DET 308: Power System II (SEM I 2011/2012)

CHAPTER 4: INTRODUCTION TO COMPONENTS OF  Meets standards of safety

TRANSMISSION & DISTRIBUTION SYSTEM IN MALAYSIA  Respect environmental standards
 Figure 1 shows an elementary diagram of a transmission and
Introduction distribution system.
 It consists of two generating stations G1 and G2, a few substations
 The transmission of electrical energy does not usually raise as and interconnecting substation and several commercial, residential,
much interest as does its generation and utilization; consequently, and industrial loads.
sometimes tend to neglect this important subject.  The energy is carried over lines designated EHV, HV, MV, and LV.
 This is unfortunate due to basically human and material resources  This voltage classification is made according to a scale of
are actually involved in transmission are much greater compared to standardized voltages whose nominal values are given in Table 1.
those employed in generation.
 Electrical energy is carried by conductors such as overhead
transmission lines and underground cable.
 Although these conductors appear very common , they possess
important electrical properties that greatly affect the
transmission of electrical energy.
 In this chapter, we will study the components of transmission and
distribution especially in Malaysia.

Principal Components of a Power Distribution System

 In order to provide electrical energy to consumers in usable form, a

transmission and distribution system must satisfy some basic
requirements.
 Thus, the system must: Fig 1: Single-line diagram of a generation, transmission and distribution system
 Provide, at all times, the power that consumers need
 Maintain the stable, nominal voltage that does not vary by more
than +/- 10%
 Maintain a stable frequency that does not vary more than +/-
0.1Hz
 Supply energy at an acceptable price

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

Table 2: Voltage classes as applied to industrial and commercial power

Voltage Class Nominal System Voltage
120/240 V
Low voltage (LV)
1 kV
Medium Voltage (MV) Up to 69 kV
High Voltage (HV) Up to 230 kV
Extra High Voltage (EHV) Up to 765 kV

 Electrical power utilities divide their power distribution systems into

two major categories:
 Transmission system: in which the line voltage is roughly
between 115 kV and 800 kV.
 Distribution system: in which the voltage generally lies
between 120 V and 69 kV. Distribution systems, in turn, are
divided into MV distribution system (2.4 kV to 69 kV) and LV
Fig 2: Overview of the electricity infrastructure (120 V to 600 V).

Table 1: Voltage classes as applied to industrial and commercial power Supply Voltage Options

 Supply may be provided at any of the declared voltages:

 275 kV, 132 kV, 33 kV, 22 kV*, 11 kV, 6.6 kV* and 415/240 V.
 Generally, supplies to domestic premises are given at single
phase 2-wire or three phase 4-wire. However, the actual supply
voltage provided depends on the magnitude of the individual
applicant’s load requirements:
Low Voltage
i. Single-phase, two-wire, 240 V, up to 12 kVA maximum
demand.
ii. Three-phase, four-wire, 415 V, up to 45 kVA maximum
demand.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

iii. Three-phase, four-wire, C.T. metered, 415 V, up to 1000  Copper has very low resistivity and is widely used as a power
kVA maximum demand. conductor, although use as an overhead conductor has become
Medium Voltage & High Voltage rare because copper is heavier and more expensive than
i. Three-phase, three-wire and 11 kV for load of 1000 kVA aluminium. It has significantly lower resistance than aluminium
maximum demand and above. by volume.
ii. Three-phase, three-wire, 22 kV or 33 kV for load of 5000  Steel cored aluminium – due to low tensile strength, aluminium
kVA maximum demand and above. conductors produce greater sag. To increased the tensile
iii. Three-phase, three-wire, 66 kV, 132 kV and 275 kV for strength, the aluminium conductor is reinforced with a core of
exceptionally large load of above 25 MVA maximum galvanized steel wires.
demands.  Electrical energy is carried by conductors such as overhead
transmission lines and underground cable.
Components of HV transmission line  Several variations of aluminium conductors area available:
 AAC – all aluminium conductor
1. Conductors  ACSR – aluminium conductor steel reinforced
2. Insulators  AAAC – all aluminium alloy conductor
3. Supporting structures  ACAR – aluminium conductor, alloy reinforced

A. Conductors Table 3: Overhead vs Underground: Advantages of Each.

Overhead Cable Underground Cable
 A wire is metal drawn or rolled to long lengths, normally Aesthetics – Less visual clutter
Less cost especially initial cost
understood to be a solid wire. Wires may or may not be (mess)
insulated. A conductor is one or more wires suitable for carrying Longer life – 30 to 50 years vs.
More safety – Less chance with
electric current. Often the term wire is used to mean conductor. 20 to 40 for new underground
public contact
 Most conductors are either aluminium, copper and steel cored works.
aluminium. Utilities use aluminium for almost all new overhead Reliability – Faster fault Reliability – Fewer short and long
installations. finding and faster repair duration interruptions.
 Aluminium is lighter and less expensive for a given current- Loading – Readily withstand Lower maintenance cost (no tree
carrying capability. Copper was installed more in the past, so overloads trimming)
significant lengths of copper are still in service on overhead Longer reach – Less voltage drop
circuits. because reactance is lower.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

 Classification of underground cables divided in 2 ways  To increase the leakage path (and hence the leakage resistance),
according to: the insulators are modelled with wave-like folds.
1. Type of insulating material used in their manufacture.  From a mechanical standpoint, they must be strong enough to
2. The voltage for which they are manufactured. withstand the dynamic pull and weight of the conductors.
 There are two main types of insulators: pin-type insulators as
Table 4: Classification of Cables
shown in Figure 4 and suspension-type insulators as in Figure 5.
Low-tension (L.T) cables Up to 1000 V
High-tension (H.T) cables Up to 11 kV
Super tension (S.T) cables From 22 kV to 33 kV Fig 4: Sectional view of a 69 kV pin-
Extra high-tension (E.H.T) cables From 33 kV to 66 kV type insulator. BIL: 270 kV; 60 Hz
Extra super voltage cables Beyond 132 kV flash-over voltage, under wet
conditions: 125 kV.
(Courtesy of Canadian Ohio Brass Co.
Ltd.)

Fig 5: Sectional view of a suspension-

type insulator. Diameter: 254mm: BIL:
125 kV, 60 Hz flash-over voltage,
under wet conditions: 50 kV.
(Courtesy of Canadian Ohio Brass Co.
Fig 3: A concentric neutral cable, typically used for underground residential Ltd.)
power delivery

 The pin-type has several has several porcelain skirts (folds) and
B. Insulators
the conductor is fixed at the top. A steep pin screws into the
insulator so it can be bolted to a support.
 Insulators serve to support and anchor the conductors and to
 For voltages above 70 kV, suspension-type insulators are used,
insulate them from ground. Insulators are usually made of
strung together by their cap and pin metallic parts.
porcelain, but glass and other synthetic insulating materials are
 The number of insulators depends upon voltage: for 110 kV,
also used.
generally use from 4 to 7; for 230 kV, from 13 to 16. Figure 6
 From an electrical standpoint, insulators must offer a high
shows an insulator arrangement for a 735 kV line.
resistance to surface leakage currents and must be sufficiently
 It is composed of 4 strings in parallel of 35 insulators each, to
thick to prevent breakdown under the high voltage stresses they
provide both electrical and mechanical strength.
have to withstand.
Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

C. Supporting structures

 The supporting structure which is called line support must keep

the conductors at a safe height from the ground and at an
Fig 6: Lineman working bare-handed on a adequate distance from each other.
735 kV line. He is wearing a special  Type of line supports:
conductive suit so that his body is not
 Wooden poles
subjected to high differences potential. In
the position shown, his potential with  Steel poles
respect to ground is about 200 kV.  Steel tower
(Courtesy of Hydro-Quebec)  For voltages below 70 kV, we can use single wooden poles
equipped with two cross-arms, but for higher voltages, two
Table 5: Comparisons between the Pin-type and Suspension-type Insulator poles are used to create H-frame.
Pin Type Suspension Type  For HV lines, steel towers are used, made of galvanized angle-
For high voltage, these are iron pieces that are bolted together.
The string is not bulky.
bulky and heavy.  The spacing between conductors must be sufficient to prevent
Replacement work is easy as arc-over under gusty wind conditions. The spacing has to be
Replacement work is difficult. only faulty unit of the string is increased as the distance between towers and as the line
to be replaced. voltages become higher.
Binding wire is used to hold
Binding wire is not used.
the conductor. Table 6: Types of Steel Towers
The string is suspended and Types of Tower Description
free to swing. So string takes A Angle of deviation lies between 0o and 30o
It is not suspended but is fixed.
position of minimum B Angle of deviation lies between 30o and 60o
mechanical stress. C Angle of deviation lies between 60o and 90o
Chances of getting affected by D Angle of deviation lies between 90o
Less affected by lightning.
lightning are more.
 Methods of arrangement of conductors:
a) Single phase circuit:
1. Single circuit
2. Double circuit

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

b) Three phase circuit Lightning Arresters / Surge Arresters

1. Single circuit
2. Double circuit  A lightning arrester is a device used on electrical power systems
 Types of line support in high voltage transmission line: to protect the insulation on the system from the damaging effect
 Wooden pole of lightning.
 Steel tubular pole  The typical lightning arrester also known as surge arrester has a
 Reinforced concrete pole high voltage terminal and a ground terminal.
 When a lightning surge or switching surge travels down the
Ground Wire power system to the arrester, the current from the surge is
diverted around the protected insulation in most cases to earth.
 Ground wire is intended to shield the line and intercept  A lightning arrester and anything connected to it can be
lightning strokes so they do not hit the current carrying dangerous to the touch; during the discharge, a very high
conductors below. voltage can exist between the protective system and ground.
 It is located at the very top of the transmission line tower.  Types of lightning arresters:
 Grounding wire normally does not carry current; consequently,  Rod gap arrester
they often made of steel. They are connected to ground at each  Horn gap arrester
tower.  Multigap arrester
 Expulsion type lightning arrester
Tower Grounding  Valve type lightning arrester

 Transmission line towers are always solidly connected to

ground.
 Great care is taken to ensure that the ground resistance is low.
 In effect, when lightning hits a line, it creates a sudden voltage
rise across the insulators as the lightning current discharges to
ground.
 Such a voltage rise may produce a flash-over across the
insulators and a consequent line outage.

Fig 7: Example arrester applications

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

Fig 10: Flash-over produced by lightning as it flows to ground.

Fig 8: Flow of electric charge along transmission line

General Classifications / Topology of Power Distribution Systems

There are three classifications:

1. Radial system
2. Ring system
3. Network system

A. Radial System

 The simplest of all distribution networks.

 A single substation supplies power to all loads in the system.

Supplementary flashovers
(a) Lightning strike and flashovers (b) Arcing downed conductor

Fig 9: Example of a lightning flash to a 132 kV distribution line and the downed wire
that carry load.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

C. Network System
Load 1 Load 2
 A combination of the radial and ring distribution system.
Although such a system is more complex than either of the
Power lines previous configurations, reliability is improved significantly.
Substation Load 3
 The network system, illustrated in figure below is one of the
most common power distribution configurations.
Power lines
Load 4

Fig 11: Radial power-transmission system. Substation Load 1 Load 2

B. Ring System

 Distribution lines encircle the service area with power being

delivered from one or more sources into substations near the
service area. Load 3 Load 4 Load 5
 Power is then distributed from the substations through the radial
Fig 13: Network power-transmission system.
transmission lines.
Substation
Power lines

 Substations are used throughout an electrical system. Starting

Substation Load 4 with the generation station, a substation raises the MV
generated by the synchronous generators to the high voltage
needed to transmit the energy economically.
 The high transmission line voltage is then reduced in those
substations located to the power consuming centers.
Load 1 Load 2 Load 3  The electrical equipment in such distribution substations is
similar to that found in substations associated with generating
Fig 12: Ring power-transmission system. plant.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

Substation Equipment A. Single busbar without separation

 A MV substation usually contains the following major

components:
 Transformer
 Circuit breakers
 Surge arresters
 Current-limiting reactors
 Horn-gap switches
 Disconnect switches
 Grounding switches
 Instrument transformers
 Relays and protective devices

Arrangement of a Substation

B. Single busbar with sectionalizer

 The arrangement and connection of incoming and outgoing
feeders in grid stations and substations and the number of
busbars have an important influence on the supply reliability of
the power system.
 Grid stations and substations and the topology of the power
system must be designed in a similar way and must therefore be
included in the context of planning as a single task.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

C. H-arrangement Substation Categories

A. Transmission Main Intake (Pencawang Masuk Utama - PMU)

 Transmission Main Intake is the interconnection point of 132

kV or 275 kV to the distribution network. The standard
transmission capacity and voltage transformation provided at
the PMU are follows:
 132/33 kV, 2 × 90 MVA
 132/22 kV, 2 × 60 MVA
 132/11 kV, 2 × 30 MVA

B. Main Distribution Sub-station (Pencawang Pembahagian Utama -

PPU)

 Main distribution Sub-station is normally applicable to 33 kV

D. Double busbar for interconnecting 33 kV networks with 11 kV networks.
 It provides capacity injection into 11 kV network through a
standardized transformation of 33/11 kV.

C. Main Switching Station (Stesen Suis Utama - SSU)

 SSU at 33 kV, 22 kV and 11 kV are established to serve the

following function:
i. To supply a dedicated bulk consumer (33 kV, 22 kV, 11
kV)
ii. To provide bulk capacity injection or transfer from a
PMU/PPU to a load center for further localized
distribution.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

D. Distribution Substation (Pencawang Elektrik – P/E) Circuit Breakers

 Distribution substations are capacity injection points from 11  Circuit breakers are designed to interrupt either normal or short
kV, 22 kV and sometimes 33 kV systems to the low voltage circuit currents.
network (415 V, 240 V).  They behave like a big switch that may be opened or closed by
 Typically capacity ratings are 1000 kVA, 750 kVA, 500 kVA local pushbuttons or by the system telecommunication signals
and 300 kVA. emitted by the system of protection.
 Conventional substation designs are of indoor type (equipment  Thus, circuit breakers will automatically open a circuit
housed in a permanent building) and out-door type (ground- whenever the line current, line voltage, frequency and so on,
mounted or pole-mounted). departs from a present limit.
Standardized M&E design of 11/0.433 kV sub-station is  The most of important types of circuit breakers are:
available at TNB offices. o Oil circuit breakers (OCBs)
 Compact substation (11/0.415 kV) has limited application and is o Air-blast circuit breakers
to be strictly applied in selective situations under the following o SF6 (sulphur hexafluoride) CB
circumstances: o Vacuum CB
o System reinforcement projects for highly built-up areas  Nameplate on the CB usually indicates:
where substation land is difficult to acquire. o The max steady-state current it can carry
o Any request to use compact substation for dedicated o The max interrupting current
supply to a single or limited group of low voltage o The max line voltage
consumers is subject to TNB approval in accordance to o The interrupting time in cycles
site constraints situation and to be considered as ‘special
feature design schemes’. A. Advantages of Oil Circuit Breakers (OCBs)
 The main reasons for the above application criteria are as
follows:  Oil is very good insulator.
o Compact design reduces future system flexibility in  Dielectric strength of oil is very high.
terms of network expansion.  Oil has great heat dissipating property.
o Compact design features, which limits the capacity of  The gases formed due to decomposition of oil during arcing
outgoing circuits. have good cooling properties.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

B. Advantages of Air-blast Circuit Breakers Factors for Selecting Circuit Breakers

 The arc in the air breaker is extinguished early. 1. Efficient (open and close in the shortest possible time under any
 Duration of arc being smaller, the contact points of the breaker network conditions).
is increased. 2. Conduct rated current without exceeding rated design
 Frequent operations of the circuit breaker are not a problem. temperature.
 Less maintenance is required. 3. Withstand, thermally and mechanically, any short circuit
 Less possibility of fire hazard. currents.
 Operates at high speed. 4. Maintain its voltage to earth and across the open contacts under
both clean and polluted conditions.
C. Advantages of SF6 Circuit Breakers 5. Not create any large overvoltage during opening and closing.
6. Be easily maintained.
7. Be not too expensive / economical.
 Dielectric strength of SF6 is higher that air and it is even more
than that of oil used in circuit breaker at high pressure.
Table 7: Differences between Fuse and Circuit Breaker
 Good ability to interrupt low and high fault currents,
magnetizing and capacitive currents Particular Fuse Circuit Breaker
 Breaker has enough overload margins. Performs interruption
It performs both
function only. The
 Maintenance needed for this breaker is minimum. Function detection and
detection of fault is made
 No risk of fire or explosion (gas is non-flammable/chemically interruption function.
by relay system.
stable).
Required elaborate
Inherently completely
Operation equipment (i.e relay) for
D. Advantages of Vacuum Circuit Breakers automatic.
automatic action.
Breaking
 High dielectric strength. Small Very large
Capacity
 Operating mechanism is very simple.
Very small (0.002 sec Comparatively large (0.1
 Arc interruption is very rapid. Operating Time
or so) to 0.2 sec)
 Suitable for repeated operation.
Required replacement No replacement after
 No possibility of explosion. Replacement
after every operation. operation.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

Fuse

Fig 17: Symbols of fuse.

Fig 14: 200 A industrial fuse.
80 kA breaking capacity.

Fig 15: A 115 kV high voltage fuse in a

substation near a hydroelectric power
plant.
Fig 18: Tripping circuit for
a circuit breaker.

Fig 16: Older medium voltage fuse for a

20 kV network.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

Fig 21: Minimum oil circuit breaker installed in a 420 kV, 50 Hz substation. Rated
Fig 19: Cross section of an oil circuit breaker. The diagram shows four of the six current: 2000 A; rupturing capacity: 25 kA; height (less support): 5400 mm; length:
bushings; the heater keeps the oil at a satisfactory temperature during cold weather. 6200mm; 4 circuit-breaking modules in series per circuit breaker.
(Courtesy of Canadian General Electric) (Courtesy of ABB)

Fig 22: Air-blast circuit breaker rated 2000 A at 362 kV. It can interrupt a current of
40 kA in 3 cycles on a 60 Hz system. It consists of 3 identical modules connected in
Fig 20: Three-phase oil circuit breaker rated 1200 A and 115 kV. It can interrupt a series, each rated for a nominal voltage of 121 kV. The compressed-air reservoir can
current of 50 kA in 3 cycles on a 60 Hz system. Other characteristics; height: 3660 be seen at the left. Other characteristics; height: 5640 mm; overall length: 9150 mm;
mm; diameter: 3050 mm; mass: 21 t; BIL: 550 kV. BIL: 1300 kV.
(Courtesy of Canadian General Electric) (Courtesy of General Electric)

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

Fig 23: Cross section of one module of an air-blast circuit breaker. When the circuit
breaker trips, the rod is driven upward, separating the fixed and movable contacts. The
intense arc is immediately blown out by a jet of compressed air coming from the
carifice. The resistor dampens the overvoltages that occur when the breaker opens.
(Courtesy of General Electric) Fig 25: MOV surge arresters protect this EHV transformer.
(Courtesy of General Electric)

Fig 24: Group of 15 totally enclosed SF6 circuit breakers installed in an underground Fig 26: MV busbar feeding eight lines, each protected by a circuit breaker.
substation of a large city. Rated current: 1600 A; rupturing current: 34 kA; normal
operating pressure: 256 kPa (38 psi); pressure during arc extinction: 1250 kPa (180
psi). These SF6 circuit breakers take up only 1/16 of the volume of conventional
circuit breakers having the same interrupting capacity.
(Courtesy of ABB)

Fig 27: Current-limiting reactors reduce the short-circuit current.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

Fig 28: Three 2.2 Ω reactors rated 500 A are connected in series with a 120 kV, 3-
phase, 60 Hz line. They are connected from ground by four insulating, and each is
protected by a surge arrester.
(Courtesy of Hydro-Quebec)

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE
 DET 308: Power System II (SEM I 2011/2012)

References:

[1] Mohammed E. El-Hawary (1995). Electrical Power Systems:

Design and Analysis. IEEE Press, New York.
[2] Previous lecture note, DET308 Power System II, Sem I
2009/2010 prepared by Madam Siti Rafidah binti Abd. Rahim,
UniMAP.
[3] Previous lecture note, MEP1623 Power System Protection, Sem
II 2009/2010 prepared by Prof. Ir. Dr. Abdullah Asuhaimi bin
Mohd. Zin, UTM.

Chapter 4: Introduction to Components of Transmission & Distribution System in Malaysia Prepared by Ms. Melaty binti Amirruddin, PPKSE