Research On Reactive Power Operation And Low Voltage Ride Through Of Hybrid Wind Farm

With the rapid increase capacity of wind power, the wind turbine generators (WTGs) should inject active power and reactive power like synchronous generators (SGs). Futhermore, the low voltage ride through (LVRT) of WTGs is more strictly than that of SGs. Considering the construction process of wind power, the wind farms may contain the fixed speed wind turbine generator (FSWTG) and different type of variable speed wind turbine generator (VSWTG). The operation region of VSWTG needs to be quantified. The VSWTG with a larger capacity of reactive power may improve the LVRT capability of FSWTG or VSWTG with a less capacity of reactive power. This paper mainly investigates the reative power operation region and LVRT of hybrid wind farms, the details are as follows:(1) The number of permanent magnet synchronous generator (PMSG) that helps LVRT of induction generators (IGs) for coordinated LVRT is newly derived. Based on the torque balance, the transient stability of IG with different voltage drops is yielded, and the critical slip yielding instability of IGs is quantified. The electromagnetic torque is given by slip and voltage, then the terms including slip and time are moved to different side of the motion equation, whose integral yields the critical voltage at the point of common coupling (PCC) to maintain stability of the IG Cosidering the active power and reactive power of PMSG and IG, the necessary number of PMSGs for coordinated LVRT to the IGs is derived with the analytical solution.(2) The series braking resistor (SBR) in the stator of SGs for dispered genertions is applied for enhancing the LVRT capability. The time domain expression of exciation system during fault and after fault is derived, and the equal area criterion (EAC) that considers the dynamic control of exciation system is built. The value of SBR is directly quantified with the given switch in rotor angle. Changing the system parameters, the effect of them on SBR is yielded.(3) The SBR in the stator of PMSGs and series compensate capacitance (SCC) between the grid side converter (VSCg) and the grid are proposed to enhance the LVRT capability during symmetrical/unsymmertrical fault. During the symmetrical fault, the SBR can avoid the oscillation of direct current (DC) link voltage. During the unsymmetrical fault, the SBR make it difficult to arrive at the current limit, and can decrease the oscillation of the reactive power that injecets to the grid; while the SCC can counteract the double-frequency active power ripple produced by the reactance of the transformer, make the DC link voltage and active power that transmitted to grid smooth.(4) The coordinated LVRT to the doubly fed induction generators (DFIGs) with help of PMSGs is newly proposed. The rotor voltage and current eliminating differential flux are given, combined with rotor voltage control equation considering stator flux transient to eliminate rotor voltage, and derivated from both sides to yield the 2nd order differential equation of rotor current. The threshold of the terminal voltage of DFIG is found by the maximum rotor current. The critical terminal voltage of DFIG is found with reactive current given by the grid code, to yield the critical voltage of PCC, active power and reactive outputs of each DFIG and PMSG For the given number of DFIGs, the necessary number of PMSGs for coordinated LVRT to the DFIGs is derived with the analytical solution.(5) Including current constraints of power winding (PW) and control winding (CW), and capacity constraints of VSCg, the analytical algorithm of the operation region of the brushless doubly fed induction generator (BDFIG) is newly quantified. Following the maximum power point tracking (MPPT), the derivatives of the upper /lower var limits with respect to the slip are derived to validate the effect of wind speeds on operation region. Following active power dispatch, the derivatives of var limits to the active output of the BDFIG are quantified to validate the effect of power regulations on the operation regions. The copper loss affects active output and var region, thus it is applied to quantify the error of the latter.