Claims:The scope of the invention is defined by the following claims:

Claims:
1. A wind energy conversion system comprising the following steps:
a) A generator is operated at a range of 500 V–700 V and is connected to the transmission network through a transformer that acts as an integral part of the WECS to interface the wind turbine and the grid.
b) A SST being used in an AC/AC system to make the setup 25% smaller in volume than traditional low frequency transformer.
c) A rotor side control ensures the variable speed operation of DFIG by enabling the generator to work in super synchronous or sub synchronous modes
2. As per claim 1, the GSC present in traditional DFIG systems is removed to make the machine setup lighter.
3. According to claim 1, the RSC in the proposed converter is controlled using a decoupled d − q synchronous frame reference. The q axis of the reference frame is aligned with the machine stator voltage.
4. According to claim 1, the torque produced by the machine can be directly controlled by controlling the q-axis rotor current iqr. The reactive power produced at the stator terminal is controlled by controlling the d-axis rotor current idr. , Description:Field of Invention
The main objective of the proposed invention to interface the turbine with the grid while providing enhanced operation and performance. In this Innovation, SST controls the active power to/from the rotor side converter, thus, eliminating the grid side converter. The proposed Fuzzy Logic Controller system meets the recent grid code requirements of wind turbine operation under fault conditions. Additionally, it has the ability to supply reactive power to the grid when the wind generation is not up to its rated value.
Background of the Invention
In wind energy conversion systems, the fundamental frequency step up transformer acts as a key interface between the wind turbine and the grid. Recently, there have been efforts to replace this transformer by an advanced power electronics based solid state transformer (SST). This Innovation proposes a configuration that combines the doubly fed induction generator based wind turbine and Fuzzy Logic Based SST operation. (IEEE Trans. Power Syst., vol. 29, no. 3, pp. 1296–1305).
In this Innovation, a new configuration is proposed that combines the operation of DFIG (Doubly fed induction generator) based WECS (wind energy conversion systems) and SST. This configuration acts as an interface between the wind turbine and grid while eliminating the GSC of DFIG. Moreover, it is essential to have fault ride through (FRT) incorporated in DFIG system to meet the grid code requirements. In the proposed work, the developed configuration allows DFIG to ride through faults seamlessly, which is the aspect (FRT) that has not been addressed in the earlier work on SST interfaced WECS.( ECCE [2012], pp. 4077‒4084)
The general DFIG based WECS proposed system configuration In the proposed configuration, the fundamental frequency transformer is replaced by the SST. The proper control of SST converter that is close to the stator of DFIG, addressed as machine interfacing converter (MIC), can aid the machine in its operation. Thus, it is proposed to eliminate the GSC in the DFIG system configuration by incorporating its role in SST. Note that this new arrangement modifies the overall operation and control of standard GSC-RSC based DFIG system. In principle, the machine terminal voltages can be maintained constant in spite of any voltage variations in the grid using MIC. (IECON [2010], pp. 631-638)
The widely used DFIG based WECS system. The stator terminals of the machine are connected directly to the grid while the rotor terminals are connected via back to back converters. The RSC allows for variable speed operation of the machine by injecting or drawing active power from the rotor. The GSC maintains the DC link by transferring the active power from the rotor to the grid or vice versa. The step up transformer is the interface between the DFIG system and grid.(IEEE Transactions on Industry Applications, vol. 27, no. 1, pp. 63-73)
The performance of the fuzzy logic based voltage and speed controller for autonomous wind energy conversion system based on permanent magnet synchronous generator is good. With the pi controller the harmonic spectrum (THD) for load voltage and load current are 1.31% and 37.92% respectively. The harmonic spectrums (THD) for load voltage and load currents with fuzzy logic are 0.47% and 35.72% respectively. Simulation results and harmonic spectrum demonstrate that the fuzzy based controller works very well and shows very good dynamic and steady-state performance (Hoshihra et al. US 2014/0042747 A1)
Summary of the Invention
The general DFIG based WECS representation is the proposed system configuration, In the proposed configuration, the fundamental frequency transformer is replaced by the SST. The proper control of SST converter that is close to the stator of DFIG, addressed as machine interfacing converter (MIC), can aid the machine in its operation. Thus, it is proposed to eliminate the GSC in the DFIG system configuration by incorporating its role in SST.
The power transfer through the high frequency stage, from the low voltage DC bus to the high voltage DC bus, is controlled by introducing a phase shift between the two high frequency AC voltages with the objective of regulating the DC bus voltage. The grid interfacing converter (GIC) connects SST to the grid and maintains the DC link by exchanging active power with the grid.
Brief Description of Drawings
The invention will be described in detail with reference to the exemplary embodiments shown in the figures wherein:
Figure 1 Block diagram representation of fuzzy control system
Figure 2.Pictorial representation of regular DFIG configuration and proposed SST based DFIG configuration.
Figure 3.Graphical representation of total Harmonic distortion (THD) of the grid cuurnet
Detailed Description of the Invention
The penetration of renewable energy sources has been increasing steadily in the power system. In particular, wind energy installations have grown rapidly with global installed capacity increasing from 47.6 GW in 2004 to369.6 GW in 2014. Amongst the many technologies that exist for wind energy conversion systems (WECS), doubly fed induction generators (DFIG) have been prevalent due to variable speed operation, high power density and lower cost. DFIG based WECS consist of an induction generator whose stator is directly connected to the grid while its rotor is connected via back to back converters known as the rotor side converter (RSC) and grid side converter (GSC), respectively.
The generator is normally operated at a range of 500 V–700 V and is connected to the transmission network (11–33 kV) through a transformer that acts as an integral part of the WECS to interface the wind turbine and the grid. Recently, there has been much interest in developing an alternative to the traditional fundamental frequency transformer using solid-state devices. The solid-state transformer (SST) achieves voltage conversion through a series of power electronics devices while offering multiple advantages, such as, smaller size, improved power quality and fault tolerant features of a power distribution system based on SST as envisioned in. Proposed in 1980, advances in solid-state technology have made SST more viable today leading to increased research in its feasibility and physical realization. A promising 10 kVA prototype has been developed. Further, the use of high voltage silicon carbide (SiC) power devices for SST has been explored and presented in. The SST can act as an interface between the grid and generation sources. However, research showing detailed configurations for integrating existing technologies is limited. In the work is reported on using SST in a micro grid based on renewable sources
it has been reported that a DFIG based wind turbines the lightest amongst the current wind systems which also explains its wide commercial use. Moreover, in the proposed configuration, the GSC present in traditional DFIG systems is removed making the machine setup further lighter. On the other hand, SST being used in an AC/AC system is expected to be 25% smaller in volume than traditional low frequency transformer. Thus, the use of SST to interface a DFIG based wind system can be expected to provide further reduction in weight and volume when compared to other wind systems with the fundamental frequency transformer.
To ensure smooth operation of the proposed configuration, the control objectives and algorithms for the RSC, MIC and the GIC are discussed below. The rotor side control ensures the variable speed operation of DFIG by enabling the generator to work in super synchronous or sub synchronous modes. In super synchronous mode, the total power generated is partially evacuated through the RSC. Under sub synchronous modes, the RSC injects active power into the rotor. The RSC in the proposed converter is controlled using a decoupled d − q synchronous frame reference. The qaxis of the reference frame is aligned with the machine stator voltage. In this innovation, the torque produced by the machine can be directly controlled by controlling the q-axis rotor current iqr. Moreover, the reactive power produced at the stator terminal can also be controlled by controlling the d-axis rotor current idr. A suitable MPPT curve is used to track the optimal rotor speed and is compared with the measured rotor speed. The error is processed by a PI controller to produce the reference torque (T∗e ) for the machine, the rotor q-axis reference current (i∗qr) is calculated.
Fuzzy logic is a form of logic that is the extension of boolean logic, which incorporates partial values of truth. Instead of sentences being "completely true" or "completely false," they are assigned a value that represents their degree of truth. In fuzzy systems, values are indicated by a number (called a truth value) in the range from 0 to 1, where 0.0 represents absolute false and 1.0 represents absolute truth. Fuzzification is the generalization of any theory from discrete to continuous. Fuzzy logic is important to artificial intelligence because they allow computers to answer ‘to a certain degree’ as opposed to in one extreme or the other. In this sense, computers are allowed to think more 'human-like' since almost nothing in our perception is extreme, but is true only to a certain degree. Through fuzzy logic, machines can think in degrees, solve problems when there is no simple mathematical model. It solves problems for highly nonlinear processes and uses expert knowledge to make decisions. The fuzzy logic controller provides an algorithm, which converts the expert knowledge into an automatic control strategy. Fuzzy logic is capable of handling approximate information in a systematic way and therefore it is suited for controlling non linear systems and is used for modeling complex systems, where an inexact model exists or systems where ambiguity or vagueness is common. The fuzzy control systems are rule-based systems in which a set of fuzzy rules represent a control decision mechanism for adjusting the effects of certain system stimuli. With an effective rule base, the fuzzy control systems can replace a skilled human operator. The rule base reflects the human expert knowledge, expressed as linguistic variables, while the membership functions represent expert interpretation of those variables. In this thesis, a new system configuration that combines DFIG and SST operation has been proposed. This configuration replaces the regular fundamental frequency transformer with advanced power electronics based SST. The key features of the proposed configuration are outlined: Replacement of regular fundamental frequency transformer with SST leading to smaller footprint. Direct interface with SST to inject active power. Elimination of GSC in a standard DFIG system as the active power to/from RSC is regulated by MIC. Simplified DFIG control as machine supports only active power. The reactive power is supported by GIC during both normal and fault conditions. Seamless fault ride through operation during both symmetrical and unsymmetrical faults as per the latest grid codes.
4 claims & 3 Figures