123may include a mechanical disconnector switching device structured to open while not conducting current or not conducting a load current. During operation of system100, a line current ILflows through galvanic isolation switching device123to DC network130.

In certain embodiments, protection system120is structured to isolate power converter110from ground faults as well as short-circuit faults. For ground fault protection, protection system120additionally includes another solid-state switching device coupled between DC capacitor114and switch arrangement111on the negative pole of DC bus113, and another galvanic isolation switching device coupled between DC link capacitor114and DC distribution line133.

Measuring device125is structured to measure electrical characteristics of the DC power flowing through galvanic isolation switching device123and transmit the measurements to controller127. For example, measuring device125may measure a magnitude of current IL, to give but one example. In certain embodiments, measuring device125includes more than one measuring device. In certain embodiments, measuring device125is structured to measure capacitor voltage VC. In certain embodiments, measuring device125may include a current sensor, a current transformer, a voltage sensor, or a voltage transformer, to name but a few examples.

Controller127is structured to operate solid-state switching device121, operate galvanic isolation switching device123, and receive measurements from measuring device125. In the illustrated embodiment, controller127includes a communication port129structured to allow controller127to communicate with a controller of another protection system, a central control system, or another device structured to monitor or control DC distribution system100. In certain embodiments, controller127includes multiple communication ports or no communication ports.

During operation of DC distribution system100, controller127is structured to receive measurements from measuring device125and determine whether a fault condition has begun to occur. In response to determining a fault is occurring, controller127is structured to open solid-state switching device121, receive measurements from measuring device125, determine a fault location using measurements corresponding to a discharging capacitor current IC, open galvanic isolation switching device123based on received measurements, close solid-state switching device121in response to determining fault135has been removed, and close galvanic isolation switching device123in response to determining DC link capacitor114has been charged.

In certain embodiments, controller127uses measurements or determined fault directions received from other protection systems or a central control system to determine a fault location. In certain embodiments, controller127transmits information such as measurements or a determined fault direction based on measurements received from measuring device125to a central control system so that the central control system can aggregate information from other protection systems and determine a fault location using the aggregated information. In certain embodiments, the central control system then transmits open commands to the protective devices closest to the fault in order to remove the fault from a DC distribution system.

In certain embodiments, controller127is structured to determine a fault is occurring within power converter110or within AC network101and open solid-state switching device121in response to determining the fault is occurring. Since solid-state switching device121blocks the flow of current from capacitor114to switch arrangement111, switch arrangement111does not need to include desaturation protection to protect switch arrangement111from discharging capacitor current. It shall be appreciated that any or all of the foregoing features of the components of system100may also be present in the other components disclosed herein.

With reference toFIG. 2, there is a flowchart illustrating an exemplary protection system process200for responding to a DC fault in a DC distribution system using an exemplary protection system, such as protection system120inFIG. 1. Process200may be implemented in whole or in part in one or more of the protection system controllers disclosed herein. It shall be further appreciated that a number of variations and modifications to process200are contemplated including, for example, the omission of one or more aspects of process200, the addition of further conditionals and operations, and/or the reorganization or separation of operations and conditionals into separate processes.

Process200begins at operation201where a controller of the protection system determines a fault is occurring within a DC network. In certain embodiments, the controller determines the fault is occurring based on measurements received from the measuring device of the protection system. In other embodiments, the controller determines the fault is occurring based on information received from another device by way of a controller communication port.

Process200proceeds to operation203where the controller opens a solid-state switching device coupled between a switch arrangement of the power converter and a DC link capacitor of the power converter. Since the fault current is flowing in a current path formed by the DC link capacitor, a galvanic isolation switching device, and the DC network, the solid-state switching device opens while conducting a low current having a magnitude less than the magnitude of the current received by the fault, also known as the fault current. The low current may include a current magnitude less than two times the nominal current magnitude. In other embodiments, the solid-state switching device opens while conducting no current.

Process200proceeds to operation205where the measuring device measures current flowing through the galvanic isolation switching device that is being discharged by the DC link capacitor to the fault. In conventional fault location, voltages and/or currents are collected to calculate the fault location before the protection device is opened. Normally, in DC systems, DC circuit breakers are opened fast, such as within 2 ms to avoid high current damages to power electronics devices. The measurements available for fault location are therefore collected in a short period of time. By contrast, when solid-state switching device121is opened, the discharging currents and induced voltages continue and may be measured to determine the fault location. In this way, more measurements can be collected in order to increase the determined fault location accuracy.

Process200proceeds to operation207where the controller determines a location of the fault based on measurements collected during operation205. The controller may determine the location of the fault based solely on measurements from the measuring device taken while the DC link capacitor is being discharged. The controller may also determine the location of the fault based on measurements received from other protection systems of the DC distribution system. In certain embodiments, the controller determines a location of the fault by transmitting the measurements or fault directions based on the measurements to a central control system. The central control system may then transmit a fault location based on a plurality of measurements received from other measuring devices of the DC distribution system.

In certain embodiments, the controller determines the location of the fault by using the measurements to calculate the inductance of the distribution line between the fault and the measuring device. Using known line inductance characteristics and the calculated inductance, the controller can determine the distance of the fault from the measuring device. In certain embodiments, the controller determines the location of the fault using a fault direction determined using the measurements, as well as other determined fault directions received from other devices of DC distribution system100.

Process200proceeds to operation209where the controller opens the galvanic isolation switching device. The controller may open the galvanic isolation switching device in response to determining the fault current magnitude has decreased to zero. It shall be appreciated that the galvanic isolation device does not need to interrupt the discharging current as discharging current will dissipate without harming the distribution lines, as the lines have higher fault current tolerances than power converter110.

Process200proceeds to operation211where the controller waits until the located fault has ended or been isolated from the healthy portion of the DC distribution system and therefore removed from the DC distribution system. In certain embodiments, the controller waits on an instruction from a central control system indicating the fault is removed, or the controller waits until the controller determines the fault has been removed based on measurements from the measuring device of the protection system.

Process200proceeds to operation213where the controller closes the solid-state switching device in response to determining the fault has been removed. The controller closes the solid-state switching device in order to charge the DC link capacitor with the switching arrangement of the power converter.

Process200proceeds to operation215where the controller determines the DC link capacitor has been charged. For example, the controller may determine the DC link capacitor has been charged by determining a voltage across the DC link capacitor has exceeded a charging threshold value.

Process200proceeds to operation217where the controller closes the galvanic isolation switching device in response to determining the DC link capacitor has been charged, allowing the DC distribution system to return to normal operation.

With reference toFIG. 3, there is a plurality of graphs300illustrating electrical characteristics of DC distribution system100during a fault. It shall be appreciated that the voltages and currents illustrated in the plurality of graphs300are but one example of the values voltages and currents of an exemplary system. Graph310illustrates capacitor voltage VC. Graph320illustrates current IC. The plurality of graphs300include time instants t1-t3.

Before time instant t1, the diodes of power converter110conduct current Idiodesat a nominal current magnitude and capacitor114is fully charged. At time instant t1, fault135begins to occur across distribution lines131and133. Between time instant t1and time instant t3, Current ILcomposed of current IC, or mainly current IC, increases to a peak before decreasing to zero and capacitor voltage VCdecreases to zero as capacitor114is discharged. Current Idiodesis zero when solid-state switching device121is opened at time instant t2between time instants t1and t3. Solid-state switching device121may conduct low current between time instants t1and t2and is not conducting current between time instants t1and t3. Solid-state switching device121is opened between time instants t1and t3, at time instant t2, allowing switch arrangement111to be isolated from fault135without solid-state switching device121having to interrupt the fault current flowing through current path103. After solid-state switching device121opens but before time instant t3, measuring device125measures current being discharged from capacitor114, and may also measure a voltage of the discharging current, such as capacitor voltage VC.

At time instant t3, current IChas decreased to zero and galvanic isolation switching device123is opened while not conducting current. It shall be appreciated that protection system120is structured to isolate fault135without opening either switching device121or123while conducting a fault current. In certain embodiments, protection system120is structured to isolate fault135by opening both switching devices121and123while not conducting the fault current.

With reference toFIG. 4, there is illustrated an exemplary DC distribution system400including a DC network440interconnecting power converters410,420, and430. Power converter410includes switch arrangement411and DC link capacitor413. Power converter420includes switch arrangement421and DC link capacitor423. Power converter430includes switch arrangement431and DC link capacitor433. System400also includes protection systems415,425, and435coupled to power converters410,420, and430, respectively. DC network440includes line inductances441for each of the distribution lines of DC network440. DC network440also includes a plurality of protective switches443.

System400also includes a central control system450structured to communicate with protection systems415,425, and435, as well a plurality of protective switches443. In response to determining fault401is occurring in DC network440, each of the protection systems415,425, and435execute an exemplary protection process, such as process200inFIG. 2.

As part of the operation for determining fault location, protection systems415,425, and435may transmit information based on measurements including the measurements themselves or fault directions determined using the measurements to central control system450. Central control system450may determine the location of fault401using the received information. For example, central control system450may determine the location of the fault using the fault directions determined by the protection systems.

In response to determining the location of fault401, central control system450transmits open commands to the switching devices closest to fault401, in this case the plurality of protective switches443. The protection devices443closest to the location of fault401execute an exemplary protection operation, such as operation209in process200inFIG. 2. In certain embodiments, the plurality of protective switches443open once the fault current magnitude decreases to nominal current levels. Once the plurality of protective switches443are opened, fault401is removed and the remaining healthy portion of system400including power converters410,420, and430may resume normal operation by recharging DC link capacitors413,423, and433and reconnecting to DC network440.

With reference toFIG. 5A, there is a circuit diagram illustrating an exemplary switch arrangement500structured to convert power transmitted between a three phase network and a DC network. Switch arrangement500includes three legs including leg510. Each leg includes semiconductor devices such as semiconductor device511of leg510. Each semiconductor device is coupled in parallel with an RC snubber circuit such as RC snubber circuit513coupled in parallel with semiconductor device511. A decoupling capacitor is coupled in parallel with each leg, such as decoupling capacitor515coupled in parallel with leg510. The plurality of RC snubber circuits and decoupling capacitors are structured to protect switch arrangement500from overvoltage conditions caused by opening a solid-state switching device of an exemplary protection system coupled to switch arrangement500.

With reference toFIG. 5B, there is a circuit diagram illustrating an exemplary switch arrangement520structured to convert power transmitted between a three phase network and a DC network. Switch arrangement520includes three legs including leg530. Each leg includes semiconductor devices such as semiconductor device531of leg530. Each semiconductor device is coupled in parallel with an RC snubber circuit such as RC snubber circuit533coupled in parallel with semiconductor device531. Another RC snubber circuit including a series coupled capacitor525and resistor523coupled across DC bus521and a diode527coupled in parallel with resistor523. The RC snubber circuits are structured to protect switch arrangement500from overvoltage conditions caused by opening a solid-state switching device of an exemplary protection system coupled to switch arrangement500.

Further written description of a number of exemplary embodiments shall now be provided. One embodiment is a protection system comprising a solid-state switching device coupled between a switch arrangement of a power converter and a direct current (DC) link capacitor of the power converter, the switch arrangement being structured to convert power transmitted between a DC network and a second network; a galvanic isolation switching device coupled between the DC link capacitor and the DC network; and a controller structured to determine a fault is occurring within the DC network, open the solid-state switching device in response to determining the fault is occurring, receive a measurement corresponding to an electrical characteristic of a fault current flowing through the galvanic isolation switching device while the solid-state switching device is open, and determine a location of the fault based on the received measurement.

In certain forms of the foregoing protection system, the controller is structured to open the solid-state switching device while the fault current is conducted in a current path including the DC link capacitor and not including the solid state switching device. In certain forms, the controller is structured to open the galvanic isolation switching device in response to a magnitude of the fault current decreasing to zero. In certain forms, the controller is structured to close the solid-state switching device in response to determining the fault has been removed from the DC network, and close the galvanic isolation switching device in response to determining the switch arrangement has charged the DC link capacitor. In certain forms, the controller is structured to open the solid-state switching device in response to determining the fault is occurring while the solid-state switching device is not conducting the fault current. In certain forms, the fault current is a discharging current flowing from the DC link capacitor to the fault. In certain forms, the controller is structured to open the solid-state switching device in response to determining a second fault is occurring within the switch arrangement. In certain forms, determining the fault location includes transmitting information based on the received measurement to a central control system, and wherein the central control system determines the fault location based on an aggregation of information received from a plurality of protection systems including the protection system.

Another exemplary embodiment is a method for protecting a direct current (DC) distribution system comprising: operating a power converter including a switch arrangement and a DC link capacitor; operating a protection system including a solid-state switching device coupled between the switch arrangement and the DC link capacitor of the power converter, and a galvanic isolation switching device coupled between the DC link capacitor and a DC network, the switch arrangement being structured to convert power transmitted between the DC network and a second network; determining a fault is occurring; opening the solid-state switching device in response to determining the fault is occurring; receiving a measurement corresponding to an electrical characteristic of a fault current flowing through the galvanic isolation switching device while the solid-state switching device is open; and determining a location of the fault based on the received measurement.

In certain forms of the foregoing method, the method comprises opening the galvanic isolation switching device in response to a magnitude of the fault current decreasing to zero. In certain forms, the method comprises closing the solid-state switching device in response to determining the fault has been removed from the DC network; charging the DC link capacitor using the switch arrangement; and closing the galvanic isolation switching device in response to determining the DC link capacitor is charged. In certain forms, opening the solid-state switching device occurs while the solid-state switching device is not conducting the fault current. In certain forms, the fault current is a discharging current flowing from the DC link capacitor to the fault. In certain forms, the method comprises opening the solid-state switching device in response to determining a second fault is occurring within the switch arrangement. In certain forms, determining the fault location includes transmitting information based on the received measurement to a central control system and determining the fault location based on an aggregation of information received from a plurality of protection systems including the protection system. In certain forms, opening the solid-state switching device occurs while the fault current is conducted in a current path including the DC link capacitor and not including the solid-state switching device.

A further exemplary embodiment is a direct current (DC) distribution system comprising: a power converter including a switch arrangement and a DC link capacitor the switch arrangement being structured to convert power transmitted between a DC network and a second network; and a protection system including: a solid-state switching device coupled between the switch arrangement and the DC link capacitor, a galvanic isolation switching device coupled between the DC link capacitor and the DC network, and a controller structured to determine a fault is occurring within the DC network, open the solid-state switching device in response to determining the fault is occurring, receive a measurement corresponding to an electrical characteristic of a fault current flowing through the galvanic isolation switching device while the solid-state switching device is open, and determine a location of the fault based on the received measurement.

In certain forms of the foregoing DC distribution system, the system comprises a central control system structured to receive information based on the received measurement from the protection system, determine the fault location based on an aggregation of information received from a plurality of protection systems including the protection system, and transmit open commands to a plurality of switching device of the DC distribution system closest to the fault location. In certain forms, the controller is structured to open the solid-state switching device while the fault current includes a current path including the DC link capacitor and not including the solid-state switching device, and wherein the controller is structured to open the galvanic isolation switching device in response to a magnitude of the fault current decreasing to zero. In certain forms, the controller is structured to open the solid-state switching device in response to determining the fault is occurring while the solid-state switching device is not conducting the fault current, and wherein the fault current is a discharging current flowing from the DC link capacitor to the fault.

It is contemplated that the various aspects, features, processes, and operations from the various embodiments may be used in any of the other embodiments unless expressly stated to the contrary. Certain operations illustrated may be implemented by a computer including a processing device executing a computer program product on a non-transient, computer-readable storage medium, where the computer program product includes instructions causing the processing device to execute one or more of the operations, or to issue commands to other devices to execute one or more operations.

While the present disclosure has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain exemplary embodiments have been shown and described, and that all changes and modifications that come within the spirit of the present disclosure are desired to be protected. It should be understood that while the use of words such as “preferable,” “preferably,” “preferred” or “more preferred” utilized in the description above indicate that the feature so described may be more desirable, it nonetheless may not be necessary, and embodiments lacking the same may be contemplated as within the scope of the present disclosure, the scope being defined by the claims that follow. In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. The term “of” may connote an association with, or a connection to, another item, as well as a belonging to, or a connection with, the other item as informed by the context in which it is used. The terms “coupled to,” “coupled with” and the like include indirect connection and coupling, and further include but do not require a direct coupling or connection unless expressly indicated to the contrary. When the language “at least a portion” and/or “a portion” is used, the item can include a portion and/or the entire item unless specifically stated to the contrary.