Systems and methods for limiting voltage on an auxiliary bus

Systems and methods for limiting voltage on an auxiliary bus are described. The voltage-limited auxiliary bus may be comprised of a DC auxiliary bus comprised of a positive conductor and a negative conductor; a chopper, wherein the chopper is normally in a non-conducting state; a resistor in series with the chopper, wherein the chopper and the resistor are connected between the positive conductor and the negative conductor of the DC auxiliary bus; and a chopper control, wherein an overvoltage on the DC auxiliary bus causes the chopper control to cause the chopper to begin conducting and the conducting limits the voltage on the DC auxiliary bus and dissipates energy from the overvoltage.

FIELD OF THE INVENTION

The present subject matter relates generally to power generation and more particularly, to a system and methods for limiting overvoltage conditions on an auxiliary bus of a power generation facility.

BACKGROUND OF THE INVENTION

Power generation facilities such as wind power plants comprised of one or more wind-turbine generators, gas-turbine plants, steam-turbine plants and the like have auxiliary electrical loads such as pumps, motors, HVAC equipment, lighting, and other balance of plant (BOP) equipment that are supplied power from one or more auxiliary busses within the facility. These auxiliary busses can be alternating current (AC) electrical busses and direct current (DC) electrical busses where the AC is converted to DC and used to supply DC loads. In some instances, these busses experience overvoltage conditions that can be caused by, for example, sudden tripping of one or more of the generators at the power generation facility. Generally, these overvoltage conditions are short in duration (e.g., lasting 1 second or less), but if not mitigated, these overvoltages may damage the BOP equipment connected to the busses.

Accordingly, described herein are systems and methods of limiting voltage on auxiliary busses during an overvoltage event.

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 of limiting voltage on an auxiliary bus. The method may generally include measuring voltage on an auxiliary bus; determining whether the measured voltage meets or exceeds a threshold voltage; and limiting voltage on the auxiliary bus by causing a chopper to change to a conducting state in response to the measured voltage meeting or exceeding the threshold voltage, wherein said chopper is connected in series with a resistor between a positive and a negative conductor of the DC auxiliary bus and said DC auxiliary bus is provided electrical energy by a converter connected to the AC auxiliary bus, said AC auxiliary bus connected to an AC electrical source.

In another aspect, the present subject matter discloses a voltage-limited auxiliary bus. An embodiment of the voltage-limited auxiliary bus may be comprised of a DC auxiliary bus comprised of a positive conductor and a negative conductor; a chopper, wherein the chopper is normally in a non-conducting state; a resistor in series with the chopper, wherein the chopper and the resistor are connected between the positive conductor and the negative conductor of the DC auxiliary bus; and a control device for controlling a conducting state of the chopper, wherein the control device causes the chopper to begin conducting when an overvoltage is detected on the DC auxiliary bus and the conducting limits the voltage on the DC auxiliary bus and dissipates energy from the overvoltage.

In another aspect, the present subject matter discloses yet another embodiment of a voltage-limited auxiliary bus. The voltage-limited auxiliary bus may be comprised of an alternating current (AC) electrical source; a DC auxiliary bus comprised of a positive conductor and a negative conductor; a converter, wherein the converter is connected to the AC electrical source through an AC auxiliary bus and the converter is connected to the DC auxiliary bus and the converter converts AC electrical energy from the AC electrical source to DC electrical energy for the DC electrical bus; a chopper, wherein the chopper is normally in a non-conducting state; a resistor in series with the chopper, wherein the chopper and the resistor are connected between the positive conductor and the negative conductor of the DC auxiliary bus; a control device for controlling a conducting state of the chopper, wherein the control device causes the chopper to begin conducting when an overvoltage is detected on the DC auxiliary bus and the conducting limits the voltage on the DC auxiliary bus and at least a portion of the AC auxiliary bus and dissipates energy from the overvoltage; a capacitor, wherein the capacitor is connected between the positive conductor of the DC auxiliary bus and the negative conductor of the DC auxiliary bus and the capacitor filters high-frequency electrical components induced on the DC auxiliary bus by the chopper conducting and helps provide low voltage ride-through for the load when the chopper is not conducting; and one or more inductors connected between the AC auxiliary bus and the AC electrical source, wherein the one or more inductors limit current flow from the AC electrical source to the converter when the chopper is conducting.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present subject matter is directed to a system and methods for determining an overvoltage condition on an auxiliary bus of an electrical system and mitigating the overvoltage by use of a chopper circuit and resistor operated in parallel to loads on a DC auxiliary bus.

FIG. 1is a simplified single-line diagram of an exemplary electrical system100including a power generation facility102that may benefit from embodiments of the present invention. Though illustrated as a single-line diagram, it is to be appreciated that and alternating-current (AC) portions of the electrical system100can be comprised of single-phase and poly-phase components such as two-phase, three-phase, and the like components and combinations thereof. This embodiment of an electrical system100is comprised of an electrical generator104that can generate power, both real and reactive. The generator104can be any device or systems for generating electrical power including, for example, one or more wind-turbine generators, one or more steam-turbine generators, one or more gas-turbine generators, one or more hydro-generators, one or more combustion-engine driven generators, one or more solar or other renewable energy sources (with conversion to AC, if necessary), and the like. Though shown as a single element, it is to be appreciated that the generator104can include other apparatus such as one or more AC to DC converters, one or more DC links, one or more DC to AC inverters, and the like. Power generated by the generator104is routed via a main power circuit breaker106to a main power bus108. While the main power circuit breaker106is generally used to isolate the generator104in case of a fault on or at the generator104, it can also be used to open the circuit between the main power bus108and the generator104when the generator104is offline so that the remainder of the facility102can be supplied power from the electric grid110. Though shown as a single element, it is to be appreciated that the main power circuit breaker106can comprise any number of electrical devices including, for example, breakers, switchgear, motor control centers, synchronizing systems, and the like. The main power bus108transfers power generated by the generator104to a main transformer112and an auxiliary (“aux”) transformer114. Similarly, when the generator104is offline, the main power bus108can be used to distribute electrical power from the electric grid110to loads within the facility102, including loads served by the aux transformer114. In one non-limiting example, the main power bus108can be operated at a voltage rating of 690 volts (line to line) and have a rating of about 1500 to about 3000 kilowatts (KW).

Generally, the main transformer112steps up the voltage on the main power bus108to the level of the electric grid110. However, the main transformer106may also step down the voltage from the electrical grid110for distribution in the facility102via the main power bus108. Generally, the main transformer112is a poly-phase transformer. The electrical grid110can be comprised of transmission and distribution level components operated at various voltage levels as needed for distributing the generated electrical power to various loads. For example, the electrical grid110may be comprised of distribution-level components that are operated at distribution voltages that can range from approximately 2 kilo-volts (KV) up to 34.5 KV (generally, these are voltages between phases of the distribution system, i.e., line-to-line (LL) voltages). Sub-transmission level and transmission level voltages can range from 35 KV and up. The electrical grid110can be comprised of a single circuit or a plurality of circuits. A circuit may be mere feet in length or several miles. The electrical grid110can serve a single load or numerous loads. The electrical grid110can be interconnected with numerous other electrical generators116.

The aux transformer114steps down the voltage from the main power bus108to a level that can be used by auxiliary loads including AC auxiliary loads118and direct current (DC) auxiliary loads120(once the AC is converted to DC). For example, the auxiliary transformer118may step down the main power bus voltage to 480 volts (LL) where it is distributed to various AC auxiliary loads118via an AC auxiliary bus122. The AC auxiliary bus122can also provide power to an AC to DC converter124that converts the AC power to DC power for distribution on a DC auxiliary bus126. Typically, the wires, cables, buswork and the like that connect to the auxiliary transformer118on the load (e.g., low-voltage) side comprise the auxiliary bus and include the AC auxiliary bus122and the DC auxiliary bus126. Typical, though non-limiting, ratings for the auxiliary transformer114are about 100 to about 150 kilovolt amperes (KVA).

In one aspect, voltage on the DC auxiliary bus126and at least a portion of the AC auxiliary bus122can be limited in the event of an overvoltage. Overvoltages caused by various disturbances in an electrical system can spike as high as about 1.5 to about 1.6 times the normal voltage value of an electrical system. These events can damage electrical equipment if not controlled and mitigated.FIG. 1illustrates a chopper and resistor circuit128on the DC auxiliary bus126that can limit overvoltage peaks on the DC auxiliary bus126and at least a portion of the AC auxiliary bus122. In one aspect, one or more inductors130are placed between the source of AC electrical energy and the AC auxiliary bus122in order to limit current inrush to the converter124when the chopper and resistor circuit128is conducting.

FIG. 2is a three-line diagram of a portion of the electrical system100shown inFIG. 1. Though shown as a three-line diagram, it is to be appreciated that embodiments of the invention contemplate any number of electrical phases of aspects of the system100, including single-phase, three-phase, and the like. As shown inFIG. 2, a voltage-limited auxiliary AC bus122and a voltage-limited auxiliary DC bus126are provided by use of the chopper and resistor circuit128on the DC bus126. Generally, the voltage-limited auxiliary bus can be comprised of a DC auxiliary bus126comprised of a positive conductor1262and a negative conductor1264. The conductors can be cables, rigid bus work, and the like. Further comprising the voltage-limited bus is a chopper1282, wherein the chopper1282is normally in a non-conducting state. The chopper1282can be comprised one or more of a gate turn-off (GTO) thyristor, gate-commutated thyristor (GCT), insulated gate bipolar transistor (IGBT), MOSFET, combinations thereof, and the like. In one non-limiting example, when conducting the chopper1282may have a frequency of about 1000 Hz. A resistor1284is in series with the chopper1282, wherein the chopper1282and the resistor1284are connected between the positive conductor1262and the negative conductor1264of the DC auxiliary bus126. The resistor1284is sized so that energy is dissipated from the DC bus126and voltage limited on the bus126in the event of an overvoltage on the bus126. For example, the resistor1284may have a rating approximately the same as the rating for the auxiliary transformer114. Non-limiting examples of ratings for the resistor1284include 100 to 150 KW. The chopper1282is controlled by a control device for controlling the conducting state of the chopper1282. Generally, the control device for controlling the conducting state of the chopper1282comprises a chopper control circuit1286. Generally, an overvoltage condition on the DC auxiliary bus126detected by the chopper control1286to cause the chopper1282to begin conducting, wherein the conducting limits the voltage on the DC auxiliary bus126and dissipates energy from the overvoltage. In one aspect, the threshold at which the chopper control1286causes the chopper1282to begin conducting and the threshold at which the chopper control1286causes the chopper1282to stop conducting can be set. As a non-limiting example, the chopper control1286may be set to cause the chopper1282to begin conducting when a 10 percent or greater overvoltage is detected on the DC bus126and to stop conducting when the overvoltage (determined during conducting periods of the chopper1282) drops below ten percent overvoltage. In one aspect, the chopper control1286can use the duration of the overvoltage as a factor to determine when to cause the chopper1282to begin conducting. As a non-limiting example, a five percent overvoltage may cause the chopper control1286to cause the chopper1282to begin conducting after 10 seconds, but a 25 percent overvoltage may cause the chopper control1286to cause the chopper1282to begin conducting after 0.5 seconds. In another embodiment, the chopper control1286can cause the chopper1282to begin conducting immediately when the detected voltage reaches or exceeds the threshold and remain conducting until the detected voltage drops below the voltage threshold. As shown inFIG. 2, the DC auxiliary power load120is in parallel with the chopper1282and resistor1284combination.

Further comprising the portion of the system100ofFIG. 2is a source of AC power. Generally, this can be the aux transformer114as supplied from the main power bus108. In one aspect, the AC electrical source is a three-phase electrical source, though single-phase and other poly-phases sources are contemplated within the scope of embodiments of the present invention. In one aspect, the AC electrical source can be a 60 Hz. electrical source though other frequencies such as, for example, 50 Hz., are also contemplated within the scope of embodiments of the present invention. The source of AC power provides electrical energy to a converter124for converting the AC electrical power to DC electrical power. The converter124is connected to the AC electrical source through an AC auxiliary bus122and the converter124is connected to the DC auxiliary bus126and the converter124converts AC electrical energy from the AC electrical source to DC electrical energy for the DC electrical bus126. While the combination of the chopper1282and resistor1284holds the DC voltage of the DC bus126at or below some predetermined threshold in the event of a high AC source voltage, the nature of the converter124forces the AC line to line voltage of the AC side of the converter124to be equal to the DC link voltage when there is current flowing in the AC side of the converter124. Therefore, clamping the DC-side voltage on the DC bus126also clamps the AC-side line to line voltage on at least a portion of the voltage-limited AC bus122. Though shown as a diode bridge, it is to be appreciated that the converter124can be any means for converting AC electrical energy to DC electrical energy including, for example, diodes, thyristors, and the like.

In one aspect, the portion of the system100ofFIG. 2can further comprises one or more inductors130connected between the AC auxiliary bus122and the AC electrical source. The one or more inductors130limit current flow from the AC electrical source to the converter124when the chopper1282is conducting. In one aspect, the one or more inductors130have a reactance at the rated frequency of the AC electrical source of about 10 percent or greater to about 30 percent or less of a reactance rating of the load on the AC bus122. In one aspect, the chopper circuit128can further comprise a capacitor1288connected between the positive conductor1262of the DC auxiliary bus126and the negative conductor1264of the DC auxiliary bus. The capacitor1288can filter high-frequency electrical components induced on the DC auxiliary bus126by the chopper1282conducting and helps provide low voltage ride-through for the DC load120when the chopper1282is not conducting. In one aspect, the capacitor1288can have a rating such that DC ripple on the DC auxiliary bus126is at 5 percent or less when the chopper1282is conducting.

FIG. 3is an exemplary flowchart illustrating an embodiment of a method of limiting voltage on an auxiliary bus. At step302, an overvoltage condition is detected on a direct current (DC) auxiliary bus. In one aspect, the DC auxiliary bus is comprised of a positive conductor and a negative conductor, a converter connected to an AC electrical source through an AC auxiliary bus and the converter is connected to the DC auxiliary bus and the converter converts AC electrical energy from the AC electrical source to DC electrical energy for the DC electrical bus, a chopper that is normally in a non-conducting state, a resistor in series with the chopper, wherein the chopper and the resistor are connected between the positive conductor and the negative conductor of the DC auxiliary bus, a load in parallel with the chopper and the resistor, a capacitor in parallel with the load, and a chopper control. At step304, voltage is limited on the DC auxiliary bus and at least a portion of the AC auxiliary bus by causing the chopper to change to a conducting state in response to detecting the overvoltage condition, wherein the overvoltage on the DC auxiliary bus causes the chopper control to cause the chopper to begin conducting. In various aspects, the method can also include filtering the output of the chopper when the conductor is in the conducting state using the capacitor, wherein the capacitor filters high-frequency electrical components induced on the DC auxiliary bus by the chopper conducting and provides low voltage ride-through for the load when the chopper is not conducting; and limiting current flow from the AC electrical source to the converter when the chopper is conducting using one or more inductors connected between the AC auxiliary bus and the AC electrical source.

FIG. 4is an exemplary flowchart illustrating an alternate embodiment of a method of limiting voltage on an auxiliary bus. At step402, voltage (V) on an auxiliary bus is measured. This can be performed using any means now known or developed including, for example, potential transformers, current transformers and resistors, and the like. The auxiliary bus can be, for example, the AC bus122or the DC bus126. At step404, the measured voltage, V, is compared to a threshold voltage (Vt). Vtcan be any value as set for operation of the auxiliary bus. For example, Vtcan be set as 10 percent over nominal voltage, 15 percent over nominal voltage, and the like. If, at step404, V is greater than or equal to Vt, then the process goes to step406. At step406, voltage is limited on the auxiliary bus by a chopper control causing a chopper to change to a conducting state in response to detecting V greater than or equal to Vt. Generally, the chopper is located between a positive and negative conductor of a DC auxiliary bus and is in a non-conducting state, a resistor is in series with the chopper, and a load is in parallel with the chopper and the resistor. In one aspect, a capacitor is in parallel with the load, wherein the capacitor filters high-frequency electrical components induced on the DC auxiliary bus by the chopper conducting and provides low voltage ride-through for the load when the chopper is not conducting. Once the chopper begins conducting at step406, the process returns to step402where V is measured on the auxiliary bus. Returning to step404, if V is not greater than or equal to Vt, the process goes to step408where, if the chopper was conducting, it is turned off using the chopper control, and the process returns to step402where V is measured on the auxiliary bus.

It is to be appreciated that the processes ofFIGS. 3 and 4can be implemented by an intelligent device associated with chopper control1286including for example a processor, a microprocessor, a computer, a field-programmable gate-array (FPGA), a programmable logic controller (PLC), and the like that has been programmed or encoded with the logic to implement the described steps.

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.