Systems and methods for dynamic AC line voltage regulation with energy saving tracking

Systems and methods for dynamic AC line voltage regulation are provided. A simple and cost-effective method for achieving AC line voltage regulation in AC systems including split-phase systems, of which the voltage for each voltage line may be regulated over a specified range, is provided. Buck and boost regulation is achieved for lowering or increasing the line voltage, respectively, with reference to the incoming grid voltage. Systems for dynamic AC line voltage regulation may comprise an AC/AC converter which uses fractionally rated switches and magnetics that handle only a fraction of the load current, resulting in lower costs. The use of an AC snubber further provides safe and robust switching of the main switching devices by eliminating failure prone switching sequences that are dependent on accurate assessment of voltage and/or current polarity for AC or bi-directional switches.

TECHNICAL FIELD

The present invention(s) relate generally to regulating voltages in an electric power system. More particularly, the invention(s) relate to dynamic AC line voltage regulation with energy saving tracking.

DESCRIPTION OF THE RELATED ART

An electric power system is a network of interconnected electrical equipment that generate, transmit, and consume electric power. Electric power is delivered to consumers through a transmission network and a distribution network from generators to consumers. The transmission network and the distribution network are often known as the transmission grid and the distribution grid, respectively. As the electric power system is highly dynamic, dynamic voltage regulation ensures the electric power system's reliability and increases its capacity and efficiency. Voltage regulation is the ability of an electric power system to provide near constant voltage over a wide range of load conditions. A voltage regulator provides such voltage regulation. Voltage regulators may be installed at a substation or along distribution lines so that all loads receive steady voltage regardless of the amount of power drawn.

Conventionally, a tapped auto-transformer with relays to select a desired voltage level is used prevalently. Typical control range is +/−15% around the input value, possibly with additional taps. While inexpensive, this approach suffers from many drawbacks, such as slow response time, the possibility of momentary load interruption during transitions, and very coarse voltage steps. The use of a triode for Alternating Current (triac) or a thyristor pair to switch the windings can yield faster response, but still results in coarse control and significant power losses, maybe higher than 1-2% of the power handled.

BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION

According to various embodiments of the invention, systems and methods for dynamic AC line voltage regulation are provided. Various embodiments provide a simple and cost-effective method for achieving AC line voltage regulation in AC systems including split-phase systems, of which the voltage for each line may be regulated over a specified range. Buck and boost regulation is achieved for lowering or increasing the line voltage, respectively, with reference to the incoming grid voltage.

Various embodiments comprise an AC/AC converter which uses fractionally rated switches and magnetics that handle only a fraction of the load current, resulting in lower costs. The use of an AC snubber further provides safe and robust switching of the main switching devices by eliminating failure prone switching sequences that are dependent on accurate assessment of voltage and/or current polarity for AC or bi-directional switches. In addition, a dynamic AC line voltage regulator may be isolated from system or internal faults. Such fail-safe operation is achieved through a bypass or ‘fail-normal’ switch comprised of a combination of a thyristor pair (or triac) and a relay.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

Fast control of ac line voltage is becoming increasingly important, as it can present the basis for controlling the alternating current (AC) line voltage and in turn controlling power drawn by the load as well as the energy consumed over a certain time.

FIG. 1illustrates an exemplary implementation of a dynamic AC line voltage regulator100in accordance with an embodiment of the invention. The example dynamic AC line voltage regulator100comprises a converter101, capacitors102-104, and varistors105-106. The converter101comprises switches110-113, an active snubber114, and a transformer115. The switches110-113are AC switches and may be realized using semiconductor switches, such as MOSFETs and IGBTs, both of which have reverse conducting capability. In one embodiment, the switches110and111are coupled in the common emitter configuration and the switches112and113are coupled in the common collector configuration.

In the illustrated example, the transformer115has windings116-118: the primary winding116is coupled to the neutral line, and the secondary and tertiary windings117-118are coupled to the AC lines L1and L2. In various embodiments, the primary winding116may have substantially higher turns than the secondary and tertiary windings117-118. One end of the primary winding116is coupled to the neutral line as shown, while the other end is coupled to the output of the converter which synthesizes an AC voltage to be injected in series with the two lines, L1and L2, through the transformer115. Due to the high-low turn ratio of the transformer115, the converter101only carries a fraction of the line currents of the AC lines L1and L2. As such, the transformer115reduces the current rating for the switches and other components of the dynamic AC line voltage regulator100, thereby reducing cost.

Various embodiments are suitable for split-phase AC systems of various voltage and/or frequency levels such as 240 Volt AC systems at 60 Hz, or 240 Volt AC systems at 50 Hz. In the illustrated exemplary implementation, a distribution transformer130has a center-tapped secondary winding with +/−120 V, with the center tap being the neutral connection N. The transformer130is a distribution transformer that steps down a medium voltage to low voltage, which may be typically installed on the pole top or on a pad. In the illustrated example, the dynamic AC line voltage regulator100is coupled between the transformer130and the load131, which receives the +/−120 volts normally. The dynamic AC line voltage regulator100injects a regulated voltage in series with the two lines L1and L2. As such, voltages of the AC lines the L1and L2lines are regulated to a desired value.

In various embodiments, the output of the AC converter101is connected to an LC filter comprising an inductor120and a capacitor121. The LC filter suppresses the switching harmonics, which result from operating the switches at a significantly higher rate than the fundamental grid frequency (i.e., fswitch>>fgrid). Accordingly, the output voltage of a dynamic AC line voltage regulator may be synthesized at the same fundamental frequency. The primary winding116of the transformer115is coupled across the LC filter, such that only the fundamental frequency is let through to the secondary sides. The injected voltage may be in phase (additive) or in opposite phase (subtractive) with reference to the AC line voltages. The amount of voltage change possible is governed by the turns ratio (e.g., 20:1:1) of the transformer115and by the duty cycle D of the converter101. In further embodiments, the winding connections of the transformer115may be configured such that the injected voltage is only in phase (additive) or in opposite phase (subtractive) with reference to the AC line voltages. The output voltages of the direct AC line voltage regulator100can be controlled over a desired range via regulation of the duty cycle D of the converter101. Various embodiments may comprise a control module (e.g., the computing module600illustrated inFIG. 6) which determines the desired duty cycle and effects changes via various control loops.

In some embodiments, the converter101may comprise a thyristor pair (or triac)122. Upon sensing an over-current situation, typically resulting from a downstream fault, the thyristor pair122may turn on. The line current can be effectively measured using a current transformer or other current sensor in series with a winding (e.g., the primary winding116, the secondary winding117, or the tertiary winding118) of the transformer115. When the thyristor pair122is turned on, the capacitor121is discharged, and the thyristor pair122provides a low-impedance path for the reflected fault current to flow. For instance, when the transformer turns ratio is 20:1:1, a 3000-Ampere fault current on the secondary winding117or the tertiary winding118may be reduced to a 150-Ampere on the primary winding116. Upon detecting the thyristor pair122is turned on, the relay124, can also be turned on. In various embodiments, the relay124is a ‘normally closed’ or NC relay, as it provides a passive automatic bypass in case the converter101fails. This improves the overall reliability of the system. Further, if the line voltage is within the nominal range and no voltage regulation is required, the converter101can be maintained off by keeping the relay124in its normally closed position, thus reducing losses.

FIG. 2illustrates an exemplary implementation of a dynamic AC line voltage regulator200in accordance with an embodiment of the invention. Similar to the dynamic AC line voltage regulator100illustrated inFIG. 1, the dynamic AC line voltage regulator200comprises an AC/AC converter201, capacitors202-204, and varistors205-206. In addition, the dynamic AC line voltage regulator200comprises an Electromagnetic Interference (EMI) filter202comprising inductors207-208and the capacitor209. One of ordinary skill in the art would appreciate that various embodiments may be applied to a single phase (not split) application or a three phase application without loss of generality.

FIG. 3illustrates control block diagrams300of various embodiments of the invention. The illustrated control block diagrams300comprise a normal mode control loop301and a fault mode control loop302. When a dynamic AC line voltage regulator operates normally (that is, the current and the voltage of the dynamic AC line voltage regulator are within the nominal ranges), the main switching devices (e.g., switches110-113inFIG. 1) of a dynamic AC line voltage regulator are controlled according to the normal mode control loop301. In the normal mode control loop301, the line voltage is measured and provided as inputs to a PI regulator303.

The fault mode control loop302, which is in parallel to the normal mode control loop301, determines if the line current exceeds a threshold that indicates a fault has occurred, in which case, the relay and the thyristors of the dynamic AC line voltage regulator are turned on while the main switching devices of the dynamic AC line voltage regulator are disabled simultaneously. As such, the dynamic AC line voltage regulator operates under the fault mode. The fault mode may be cleared by a control block304that monitors the line voltage and line current and resets the controller to the normal mode control loop301.

Various embodiments comprise an active AC snubber circuit to ensure a safe operation of the AC/AC converter of a dynamic AC line voltage regulator. As errors in measurements invariably exist that may lead to incorrect switch patterns causing large voltage spikes when a current path suddenly becomes open or causes a large and potentially detrimental, shoot-through when a capacitor is shorted. Further, in addition to use of a bypass switch, under a major fault, the AC switches must be turned off under local gate drive control to ensure protection. The active snubber circuit provides a free-wheeling path for the converter and ensures that that the converter is tolerant of both errors in measurement and timings and of faults. Moreover, the active snubber circuit may eliminate the zero crossing problems in AC/AC converters with AC switches controlled by sequenced communication strategies. In one embodiment, the active snubber circuit is implemented into the gate drivers of the switches.

FIG. 4Aillustrates an exemplary circuit diagram of an AC snubber401. The AC snubber401comprises a switch402, a resistor403, a diode404, and a capacitor405. The snubber capacitor405may synthesize an AC source that tracks the incoming AC line voltage (e.g., Vin inFIGS. 1 and 2). In one embodiment, the snubber capacitor405tracks the voltage when the polarity of the voltage is positive. As the line voltage reaches a peak value and begins to decrease, the switch402is controlled so as to discharge the capacitor405in a manner such that the voltage across the capacitor405essentially tracks the AC line voltage, but is controlled to be marginally higher than the line voltage value.

The AC snubber401may further comprise a control module that regulates the switching on and off of the switch402such that the snubber capacitor405provides a half-wave rectifier voltage. Further, the voltage spike on the output due to snubber operation may be eliminated by regulating the snubber voltage401appropriately, thus improving the total harmonic distortion (THD) of the converter of a dynamic AC line voltage regulator, thereby ensuring that volt-second balance across the output filter is well maintained.

FIG. 4Bdepicts an exemplary waveform based on hysteresis-based control of the snubber capacitor voltage in accordance with an embodiment. The AC snubber operates entirely separate from the main controller of the converter of a dynamic AC line voltage regulator. The control is based on hysteresis control where the voltage across the snubber capacitor405is regulated within two bands through control of the switch402.

As illustrated, the waveform452is the snubber voltage trajectory. Waveforms450and451are the upper and lower boundaries for defining the snubber voltage, respectively. The lower boundary is selected so that it is slightly higher than the half-wave of the AC input voltage in order to reverse bias the diode404during normal operation. The upper boundary is selected in accordance with the RC time constant associated with the resistor403and snubber capacitor405to provide a certain effective switching frequency while discharging the snubber capacitor405. In one embodiment, the size of the snubber capacitor405is very small (e.g., in the order of 0.1 to 1 μF), the dissipated energy is typically a very small fraction of the total energy handled by the converter of a dynamic AC line voltage regulator.

In addition to the ability to restore a line voltage to a normal value (e.g., the nominal value), even as the incoming line voltage fluctuates, thus creating the desired environment for operation of connected loads and equipment; some embodiments may comprise an energy saving tracking module to track the variation of real and reactive power consumed when the voltage varies thereby providing energy saving tracking. For typical loads, a 1% drop in voltage may provide a 0.5-2% drop in power consumption. When these savings are integrated over longer periods, such as a month for utility billing purposes, sufficient energy savings may be obtained.

FIG. 5Aillustrates an exemplary conservative voltage regulation to reduce energy consumption in accordance with an embodiment of the invention. As illustrated, the curve501depicts the nominal voltage VNOM. The region502is the conservative voltage regulation (CVR), in which the line voltage is regulated between (VNOM−ΔVh) and VNOM. The curve503depicts an unregulated line voltage. The energy being saved,

VNOM2Load-(VNOM-Δ⁢⁢Vh)2Load,
is proportional to 2*VNOM*ΔVh.

Various embodiments may measure and track the amount of power and energy consumption saved. In one embodiment, a dynamic AC line voltage regulator may be periodically turned off and on, and power consumption is measured in both scenarios. The voltage is shifted between the two values at a rate corresponding to the fastest load.

In further embodiments, a sensitivity metric (Sp and Sr) that measures the percent change in real (Sp) and reactive (Sr) power to a small imperceptible change (e.g., 1%) in line voltage, may be determined. The value for Sp can be computed periodically, or every time a change in load current is measured, indicating that a new load has been turned on or off. In one embodiment, Sp can be calibrated over longer periods (e.g., one hour) or when a substantial load change is detected. A dynamic AC line voltage regulator may change the voltage gradually to the nominal value, and then back to an optimal value (e.g., a value at which energy consumption is minimized.) This change takes place over a period of time which is short enough when compared to the overall energy consumption, and is undetectable to the customer because the rate of change is slow and the amount of change is small enough. This capability is demonstrated inFIG. 5B.

FIG. 5Billustrates an exemplary optimal voltage regulation point tracking and convergence in accordance with an embodiment of the invention. As illustrated, in one embodiment, a dynamic AC line voltage regulator may sweep the line voltage very slowly (e.g., within 5 to 10 minutes time interval) over the allowed voltage band as dictated by such standards as American National Standards Institute (ANSI) (e.g. +/−5%). At each discrete voltage point, real and reactive power consumed by the load may be assessed. The dynamic AC line voltage regulator, upon completing the sweep of the voltage, determines a voltage point corresponding to the minimum power consumption by the load. Subsequently, the line voltage is slowly converged to the voltage point corresponding to the minimum load consumption. In various embodiments, with known sensitivity metric (Sp and Sr) and a calibrated load model, a dynamic AC line voltage regulator may estimate the actual real and reactive power reduction being realized.

As used herein, the term set may refer to any collection of elements, whether finite or infinite. The term subset may refer to any collection of elements, wherein the elements are taken from a parent set; a subset may be the entire parent set. The term proper subset refers to a subset containing fewer elements than the parent set. The term sequence may refer to an ordered set or subset. The terms less than, less than or equal to, greater than, and greater than or equal to, may be used herein to describe the relations between various objects or members of ordered sets or sequences; these terms will be understood to refer to any appropriate ordering relation applicable to the objects being ordered.

Computing module600might include, for example, one or more processors, controllers, control modules, or other processing devices, such as a processor604. Processor604might be implemented using a general-purpose or special-purpose processing engine such as, for example, a microprocessor, controller, or other control logic. In the illustrated example, processor604is connected to a bus602, although any communication medium can be used to facilitate interaction with other components of computing module600or to communicate externally.

Computing module600might also include one or more memory modules, simply referred to herein as main memory608. For example, preferably random access memory (RAM) or other dynamic memory, might be used for storing information and instructions to be executed by processor604. Main memory608might also be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor604. Computing module600might likewise include a read only memory (“ROM”) or other static storage device coupled to bus602for storing static information and instructions for processor604.

The computing module600might also include one or more various forms of information storage mechanism610, which might include, for example, a media drive612and a storage unit interface620. The media drive612might include a drive or other mechanism to support fixed or removable storage media614. For example, a hard disk drive, a floppy disk drive, a magnetic tape drive, an optical disk drive, a CD or DVD drive (R or RW), or other removable or fixed media drive might be provided. Accordingly, storage media814might include, for example, a hard disk, a floppy disk, magnetic tape, cartridge, optical disk, a CD or DVD, or other fixed or removable medium that is read by, written to or accessed by media drive612. As these examples illustrate, the storage media614can include a computer usable storage medium having stored therein computer software or data.

In alternative embodiments, information storage mechanism610might include other similar instrumentalities for allowing computer programs or other instructions or data to be loaded into computing module600. Such instrumentalities might include, for example, a fixed or removable storage unit622and an interface620. Examples of such storage units622and interfaces620can include a program cartridge and cartridge interface, a removable memory (for example, a flash memory or other removable memory module) and memory slot, a PCMCIA slot and card, and other fixed or removable storage units622and interfaces620that allow software and data to be transferred from the storage unit622to computing module600.

Computing module600might also include a communications interface624. Communications interface624might be used to allow software and data to be transferred between computing module600and external devices. Examples of communications interface624might include a modem or softmodem, a network interface (such as an Ethernet, network interface card, WiMedia, IEEE 802.XX or other interface), a communications port (such as for example, a USB port, IR port, RS232 port Bluetooth® interface, or other port), or other communications interface. Software and data transferred via communications interface624might typically be carried on signals, which can be electronic, electromagnetic (which includes optical) or other signals capable of being exchanged by a given communications interface624. These signals might be provided to communications interface624via a channel628. This channel628might carry signals and might be implemented using a wired or wireless communication medium. Some examples of a channel might include a phone line, a cellular link, an RF link, an optical link, a network interface, a local or wide area network, and other wired or wireless communications channels.