FIELD

The present disclosure relates generally to voltage source converters, and more particularly to systems and methods that provide coordinated neutral point voltage balancing for voltage source back-to-back three-level neutral point clamped (NPC) converters.

BACKGROUND

Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. Generally, during operation of a wind turbine, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that drives a low-speed shaft. The low-speed shaft drives a gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed, wherein the high-speed shaft rotatably drives a generator rotor.

In many wind turbine configurations, the generator is electrically coupled to a bi-directional voltage source power converter that includes a rotor-side converter (RSC) joined to a line-side converter (LSC) via a regulated DC link. Each of the RSC and the LSC typically includes a bank of pulse width modulated switching devices, for example, insulated gate bipolar transistors (IGBTs). As such, the LSC converts the DC power on the DC link into AC output power that is combined with the power from the generator stator to provide multi-phase power having a frequency maintained substantially at the frequency of the electrical grid bus (e.g. 50 HZ or 60 HZ).

The above system is generally referred to as a doubly-fed induction generator (DFIG) system, whose operating principles include that the rotor windings are connected to the grid via slip rings and the power converter controls rotor current and voltage. Control of rotor voltage and current enables the generator to remain synchronized with the grid frequency while the wind turbine speed varies (e.g., rotor frequency can differ from the grid frequency). In general, the LSC is used to control the power factor at the input mains along with DC bus voltage regulation, whereas the RSC is used to control active/reactive power for variable speed operation of the wind turbine.

For certain power converters, such as three-level neutral point clamped (NPC) converters, fluctuating neutral point voltage (i.e. the DC Link voltages of upper and lower capacitors) can be different in some cases, which is termed as DC unbalance. Also, in other cases, the neutral point voltage tends to oscillate with the triplen frequency of the converter's fundamental voltage, which is termed as AC unbalance. If such at least one of the DC unbalance or the AC unbalance is not balanced through any pulse width modulation (PWM) technique or external hardware circuitry, the voltages of the upper and lower capacitors can begin to drift away, thereby resulting in charging of one capacitor to the total DC link voltage and the other capacitor to zero voltage. Even if the DC unbalance problem is solved, there exists the triplen frequency neutral point voltage oscillation (AC unbalance), which is also an inherent problem that needs attention from either controls or by placing a large capacitor. This problem is aggravated for back-to-back three-level NPC converters applied to DFIG systems for wind applications where the frequency of the LSC is fixed to 50/60 Hz and the frequency of the RSC is variable at the slip frequency.

Conventional control schemes for providing voltage balance include common mode voltage injection with carrier-based PWM techniques. However, such solutions are restricted to single converter systems or back-to-back systems with a close match of frequency on both sides of the converters.

Thus, an improved system and method for controlling voltage source converters of renewable energy power systems, such as wind turbines, that address the aforementioned issues would be welcomed in the art. Accordingly, the present disclosure is directed to a system and method that provides coordinated neutral point balancing for back-to-back three-level NPC converters with adaptive controller gains for both the RSC and the LSC based on, for example, slip, modulation index, power factor, and/or active power.

BRIEF DESCRIPTION

Aspects and advantages 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 disclosure is directed to a method for controlling a three-level back-to-back voltage source power conversion assembly. The voltage source power conversion assembly has a first power converter coupled to a second power converter via a direct current (DC) link. The method comprises receiving, via a controller, an indication of at least one of a direct current (DC) unbalance or an alternating current (AC) unbalance occurring in voltage of the DC link. In response to receiving the indication, the method includes activating, via the controller, a balancing algorithm for the voltage source power conversion assembly. The balancing algorithm includes determining a deviation of a midpoint voltage of the DC link as a function of a total voltage of the DC link, calculating a voltage compensation needed for pulse-width modulation signals of the power conversion assembly based on the deviation in the midpoint voltage, and coordinating common mode voltage injection from each of the first and second power converters independently at a neutral point of the voltage source power conversion assembly based on the voltage compensation, thereby minimizing the at least one of the DC unbalance or the AC unbalance at any given operating condition.

In an embodiment, the balancing algorithm may also include determining a frequency of the AC unbalance.

In further embodiments, coordinating common mode voltage injection from each of the first and second power converters independently at the neutral point of the voltage source power conversion assembly based on the voltage compensation may include applying adaptive controller gains to each of the first and second power converters based on one or more operating parameters of the voltage source power conversion assembly.

In one embodiment, for example, the operating parameter(s) may include, for example, frequency of the AC unbalance, a modulation index, a power factor, a slip frequency, or an active power.

For example, in particular embodiments, coordinating common mode voltage injection from each of the first and second power converters independently at the neutral point of the voltage source power conversion assembly based on the voltage compensation may include providing first and second balancing proportional integral (PI) regulators tuned at a first bandwidth and a different, second bandwidth, respectively and independently determining the adaptive controller gains for each of the first and second power converters using the first and second PI regulators, respectively.

In another embodiment, coordinating common mode voltage injection from each of the first and second power converters independently at the neutral point of the voltage source power conversion assembly based on the voltage compensation may include selectively switching between the first power converter and the second power converter for providing the common mode voltage injection at the neutral point based on at least one of a set of rules or one or more look-up tables. For example, in such embodiments, selectively switching between the first power converter and the second power converter for providing the common mode voltage injection at the neutral point may include providing the common mode voltage injection at the neutral point via at least one of the first power converter only, the second power converter only, or both the first and second power converters based on the one or more operating parameters.

In further embodiments, the first and second sets of switching devices may be arranged in a neutral point clamped (NPC) topology. Alternatively, the first and second sets of switching devices may be arranged in an active neutral point clamped (ANPC) topology.

In several embodiments, the voltage source power conversion assembly may be part of a renewable energy power conversion system that includes a doubly-fed induction generator coupled to the voltage source power conversion assembly. Moreover, in an embodiment, the renewable energy power conversion system may be a wind turbine power system, a solar power system, or an energy storage system.

For example, where the renewable energy power conversion system is the wind turbine power system, the first power converter may be a rotor-side converter and the second power converter may be a line-side converter. Moreover, in such embodiments, the DC unbalance and/or the AC unbalance occurring in the voltage of the DC link may occur in at least one of a triplen frequency of an asynchronous generator slip frequency or a synchronous generator fundamental frequency

In another aspect, the present disclosure is directed to a three-level back-to-back voltage source power conversion assembly. The power conversion assembly includes a first power converter having a first set of switching devices and a second power converter coupled to the first power converter via a direct current (DC) link. The second power converter includes a second set of switching devices. The power conversion assembly also includes a converter controller communicatively coupled to the first and second power converters. The converter controller is configured to implement a balancing algorithm for the power conversion assembly, including but not limited to, in response to receiving an indication of at least one of a direct current (DC) unbalance or an alternating current (AC) unbalance occurring in upper and/or lower capacitor voltages of the DC link, determining a deviation of a midpoint voltage of the DC link as a function of a total voltage of the DC link, calculating a voltage compensation needed for pulse-width modulation signals of the power conversion assembly based on the deviation in the midpoint voltage, and coordinating common mode voltage injection from each of the first and second power converters independently at a neutral point of the voltage source power conversion assembly based on the voltage compensation, thereby minimizing the at least one of the DC unbalance or the AC unbalance at any given operating condition. It should be further understood that the power conversion assembly may further include any of the additional features described herein.

In yet another aspect, the present disclosure is directed to a wind turbine power system. The wind turbine power system includes a generator having a rotor and stator and a three-level back-to-back voltage source power conversion assembly electrically coupled to the generator. The power conversion assembly includes a rotor-side converter having a first set of switching devices and a line-side converter coupled to the rotor-side converter via a direct current (DC) link. The DC link further includes upper and lower capacitors. The line-side converter includes a second set of switching devices. The wind turbine power system also includes a converter controller communicatively coupled to the rotor-side and line-side converters. The converter controller is configured to implement a balancing algorithm for the power conversion assembly, including but not limited to, in response to receiving an indication of an unbalance (DC or AC) occurring in voltage of at least one of the upper or lower capacitors of the DC link, determining a deviation of a midpoint voltage of the DC link as a function of a total voltage of the DC link, calculating a voltage compensation needed for pulse-width modulation signals of the power conversion assembly based on the deviation in the midpoint voltage, and coordinating common mode voltage injection from each of the first and second power converters independently at a neutral point of the voltage source power conversion assembly based on the voltage compensation, thereby minimizing the at least one of the DC unbalance or the AC unbalance at any given operating condition. It should be further understood that the wind turbine power system may further include any of the additional features described herein.

These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

