Model‐based control addition to prescribe DFIG wind turbine fast frequency response

Abstract

This paper investigates the physical capability of double-fed induction generator (DFIG) wind turbines for inertial support of frequency response. Frequency stability is modeled using the DFIG electromechanical and generator controller dynamics, and a destabilizing effect is demonstrated in low-inertia systems. To improve response, a synchronous reference frame DFIG controller is proposed that acts by following low-frequency grid dynamics and adds a fast acting proportional plus integral (PI)-controlled frequency-responsive component to existing qd current commands. The proposed controller is derived in a straightforward manner using only the DFIG dynamic equations and is designed using pole/zero placement techniques. Laboratory experiments using a micro-scale DFIG wind turbine with hub-emulating flywheel prove better capability for transient frequency regulation even under extreme load change. The result is a DFIG controller that balances the appearance of transients in electrical and mechanical systems.Value is achieved in providing immediate continuous inertial response to support load change.The proposed frequency response can improve the use of existing physical inertia from wind turbines. KEYWORDS flywheel, frequency control, inertia, wind energy generation, wind power control, wind turbine generators 1 INTRODUCTION Decreasing inertia is one of the major obstacles to enabling very high penetration of renewable energy sources in future power systems. 1 Renewable energy resources with power electronic (PE) interface are reducing power system physical inertia and increasing susceptibility to voltage angle instability. 2,3 Wind turbines with a double-fed induction generator (DFIG) and PE back-to-back (B2B) converter are so-called ''Type-III'' machines. The power system is partially coupled to the massive rotor hub assembly by stator windings; the other portion is coupled to PE-connected rotor terminals. Wind turbines with DFIGs are a popular resource and may become the only source of electromechanical coupling between the power system and rotating mass. Their physical dynamics and control dynamics both influence their frequency response, and their characteristics are critical to stability. This paper seeks to understand the problems associated with DFIG wind turbine frequency response and aims to improve the use of their physical inertia in frequency regulation.Prior work in this field suggests that DFIGs possess sufficient capability for inertial frequency response when neglecting influence of control. 4It is now hypothesized that through the flexibility of PE control, the DFIG can be made to offer its inertia to frequency response in an innovative and more effective way. This paper goes beyond generator modeling and evaluates the frequency response of DFIG control. Linearized transfer functions are derived using the DFIG dynamic model and its respective control laws. They reveal how conventional DFIG control action degrades the response. A PI controller is then derived from the DFIG dynamic model to correct a...