
South African Standard SANS 10142 - The Wiring of Premises is designed for electrical and instrumentation personnel, who have prior knowledge of electrical engineering this latest requirements of the standard. This manual is a must have for those working in the residential, commercial, or industrial electrical industry. Each Article of the Code is thoroughly discussed and reviewed in easy-to-understand language. This manual is designed to provide up to date information and training on the latest edition of South African Standard SANS 10142 - 'The Wiring of Premises'. Note: This manual is NOT the standard itself, it is a guide to the implementation of the standard.
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Distribution System Overview
1 Distribution System Overview
This chapter gives brief information on 3-phase electrical systems and why they are used.
Different methods of generation, transmission and distribution of electricity are discussed. Different types of transformer and its connections are illustrated. Use of switching equipment and their different types are given. Circuit breakers for low voltages and for high voltages are discussed. The dangers of electricity and the need for safety in operations and maintenance is covered in detail.
Learning objectives
1.1 Introduction
In distribution systems, three-phase systems are the most common; although for certain special jobs, a greater number of phases is also used. All modern generators are practically three-phase. For transmitting large amount of power, three-phase is invariable used. The reason for this is:
For larger installations all three phases and the neutral are taken to the main distribution panel. From the three-phase main panel, both single and three-phase circuits may lead off.
This delay between ‘phases’ has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor. Figure 1.1 shows 3-phase connections of transmission lines.
Figure 1.1
3-phase connections
1.2 Methods of generation of electricity
At the power station, an electrical generator converts mechanical power into a set of alternating electric currents, one from each electromagnetic coil or winding of the generator.
The popular methods of power generation by conventional methods are:
The alternative methods of generating electrical energy without the use of prime movers are called the non-conventional methods of power generation. For example:
As an example, at a coal-fired power plant in Laughlin, Nevada USA, owners of the plant ceased operations after declining to invest in pollution control equipment to comply with pollution regulations.
1.2.1 Electric generators
An electric generator consists of two main sections – the revolving section called the rotor, which is directly coupled to the steam turbine's drive shaft, and the stator, a series of wire coils, which form a cylinder around the rotor. The rotor, which is really an electro-magnet, revolves at high speed to generate electricity (alternating current) in the stator. A separate static exciter energizes the wire coils of the rotor.
A generator produces electricity. Figure 1.2 shows production of electricity.
Figure 1.2
Production of electricity
1.2.2 How electricity is made
Electricity has traditionally been generated from coal – a fossil fuel. The process to produce electricity from coal comes through stages, which are:
Mining coal
Coal is mined at open cut or underground mines, then crushed, washed and transported to power stations to be stockpiled and used as fuel.
The boiler
The pulverized coal is burned at very high temperature, converting water circulating in the boiler tubes into high-pressure steam.
Steam turbine
The steam produced by the boiler is injected at very high pressure into the turbine, spinning the fan-like blades mounted along the main drive shaft. This shaft continues like an axle from one end of the turbine to the other.
Hydroelectric generation
The principle of electricity generation is same in both hydro and thermal (steam) power stations. In a thermal station high-pressure steam produced drives horizontal turbines; water drives vertical turbines in a hydro station.
The generator produces alternating current (AC), which, after being increased in voltage via a transformer, is passed through a switchyard into the electricity grid.
1.3 Transmission of electricity
Generators output at a voltage that ranges from hundreds of volts to 30,000 volts. At the power station, transformers ‘step-up’ this voltage to one more suitable for transmission.
Transmission is done between the power plant and a substation near a populated area. Transmission normally takes place at high voltage (110 kV or above). Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages such as 66 kV and 33 kV are usually considered sub-transmission voltages but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 230 kV are considered extra high voltage (HV) and require different designs compared to equipment used at lower voltages.The power lost is proportional to the resistance and inversely proportional to the square of voltage.
DC systems require relatively costly conversion equipment which may be economically justified for particular projects. Single phase AC is used only for distribution to end users since it is not usable for large polyphase induction motors.
At the generating plants the energy is produced at a relatively low voltage (LV) of up to 30 kV then stepped up by the power station transformer to a higher voltage (138 kV to 765 kV AC, ± 250-500 kV DC, varying by country) for transmission over long distances to grid exit points (substations).
Transmitting electricity at high voltage (HV) reduces the fraction of energy lost to Joule heating. However, at extremely high voltages, more than 2000 kV between conductor and ground, corona discharge losses are so large that they can offset the lower resistance loss in the line conductors.
Electrical power is always partially lost by transmission. This applies to short distances such as between components on a printed circuit board as well as to cross country high voltage (HV) lines. The major component of power loss is due to ohmic losses in the conductors and is equal to the product of the resistance of the wire and the square of the current:
Ploss = RI2
For a system which delivers a power, P, at unity power factor at a particular voltage, V, the current flowing through the cables is given by
Thus, the power lost in the lines,
Therefore, the power lost is proportional to the resistance and inversely proportional to the square of the voltage. A higher transmission voltage reduces the current and thus the power lost during transmission.
In addition, a low resistance is desirable in the cable. While copper cable could be used, aluminum alloy is preferred due to its much better conductivity to weight ratio making it lighter to support, as well as its lower cost. The aluminum is normally mechanically supported on a steel core.
1.4 Electrical distribution
Electricity distribution is the penultimate stage in the delivery (before retail) of electricity to end users. It is generally considered to include medium-voltage (less than 50 kV) power lines, electrical substations and pole-mounted transformers, low-voltage (less than 1000 V) distribution wiring and sometimes electricity meters.
Distribution networks are typically of two types,
A radial network leaves the station and passes through the network area with no normal connection to any other supply. This is typical of long rural lines with isolated load areas. An interconnected network is generally found in more urban areas and will have multiple connections to other points of supply.
Long feeders experience voltage drop requiring capacitors or voltage regulators to be installed, and the phase physical relationship to be interchanged.
Virtually all public electricity supplies are AC today. Users of large amounts of DC power such as some electric railways, telephone exchanges and industrial processes such as aluminum smelting either operate their own or have adjacent dedicated generating equipment, or use rectifiers to derive DC from the public AC supply.
Figure 1.3 shows the distribution of 3-phase electricity.
Figure 1.3
Distribution of electricity 3-phase connections
1.4.1 Modern distribution systems
The modern distribution system begins as the primary circuit leaves the sub-station and ends as the secondary service enters the customers meter socket. A variety of methods, materials, and equipment are used among the various utility companies , but the end result is similar. First, the energy leaves the sub-station in a primary circuit, usually with all three phases.
The most common type of primary is known as a wye configuration (so named because of the shape of a "Y".) The wye configuration includes 3 phases (represented by the three outer parts of the "Y") and a neutral (represented by the center of the ‘Y’.) The neutral is grounded both at the substation and at every power pole.
The other type of primary configuration is known as delta, this method is older and less common. Delta has only 3 phases and no neutral. In delta there is only a single voltage, between two phases (phase to phase), while in wye there are two voltages, between two phases and between a phase and neutral (phase to neutral). Wye primary is safer because if one phase becomes grounded, i.e. makes connection to the ground through a person, tree, or other object, it should trip out the fused cutout similar to a household circuit breaker tripping. In delta, if a phase makes connection to ground it will continue to function normally.
1.5 Transformers
A transformer is a device that transforms voltage from one level to another. They are widely used in power systems. With the help of transformers, it is possible to transmit power at an economical transmission voltage and to utilize power at an economically effective voltage.
1.5.1 The ideal transformer
The following assumptions are made in the case of an ideal transformer:
1.5.2 Types of transformers
Transformers can be classified in various ways:
Here are a few important points about transformers:
1.5.3 3-phase transformers
Previously, it was common practice to use three single-phase transformers in place of a single 3-phase transformer. However, the consequent evolution of the 3-phase transformer proved space saving and economical as well.
A 3-phase transformer is a combination of three single-phase transformers with three primary and three secondary windings mounted on a core having three legs. Commonly used 3-phases are:
Delta connection
Generally, the Delta 3-wire system is used for an unbalanced load system. The 3-phase voltages remain constant regardless of load imbalance (see Figure 1.4).
Figure 1.4
3- phase transformer delta connection on primary side
The relationship between line and phase voltages is:
VL = Vph
where
VL Line voltage
Vph Phase voltage
The relationship between line and phase currents is:
IL = Ö3 Iph
where
IL Line current
Iph Phase current
3-Phase 4-wire star connections
The star type of construction allows a minimum number of turns per phase (since phase voltage is 1/Ö3 of line voltage), so it is the most economical method. Each winding at one end is connected to a common end, like a neutral point; therefore, on the whole there are four wires.
This connection works satisfactorily only if the load is balanced. With unbalanced load to the neutral, the neutral point will drift causing unequal phase voltages (see Figure 1.5).
Figure 1.5
Three phase 4-wire transformer star connection
The relationship between line and phase voltages is:
VL = Ö3 Vph
where
VL Line voltage
Vph Phase voltage
And the relationship between line and phase currents is:
IL = Iph
where
IL Line current
Iph Phase current
For the output power of a transformer in kW, we use:
where
VL Line voltage
IL Line current
Cos f power factor
Possible combinations of star and delta
The primary and secondary windings of three single-phase transformers or a 3-phase transformer can be connected in the following ways:
Figure 1.6 shows the various types of connections of 3-phase transformers. On the primary side, V is the line voltage and I the line current. The secondary sideline voltages and currents are determined by considering the ratio of the number of turns per phase
(a = N1/N2) and the type of connection.
Table 1.1 gives a quick view of primary line voltages and line currents and secondary phase voltages and currents.
The power delivered by the transformer in the ideal condition irrespective of the type of connection = √3.VL. IL assuming cosf = 1.
Figure 1.6
Types of connections for 3-phase transformers
Table 1.1
View of primary line voltages and line currents and secondary phase voltages and currents
Connection
|
Line Voltage |
Line Current |
Phase Voltage |
Phase Current |
(a) Delta-Delta |
|
|
|
|
Primary Delta |
V |
I |
V |
I / 1.732 |
Secondary Delta |
V/a |
Ia |
V/ a |
Ia / 1.732 |
(b) Delta-Star |
|
|
|
|
Primary Delta |
V |
I |
V |
I / 1.732 |
Secondary Star |
1.732V/a |
Ia / 1.732 |
V/a |
Ia / 1.732 |
(c) Star-Star |
|
|
|
|
Primary Star |
V |
I |
V / 1.732 |
I |
Secondary Star |
V/a |
Ia |
V / 1.732 a |
Ia |
(d) Star-Delta |
|
|
|
|
Primary Star |
V |
I |
V / 1.732 |
I |
Secondary Delta |
V/ 1.732 a |
1.732 Ia |
V / 1.732 a |
Ia |
1.6 Switching
Switching is an operation intended to switch on or off or vary the supply of electrical energy to all or part of an installation for normal operating purposes, for
example a contactor. Table 1.2 shows different switching methods.
Table 1.2
Different switching methods and its purpose
Provision |
Purpose |
For |
Switching off for mechanical maintenance |
To enable non-electrical work to be carried out on switched circuit safely |
Non-electrical skilled persons |
Emergency switching |
To rapidly cut off electrical energy to remove any unexpected hazards |
Anyone |
Functional switching |
To enable proper functioning and control of current using equipment |
Installation user |
1.6.1 Switching Equipment
We will now take a look at some of the switching equipment you may come across.
Elementary switching devices
Figure 1.7
Symbol of disconnector
Figure 1.8
Symbol of load breaking switch
Figure 1.9
Symbol of bistable remote control switch
Figure 1.10
Symbol for contactor
1.7 Circuit breakers
A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage (HV) circuits feeding an entire city.
The circuit breaker is the only item of switchgear capable of simultaneously satisfying all the basic functions necessary in an electrical installation. It can provide a wide range of other functions, for example: indication (on-off - tripped on fault); under voltage tripping; remote control, etc. These features make a circuit breaker the basic unit of switchgear for any electrical installation. Table 1.3 shows circuit breakers functions.
Table 1.3
Circuit breaker function
Function |
Possible condition |
|
Isolation |
|
Y |
Control: |
Functional |
Y |
|
Emergency switching |
Y |
|
Switching off for mechanical maintenance |
Y |
Protection: |
Overload |
Y |
|
Short circuit |
Y |
|
Insulation fault |
(With differential current relay) |
|
Under voltage |
(With undervoltage tripcoil) |
1.7.1 Types of circuit breakers
Just like transformers, circuit breakers too can be classified in various ways.
Depending on operation
Depending on voltage
Many designs of LV circuit-breakers feature a short-circuit current limitation capability, whereby the current is reduced and prevented from reaching its (otherwise) maximum peak value .The current- limitation performance of these CBs is presented in the form of graphs, typified by that shown in Figure 1.11.
Figure 1.11
Performance curves of a typical LV current-limiting circuit breaker
H V breakers can also be classified by the medium used to extinguish the arc:
High voltage (HV) breakers are routinely available up to 765 kV AC.
1.7.2 Selection of a circuit breaker
The choice of a CB is made in terms of:
Figure 1.12
A typical LV current-limiting circuit breaker
1.8 Electrical hazards
Hazards from electrical equipment can be any of the following:
Primary hazards
Secondary Hazards
Table 1.4 shows the safety hazards posed by electrical equipments commonly used in electrical generation and distribution systems and substations.
Table 1.4
Types of equipment and hazards associated with it
Type of Equipment |
Hazards |
Generation equipment |
Electric shock, arc flash, mechanical hazards |
Transformers |
Electric shock, arc flash, fire hazard |
Overhead transmission/distribution lines |
Electric shock, arc flash, fall from heights |
Cables |
Electric shock, arc flash, fire hazard |
Bus ducts |
Electric shock, arc flash, thermal hazard |
Distribution equipment |
Electric shock, arc flash, thermal hazard, fire hazard |
Motive equipment |
Electric shock, arc flash, thermal hazard, mechanical hazards |
Heating equipment |
Electric shock, arc flash, thermal hazard |
Lighting equipment |
Electric shock, arc flash, thermal hazard, fall from heights |
Uninterruptible power supplies with battery |
Electric shock, arc flash, hazards from corrosive liquids and explosive gases |
1.8.1 Hazardous conditions
Direct contact: Contact with exposed current carrying parts
Indirect Contact: Contact with energized conductive parts
1.9 Electrical accidents and safety measures
We will briefly discuss in this section why electrical accidents happen and how we can avoid them. These points will be elaborated in subsequent chapters in further detail. Electrical accidents happen mostly as a result of the following:
Isolation measures and work on/near live equipment
Isolating normally live equipment before starting any work on it can improve safety substantially in any system. We must however bear in mind that there are certain kinds of equipment where live work is possible and certain kinds of activities where work in the vicinity of exposed live parts is unavoidable. But such work must be carried out according to well laid safety procedures.
Eliminate faults to improve safety
The other major cause of accidents is faulty equipment (which can include both poorly designed or improperly operating equipment). Unless safety is built into the design of the equipment, it can result in accidents and injury. Similarly, improperly maintained equipment too can result in failures and thereby cause accidents.
Improved knowledge level
Operating personnel with insufficient knowledge, lack of familiarity with equipment and system can also result in unsafe situations. Absence of proper operational safety procedures and violations of existing procedures can both result in accidents.
1.9.1 Safety measures
The following are some general safety measures, which should be adopted to reduce the possibility of accidents in electrical equipment.
Technical measures
Accident prevention measures
Organizational measures
We will discuss these measures in detail in the ensuing chapters.
1.10 Periodic inspection and maintenance
The objective of periodic inspection and maintenance is to determine whether an installation is in a satisfactory condition for continued service. Periodic inspection should comprise careful scrutiny of the installation without dismantling or with partial dismantling as per the scope decided by a competent person based on availability of records and the condition of the installation. Inspection will generally be along the lines followed for initial verification. The following aspects need to be carefully examined:
The person carrying out the work should give a Periodic Inspection report together with the schedule of inspection and the schedule of tests to the person ordering inspection. The record of defects/damage/non-compliance with regulations, etc. should be included in this report. The person carrying out the inspection will record the recommendation regarding the next appropriate date of inspection.
Circumstances, which require a periodic inspection and test can include:
General areas of inspection should be:
Safety measures for inspection:
Follow up measures
The defects revealed by periodic inspection reports should be attended to without delay to avoid unsafe situations. Apart from defect resolution, the following actions are also needed:
While planned preventive maintenance is done according to a fixed schedule using a recommended list of maintenance works, condition based maintenance is pro-active and relies on early warning of problems. Even though this practice is well established in specific segments of mechanical machinery (such as vibration signature analysis in high speed machines), the applications in the electrical field are gradually becoming popular. The main benefit is need-based maintenance and preventing major unforeseen failures, both of which have major cost implications.
1.11 Summary
In 3-phase distribution systems, three circuit conductors carry three alternating currents (of the same frequency). This is a common method of electric power transmission. This system uses less conductor material to transmit electric power.
There are different methods available for power generation. Conventional methods are hydraulic, steam and diesel generation; non-conventional methods are nuclear, solar and wind power. Electric power is normally generated at 11-25kV in a power station.
A transformer is a device that transforms voltage from one level to another. With the help of transformers, it is possible to transmit power at an economical transmission voltage and to utilize power at an economically effective voltage. There are different types of transformer available depending on cooling type, construction and application.
Circuit breakers can provide a wide range of other functions, for example: indication (on-off - tripped on fault); under voltage tripping; and remote control. There are low voltage (LV) circuit breakers and high voltage (HV) circuit breakers available.
Electrical system hazards can be classified as primary i.e. electric shock and burns, and secondary hazards i.e. arc flash or internal organ damage. Safety precautions need to be taken whilst working on electrical systems. Electrical systems should be inspected periodically to avoid accidents. The objective of periodic inspection and maintenance is to determine whether an installation is in a satisfactory condition for continued service.
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