Chapter N

Characteristics of particular sources and loads


Availability and quality of electrical power

From Electrical Installation Guide

The disturbances presented above may affect:

  • Safety of human life
  • Safety of property
  • The economic viability of a company or production process

Disturbances must therefore be eliminated.

A number of technical solutions contribute to this goal, with varying degrees of effectiveness. These solutions may be compared on the basis of two criteria:

  • Availability of the power supplied
  • Quality of the power supplied

The availability of electrical power can be thought of as the time per year that power is present at the load terminals. Availability is mainly affected by power interruptions due to utility outages or electrical faults.

A number of solutions exist to limit the risk:

  • Division of the installation so as to use a number of different sources rather than just one
  • Subdivision of the installation into priority and non-priority circuits, where the supply of power to priority circuits can be picked up if necessary by another available source
  • Load shedding, as required, so that a reduced available power rating can be used to supply standby power
  • Selection of a system earthing arrangement suited to service-continuity goals, e.g. IT system
  • Selectivity of protection devices (selective tripping) to limit the consequences of a fault to a part of the installation

Note that the only way of ensuring availability of power with respect to utility outages is to provide, in addition to the above measures, an autonomous alternate source, at least for priority loads (see Fig. N15).

Fig. N15 – Availability of electrical power

This source takes over from the utility in the event of a problem, but two factors must be taken into account:

  • The transfer time (time required to take over from the utility) which must be acceptable to the load
  • The operating time during which it can supply the load

The quality of electrical power is determined by the elimination of the disturbances at the load terminals.

An alternate source is a means to ensure the availability of power at the load terminals, however, it does not guarantee, in many cases, the quality of the power supplied with respect to the above disturbances.

Today, many sensitive electronic applications require an electrical power supply which is virtually free of these disturbances, to say nothing of outages, with tolerances that are stricter than those of the utility.

This is the case, for example, for computer centers, telephone exchanges and many industrial-process control and monitoring systems.

These applications require solutions that ensure both the availability and quality of electrical power.

The UPS solution

The solution for sensitive applications is to provide a power interface between the utility and the sensitive loads, providing voltage that is:

  • Free of all disturbances present in utility power and in compliance with the strict tolerances required by loads
  • Available in the event of a utility outage, within specified tolerances

UPSs (Uninterruptible Power Supplies) satisfy these requirements in terms of power availability and quality by:

  • Supplying loads with voltage complying with strict tolerances, through use of an inverter
  • Providing an autonomous alternate source, through use of a battery
  • Stepping in to replace utility power with no transfer time, i.e. without any interruption in the supply of power to the load, through use of a static switch

These characteristics make UPSs the ideal power supply for all sensitive applications because they ensure power quality and availability, whatever the state of utility power.

A UPS comprises the following main components:

  • Rectifier/charger, which produces DC power to charge a battery and supply an inverter
  • Inverter, which produces quality electrical power, i.e.
    • Free of all utility-power disturbances, notably micro-outages
    • Within tolerances compatible with the requirements of sensitive electronic devices (e.g. for Galaxy, tolerances in amplitude ± 0.5% and frequency ± 1%, compared to ±10% and ± 5% in utility power systems, which correspond to improvement factors of 20 and 5, respectively)
  • Battery, which provides sufficient backup time (8 minutes to 1 hour or more) to ensure the safety of life and property by replacing the utility as required
  • Static switch, a semi-conductor based device which transfers the load from the inverter to the utility and back, without any interruption in the supply of power
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