This page contains a Flash digital edition of a book.
CPD PROGRAMME


Professional development


The CIBSE Journal CPD Programme


Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers.


Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a long-term commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK).


CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSE’s requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institution’s guidance.


Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at www.cibsejournal.com/cpd while the information they contain remains current.


You can also complete the questionnaire online, and receive your results by return email.


This CPD module looks at the principles of power factor and power quality on buildings Power quality for building electrical supplies


At the recent CIBSE ASHRAE Technical Symposium, ‘power factor’ correction was raised in a plenary session, and it was clear that there was some uncertainty about some of the concepts being discussed. This CPD article aims to provide an introduction to the principles of power factor, and some wider issues of power quality on buildings. Electrical systems supply power to equipment incorporating magnetising actions – such as many motors driving fans, pumps and compressors, and certain types of lighting – will require additional current above that needed to do useful work; this current is used to produce the magnetic flux in the device. In addition, the basis for calculating electrical loads is based on the concept of ‘linear’ loads, which means that they draw a sinusoidal current wave-shape to match the supply voltage. However, increasingly there are a numbers of non-linear loads in buildings that will also de-rate the performance of the distribution network by adding harmonics into the supply system (due to their pulsing actions) potentially affecting the performance and longevity of equipment. This reduction in power quality does


not necessarily cost the consumer money directly, but for commercial and industrial users it may well affect the electrical tariff costs, and will alter the effective capacity of the electrical distribution system.


www.cibsejournal.com


Load types In an alternating current (AC) electrical system there are resistive, inductive and capacitive components. Resistive loads include incandescent


lamps, resistance heaters and the cable itself. If a circuit were purely resistive, the current and the voltage would be in phase. A representation of the voltage and current waveforms for a resistive system is shown in Figure 1. No real power system is purely resistive, as it will certainly also have some capacitive characteristics (the ability to store energy briefly). Inductive loads would typically


include induction motors, the ballast coils used with fluorescent lamps, many low energy lamps, and any systems that include magnetic components – such as transformers, welding equipment, and many electrical goods. In an inductive circuit, the current will ‘lag’ the voltage waveform, as shown in Figure 2. Capacitive loads include such components as digital electronic equipment – a load that is becoming increasingly prevalent in commercial buildings. In a capacitive circuit, the current will ‘lead’ the voltage waveform. Capacitive loads (in the form of capacitors) are added to systems as a means of counteracting inductive loads. Most commercial and industrial buildings are likely to have an overall load


Figure 1: Voltages and current in a purely resistive system


Current lagging the voltage by 30°


Figure 2: Voltages and current in a simple inductive system with a lag of 30° (and a resulting displacement power factor of cos30° = 0.87)


profile that is inductive. Both inductive and capacitive loads are known as ‘reactive loads’ and, preferably, the goal should be to have almost zero reactive currents (meaning that the current and voltage are in phase) at the point that the electrical supply enters the building. Inductive loads require a current to


create a magnetic field (flux), and this field June 2012 CIBSE Journal


51


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68