reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

The maximum starting current

1ST = 1.2 x 'FL X ratio x x cos0

=1.2 x 180 x 5.4 x 0.962 x 0.91

=1021 A

(a)Continuous operation The current rating of a single-core 300 mm 2 cable laid in air is given in Appendix C as 675 A at 25 ° C.

At 35 ° C the current rating

=675 x 0.91 (from Appendix E)

=614A

(b) Fault conditions The conductor temperature during normal full-load operation is given by:

°FL = 0A + (0),4 - OA) x (IFL/Ic)2

substituting = 35 -4-(90 - 35) x (180/614)2

Vs = 47rft 10 -7 1og (s/r) V/m

Taking the armour radius for a 300 mm 2 cable as 18 mm and

substituting V s = 4 X 7r. x 50 x 180 x 10 -7 x log r, (80/18)

= 0.017 V/m

The magnitude of sheath voltage on the outer cables is given by:

1 Vs = 2r fl 10 -7 [(2 log, (V2 x s/r) 2 +

1.44] 2

= 2 x r x 50 x 180 x 10 -7 x

[ (2 log, Oh x 80/18)) 2 + l.44]2

As the cable length is 100 m, the standing voltage on the centre cable is 1.7 V and on the two outer cables 2.2 V, and is therefore satisfactory.

During a short-circuit fault the sheath voltage is given by:

Vsc = V s x

substituting for outer cable Vsc

= 2.2 x 39400/180

— 482 V

The conductor temperature after two hot starts is given by:

(27 + 228)] e 2ST = 29 ° C

The short-circuit sheath voltage is less than 2 kV and is therefore satisfactory.

In summary, one 300 mm 2 single-core cable per phase is required.

3.3 kV motor

A 3.3 kV pump motor has a rating of 300 kW. The supply is taken from a 3.3 kV FSD with a 400 A fuse. The motor manufacturer has quoted a starting time of 15 seconds. The cable is to be laid in air at an ambient temperature of 25 ° C and a route length of 110 m. Backup protection is provided by the fuse in the zone protected by the high set instantaneous overcurrent relay. Determine the size of cable required.

From Table 6.18, t = 0.945 and cos (/) = 0.91 and from Table 6.17 the motor starting ratio = 5.8. The full load current is given by:

kW output x 1000

'FL — ▪ x VL x x cost)

300 x 1000

▪x 3300 x 0.945 x 0.91

'FL = 61 A

The maximum starting current

= 1.2 x FL x ratio x x cos 43

1.2x 61 x 5.8 x 0.945 x 0.91

=365 A

(a) Continuous operation The smallest three-core 3.3 kV cable in the standard range is 150 mm 2 (because of short-circuit requirements) and it can be seen from Appendix C that this has a rating of 265 A in air at 25 ° C.

(b)Fault condition The conductor temperature during full-load operation is obtained from:

0 FL = OA + (OM — OA) (I FL /iC) 2

25 + (70 — 25) (61/265) 2 =27 ° C

The 1 2 t adiabatic line for a short-circuit temperature rise from 29 ° C to 160 ° C is now determined from:

I 2 t = K 2 S 2 log, [(Or + 3)/(Ot + 3)1

---(148) 2 x (150) 2 log, [(160 + 228)/

(29 + 228)]

= 203 x 106

The short-circuit I 2 t adiabatic line is superimposed on the fuse time versus current characteristic as shown in Fig 6.36. For the purposes of this example it is assumed that the cable is protected.

(c) Voltage regulation The motor starting voltage regulation is obtained first. The conductor resistance at the end of two hot starts temperature from

(b) is 29 ° C. From Appendix B, for a 150 mm 2 cable at 20 ° C the conductor resistance is 206 /.4.11/m and the equivalent star reactance is 80 giI/m. From the manufacturer, power factor cogb on starting is 0.5.

Conductor resistance RL

= Rc20 [1 + a20(02ST — 20)]

= 206 [1 + 0.00403(29 — 20)]

= 213 itf2/m

G/o R = 100 IL (RL cosy& + XL sin) .s./3/ 14.

=100 x 365 x 110 (213 x 0.5 + 80 x 0.87) 10_6 x V3/3300

=0.37%

This regulation is small compared with the 20% allowed at motor terminals during starting and therefore is acceptable. Because of the small value of regulation during starting, in this case there is no need to check the regulation under full load conditions.

In summary a three-core 150 mm 2 cable is required.

415 V motor

A 3 kW 415 V pump motor fed from a 415 V contactor starter has a cable route length of 80 m. The cable is

The motor starting voltage regulation is calculated using the conductor temperature after two hot starts which from (b) is 51 ° C. Correcting the conductor resistance for this temperature gives RL = 8313 A/m. From Table 6.20 the motor power factor during starting, cos 0= 0.28. Now:

(b)Digital signals defined as voltages or current whi ch are normally at one voltage or another with a relatively rapid change between states. Examples are plant orientated alarm signals, sequence control input and output signals (e.g., 0-48 V). They also include switched 110 V DC and 110 V AC circuits.

These maximum lengths for voltage regulation are both considerably in excess of the actual route length of 80 m.

In conclusion, a 3-core, 2.5 mm 2 cable is required to supply this 3 kW motor.

In this example, the limiting factor for route length is the requirement to ensure sufficient current flows

during an earth fault to operate the fuse. As discussed under 415 V fuse/contactor in Section 4.6.2 of this

chapter, the accurate but more time-consuming method for obtaining the minimum earth fault current is to construct the cable I 2 t adiabatic line (hot) on the relevant fuse characteristic. This will invariably give a longer route length and may be worthwhile determining in instances where this requirement dictates the conductor size.

5 Control and instrumentation cable systems

This section deals with the cabling systems that are necessary for the following functions:

•Control and instrumentation.

•Protection, intertrips and interlocks.

•Metering.

•Telecommunications.

•Alarms.

•Computers and data logging equipment.

5.1Signal levels

The types of signal being considered can be broadly split into two classes:

(a)Analogue signals consisting of voltages that vary relatively slowly and currents such as those present in transmitter outputs (e.g., 4-20 mA), ther-

mocouple outputs (e.g., 0-40 mV) and position indicating potentiometers (e.g., 0-10 V). They also include current transformer (CT) and voltage transformer (VT) circuits for instrumentation.

(a)Multipair control cables (as described in Section 3.6 of this chapter) which are suitable for use at voltages up to 110 V AC or 150 V DC. How-

ever, these cables should not be used for circuits which contain unsuppressed 110 V AC contactor or relay coils of such a rating that they are likely to give rise to switching transients that are in excess of the 2 kV test voltage. These cables have a cross-sectional area of 0.5 mm 2 and it is recommended that the maximum current in any conductor be limited to I A, and that no more than 40% of the pairs be loaded with this current at one time.

(b)Multicore control cables (as described in Section 3.5 of this chapter) which are rated at 600/1000 V and have a conductor cross-sectional area of 2.5

mm 2 . These cables are used where the circuit voltage (continuous or transient) or circuit current is in excess of the capabilities of multipair cables. However, these cables are more expensive and also more prone to interference (see Section 5.3 of this chapter) and should not be used unless essential.

(c)Special cables such as coaxial, triaxial and low loss individually screened pair cables are necessary for particular applications. The performance of such special cables is normally prescribed by the plant contractor involved and they are frequently provided by him as free issue for the cable contractor to install. One problem with many of these special cables is that they are normally not armoured and are therefore unable to withstand the rigours of installation and service in a power station environment; so, in many cases, it is necessary to provide mechanical protection by conduit or trunking.

5.3 Cable interference

This section is intended to give an insight into basic interference theory which will be found useful in understanding why it is important to give careful consideration to signal types and the type of cable to which they are allocated. Table 6.21 contains the abbreviations, descriptions and units that will be used throughout the following interference theory.

TABLE 6.21

Descriptions and abbreviations used in interference theory

CfpNOLIC TOR 15

CARRYING AMP

ATO PLANE OF PAGE;

FiG. 6.39 Magnetic field around an isolated single conductor

5.3.1 Interference in multipair cables

Bay current carrying conductor produces a magnetic field and an electric field. If we consider the hypo- thetical case of an isolated single conductor, see Fig 6.39, it will be surrounded by a magnetic field inch- ated by the dotted lines and an electric field indicated by the solid lines. The spacing of the lines gives an indication of the relative strength of the field, where

(a)Magnetic flux density at a point P, r metres from the centre of the conductor is

2-rr

(b) Electric field intensity

e2rr

These two expressions show how B and E decrease as r increases.

If we now consider a balanced pair of conductors, i.e., carrying

in the same way expressions can be stated which give the magnetic flux density and electric field strength.

It can be shown that the magnetic flux density and the electric field strength at any point P vary as the inverse of the square of the distance between P and the cable system centre. In fact the field strengths at P1 and Pi will not be identical when ri and r2 are equal because of the geometry of the arrangement, but providing that r is considerably larger than d (conductor diameter), then this approximation may be made. In our calculations we will be able to assume, generally, that r is greater than d and so the simplification will be invoked:

(a)Magnetic flux density at a point P, r metres from the centre of the twin system.

=Ald

Note the two implications of these formulae; firstly the flux density and field intensity vary inversely with the square of the distance from the system (and not directly as the inverse of the distance as in the isolated conductor case), and secondly that the smaller d is made the smaller the flux density/field intensity at a given point becomes.

Before we turn from looking at the magnetic/electric field produced by current carrying conductors to the interference induced in other conductors by those fields, there is one further point to consider.

The last example considered long, straight, parallel conductors. If we twist those conductors to form a twisted-pair then there is a cancellation of the magnetic/electric fields which is most easily explained by the diagram shown in Fig 6.41.

Field at P due to A is cancelled by that due to B C is cancelled by that due to D E is cancelled by that due to F,

etc.

Assuming the twisted cable to be made up of very short, straight lengths joined by transpositions it can be shown that (for short enough parallel lengths) the magnetic and electric fields, at a point P outside the cable will each be cancelled.

Since we are dealing with a twisted pair, there ar e many small geometrical considerations which will affect the amount of cancellation and thus there residual fields. These considerations include conductor geometry, twinning lay and spacing variation. usual to apply a reduction factor of about 10 for the case of a normal twisted-pair cable compared with a straight parallel-pair.

We shall now consider the way in which the magnetic field produced by (a) a single conductor and (b) a pair of conductors, affects another pair of conductors in their vicinity. In both cases straight parallel conductors are considered initially, the reduction factor above being applied at the end of the calculation.

In this situation we are primarily concerned with the magnetic coupling between the two circuits since we are working at normal power frequencies. Electrostatic coupling becomes more important as frequency rises (l0 4 kHz and above), but at mains frequency magnetic coupling dominates.

Referring to Fig 6.42, the induced voltage on a pair is a function of the magnetic field cutting them which in turn depends on the separation of the pair, their mutual separation from the current carrying conductor and other geometric considerations. Mathematically the amount of flux linking the cables may be found in several ways. To do this exactly is a relatively complicated procedure, but an approximation may be made by finding the average flux at the centre of the pair and assuming this to be constant over the area between them (safe assumption if d is small). To find the total flux cutting the pair per metre, we must then multiply this average by (2d x 1) square metres which is the area enclosed by a 1 metre length of the pair.

The voltage induced on the pair is then proportional to this flux and its rate of change, i.e., frequency. The induced voltage per metre is given by:

which simplifies to

V— 2Adfl

(6.6)

The basic implications of this are that as we make d smaller and/or R larger the induced voltage drops.

he pair cable is a twisted pair then a reduction I f t or of about 10 may be applied to this induced

fact

wItage as mentioned previously.

Following through a similar logic for a pair cable

where d = 7 x 10 m R = 6 x 10 -1 m

f = 50 Hz

I = 840 A

2 x 4r X 10 -7 X 7 x 10 -4 X 50 X 840

Thus V -

6x 10'

FIG. 6.43 One pair relative to another pair

this means is that the induced voltage now drops off much more quickly as d is reduced and also as R is increased. Note that in both Equations

(6.6) and (6.7) a frequency term appears on the top li ne indicating that induced voltage is directly propor-

tional to frequency. This is why transient phenomena (basically high frequency harmonics) may give rise to

interference.

If both the pair cables are twisted-pairs then a reduction factor of (10 x 10) may be applied.

The type of interference that we have calculated here is manifested as a potential difference between the cores, i.e., a voltage source connected in series ith the cable. Thus this type of interference is known mode voltage (or transverse mode voltage). It from the previous equations that both suffer a rise in potential above the local earth

potential, again dependent upon the strength of the interfering field in which the cores are situated and this is known as common mode voltage (or longitudinal oltage).

In general, the equipment at the end of the cable be examining the potential difference/current/ between the two cores and thus will sensitive to series mode interference. Common interference does not pose such a problem unless

of very large magnitude, when special isolation methods must be employed. In this examination we are concerned mainly with series mode voltages.

Two examples of interference calculations for paired

,

:ables will now be given:

(a)A twisted pair suffering interference from a single Power cable carrying 840 A, situated 600 mm away

(effects of armour, other cables and supporting steel work are ignored).

= 1.23 x 10 -4 V/m

Allowing a reduction factor of 10 since the pair is twisted,

Induced voltage = 12.3 AV/M

Note: d is calculated on the basis of a 1/0.8 mm diameter conductor with an insulation radial thickness of 0.3 mm

(b)A twisted pair suffering interference from (i) a single core, 25 mm away in the same cable and (ii), a twisted pair carrying balanced current also 25 mm away. In both cases the interfering conductor(s) is (are) carrying 1 A.

4 X 47 x 10 -7 X (7 x 10 -4 ) 2 x 50 x 1

V=

(2.5 x 10 -2 ) 2 - (7 X 10 -4 ) 2

V = 1.9 x 10 -7 V/m

Allowing a reduction factor of 100 since both pairs are twisted:

V = 0.0019 AV /m

(ii) represents an improvement of about 45 dB over (i), illustrating the importance of maintaining balanced pair working.

5.3.2 Interference in multicore cables

The basic theory given in the previous section for multipair cables is equally relevant to rnulticore cables with the obvious exception that no factors have to be taken into account for twisting.

Two examples of interference calculation will now be given:

(a)Two adjacent cores suffering interferences from a single power cable carrying 840 A, situated 600 mm away (effects of armour, other cables and cable supporting steelwork are ignored).

where d = 1.7 x 10 -3 m r = 6 x 10 -1 m

f = 50 Hz

I = 840 A

2 x 41- x 10 -7 x 1.7 x 10 -3 x 50 x 840

Thus V

6 x 10 -1

V = 2.99 X 10 -4

Induced voltage = 299 ILV/M

Note: d is calculated on the basis of 7/0.67 mm stranded conductor with an insulation radial thickness of 0.7 mm.

(b)Two widely spread cores within a 37-core cable suffering interference from a single power cable situated 600 mm away (effects of armour, other cables and cable supporting steelwork are ignored).

where d = 10.2 x 10 - 3 m R = 6 x 10 -1 m

f = 50 Hz

1 = 840A

Induced voltage = 1795 AV/M

Note: d is calculated for diametrically opposite cores in the outer layer of a 37-core cable, with conductor and insulation parameters as in the previous example.

5.3.3Circuit considerations

From the previous sections it can be seen that multi- core cables can be up to 150 times more susceptible

to interference than multipair cables under the con. ditions considered. The actual level of interference within multicore cables is dependent on the phys ical location of the cores which are used to complete the

electrical circuit. If the cores used are widely spaced within the cable, then the level of interference will be

greater than if adjacent cores were used. In general, little control is used over the selection of cores used

and, in practice, cores forming an electrical circuit may even be in different cables in which case the levels of interference can be significantly greater.

From Section 5.3.1 of this chapter, it can he seen that in order to reduce interference between circuits (cross-talk) and from external sources to a minimum, it is essential that balanced pair working is used. This means that the current in one core of a pair must be equal and opposite in direction of flow to that in the other core of the same pair. This is a mandatory re. quirement for analogue signals and should be adopted whenever possible for digital signals.

Also from Section 5.3.1 it can be seen that inter. ference within a control cable is inversely pro portional

least 600 mm and from multicore power cables by at least 300 mm. Interference is directly proportional to the distance over which the control cable and power cables are parallel. It is therefore possible to waive these separation requirements over short distances and the requirement is not applied to cable ends where they are terminated into equipment. A general rule used is that power and control cables can be run at less than the foregoing stated separation distances provided that the summated total length does not exceed five metres.

Analogue and digital signals are normally segregated into separate cables, but separation is not required between such cables.

A further consideration when designing control cable systems should be the effects of cable capacitance. One particular aspect that needs attention is to ensure that the capacitance due to cable length or type is not such that the leakage current is sufficient to cause 'sealing in' of relay coils. Further information on the effects of capacitance is given in Volume F.

5.4 Control and instrumentation cable system design

This section deals with the design of cable systems to handle circuits used for control, protection, instrumentation and communications within a power station. This design work is now generally carried out by the CEGB who also issue the detailed working instructions and drawings to the cable installer.

The work involved on a major project such as Heysham 2 covers 36 000 control cables and well over 1 million wire terminations. Each 'cable' and each 'wire' has to be uniquely designed and identified to ensure

these factors was that approximately 2 07o of wires were wrongly identified or terminated.

The fourth and final phase of the evolution of the cable network system began with the introduction of 660 MW units in the early 1970s, which had associated with them more sophisticated control and data logging systems. There was clearly a need to review and if possible simplify the design and installation of control cables and terminations. It was also considered desirable to try and spread the load factor on the design and site labour forces. Such considerations brought about the adoption of cable networks which utilised jumpering facilities to complete the circuitry. This type of network has been used at stations such as Littlebrook D, Dinorwig and Heysham 2; a description of the principles and equipment used is given in the next section.

5.5 Cable network system using jumpering

5.5.1 Basic principles of cable network

The cable network is a hierarchical system formed to route circuits from the 'field' (plant areas, switchrooms, etc.) to the control room area. A simplified arrangement to demonstrate the basic principles is shown in Fig 6.46. As can be seen, the network is built up using 20-pair modules to match the type of multipair cables used. These multipair cables, which are fully described

in Section 3.6 of this chapter, are constructed on a unit basis. This means that each cable consists of a number of 20-pair units, the pairs of each unit having the same colour code identification sequence, the units themselves being identified by numbers.

The network is built up in 20-pair modules from field marshalling boxes or local panels around the plant, via network marshalling boxes and trunk cables into a marshalling centre.

Similarly, central equipment such as control desks or alarm equipment is connected in 20-pair modules via trunk cables into the marshalling centre. Therefore each 20-pair module will start in a field marshalling box or in an item of control and instrumentation equipment. All pairs of the field end of the module will be made-off onto terminals and the whole module will be extended back to the marshalling centre.

Each 20-pair module is given a unique number that will appear above every block of 40 terminals that are used to terminate the module. Each wire in every module is terminated in the same terminal position in a block as shown in Fig 6.47. This means that a signal onto terminal 1A of, say, a field network box will end up on terminal 1A of the same module in the marshalling centre. Since each module is uniquely identified by a number and each pair within a module is uniquely identified by colour code, a system of attaching ferrule numbers to core terminations is not considered necessary. Since . the terminations within 20-pair modules should never need to be disturbed for correction or