Model and simulation of a stand alone power system comprising a diesel engine driven synchronous generator and a power conditioner

International Journal of Electrical Engineering Education, Jul 1998 by Perahia, J, Nayar, C V

Abstract A model of a stand alone power supply comprising a diesel engine driven synchronous generator, a power conditioner, a bank of batteries, and an adjustable dump load is presented. The response of the power system to two different load profiles is included.

1 INTRODUCTION

As the applications and uses for control systems have increased, so also have the demands upon the performance of these systems increased. The major concern now is the design and development of newer equipment with greater usefulness and capabilities. Described is a stand alone power system with significant advantages over comparable systems, and included is a model of the system and simulation results.

A schematic of the stand alone power system is shown in Fig. 1. The system comprises a diesel engine driven synchronous generator, a power conditioner which drives two switches SW^sub G^ and SW^sub D^, a bank of batteries, and an adjustable dump load. In this system the diesel engine driven synchronous generator fulfills the role of a conventional power source, while the other components improve diesel engine fuel efficiency and add flexibility to the power system.

As seen in Fig. 1, the power conditioner either transforms d.c. power from the batteries into a.c. power to supply the a.c. electricity grid (inversion), or transforms a.c. power at the a.c. bus into d.c. power to re-charge the batteries (conversion). The power conditioner also drives two switches, one switch labelled SW^sub G^ to connect the synchronous generator to the a.c. bus, and the other switch labelled SW^sub D^ to connect the adjustable dump load to the d.c. bus. The adjustable dump load removes unwanted electrical energy from the power system and is used to taper re-charge the bank of batteries.

The controller of the stand alone power supply is housed in the power conditioner. It is the function of the controller to select between inversion and conversion modes of the power conditioner and to switch on or switch off either the diesel or the dump load. The controller also varies the size of the dump load.

The configuration depicted in Fig. 1 takes advantage of the combination of a power conditioner and batteries to permit the diesel engine to achieve maximum diesel fuel efficiency. A partially loaded diesel engine has poor fuel efficiency but battery re-charging increases the load on the diesel engine and thereby raises fuel efficiency. Diesel engine loading is described with reference to Fig. 1 which shows battery re-charging draws d.c. power from the converter portion of the power conditioner whose a.c. power is supplied by the synchronous generator. The synchronous generator is powered by the diesel engine and thus the power required to battery re-charge is taken-up by the diesel engine.

Another benefit offered by the system in Fig. 1 is found in the power conditioner's inversion facility, allowing electrical energy to be supplied to the a.c. bus. For example, depending on the size of stored electrical energy, electrical loads incompatible with the capacity of the diesel engine suddenly may become compatible with the combined electrical capacity of the diesel engine, the inversion facility, and the bank of batteries. This benefit is contingent on the rated capacity of the inversion facility and on the rated current of the bank of batteries.

In regard to usefulness, the system shown in Fig. 1 also offers a convenient electrical connection point for d.c. electrical devices powered by renewable energy sources such as solar panels or a wind turbine driven electric generator1. Capital costs of the stand alone power supply may inhibit the purchase of devices to convert renewable energy into electrical energy initially. However, at a later stage these devices can be purchased and then easily coupled to the system.

Section two of the paper presents the block diagram model of the diesel engine, the model of the synchronous generator, and their time domain representation. Section three presents the model of the bank of batteries and the power conditioner while section four contains the model of the adjustable dump load. Section five describes the control of the diesel engine and the adjustable dump load. Section six presents the response of the power system to two different load profiles and section seven concludes.

2 MODEL OF DIESEL ENGINE DRIVEN SYNCHRONOUS

GENERATOR

The block diagram of the diesel engine driven synchronous generator is shown in Fig. 2. Fig. 2 portrays a typical speed control system where the actuating signal, the difference between set point speed and actual engine speed, causes a change in the torque of the prime mover. The system has been linearized and the linear differential equations describe the system's variables as a function of time because time is required for the engine to accelerate or decelerate to its desired speed. In brief, the method employed obtained the block diagram for each component or part and then hooked-up or connected the corresponding inputs and outputs for each diagram to obtain one overall representation of the diesel engine driven synchronous generator. A summary of the results is presented as follows.