Unit 67 Further Electrical Power

Unit 67:

Further Electrical Power

Unit code:

QCF level:

Credit value:


The aim of this unit is to extend learners’ understanding of the distribution of electrical power and help them to meet the energy deployment needs of the future.

Unit abstract

Energy, either from traditional fossil fuels or sustainable alternative energy sources, needs to be converted into an appropriate format to allow for efficient reliable transmission and distribution to the various users, at acceptable quantities, to meet their requirements.

The dissemination of electrical energy is a problem of ever-growing complexity as our dependency grows on its use and consistency of availability. Our communications, transport and commercial operational systems, to name but a few, would all come to an abrupt halt, should it fail to deliver. Historically, ‘heavy current’ engineers have focused broadly on thermal/current, voltage and system operation constraints. Now with environmental concerns increasing, aesthetic issues, maximising use of existing systems through upgrades, preventive and fault management and reduction of energy loss throughout the transmission and distribution system all need to be taken into account.

This unit develops an understanding of transmission and distribution topics and focuses on the use of overhead lines and cables within power systems. The origin and propagation of surges and transients are analysed. The subject matter of power system faults is, for simplicity, limited to analysing symmetrical faults and logically relates to aspects of power system protection schemes. The synchronisation, operation and use of synchronous machines are also investigated.

Learning outcomes

On successful completion of this unit a learner will:

1     Understand the construction and properties of overhead lines and cables

2     Understand symmetrical faults and protection schemes

3       Be able to analyse power system transients

4       Understand the synchronising and control of synchronous machines.

Unit content

1      Understand the construction and properties of overhead lines and cables

Construction and properties: tower; post; ASCR conductor; disc and pin insulators; string efficiency; grading rings; arcing horns; corona; bundled conductors; dampers; transposition; span and sag

Cable types: construction of single core and three core cables; properties eg capacitance, dielectric, voltage rating, electric stress, thermal resistance, losses, heating and cooling, belted type, screens, superconducting cables; comparison of different types used in power systems

Fault location: description of methods used in cables and lines eg resistance bridges, time domain reflectometry, tracing methods, energy discharge, thermal imaging

Performance evaluation: short and medium length eg series impedance, ‘T’ and ‘π’ models, voltage drop, current drop, power losses, Ferranti effect

2      Understand symmetrical faults and protection schemes

Components: current transformers eg burden, open circuit operation; over-current relays eg induction disc, thermal, solid state; Buchholz relay; circuit breakers eg air blast, oil, vacuum; fuses

Fault analysis: symmetrical faults in three phase systems eg fault limiting reactors, ring and tie bar reactors, per unit values, fault level, fault current, simulation of faults

Protection schemes: type eg unit protection, time graded over-current, distance protection, transformer protection, feeder protection, motor protection; properties eg co-ordination, discrimination; testing eg CT polarity, CT knee-voltage, CT magnetising characteristic, commissioning, primary and secondary injection

3      Be able to analyse power system transients

Surges: origin eg lightning and switching operations; propagation and effects of surges eg surge impedance, surge velocity, basic impulse level (BIL); voltage and current surges; reflection coefficient; propagation and reflection of surges at junctions of lines and cables; use of Bewley lattice diagram to analyse multiple reflections; circuit breaker transients

Surge control: description of methods and components eg surge diverter, rod gap, expulsion tube

4      Understand the synchronising and control of synchronous machines

Synchronising: requirements on the ‘running and incoming’ voltages eg magnitude, frequency, phase, phase sequence; synchronising methods eg three lamp methods, synchroscope

Control of synchronous machines: methods of operational control eg voltage, frequency, power, power factor, infinite bus-bars, V-curves

Learning outcomes and assessment criteria

Learning outcomes
Assessment criteria for pass

On successful completion of
The learner can:

this unit a learner will:

LO1 Understand the construction

explain the construction and properties of an overhead

and properties of overhead


lines and cables

compare different types of cable used in power systems

explain methods of fault location

use T and π models to evaluate performance

LO2 Understand symmetrical

explain the function of the components in a protection

faults and protection



use one-line diagrams to solve fault analysis problems

solve problems involving the use of fault-limiting


analyse the protection scheme used in a given system

LO3 Be able to analyse power

analyse the propagation of surges

system transients

use a Bewley lattice diagram to analyse multiple


explain how surges occur and compare two methods

used for surge control

LO4 Understand the

analyse and compare two methods of synchronising

synchronising and control of

explain how the control of voltage, frequency and power

synchronous machines

factor of a synchronous machine can be achieved.


This unit may be linked with Unit 1: Analytical Methods for Engineers, Unit 5: Electrical and Electronic Principles and Unit 63: Electrical Power.

Essential requirements

Sufficient laboratory and test equipment will need to be available to support a range of practical investigations (eg protection relays, current transformers, synchronous machine, synchroscope, etc).

Employer engagement and vocational contexts

Delivery of this unit will benefit from centres establishing strong links with employers willing to contribute to the delivery of teaching, work-based placements and/or detailed case study materials.