Patent Application
20140217832
The present disclosure relates to disconnect switches in DC power systems.
This section provides background information related to the present disclosure which is not necessarily prior art.
Disconnect switches are commonly used in electrical circuits for interrupting and/or preventing the flow of current between an electric power source and an electric load. For example, and as shown in
However, when there is a hard fault in the inverter, such as a short circuit across its input terminals (internally or externally), the PV power source is also short circuited. If the disconnect switch S1 is opened when a short circuit current from the PV source is flowing, a large voltage may develop across the switch. This large voltage across the switch, coupled with any wiring inductance L1, may result in extended arcing across the switch contacts.
One particular example of this is illustrated in
The extended arcing from time t0 to time t1 can produce a large amount of heat in the switch, which reduces its life. It can also permanently weld the contacts in the switch and thus prevent the switch from operating as intended.
It is also known to use a disconnect switch S1 having several pairs of switch contacts connected in series (e.g., a triple pole, single throw switch), as shown in
This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
According to one aspect of the present disclosure, a system includes a soft DC power source having an output terminal, a DC load, a disconnect switch having a power side and a load side coupled between the output terminal of the soft DC power source and the DC load, and a capacitor coupled between the power side of the disconnect switch and a reference potential. The capacitor inhibits a rise in voltage across the disconnect switch as the disconnect switch is opening to inhibit arcing in the switch.
According to another aspect of this disclosure, a DC disconnect switch assembly includes a pair of input terminals for coupling to a DC power source, a pair of output terminals for coupling to a DC load, a disconnect switch coupled between one of the input terminals and one of the output terminals, and a capacitor coupled between the pair of input terminals.
Further aspects and areas of applicability will become apparent from the description provided herein. It should be understood that various aspects of this disclosure may be implemented individually or in combination with one or more other aspects. It should also be understood that the description and specific examples herein are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.
Example embodiments will now be described more fully with reference to the accompanying drawings.
Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.
The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.
Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.
Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.
A system according to one example embodiment of the present disclosure is illustrated in
In the particular example shown in
The DC power source 102 is preferably a “soft DC power source,” meaning the DC power source has a defined open circuit voltage and a defined short circuit current, with its output voltage decreasing (linearly or otherwise) with increasing output current, and vice versa. One example of a soft DC power source is a photovoltaic power source (e.g., formed of one or more solar panels or cells). Therefore, when there is a short circuit condition in the system 100 (e.g., due to a fault in the DC load 108, because the soft DC power source is connected in reverse polarity, etc.), the voltage on the power side (and the load side) of the disconnect switch S1 drops to about zero volts.
The DC power source may be configured to supply high dc voltages, such as up to 600 VDC, up to 1200 VDC, etc.
The inductance L1 may represent various sources of inductance in the system 100, including the parasitic inductance of one or more electrical conductors (e.g., wires) coupled between the DC power source 102 and the DC load 108 and/or any inductance in the DC load 108 coupled between its input terminals 110, 112.
As shown in
Further, the system 100 may or may not include circuit breakers (e.g., current fuses) in addition to the disconnect switch S1.
In the system 100 of
The DC load 108 may be, for example, a switch mode power supply (SMPS). Further, if the DC power source is a photovoltaic power source, the SMPS load may be configured to implement a maximum power point tracking (MPPT) function. The SMPS may be, e.g., a DC/DC converter or a DC/AC converter (also referred to as an inverter). If the SMPS is an inverter, the inverter may be configured to implement an MPPT function and/or may be a grid-tie inverter for supplying AC power to a utility grid. Alternatively, other types of DC loads may be employed without departing from the scope of the present disclosure.
While not shown in
When the disconnect switch S1 is opened during a short circuit condition, a short circuit current will flow through the capacitor C1, causing the capacitor C1 to absorb any energy discharged by the inductance L1. During this time, the voltage across the disconnect switch S1 will slowly rise, as the capacitor C1 charges. The value of C1 can be selected to prevent the voltage across the disconnect switch S1 from exceeding a defined voltage before the disconnect switch S1 is fully opened, so as to inhibit extended arcing in the disconnect switch S1. The defined voltage may be, for example, 100 VDC, or any other suitable voltage.
The value of the capacitor C1 may be calculated based on the value of the inductance L1 and the maximum short circuit current. At maximum short circuit current (Isc), the energy stored in the inductance L1 is about 0.5*L1*Isc2. The parasitic inductance of wiring is typically about 10 nH/inch. Therefore, if the inductance L1 is primarily attributable to the parasitic inductance of the wiring, and if the wiring is about one hundred feet in length, the value of the inductance L1 may be about 12 microH. In that event, if the maximum short circuit current Isc is limited to about 12 ADC, the value of the capacitor C1 may be selected to be about 0.47 uF.
In the particular example shown in
As shown in
The assembly 300 may also include a housing 310 for enclosing the disconnect switch S1, the capacitor C1 and/or the resistor R1.
The assembly 300 may also include additional components not shown in
For a typical residential 5 kW photovoltaic array having an open circuit voltage of 600V, the disconnect switch may have a maximum current rating in the range of 15 ADC to 40 ADC and a maximum voltage rating of 750 VDC. The capacitor may have a capacitance of, e.g., about 0.47 uF to about 3.3 uF. Further, the resistor R1 (if employed) may have a resistance of, for example, about 5 ohms to about 50 ohms. It should be understood, however, that other ratings and/or component values may be employed in any given implementation without departing from the scope of this disclosure.
The teachings of this disclosure may be applied to a variety of applications including, without limitation, residential and/or grid-tied PV power applications.
The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.
