Patent Application
20230420979
Variable or adjustable Frequency Drives (VFD, AFD) play a vital role in controlling motors in industrial environments, e.g., in the oil and gas industry. The benefits include, but are not limited to a lower inrush current, controllability, and energy savings. However, VFDs tend to be sensitive to power quality issues such as momentary voltage sagging and blackouts. This may be a major limitation in industrial environments in which power quality issues are not uncommon. Operation disruptions and extensive losses may be the result.
This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.
In general, in one aspect, embodiments relate to a system for increasing immunity against power quality problems, the system including a battery bank configured to supply an uninterruptible power supply (UPS) with direct current (DC) power; a variable frequency drive (VFD) comprising: a DC bus with a DC bus voltage; a VFD inverter configured to invert the DC bus voltage to an AC VFD output voltage of a desired frequency; a rectifier configured to: convert an alternating current (AC) supply voltage to the DC bus voltage, and supply the DC bus with the DC bus voltage; an electrical connection between the battery bank and the DC bus, the electrical connection configured to provide a supplemental DC current from the battery bank when a DC current provided by the DC bus fails to match a DC current demand by the VFD inverter.
In general, in one aspect, embodiments relate to a method for increasing immunity of a variable frequency drive (VFD) against power quality problems, the VFD comprising: a direct current (DC) bus with a DC bus voltage; a VFD inverter configured to invert the DC voltage to an AC VFD output voltage of a desired frequency; a rectifier configured to: convert an alternating current (AC) supply voltage to the DC bus voltage, and supply the DC bus with the DC bus voltage, wherein the DC bus is electrically connected to a battery bank, the battery bank configured to supply an uninterruptible power supply with DC power, the method comprising: in presence of a deterioration of the AC supply voltage, providing a supplemental DC current from the battery bank to the VFD inverter.
In light of the structure and functions described above, embodiments of the invention may include respective means adapted to carry out various steps and functions defined above in accordance with one or more aspects and any one of the embodiments of one or more aspect described herein.
Other aspects and advantages of the claimed subject matter will be apparent from the following description and the appended claims.
Specific embodiments of the disclosed technology will now be described in detail with reference to the accompanying figures. Like elements in the various figures are denoted by like reference numerals for consistency.
In the following detailed description of embodiments of the disclosure, numerous specific details are set forth in order to provide a more thorough understanding of the disclosure. However, it will be apparent to one of ordinary skill in the art that the disclosure may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description.
Throughout the application, ordinal numbers (e.g., first, second, third, etc.) may be used as an adjective for an element (i.e., any noun in the application). The use of ordinal numbers is not to imply or create any particular ordering of the elements nor to limit any element to being only a single element unless expressly disclosed, such as using the terms “before”, “after”, “single”, and other such terminology. Rather, the use of ordinal numbers is to distinguish between the elements. By way of an example, a first element is distinct from a second element, and the first element may encompass more than one element and succeed (or precede) the second element in an ordering of elements.
In general, embodiments disclosed herein relate to systems and methods for increasing immunity of variable frequency drives (VFDs) against power quality issues. Electrical substations or other types of electric power distribution facilities in industrial or commercial environments may be equipped with switchgear such as disconnect switches, fuses, circuit breakers and other equipment for controlling, protecting, and isolating electrical equipment. In addition, equipment such as uninterruptible power supplies (UPS), battery chargers supplying DC loads, VFDs controlling electric motor speed, etc. may be present.
In one or more embodiments the VFD is configured to rely on an existing battery bank (e.g., batteries of the UPS) to compensate for voltage dips or momentary blackouts to keep the VFD and the motor associated with the VFD operational. The battery bank may, thus, provide backup power to the VFD, at times when the incoming power supply is temporarily compromised. Configurations in accordance with embodiments of the disclosure have various benefits. The availability of power from the battery bank, whenever a voltage dip or other power quality problem is taking place, helps extend the VFD voltage ride-through capability and thereby reduces process interruptions. Further, while many industrial and commercial facilities are equipped with battery banks (e.g., for UPS equipment and battery chargers), these battery banks tend to be expensive and require costly maintenance with finite lifecycles. Frequently, battery banks reach end of service before exploiting all available energy due to temperature or aging resulting in low utilization factor. By interfacing the VFD with a battery bank, the utilization factor increases, thereby making better use of the existing battery bank at a low cost.
Turning to
The VFD system (110) may drive an electric motor in a controllable manner. The VFD drive system (110) may drive an AC motor. By varying the frequency of the VFD output voltage, the VFD system (110) may control the speed of the AC motor. The operation of the VFD system (110) is discussed in detail below. The VFD system (110) may receive power from a power supply, e.g., an AC power supply (170). The AC power supply (170) may be a commercial or industrial power supply, e.g., a 400V 3-phase AC power supply. The AC power supply (170) may have a different configuration, without departing from the disclosure. For example, the power supply may be single-phase and/or may have any voltage. In one or more embodiments, the VFD system (110) further receives a DC power supply (180), from the battery bank (120). The DC power supply may have any voltage, based on the voltage of the battery bank (120). A detailed discussion of the use of the AC power supply (170) and the DC power supply (180) by the VFD system (110) is provided below.
The battery bank (120) may include secondary batteries such as lead-acid, nickel-cadmium, nickel-metal hydride, and/or lithium-ion batteries. Any number of secondary batteries may be combined to provide the desired storage capacity and the desired battery bank voltage. Depending on the size of the battery bank (120), it may be kept in a separate room or a more compact enclosure. The battery bank (120) is designed to provide backup power for various loads that are powered through the UPS system (130) or to DC loads. These loads may include, for example, information technology and/or telecommunication equipment that may, under regular operating conditions, be powered by an AC power supply, and that may be powered via the UPS system (130) upon failure of the AC power supply. The UPS system (130) may include an inverter to convert DC (from the battery bank) to AC, and may further include a rectifier to convert AC (line voltage) to DC. A charger may be included to charge the battery bank (120). Used conventionally, the battery bank (120) may have been designed to primarily or even exclusively provide backup power via the UPS system (130). In such a configuration, the utilization factor of the battery bank (120) may be low. Additionally connected DC loads (140) may slightly increase the utilization factor. By interfacing the VFD with a battery bank, the utilization factor increases, thereby making better use of the existing battery bank. A more detailed discussion of the interaction of various components shown in
Turning to
The VFD (210) includes a rectifier (212) that receives an AC supply voltage (202), e.g., a 3-phase AC voltage as shown, and converts the AC supply voltage into a DC voltage. The DC bus voltage obtained in this manner may be stabilized by a DC bus capacitor (214). A VFD inverter (216) may generate an AC VFD output voltage from the DC bus voltage. The AC VFD output voltage may have three phases and may drive an AC motor (222). By varying the frequency of the AC VFD output voltage, the speed of the AC motor (222) may be modulated. The VFD inverter (216) may include various electronic circuits, including power circuits for generating the AC VFD output voltage and control circuits for controlling the power circuits to generate the AC VFD output voltage at a desired frequency.
The UPS (240) includes a rectifier (242) that receives an AC supply voltage. The AC supply voltage may be the AC supply voltage also supplied to the VFD (210) or a different supply voltage. The rectifier (242) converts the AC supply voltage to a DC voltage suitable for charging the battery bank (244). The DC battery bank voltage may be supplied to a load (246). The load may be a DC load or an inverter configured to generate an AC voltage, e.g., for information technology, instrumentation and/or communication equipment connected to the UPS.
Continuing with the discussion of the VFD (210), during regular operation, a DC current provided by the DC bus (I1) is sufficient to meet a DC current demand (I3) by the VFD inverter (216). In other words, I1=I3. This may even be the case0 when a very brief voltage dip occurs in the AC supply (202). In this case, the DC bus capacitor (214) may be sufficient to stabilize the DC but voltage. Larger DC bus capacitors may be able to buffer longer and/or more significant voltage drops than smaller DC bus capacitors.
However, when the quality of the AC supply (202) is more significantly compromised (e.g., due to a prolonged and/or more significant voltage drop), this may not be the case. Specifically,
To prevent a reverse flow of current from the DC bus to the battery bank, the electrical connection (218) may include a blocking diode (220).
To further illustrate the configuration of the examples (200, 250) of
Assuming a relatively common industrial system with 240 lead acid cells at 2.25 VDC/cell in series, the DC battery bank voltage may be 240*2.25=540 VDC or 95% of the nominal DC (95% of the 564 VDC). In this case, the battery bank may directly feed into the DC bus of the VFD, without requiring a DC/DC converter such as a buck/boost converter. In the described configuration, the capacity of the battery bank (e.g., measured in watts hours) may determine the time interval of AC supply disruption that may be covered.
While
Turning to
In Block 402, an AC supply voltage is determined. The AC supply voltage being determined is the AC supply voltage present at the input of the VFD.
In Block 404, if the AC supply voltage is less than 80% of the nominal AC supply voltage, in Block 406 a supplemental DC current is provided from the battery bank to the VFD inverter of the VFD, in addition to the current being provided by the rectified AC supply. A boost conversion may be performed to increase the DC battery bank voltage. A buck conversion may be performed to lower the DC battery bank voltage. If the AC supply voltage is greater than 80% of the nominal AC supply voltage, no supplemental DC current is provided because the VFD may drive the AC motor without interruption in presence of an AC supply voltage that is 80% of the nominal AC supply voltage.
In Block 408, the DC supply is provided to the VFD inverter.
In Block 410, the AC motor is driven by the VFD inverter. The frequency of the VFD inverter output may be modulated to control the speed of the AC motor.
Embodiments disclosed herein provide long time ride-through covering power and control aspects in the same facility where AFD is needed, thereby increasing the overall system efficiency. Further, embodiments disclosed herein reduce mean time between failure of AFD by enhancing the ride-through capabilities, and results in reduction of production losses in the oil and gas industry.
Embodiments of the disclosure may be used in many different environments and for many different applications. Broadly speaking, embodiments of the disclosure may be used in any environment, e.g., in the oil and gas or other industries where equipment such as VFDs, UPS systems, charger systems, etc. are used. The VFD may be used for an electrical submersible pump (ESP), or any other system or device that includes an AC motor.
Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.
