Power flow analysis of distribution systems with embedded induction generators

Small-scale induction machines are being increasingly used as generators in low-and-medium voltage distribution systems to generate electricity and enhance the main grid. Such nonconventional sources of energy are commonly known as Distributed Generators (DGs). These DGs are usually modeled as PV or PQ models in the steady-state and power flow analysis of distribution systems. However, when connected to a weak distribution network, modeling of induction generators (IGs) as PV nodes is not always appropriate due to the unregulated terminal voltage as well as the poor voltage regulation. On the other hand, handling them as PQ nodes has long been considered as an over simplified approach. This paper discusses the modeling of the induction machine as an admittance load. For this purpose, a grid-connected Cage Rotor Induction Generator CRIG is used. The generator was also assumed to supply a local load, therefore its minimum capacitor bank value required for self-excitation is estimated. The machine´s active power is assumed constant and the reactive power dependency on machine´s slip is ignored. In this paper, operating at maximum torque is considered. The slip at which maximum regenerative torque occurs is computed from the machine´s data and the operating conditions, and the admittance model is then obtained. This model is integrated into the commonly used backward/forward method and the results are reported. Compared to the PV model, reasonable results were obtained with less computational efforts.