Development and implementation of hydro turbine and governor models in a free and open source software package

Abstract

This article studies accurate and detailed hydro turbine and governor models, and implements these models in Power System Analysis Toolbox (PSAT)—a free and open source software. The implementation of these models in PSAT is approached using a general methodology, which is described in detail. To evaluate the performances of the developed hydro turbine and governor models, simulation studies are carried out on test systems of different scales, from a single-machine infinite-bus (SMIB) system to a larger system which includes 20 generators. Further more, transient stability analysis and small signal stability analysis are carried out to assess the performance of the implemented models.

Introduction

Hydro-power in Sweden accounts for one half net production of electrical power generation [1]. Furthermore, as one of the most important renewable energy sources, the exploitation of hydro-power naturally attracts more and more attention worldwide. In hydro power production systems, the functions of hydro turbine and governors cannot be neglected because they participate in primary frequency control of power systems. To highlight this fact, Fig. 1 shows the relationship of a generator’s turbine and governor (indicated by the dashed block) with the overall power system.

Different power sources determine the physical features and categories of hydro turbine and governors, thus each type requires its unique modeling. For example, time delays in hydro turbine and governor models are due to transient droop compensation, pilot valves and gate servomotors. Additionally, the effect of water inertia in hydro turbines significantly influences the governor’s requirements and must be compensated by a rate feedback, which must be reflected in the modeling of these devices [2]. In this case, it is important that hydro turbine and governors are adequately modeled so that the models can accurately represent the behaviors and capacities of hydro turbine and governors as close as possible. To examine model performance, the developed models are implemented in test power systems of different scales and their stabilities are evaluated: i.e. if the system remains in equilibrium after being subjected to a disturbance [3]. To this end transient (large signal) stability and small signal (small disturbance) stability analysis are frequently utilized as system stability analysis approaches [4]; in this study both approaches are used.

On the other hand, computer-based simulation tools are used by engineers for modeling and simulation to assess the stability dynamic and performance of the power system. With advances in computer software, there has been a large number of power system simulation software packages developed, which can be basically divided into three categories: proprietary software, open source software and free software [5]. Proprietary software are conceived by the general public to be well-tested and computationally efficient. However, license agreements restrict their use by imposing different conditions. Users are prohibited and generally have no possibility to modify the source code nor to distribute binaries (i.e. executables). Additionally, they seldom allow to import or export data in different formats [6].

Nevertheless, free and open source software are usually freely (at no cost) distributed on line and are less cumbersome for education and research purposes. More importantly, they allow users to change the source code, add new algorithms, or implement new components. In addition, free software stands on an ethical pillar which aims to warrant intrinsic freedom of computer uses that are jeopardized by proprietary software [7]. Despite theses advantages, there still exist some implementation challenges; particularly for new users. One of these challenges is that it is seldom documented in a detailed fashion the necessary approaches for implementing new components or algorithms in free and open source software. This severely restricts free and open source software to be accepted and popularized by engineers, and also decreases individuals’ cognition of the software’s flexibility characteristics.

The objective of this paper is to develop hydro turbine and governor models and implement them in Matlab-based Power System Analysis Toolbox (PSAT)—a free and open source software. To evaluate models’ performance, the relevant simulation will be performed on a single-machine infinite-bus (SMIB) system and another larger system, e.g. the KTH-NORDIC 32 system, for both transient and small signal stability analyses.

In view of the description above, the contributions of this paper can be summarized as follows:

• To develop four detailed hydro turbine and governor models which can represent the behaviors of hydro turbine and governor precisely, e.g. water hammer effect.

• To exhibit a systemic, clear and general approach for implementing new components in PSAT—a free and open source software.

• To illustrate a detailed description for how to implement new hydro turbine and governor models in PSAT.

• To perform different system stability analysis approaches and their application in PSAT, e.g. transient stability analysis and small signal stability analysis.

The remainder of this paper is organized as follows. Section 2 describes how to develop models for hydro turbine and hydro governor, respectively, and presents an example model with its structure and realization description. In Section 3, a general approach for implementing components in PSAT is described, as an example, this approach is illustrated with the implementation of a hydro turbine and governor model in PSAT. Next, Section 4 shows simulation results and discusses the performance of the implemented models in the SMIB system as reflected by transient stability analysis and small signal stability analysis. Implementation of turbine and governor models in a large power system is carried out in Section 5. Conclusions are drawn in Section 6 highlighting the contributions reported in this paper.