REPLACING THE GRID INTERFACE TRANSFORMER IN WIND ENERGY CONVERSION SYSTEM WITH SOLID- STATE TRANSFORMER
Abstract
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 paper proposes a configuration that combines the doubly fed induction generator (DFIG) based wind turbine and SST operation. The main objective of the proposed configuration is to interface the turbine with the grid while providing enhanced operation and performance. In this work, SST controls the active power to/from the rotor side converter (RSC), thus, eliminating the grid side converter (GSC). The proposed 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. A detailed simulation study is conducted to validate the performance of the proposed configuration.
EXISTING SYSTEM:
A promising 10 kVA prototype has been developed and presented in [6]. Further, the use of high voltage silicon carbide (SiC) power devices for SST has been explored and presented , SST can act as an interface between the grid and generation sources. However, research showing detailed configurations for integrating existing technologies is limited. In, work is reported on using SST in a microgrid based on renewable sources. In, SST is used to interface a wind park based on squirrel cage induction generator (SCIG) with the grid. However, a detailed analysis on fault ride through requirement and reactive power support has not been conducted . In this paper, a new configuration is proposed that combines the operation of DFIG based WECS 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)
PROPOSED SYSTEM:
Motivation for DFIG , it has been reported that a DFIG based wind turbine is the lightest ast 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. B. Background The widely used DFIG based WECS configuratio. 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 T1, is the interface between the DFIG system and grid Three stage SST configuration , where it connects the grid to a distribution load. Conv-1 is a fully controlled three-phase converter connected to the high voltage grid (11-33 kV). It draws real power from the grid and maintains the high voltage DC bus ( . This high voltage DC is converted to high frequency AC voltage by a half bridge converter (HB-1) which is then stepped down using a smaller sized high frequency transformer. This transformer provides the galvanic isolation between the grid and load. A second half bridge converter (HB-2) converts the low voltage AC to low voltage DC voltage ( This DC bus supports conv-2 which maintains the three-phase/single phase supply voltage to the load by producing a controlled three phase voltage. The configuration thus performs the function of a regular transformer allowing for bi-directional power flow using a series of power electronics devices [5-10].
CONCLUSION
In this paper, 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 below: – 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.
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