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close this book Village Electrification
close this folder Part 3: Electrical control systems
View the document 1. Fundamental principles of electrical control systems
View the document 2. A concept for standardized electrical control systems
View the document 3. Specific problems with isolated, rural electrification schemes

1. Fundamental principles of electrical control systems

1.1 System Control

1.2 Protection

1.2.1 Protection of Human Beings

1.2.2 Protection of Property

1.3 Instrumentation

We need first to specify what the term control means in our context. For a small grid with a single generator and a simple power distribution, controlling mainly includes the stabilization of the line voltage and the frequency within certain given limits. This is normally done by measuring these values and then adjusting either the generating gear or the load.

Besides this the control must ensure the safety of people and equipment, shutting down the plant or set off an alarm if any danger occurs.

To control the electrical and mechanical system at least the control sub-systems shown in Table I are required.

Although references will be made at various points to the others, this section is basically dealing with the system control.

The electrical control system generally provides not only the 'control', but also 'protection' and 'instrumentation'. These three sub-systems have to provide a number of functions, the most important of which shall be outlined in the following paragraphs.

1.1 System Control

Electrical control signals enable and trigger essential electrical functions like voltage build-up, load control and management, normal and emergency deexcitation of the generator or shut down of the plant.

In basic electrical control systems like the designs shown in section 3) control is done manually through push buttons and switches. In schemes that are designed for a higher degree of automation, electronic, programmable logic controllers (PLCs) can provide fully automatic start-up and shutdown procedures.

1.2 Protection

While electricity is probably the most convenient form of energy presently available, its use involves certain risks. reduce danger to a minimum, national rules and international standards form a base for electrical safety. Protection systems have to be designed accordingly to fulfil the specified requirements which can basically be categorized into two groups: the protection of human beings (operators, consumers...), and the protection of property (generating equipment, appliances...).

1.2.1 Protection of Human Beings

Touching life parts is extremely dangerous and often even causes loss of life.

The effects of various intensities of earth fault currents through a human body are indicated in Table 2

Table 1 Control Sub-Systems

Table 2 Damaging effects of electrical currents on human bodies

Protective measures are normally divided into three categories: basic, direct and indirect protection.

a) Basic Protection

Is ensured by the insulation of all life parts to prevent from a direct contact.

b) Direct Protection

Is ensured by simply placing electrical circuits and installations out of reach and by prevention of direct contact through enclosures, barriers or covers and housing. The degree of protection is best indicated with reference to the international IP classification. The IP-code consists of two figures: the first one indicates the degree of protection of persons from contact (or the penetration of solids), the second specifies protection against penetration of water. Widely adopted IP classes are summarized in Table 3. In spite of providing basic protection, enclosures and barriers, accidents still may occur for instance in case of an insulation failure. In view of these cases direct protection can be enhanced by the use of residual current operated circuit breakers (RCCB) or earth leakage breakers/relays (ELB/R).

c) Indirect Protection

Is provided by a number of measures briefly mentioned hereunder. Earthing: electrical connection of all accessible, conducting parts like covers, frames or housings together and to earth (neutral potential see also chapter earthing). Also provide effective, automatic disconnection of the supply before a shock is likely to prove fatal. Use of Class 11 Equipment: appliances with double insulation, which have no exposed conductive parts. Electrical Separation: usually provided by isolating transformers with the secondary side floating (non-earthened). This is, however, limited to certain circuits and appliances only. The first method is mostly applied.

1.2.2 Protection of Property

This term generally covers the protection of generating equipment, including generator, electrical control system and power cables inside of the plant, the protection of transformers and the distribution system outside of the plant and the protection of consumer appliances.

Table 3 Standards of Protection of el. Apparatus against Contact of Persons and Water

Most of these subjects are covered by standard text books or manufacturers' data sheets. Specific aspects to which some attention is paid here are: The dimensions of copper wires, the aerating factors for cables, the coordination of current breaking devices and conductors, the breaking capacities and time/ current characteristics of breaking devices and lightning protection.

a) Capacity and Derating Factors for Cables

The permissible current rating of any current carrying device (e.g. generator, cable...) is of course dependent on the actual layout (e.g. the effective cabling used. Table 4a shows correct cross sections for Copper wires) but also on ambient/atmospheric conditions.

Table 4a Copper wire diameters for different currents and installations.

While designing protection for an electrical system, the operating conditions need therefore to be taken into account. Of particular importance are aerating factors for power cables and generators.

Elevated temperatures considerably reduce the rated currents. Correction factors for different ambient temperatures are given in Table 4.

Bundeled cables do not dissipate heat very well therefore, their rated current drops (or for a given current the cross-section must increase).

Table 4 Correction factors k for cable ratings at different ambient temperatures

Some correction factors for grouping single and multicore cables are shown in Table 5.

b) Coordination of Current Breaking Devices and Conductors

As a general rule, conductors in isolated or small grids should be designed for the maximum power that can be generated.

Still, overload conditions can be created for instance by load imbalance in a three phase system. Such overload or short-circuit currents must be interrupted before they cause temperature rises harmful to insulations, joints, terminations and surroundings of the conductor. There must be proper coordination between the protective device and the rating of the conductor.

Notes: These factors are appealable to groups of cables all of the same size and equally loaded, including groups bunched in more than one plane. Where spacing between adjacent cables exceeds twice their overall diameter, no reduction factor needs to be applied Example: a group with 10 single core cables protected by a 60 A fuse. The minimum rating for each cable is 60 A / 0.59 = 101.7 A.

Table 5 Correction Factors for Groups of More than Three Single Core Cables or More than One Multicore Cable

As a rule of thumb the following coordination guidelines can be applied:

Table 6 Correction factor for rated currents

It is understood that such design practice might lead to slightly overdimensioned, but safe!, power cables. On the other hand, this practice takes into account tolerances of fuses and other breaking devices that do not necessarily operate at their rated value.

c) Breaking Capacity and Time/Current Characteristics of Current Breaking Devices

Special attention has to be paid to ensure that a protective device is in fact capable of breaking a short circuit current in due time. The designer therefore has to assure that the breaking capacity is higher than the possible maximum current, a value that should be ascertained by the generator manufacturer.

Effectiveness of short circuit protection can be checked by applying the adiabatic equation.


where t = duration of short circuit [s]

A=conductor cross section [mm2]

I = fault current [A]

k = factor for conductors & insulating material {13}

Table 7 k values to be used in the adiabatic equation for short circuits up to 5 second duration (source: Kempe Engineer's Yearbook)

The calculated time t must be equal or greater than the time taken by the protective device to clear the fault. As a basis for such comparisons, exact time/current characteristics of fuses or breakers used are indis

pensable. As an example a characteristic curve for fuses is shown in Fig 1.

Fig 1 Time/current characteristic for fuses - a sample

d) Lightning Protection

In lightning-prone areas, persons and equipment need to be protected against atmospheric overvoltage. The basic measure for lightning protection is the use of lightning arrestors that divert the surge to the ground.

For high tension transmission systems, it is common practice to place a set of suitably sized surge arrestors at the outgoing lines of a power station and further sets every kilometer of a transmission line. For small low tension systems, 500 V arrestors should be installed close to the equipment.

Adequate earthing must be provided for every arrestor (see part 'Earthing').

1.3 Instrumentation

The instruments incorporated in the control panel shall basically enable the operator to monitor simple electrical system parameters like voltage, currents, frequency and to record load and energy generation! supply-curves. Furthermore they should indicate abnormal conditions and facilitate operation (e.g. load prediction and management).

Instruments should be easily readable and ergonomically installed. They should be grouped logically to avoid confusion. Is an instrument part of the control (for instance speed), and any action has to be taken by the operator, she shouldn't have to move at all. All elements of a control system have to be within reach. The degree of instrumentation will mainly depend on the requirements of the owner and on the complexity of the scheme.