Electrical Load-Curve Coverage

Proceedings of the Symposium on Load-Curve Coverage in Future Electric Power Generating Systems, Organized by the Committee on Electric Power, United Nations Economic Commission for Europe, Rome, Italy, 24 – 28 October 1977

Robert Maxwell

• Publisher: Pergamon
• Year: 2013
• ISBN:
  9781483280936
• Topic: Business
• Subtopic: Infrastructure
• Index: Business

Electrical Load-Curve Coverage covers the proceedings of the Symposium on Load-Curve Coverage in Future Electric Power Generating Systems, organized by the Committee on Electric Power, United Nations Economic Commission for Europe, Rome, Italy, on October 24-28, 1977. This symposium considers the opportunity for an international exchange of economic and technical information on problems connected with the coverage of the full load curve. This book is composed of five parts encompassing 48 chapters, and begins with the optimum composition of the generating system. The first part considers the use of mathematical models for optimization of energy sources in some countries. The succeeding parts deal with the means of electricity generation for the variable part of the load curve. This part examines the electric consumption of steam and gas turbines and the concept of peak-power load. Other parts discuss the means of storage at user level. The final parts consider natural characteristics of the energy demand (load curves). This book will prove useful electrical engineers and researchers.

Optimum Composition of the Generating System

Consolidated Report

Valtorta Marino, Deputy Director of Programming, Ente Nazionale per l’Energia Elettrica (ENEL), Rome, Italy

Publisher Summary

This chapter describes the composition of the generating system in the way which best matches the load from the technical and economic points of view with the quantities of electric power required at any time by consumers. The load curve coverage is to be considered from two points of view: planning and operation. To interpret the various patterns of load variations, and in particular the associated rates, the chronological diagram is needed: from a yearly diagram of this kind, it can be realized at once that high variation rates appear in daily cycles. The shape of the daily diagram, its weekly and seasonal diversification and their statistical features, depend on the behavior of different consumer groups in their totality and on their relative participation. In regard to hydro units, the head variation is reflected in the maximum capacity, and the constraint on their capability imposed by the availability of water has a stochastic nature possibly modified by the policy or reservoir exploitation. The transmission network can provide valuable help to the load coverage problem, but possible bottlenecks may require special flexibility characteristics in the generating units.

FOREWORD

The composition of the generating system in the way which best matches the load from the technical and economic points of view with the quantities of electric power required at any time by consumers in their totality is in fact the subject-matter of the whole symposium. The optimum coverage of the load curve, in the short, medium and long term is indeed the main objective of electricity supply systems.

In order to attain such an optimum composition, it is necessary to identify the relevant characteristics of the load-curve on one hand and of the means of generation on the other. The performance of different means in the load curve coverage are interrelated. Therefore, in the search for the optimum, the whole system of generating units available for orchestration in the supply of the whole load should be considered.

This report is concerned with those papers dealing specifically with the general problem of the optimum composition of the generating system. Due to the above-mentioned interrelationships, reference will however be made to many other papers dealing with more particular items.¹

The subject-matter to be considered is so large and manifold that this report really cannot be exhaustive. It is intended on one hand to establish through a general survey a common glossary (the wording of different authors having more or less different meanings) and on the other to indicate some discussion topics, the various papers touching more or less deeply on a very wide range of problems.

The Load-curve Characteristics

The load curve coverage is to be considered from two points of view: planning and operation. In both perspectives different time intervals have to be examined, ranging from a year to a few minutes or seconds.

Looking at the year as a whole, the synthetic picture of the yearly load duration curve gives the first information about the occurrence of different demand levels, and conversely about the utilization factor of power layers at different levels.

But the yearly load duration curve is obviously too concise: the main shortcoming is that the chronological sequence is lost and therefore demand values are brought close to each other and are associated in a rather misleading way (R.23). Therefore special care should be taken in handling such curves of such duration.

In order to interpret the various patterns of load variations, and in particular the associated rates, the chronological diagram is needed: from a yearly diagram of this kind, it can be realized at once that high variation rates appear in daily cycles.

The daily diagram of load as a function of time (after filtering the noise of very small, short and frequent fluctuations due to the random change of load structure and of single consumer demand) shows several ripples R.11, R.39, R.41) and then several peaks and hollows. Among these the maximum peak and the minimum hollow and their ratio (R.21, R.42) have a particular interest. But also the number of ripples, the amplitude of variations both upward and downward and the rates of rise of fall (variable during each rise or fall) are important characteristics. It should be pointed out that the largest variation in amplitude is not necessarily related to the maximum peak and the highest rate is not necessarily related to the largest variation (R.23, R.42). On the other hand, the width of the intervals of continuous persistency of different demand levels or demand layers at different levels, and of spans between them should be considered. Furthermore, high variation rates have different importance in relation to their persistency.

The pattern of daily diagrams and all the above mentioned characteristics are different for various days of the week: working days (separate consideration is generally needed for Monday), Saturday and Sunday (likewise holidays). And for these typical days the pattern can be different according to the season.

The instantaneous values of load during a given weekday, in a given season, even after noise filtering, have a further random variability due to occasional load concentrations or reductions of larger amplitude and duration¹ than the noise fluctuations; moreover, differences affecting the whole daily diagram, perhaps for several consecutive days, may appear from one year to another as a result of the influence of speci...