Wind energy penetration has been increasing annually in electric power grids because of its sustainability though it is capital intensive. Accordingly, the concern for impacts of significant wind power integration on stabilities of electric power grids has been on the rise since it replaces a sizeable fraction of conventional power plants, which are stability-aid units. This concern has been dealt by extending national grid codes including requirements for wind power integration.
Grid codes solely stated a set of integration rules and operation specifications for conventional power plants. However, grid codes have been extended to wind energy integration and modified as the worldwide share of wind energy has been increasing. Generally, the extended grid codes include the operation specifications that secure power grid resilience mainly in terms of grid voltage and frequency in the steady-state and transient-state.
To stabilize the grid voltage under disturbances, the grid codes require a wind turbine generator (WTG) or wind power plant (WPP) to provide a voltage-dependent reactive power (Q) control that offers an ability to stabilize the voltage at the connection point of a WTG or WPP. Particularly, the most challenging requirement is the low voltage ride-through (LVRT) capability of a WTG or WPP. This requirement states that a WTG or WPP should rapidly provide Q above a specified Q or reactive current (IQ) according to the voltage profile under a low voltage condition to support the voltage stability. Thus, for a WPP consisting of multiple WTGs to comply with these requirements, a hierarchical voltage support scheme that can secure the ability of voltage-dependent Q or IQ control at the point of interconnection (POI) is required.
This thesis proposes a hierarchical voltage support scheme that can enhance voltage support capability of a doubly-fed induction generator (DFIG) based WPP by utilizing its Q/IQ to rapidly respond to voltage changes at the POI for low voltage conditions. First, to support the POI voltage and terminal voltage of a DFIG, the voltage-dependent Q/IQ injecting functions are designed in both the WPP and DFIG controllers.
The WPP controller in the proposed scheme aims to rapidly support the POI voltage for voltage dips by dispatching partial voltage set points to the DFIGs in proportion to their voltage margins at RSC, and thereby it enables the DFIGs with large voltage margins to participate in the voltage support with larger voltage set points. Further, to prevent an overvoltage at the POI caused by the saturation of the WPP controller, the WPP controller employs an overvoltage detector that initializes the integrator of the WPP controller.
The DFIG controller in the proposed scheme aims to rapidly support the POI voltage and the terminal voltage of the DFIG by coordinating the RSC and GSC controllers using an IQ management system (IMS). To achieve this, the IMS acquires terminal voltage at the DFIG and IQ capabilities of the RSC and GSC, and it determines voltage set points and flexible IQ-V characteristics, which have a IQ-V gain and IQ limits, of the RSC and GSC controllers; finally, the IQ references for each controller are determined by the product of a voltage set point and IQ-V gain. In the IMS, the voltage set points are determined in proportion to available IQ capabilities of the RSC and GSC; in detail, the voltage set points are defined as the ratio of available IQ capability of the RSC or GSC to the sum of the available IQ capabilities of the RSC and GSC. Then, the IMS determines the IQ-V gains and IQ limits for the RSC and GSC controllers depending on the available IQ capabilities and voltage set points; in detail, the IQ-V gain is determined in proportion to the IQ capability and voltage set point, and the IQ limit is set to the IQ capability at a certain operating point of the DFIG. In addition, the IQ capability of the RSC is intentionally extended by degrading active current reference in the RSC controller for severe voltage dips. Thus, can enhance the voltage support capability of a DFIG-based WPP for voltage dips by extending IQ capabilities of DFIGs and providing IQ through the flexible IQ-V characteristics. In addition, to prevent an overvoltage at the POI and DFIG terminal caused by the excessive IQ injection from the DFIGs, the IMS employs an overvoltage detector that initializes the voltage set points for the RSC and GSC.
The performance of the proposed scheme is investigated under small and large disturbances with various scenarios: Different wind condition, grid stiffness, voltage dip and fault location. To simulate the scenarios, a model system that consists of 20 units of 5-MW/6-MVA DFIG and voltage divider is modeled using an EMTP-RV simulator.
The advantages of the proposed scheme are that it helps secure the voltage stability of a transmission system where a MW-range WPP is integrated by enhancing the voltage support capability and it ensures less Q/IQ compensating devices required to comply with desired operating specifications in grid codes by a self-regulating IQ capability of a WPP according to the POI voltage.