Page 42 - Power Electronic Control in Electrical Systems

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH002.3D ± 32 ± [31±81/51] 17.11.2001 9:49AM

32 Power systems engineering ± fundamental concepts

important. This is the subject of reactive compensation. Where the focus is on
individual loads, we speakof load compensation, and this is the main subject of this
chapter along with several related fundamental topics of power systems engineering.
Chapter 3 deals with reactive power control on long-distance high-voltage transmis-
sion systems, that is, transmission system compensation.
Load compensation is the management of reactive power to improve the quality of
supply at a particular load or group of loads. Compensating equipment ± such as
power-factor correction equipment ± is usually installed on or near to the consumer's
premises. In load compensation there are three main objectives:
1. power-factor correction
2. improvement of voltage regulation 2
3. load balancing.
Power-factor correction and load balancing are desirable even when the supply
voltage is `stiff ': that is, even when there is no requirement to improve the voltage
regulation. Ideally the reactive power requirements of a load should be provided
locally, rather than drawing the reactive component of current from a remote power
station. Most industrial loads have lagging power factors; that is, they absorb
reactive power. The load current therefore tends to be larger than is required to
supply the real power alone. Only the real power is ultimately useful in energy
conversion and the excess load current represents a waste to the consumer, who
has to pay not only for the excess cable capacity to carry it, but also for the excess
Joule loss in the supply cables. When load power factors are low, generators and
distribution networks cannot be used at full efficiency or full capacity, and the
control of voltage throughout the networkcan become more difficult. Supply tariffs
to industrial customers usually penalize low power-factor loads, encouraging the use
of power-factor correction equipment.
In voltage regulation the supply utilities are usually bound by statute to maintain
the voltage within defined limits, typically of the order of 5% at low voltage,
averaged over a period of a few minutes or hours. Much more stringent constraints
are imposed where large, rapidly varying loads could cause voltage dips hazardous to
the operation of protective equipment, or flicker annoying to the eye.
The most obvious way to improve voltage regulation would be to `strengthen' the
power system by increasing the size and number of generating units and by making
the networkmore densely interconnected. This approach is costly and severely
constrained by environmental planning factors. It also raises the fault level and the
required switchgear ratings. It is better to size the transmission and distribution
system according to the maximum demand for real power and basic security of
supply, and to manage the reactive power by means of compensators and other
equipment which can be deployed more flexibly than generating units, without
increasing the fault level.
Similar considerations apply in load balancing. Most AC power systems are three-
phase, and are designed for balanced operation. Unbalanced operation gives rise to
components of current in the wrong phase-sequence (i.e. negative- and zero-sequence

2
`Regulation' is an old-fashioned term used to denote the variation of voltage when current is drawn from
the system.