Starting of induction motor fed with stand-alone DFIG

Abstract

This paper presents dynamic simulation and control of stand-alone doubly fed induction generator (DFIG) loaded with 3-phase induction motors (IMs). The study reveals that direct on-line starting of large IMs causes a large voltage sag across the generator terminals as the starting current drawn reaches up to 8-9 times the rated load current. Traditionally, this problem has tackled by oversizing of the generator or employment of special starters, under the pretext of mitigating voltage sag. This work explores ways that the starting current can be reduced economically by applying constant V/f control side by side with indirect field-oriented control (FOC) applied on the rotor side converter of the DFIG. This methodology enables starting of larger IMs and mitigation of voltage sag that occurs during the start-up period. Two different rating of IMs loaded with speed-squared mechanical torque are mainly considered. Simulation results of the system behavior under study confirm the capability of the proposed control.

Maximum allowable hp rating of 3-phase induction motor fed through a stand-alone constant V/f controlled DFIG via RSC

“…DFIG loading capacity limits in steady state operation are deduced at the rotor and stator maximum permissible currents, nominal values [26], and at the rotor maximum permissible voltage [32]. For constant V/f operation, the constant of proportionality (c) is given by: 𝑐 = 𝑉 𝑠𝑟 𝑓 𝑠𝑟 (18) Where Vsr and fsr are the nominal values of the DFIG stator output VF. For any given stator frequency (fs) the voltage of the stator is expressed by:…”

“…The output VF control of the stand-alone DFIG can be obtained by adjusting the magnitude of currents and frequency of the rotor via the RSC from the reference values, Vs * , and ωs * respectively. Figure 7(a) shows the proposed control method to choose the DFIG reference values of stator VF outputs by the help of control switch [18]. At position (a), the DFIG operates at nominal stator VF outputs.…”

“…Stand-alone wind energy conversion systems employing doubly fed induction generators (DFIG) have become a suitable choice for feeding low power consumers in remote areas [1]- [16]. DFIG can produce a constant outputs, terminal voltage and frequency (VF), using field-oriented control (FOC) when supplying isolated loads regardless of the loading conditions and generator operating speed [1], [17], [18]. In remote area applications, where the DFIG is used as a power source, dynamic loads such as induction motors (IM) are used in farm irrigation pumps [19].…”

“…Hence, a larger rating of IM has not been used. Sharawy et al [18] has proposed a new method when the IM is directly connected to the DFIG. The output VF of the DFIG is firstly reduced to certain value before connection of the IM while maintaining the V/f ratio constant.…”

“…This procedure contributes to the reduction of motor starting current and makes it possible to start larger IMs rating with respect to the DFIG rating. Furthermore, the procedure mitigates the voltage sag which occurs during the starting period within allowable limit values [25] without oversizing the rating of DFIG or employing any special starters [18].…”