Patent Application
20120049805
The invention relates generally to a power generation system and more particularly to a fault ride through switch and method of use in a power generation system.
Power generation systems typically include power generators that generate power from various sources and transfer the generated power to a power grid connected to the power generators. The power grid collects the power generated from multiple power generators and transmits the power to different locations. Generally, in order to connect any power generator to the power grid, the power generator must comply with grid code requirements of that particular location. One such grid code requirement is a fault ride through operation of the power generator for the entire duration of the grid event.
Several kinds of faults may occur during operation of the power generation system such as “voltage dip,” “voltage sag,” or a short circuit at a point of interconnection with the power grid. In an event of a fault, certain types of power generators accelerate rapidly, loose synchronism during operation, and, consequently, fail the grid code “ride through” requirement. In a non-limiting example, certain types of generators include small rotor generators. These kinds of power generators are often not allowed to connect to the power grid as they tend to be non compliant with the grid codes.
Various approaches are employed to overcome the issue of rapid acceleration in power generators during fault conditions. One such mechanical approach is to provide a braking mechanism to halt a prime mover in the power generator. This approach results in power generation losses, inefficiency, and high maintenance costs. Another approach provides an electrical brake concept to halt the power generators during fault conditions. However, most of the electrical brake concepts include expensive power electronics.
Hence, there is a need for an improved system and method for an inexpensive fault ride through power generation system to address one or more aforementioned issues.
In an embodiment of the invention, a power generation system is provided. The power generation system includes a prime mover configured to generate mechanical energy. The power generation system also includes a power generator configured for generating electrical power from the mechanical energy received from the prime mover. The power generation system further includes a fault ride-through switch electrically coupled in series between the power generator and a power grid. The fault ride-through switch includes a first branch configured to carry the electrical power during normal operating conditions and includes an LC resonance circuit at each phase of the power generation system. The fault ride-through switch also includes one or more multiphase transformers configured for providing voltage phases of different polarities to the LC resonance circuit at each phase of the power generation system. The fault ride-through switch further includes a second branch coupled in parallel with the first branch and including a resistive element and an inductive element electrically coupled in series at each phase of the power generation system wherein the resistive element is configured to absorb the electrical power during fault conditions.
In another embodiment of the invention, a fault ride-through switch electrically configured for being coupled in series between a power generator of a power generation system and a power grid is provided. The fault ride-through switch includes a first branch configured to carry electrical power from the power generator during normal operating conditions and includes an LC resonance circuit at each phase of the power generation system. The fault ride-through switch also includes one or more multiphase transformers configured for providing voltage phases of different polarities to the LC resonance circuit at each phase of the power generation system. The fault ride-through switch further includes a second branch coupled in parallel with the first branch and including a resistive element and an inductive element electrically coupled in series at each phase of the power generation system wherein the resistive element is configured to absorb the electrical power during fault conditions.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Embodiments of the present invention include a fault-ride through switch and a power generation system including a fault ride-through switch. The power generation system includes a prime mover configured to generate mechanical energy from various sources. The prime mover transfers the mechanical energy to a power generator configured to convert the mechanical energy to electrical power. The electrical power is transmitted to a power grid. A fault ride-through switch is coupled between the power generator and the power grid and includes a first branch and a second branch each having the same number of legs as the power generation system phases (herein after referenced as “system phases”). The first branch further includes a LC resonance circuit at each leg of the first branch. The first branch also includes one or more multiphase transformers at each leg electrically coupled in series to the LC resonance circuit.
The power from the power generator flows from the first branch during normal operating conditions. In an event of fault, the power from the power generator is diverted from the first branch and flows through a second branch. The second branch includes an inductive element as well as a resistive element that absorbs the power received from the power generator during the fault conditions. This absorption enables synchronized operation of the prime mover and power generator during fault conditions. As used herein, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The fault ride-through power generation system is described below in detail with reference to
As shown in
In the example shown in
For ease of illustration, flow of the input electrical power P1a through branch leg 32 will be described in detail. During normal operating conditions the input electrical power P1a includes voltage V1a and current Ia. The current Ia flows through the branch leg 32. LC resonance circuit 24 of branch leg 32 has a resonance frequency substantially equal to that of power generation system 10. The LC resonance circuit 24 does not behave as a resonant circuit during the normal operating conditions. The non-resonant behavior of LC resonance 24 circuit occurs when LC resonance circuit 24 is subject to voltages that are almost equal in amplitude but are of different polarities. Such voltages are generated by multiphase transformers 26 coupled in series with LC resonance circuit 24. Multiphase transformers 26 are configured for providing voltage phases of different polarities to the LC resonance circuit 24 at each phase of the power generation system 10. In the embodiment shown in
Multiphase transformer 26 at branch leg 32 includes a primary winding 38, a secondary winding 40, and a tertiary winding 42. In the embodiment of
In the embodiment of
The LC resonance circuit 24 at the branch leg 32 includes an inductor current Il and a capacitor current Ic. The voltages are applied to the LC resonance circuit 24 at such a phase that the inductor current Il and the capacitor current Ic combine to form the phase branch current Ia in phase with V1a and an output voltage V2a received at an output of the branch leg 32 in the fault ride through switch 18 during operating conditions.
During fault conditions, in contrast, at zero or low voltage at the output the voltages across the inductor 44 and the capacitor 46 of the LC resonance circuit 24 become almost in phase at about the same amplitude, and high impedance or an infinite impedance or resonance state results in LC resonance circuit 24. Under this in phase state, branch leg 32 current Ia is unable to flow from the first branch 20, and thus the flow of the input electrical power P1a is diverted to branch leg 48 of second branch 22. In one embodiment, each leg of branch 22 includes a respective resistive element 28 and a respective inductive element 30 electrically coupled in series to each other. The resistive element 28 absorbs the input electrical power P1a received from power generator 14 during the fault conditions. The inductive element 30 provides a delay in the voltage drop thereby enabling the resistive element 28 to absorb the input electrical power P1a steadily and uniformly over a period of time. The resistive element 28 is selected based on the magnitude of power and voltage to be handled by the power generation system.
The absorption of the input electrical power P1a received from the power generator 14 during fault conditions results in synchronized operations of power generator 14 and prime mover 12. Once the fault is rectified and input voltages V1a, V1b and V1c are normal, the state of LC resonance circuit 24 is changed to the non-resonant state, thus allowing the input electrical power P1a to flow from the first branch 20 as described above.
The flow of the input electrical power P1b and P1c in branch leg 34 and 36 occurs in a similar manner as described above with reference to the flow of the input electrical power P1a in branch leg 32. The operation of the fault ride through switch 18 may be better understood with reference to
The current I in the fault ride through switch 18 is illustrated below as current vector Ia of the current in phase “a” depicted by 80, a current Iar through the integrated resistive element 58 in the integrated second branch 56 under normal operating conditions is represented by a current vector depicted by 82 and the current Iac and Ial through the inductor 44 and the capacitor 46 provided in the first branch 20 at each phase is depicted by 84.
As illustrated by the phasor diagram, the voltages V2bc and V2cb are added in series to the voltage V1a in the LC resonance circuit 24 resulting in currents Ial and Iac at the inductor 44 and the capacitor 46 of the LC resonance circuit 24 respectively. The inductor current Ial and the capacitor current Iac are almost in phase and almost equal in amplitude when the voltage difference Vaa between the input voltage V1a and output voltage V2a is zero. For example, at one instant, V1a is substantially equal to V2b. In such cases the flow of power P1 results in a small phase shift between the input voltage of one phase V1a and the output voltage of another phase V2b. However, the phase shift between V1a and V2b is negligible, leading to in phase inductor current Ial and the capacitor current Iac with almost equal magnitudes. Furthermore, the current Iar through the integrated resistive element 58 in the integrated second branch 56 is close to zero as the difference Vaa between V1a and V2b is close to zero.
The current I in fault ride through switch 18 is illustrated below as current vector Ia of the current in phase “a”. current Iar through the integrated resistive element 58 (
As illustrated herein, the fault condition results in a zero impedance three-phase short circuit at the output 60. The particular fault condition results in zero output voltages at each phase (V2a=V2b=V2c=0). However, the input voltages V1a, V1b and V1c are supplied to the fault ride through switch by the power generator. The zero output voltage V2a=0 results in Vaa being equal to V1a. Furthermore, the inductor current Ial and the capacitor current Iac become 180 degrees phase shifted to each other having equal amplitude resulting in zero current at the output. Therefore, the current flows through the integrated second branch via the inductive element and the resistive element.
The various embodiments of the fault ride through switch and method for power generation described above includes a prime mover, a power generator, a fault ride through switch and a power grid. Thus, these tools and techniques enable the power generation system to operate in a faulty condition and to maintain synchronism between the prime mover and the power generator during faults. The fault ride through switch also enables the power generators to fulfill the grid code requirements of a “fault ride through” thereby allowing vendors to attach their power generators to the power grid. The fault ride through operation of the power generation system provides an increased efficiency at a lower cost.
Of course, it is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. For example, a three-phase transformer with respect to one embodiment can be adapted for use with two windings described with respect to another embodiment of the invention to provide a fault ride through operation. Similarly, the various features described, as well as other known equivalents for each feature, may be mixed and matched by one of ordinary skill in this art to construct additional systems and techniques in accordance with principles of this disclosure.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
