Optimal Solar Array Configuration and DC Motor Field Parameters for ...

the APCC Sunsine manufactured by American Power Conversion Co., Inc. The Gemini is a line-commutated inverter, while the APCC and Teslaco are self-commutated. Several observations can be made from the case studies. Whether self- or line-commutated, all PCSs are intrinsically prone to islanding. However, the Teslaco and APCC are equipped with feedback controls that are unstable in the absence of a strong utility source. This instability causes frequency to drift and ultimately the PCS shuts down. It is shown that this protective scheme is effective even in cases involving multiple PCSs. The Gemini PCS studied does not utilize such a scheme and islanding is quite likely in such PCSs if the distribution feeder is capacitively compensated. It is suggested that the destabilized feedback scheme should be required on all PCSs intended for utility-connected operation. Discussers: A. F. Imece and R. A. Jones Reference

1I1 Ranade, S. J., Prasad, N. R., Omick, S., and Kazda, L. F., "A Study of Islanding in Utility-Connected Residential Photovoltaic Systems Part I-Models and Analytical Methods," Companion paper.

89 WM 027-4 September 1989

89 WM 020-9 September 1989

Optimal Solar Array Configuration and DC Motor Field Parameters for Maximum Annual Output Mechanical Energy Mohamed Mostafa Saied, IEEE Senior Member, and Monji G. Jaboori, IEEE Student Member Electrical and Computer Engineering Department College of Engineering and Petroleum Kuwait University P.O. Box 5969 13060 Safat, Kuwait Abstract-This paper presents a method for the design of systems comprising photovoltaic generators directly supplying DC motors. It is based on maximizing the total annual gross mechanical energy delivered to a mechanical load with a given torque speed characteristic. Seasonal variations of the insolation level and the ambient temperature are taken into account. Using an optimization technique, it is possible to determine the array configuration, and accordingly the optimal motor voltage and current ratings, as well as the motor field parameter that yields the maximum output energy for a given annual radiation and ambient temperature variation curves. The proposed technique is applied to two practical case studies, and the resulting optimal design parameters are compared with those available in the literature. A comparison is also made between the optimal separately-excited and series motor design alternatives regarding the annual gross mechanical output energy.

Understanding Modern Generator Control

Introduction

R. M. Wright Tri-Sen Systems P.O. Box 578 La Marque, TX

This paper deals with the utilization of solar energy, after its conversion in a photovoltaic array, to supply direct current motors used to operate pumping loads. This represents one of the promising applications suitable for farm irrigation and other similar situations. The proper matching of the solar array to the DC motor is a crucial issue in such applications. There are several factors affecting the operating points of this array/motor system, and accordingly enhance or offset the matching condition. Most important are: 1. The radiation and temperature dependent voltage current characteristics of the solar cell. 2. The array configuration, i.e., the number of cells in series and strings in parallel (Ns, Np). 3. The kind of DC motor used and the type of mechanical load, namely torque speed relation. 4. Due to the motor armature losses, the optimal points corresponding to maximum electrical power do not coincide with those relating to maximum mechanical power.

The need exists for wider understanding of the control principles traditionally used for power generation. Since these same control principles (sometimes in a modified form) are incorporated in the new microprocessor-based digital control systems, it is paramount that those of us who design, apply, operate and manage this equipment understand how both traditional and contemporary generator control works. These principles operate our equipment today and in a modified and improved form will operate our equipment tomorrow. It is vital that we understand the beast when we change horses. Definitions of control modes, PROPORTIONAL ONLY, DROOP and SPEED/LOAD, for instance, have traditionally varied from one discipline to another. Now that different disciplines are being forced closer together by the computer revolution, we should understand that these terms are only names used by different trades to define the

same function. Other control mode definitions fall in the same

category.

DROOP control (as performed by the age-old mechanical flyball governor) has many valuable attributes. DROOP mode performance is incorporated in the newer digital governors as well because we have not devised anything better for the purpose of controlling synchronous generator drivers. Most of us do not understand DROOP control

as well as we need to. The same goes for other control modes. These are discussed and portrayed at length. The new digital governors provide many features that improve on

and add to synchronous generator driver control. These need to be tabulated as well. Induction generators are fast becoming more common. Control of synchronous generators and of induction generators must be quite different because of the nature of the electrical equipment. We need to understand why and how to differentiate between the two when choosing controls. In this age of technical complexity, it is easy to concentrate on the complex and how it interacts with itself. It has become too easy with the press of modernity to ignore the basics. We need to go back and review them. Discusser: R. P. Schulz

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Computation Procedure and Results The procedure determines the optimal solar array configuration as well as the optimal value of the flux coefficient c of a separatelyexcited motor, or the armature field mutual inductance M of a series motor for a particular pumping load. As for the energy output at a given field parameter, the program finds the actual operating point of the motor, i.e., the point of intersection of the VI characteristic curve at a certain radiation level, ambient temperature and the motor characteristic equation. The resultant voltage and current values represent the actual operating point denoted (la, Va) which in turn yield the actual output power denoted (Pa). Output power multiplied by the duration time of a certain radiation level and summed up along the year gives the gross mechanical energy for the given field parameter. To get at an optimum field parameter of the motor, for a given motor voltage rating (i.e., array configuration), a certain range for the aforementioned target variable is assumed and then searched. The execution of the program proceeds until the specified accuracy is met.

Fig. 1 depicts

some

of the results obtained for the separately-

IEEE Power Engineering Review, September 1989

excited motor. It gives the dependence of the annual output energy in millions of joules on the motor flux coefficient c, for different solar array configurations having the same total number of cells (about 5832). The first number of each curve, Ns, gives the number of cells per string, while the second, Np, gives the number of strings in parallel. It is seen that for each cell configuration there is an optimal value of the flux coefficient c yielding maximum output energy per year.

The

same statement

armature mutual inductance M as seen from Fig. 2 which depicts its effect on the annual output energy. A comparison of the peak values of the annual energy in Fig's. 1 & 2 clearly shows the superiority of the separately-excited motor. Specifically, the maximum output energy of the separately-excited motor is seen to be 2.3 x 106 Joules/year, while the corresponding figure for the series motor is seen to be 2.0 x 1 06 Joules/year, i.e., 15% lesser solar energy utilization. Discusser: A. M. Sharaf

is also valid regarding the series motor field-

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Fig. 1. The Annual Output Gross Mechanical Energy for SeparatelyExcited Motors, as a Function of their Flux Coefficient c, for Different SolarArray Connections. The First Number on Each Curve (Ne) gives Cells per String, the Second is the Number of Strings in Parallel (Np).

2

1.9 1.8

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UAuol Induconca (U)

Fig. 2. The Annual Output Gross Mechanical Energy for Series Motors, as a Function of their Mutual Inductance M, for Different Solar Array Connections. The First Number on Each Curve (Ne) gives Cells per String, the Second is the Number of Strings in Parallel (Np).

IEEE Power Engineering Review, September 1989

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