ELEC-1104

Lecture 5:Lecture 5:

T i i dTransmission and Di t ib tiDistribution

Power System Layouty y

Transmission and Distribution

The transmission system is to transmit aThe transmission system is to transmit a large amount of energy from the power stations to the load centres.stat o s to t e oad ce t es.

Th di t ib ti t i t t kThe distribution system is to take energy from the transmission network and supply it t th dto the end consumers.

Transmission and Distribution

To transmit a given power, the higher theTo transmit a given power, the higher the voltage, the smaller is the current, and hence lower is the resistance loss.e ce owe s t e es sta ce oss.

Power ∞ Voltage × CurrentLoss = current2×ResistanceLoss = current2 ×Resistance

To raise the voltage, AC transformer is i d H th t i i drequired. Hence the transmission and

distribution system normally operates in AC.

Three phase transmissionp

Advantages of three phase transmission:Advantages of three phase transmission:Only 3 conductors are required to supply a 3-phase line hence better transmission3-phase line, hence better transmission efficiency.Th t t l 3 h i t t thThe total 3-phase power is constant rather than pulsating, hence smoother operation.For a machine of given size, higher rating can be obtained with 3-phase design.

Transmission Systemy

The transmission network can take onThe transmission network can take on different forms, it’s a compromise between reliability and cost.e ab ty a d cost.

The characteristic of the transmission network is that direction of energy flow is not fixed but depend on generation and load conditions.

Transmission Systemy

Transmission refers to the highest voltageTransmission refers to the highest voltage levels in the system network and could vary from 132 kV (66 kV in HK) upward.o 3 V (66 V ) upwa d.» UHV – above 750 kV» EHV – 400 to 750 kV» EHV 400 to 750 kV

The higher level transmission network isThe higher level transmission network is sometimes refer to as the grid.

Today’s Transmission Systemy y

Power plants may be far from the load (e.g. p y ( gto avoid fuel transportation, building power plants near the coal mines).

To enhance reliability of power supply, network structure is adopted for the ptransmission system.

As the transmission capacity is increasedAs the transmission capacity is increased (in GW order), the transmission voltage is getting higher and higher.getting higher and higher.

Today’s Transmission Systemy y

Standard voltages are used in each country, hi h l d d d d i fwhich leads to standard design of

equipment (transformers, generators, motors circuit breakers etc ) beneficial tomotors, circuit breakers etc.) beneficial to investment, operation & maintenance.

Higher voltage networks are built on top of existing networks resulting in layers ofexisting networks, resulting in layers of transmission and sub-transmission systems.

Representation of a electric power system:One-line diagramOne-line diagram

One-line diagram-- for system diagram, and show parameters, connections.

generator

transformer

impedanceimpedance L Cline

bus (node)circuit breaker

grounding

load

grounding

Transmission lines

Distribution Systemy

Unlike transmission, the energy flow inUnlike transmission, the energy flow in distribution is always unidirectional.The sub-transmission system can be simplyThe sub-transmission system can be simply regarded as a high voltage level distribution.Th di t ib ti t b di id d i tThe distribution system can be divided into:» Primary distribution (1.1~6.6 kV, 3-phase)» Secondary distribution (380/220V)

Distribution

Domestic and small consumers normallyDomestic and small consumers normally received their supply from the service mains of secondary distribution system.o seco da y d st but o syste .Large consumers may receive their 3-phase supply directly from primary distribution atsupply directly from primary distribution at a higher voltage.V l i th iVery large consumers may receive their supply from the sub-transmission system.

Distribution

The primary distribution system receives itsThe primary distribution system receives its supply from the distribution substations and branched out as 3-phase feeders.b a c ed out as 3 p ase eede s.The feeders then branched out as sub-feeders and single-phase lateralsfeeders and single-phase laterals.The laterals supply the service mains of the

d di t ib ti t k th hsecondary distribution network through distribution transformers.

Radial Distribution Systemy

distributordistributorDistribution transformer

Sub-feederfeeder

Substation

To consumers

lateral

Ring Distribution Systemg y

Distribution transformer

feeder

Substationdistributor

To consumersTo consumers

Distribution systemy

D.C. vs A.C. in Utilization

Most appliances can work equally well onMost appliances can work equally well on A.C. or D.C.Electronic equipment work on low voltageElectronic equipment work on low voltage D.C. (voltage reduction then rectification)A C t ffi i t th D CA.C. motors are more efficient than D.C. motors.Lighting at low frequency A.C. would flicker.

A.C. Transmission

Average powerAverage power

factorpowerIVPower ×= ˆˆ21

A.C. generators are more efficient and can 2

be designed for a higher generating voltage.Voltage drop caused by line inductance.Charging current to line capacitance reduces the useful current carrying capacity of lines.y g p y

A.C. Transmission

Voltage level can be changed easily by theVoltage level can be changed easily by the use of transformers.Current value passes through zero twice aCurrent value passes through zero twice a cycle facilitates current interruptionA C t t t i h iA.C. system must operate in synchronism, and hence can easily run into stability

blproblems.

D.C. Transmission

Average PowerAverage PowerPdc = VdcIdc

N t ff t d b li i d tNot affected by line inductance nor capacitanceNo easy voltage transformationNo easy way to cut off D.C. current

D.C. vs A.C. in Transmission

D.C. transmission is more efficient.Operationally D.C. is not affected by line capacitance and inductance.pD.C. has no stability problem.A C system is preferred mainly because ofA.C. system is preferred mainly because of the readiness in transforming the system voltage from one level to another.gThe difficulty in the cut off of D.C. fault current also works against D.C.g

H.V.D.C. Transmission

To transmit large amount of power at highTo transmit large amount of power at high voltage, advantage of D.C. is significant.To integrate D C transmission with the A CTo integrate D.C. transmission with the A.C. system, expensive converter stations must be installed at both ends of the D C linebe installed at both ends of the D.C. line.Becomes economically viable as distance of t i i itransmission increases.

HVDC Systemy

H.V.D.C. Applicationspp

To link up two A.C. system operating atTo link up two A.C. system operating at different frequencies.As a asynchronous link within an A CAs a asynchronous link within an A.C. system to improve stability.F b i blFor submarine cables.For transmission of large amount of power over a long distance.

System Voltagey g

To transmit a given power, the higher theTo transmit a given power, the higher the voltage, the smaller is the current, and hence lower is the resistance loss.e ce owe s t e es sta ce oss.

Power ∞ Voltage × CurrentLoss = current2×ResistanceLoss = current2 ×Resistance

Hence for same loss,2acceptable resistance ∞ Voltage2

i.e. 2voltage1

cetanresis1sizeconductor ∝∝

voltagecetanresis

System Voltagey g

On the other hand, there are costs thatOn the other hand, there are costs that increases with system voltage.

» Insulation costs» Transformer costss o e cos s» Supporting structure costs

Ci it b k d it h t t» Circuit breaker and switch costs etc.

System Voltagey g

Hence corresponding to the amount ofHence corresponding to the amount of power to be handled, there is a certain optimum system voltage.opt u syste vo tage.Thus the optimum transmission voltage for CLP is higher than HEC because the loadCLP is higher than HEC because the load capacity of CLP is greater than HEC.A d th di t ib ti l l thAs we go down the distribution level, the amount of power to be handled reduces, hence the oltage le el also red ceshence the voltage level also reduces.

System Frequencyy q y

Electromagnetic machines (generators,Electromagnetic machines (generators, motors, transformers)

voltage ∝ frequency × fluxvoltage ∝ frequency × fluxFor same system voltage,

higher frequency → lower flux→ smaller core size

→ lower cost

System Frequencyy q y

Transmission linesThere is a component in voltage drop due to line inductance

ΔV = (2πf×L)IThere is also a charging current supplied to line capacitanceline capacitance

ΔI = (2πf×C)V

A l f ill d h lA lower frequency will reduce the voltage drop along the line and reduce the charging current to charge up the line capacitancecurrent to charge up the line capacitance.

System Frequencyy q y

Lamp outputs varies at twice the supplyLamp outputs varies at twice the supply frequency. Too low a frequency can cause lamp flickering.a p c e g.Motor speed varies directly with frequency. For constant torque output ∞ frequencyFor constant torque, output ∞ frequency.Two main frequency employed:

50 Hz – Hong Kong, China, Europe60 Hz – USA, Canada.

Power system network in Hong Kongy g g

The power system network in Hong KongThe power system network in Hong Kong operates in» 3-phase A C at 50 Hz» 3 phase A.C. at 50 Hz

The highest transmission voltage is» 400 kV for CLP» 400 kV for CLP» 275 kV for HEC.

Th l iThe consumer voltage is» 380V/220 V line/phase voltage

Overhead Transmission

Conductors are bare conductors with no insulation (insulated by air).Conductors are supported from supporting pp pp gstructures (steel towers) by insulator strings.There may be one 3-phase circuit (single y p ( gcircuit line) or two 3-phase circuits (double circuit line) supported by the same tower.Earth wire to protect line from lightning strike.

Overhead Transmission

Corona dischargegCorona is the ionization of air surrounding line conductors due to high voltage stress.line conductors due to high voltage stress.Its occurence depends on:» System voltage» System voltage» Conductor size

C d t f diti» Conductor surface condition» Atmospheric condition

Overhead Transmission

Corona dischargeCorona dischargeCorona is the ionization of air surrounding line conductors due to high voltage stressline conductors due to high voltage stress and would result in corona loss, radio interference as well as noise pollutioninterference as well as noise pollution.Concern of EM field on health hazards.Sight of supporting towers

Underground Transmissiong

Energy is transmitted over insulated cablesEnergy is transmitted over insulated cables normally buried underground.At transmission voltage cables are muchAt transmission voltage, cables are much more expensive than overhead lines.C l d i l tiCommonly used insulations:

Oil-filled paperCross-linked polyethylene (XLPE)SF6 Gas insulated ducts6

Underground Transmissiong

The conductors in underground cables areThe conductors in underground cables are much closer together and hence the cable capacitance is much higher. capac ta ce s uc g e .Charging current to cable capacitance is a major operation concernmajor operation concern.Systems with many cables may have

i ti ti d iexcessive reactive power generation during light load periods.