I- Introduction

II- Static VSA Fundamentals and the CPFLOW

III- Proposed Equivalent Distribution System Model to Account For Operating Discontinuities

IV- Proposed Algorithm

V- Tests and Results

References

Abstract

The reduction in bus voltage magnitudes as the load demand grows may lead to sudden disconnection of loads and/or distributed generation units, in distribution grids, caused by undervoltage protection schemes. As proposed in this paper, this discontinuous behaviour of distribution grids can be modeled as a sudden load variation in traditional static Voltage Stability Assessment methods, such as the continuation power flow (CPFLOW). A discussion on the impacts of these discontinuities on the equilibrium diagram of the system is presented in this paper, as well as a set of numerical simulations showing that the traditional CPFLOW algorithm presents convergence problems caused by the discontinuities under analysis. From this perspective, this paper proposes an algorithm based on novel predictor/corrector and identification schemes, which are capable of successively calculating the discontinuities that exist in the equilibrium loci of the system under analysis, as well as the Maximum Loadability Point and the type of bifurcation. A simplified modeling approach that eliminates the need for a complex (and computationally expensive), detailed description of distribution grids is also elaborated and incorporated into the proposed algorithm. The simulated examples show that the proposed algorithm adequately handles the problem, yelding more accurate results than the traditional CPFLOW algorithm.

INTRODUCTION

POWER systems are nowadays operated closer to their limits in an overall sense, which makes them more prone to voltage stability problems. One example of voltage instability was seen in Brazil in 2009, when the three transmission lines that deliver power from the Itaipu power plant to the bulk power grid were disconnected. This disturbance caused voltage sags in the state of Sao Paulo, which resulted in disconnection of the HVDC link between Brazil and Paraguay [1]. Problems like this motivated this research, with the aim of developing more robust and accurate voltage stability assessment algorithms, that can upgrade or even replace the traditional voltage stability assessment tools [2]. Traditional Voltage Stability Assessment (VSA) methods typically rely on algorithms that assume a continuous behaviour of the load [3], which is an assumption that may not hold in practice, taking into account several particular characteristics of current power systems operation, such as the presence of undervoltage load shedding (ULS), distributed generation (DG) undervoltage protection, and modern demand side management schemes [4]. In the case of DG, these small generators may exhibit discontinuous behaviours (due to protection actuation) during disturbed operational regimes, in order to comply with the grid codes. Also, typical load shedding and demand side management schemes clearly produce discontinuous behaviours of the loads in distribution grids. Several authors have tried to address this issue as reported in [5] and [6] where the mandatory disconnection of DG was considered during dynamic simulations. In [5], it was demonstrated that inadvertent undervoltage trip of a DG unit may cause instability in bulk power systems. In [6], a transmission system Maximum Loadability Point (MLP) was estimated from small successive increments in its load.