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

operate is required to be not more than 1.3 times

to

he current rating of the cable.

t

4,4

ith motor circuits, cable sizing has to take account

W

f the starting condition as well as full load running.

o

to obtain information on the starting requirements of motors for power station use, reference is made to ESI Standard 44-3 [10] for 3.3 kV motors and boe and ESI Standard 44-4 [11] for 415 V motors.

d

Both standards call for motors to comply with BS5000: part 40 [12], and for general applications a cage induction motor suitable for direct-on-line starting is provided. The standards also specify the frequency of ,tariing, stating that the motor shall be suitable for

starts in succession under specified conditions of Load, torque and inertia, with the motor at its normal running temperature followed by a cooling period of at least 30 minutes before attempting another starting

sequence.

The conductor temperature rise due to two succes- sise motor starts increases with motor size. Typically at 415 V, the temperature rise is approximately 15 ° C for a 5.5 kW motor (4 mm 2 cable) and 50 ° C for a 150 kW motor (185 mm 2 cable).

It is first necessary to be able to calculate the conductor temperature rise due to motor starting. For calculation purposes it is convenient to regard motor starting as equivalent to a low value short-circuit and use the formula discussed earlier for this purpose. In practice this will give a slightly pessimistic result as some heat will he lost from the conductors with the longer durations involved. In order to apply the formula for short-circuit temperature rise, the starting current and duration must first be found.

4.4.1 Motor starting current

The starting characteristic of a typical general purpose nduction motor is shown in Fig 6.32.

The standstill starting current (100 07o slip) at rated

\ °Rage and frequency is termed the 'locked rotor cur- rent'. As can be seen, the starting current remains

substantially equal to this value until the motor is up R.) approximately 80 07o of its rated speed. It is there- tore convenient to assume that the starting current remains at the locked rotor current for the whole of he starting period, and to use this value for tempera- ture rise calculation purposes.

3 The rated output of a motor is the mechanical power

,ailable at the shaft expressed in watts. For power station, use, motor rated outputs are assigned to voltage le ,els on the following basis:

•415 V — up to 150 kW.

•3.3 kV — 150 kW to 1800 kW.

•11 kV — 2000 kW and above.

Power cable system design

MOTOR

STAR TING STARTING CURRENT

CURRENT

6

RA TEC SPEED

So CURHENTLUAU FULL J;)

TIP.1E - s

FtG. 6.32 Starting characteristic of a typical general purpose induction motor

Motor starting performance is specified in kVA and expressed as a ratio where:

starting (locked rotor) kVA

Ratio -

rated output kW

ESI Standard 44-3 and 44-4 specify that the starting (locked rotor) kVA be in accordance with BS4999: Part 41 [13]. These are given in Tables 6.16 and 6.17 for the range of motor sizes/voltage level concerned.

TABLE 6.16

415 V motor starting (locked rotor) ratio

TABLE 6.17

3.3 kV and larger motor starting (locked rotor) ratio

For ratings in excess of 10 MW the ratio is required not to exceed 5.0

FL

N13 x VL x x coso

phase angle at rated load

Re-arranging the second equation and substituting for starting kVA gives:

Ratio x kW output x 1000

vi 3 X VL

and by dividing the two equations for current

A starting current tolerance of +20% is permitted in B54999: Part 101 [14]. Hence, maximum starting

(locked rotor) current 'ST is given by:

IST = 1.2 'FL x Ratio x x cos4)

Currents are derived using average values of efficiency and power factor as given in Table 6.18.

TABLE 6.18

4.4.2 Motor starting times

Motor starting times are determined on the basis shown in Table 6.19.

4.5 Cable voltage regulation

In any system carrying current, there is a voltage drop between the supply end and the receiving end due to the impedance of the system. For AC systems, the supply end is the feeder transformer secondary terminals, and the receiving end is the incoming terminals of the item of plant or equipment concerned.

The voltage drop is normally expressed as a percentage of the nominal system voltage called voltage regulation, i.e.,

(Vs — VR) x 100

% Reg —

Vs

where Vs = nominal system voltage at supply end.

VR = voltage at receiving end.

All power station electrical plant and equipment is designed to operate within specified voltage regulation limits during steady state full load and under motor starting conditions, and consequently may not function or operate correctly if these limits are exceeded. These li mits are specified as:

•Steady state full load, +6% and — 10% nominal system voltage.

•Motor starting, — 20% nominal system voltage for a period up to 90 s.

At voltage levels above 415 V, voltage regulation only becomes a concern on the occasional circuit with a long route length. The main reason is that the denominator is now much larger, permitting a corresponding increase in voltage drop for the same value of voltage regulation. For example, for a voltage regulation of 3% at 415 V, 3.3 kV and 11 kV the permitted voltage drops are 12.5 V, 99 V and 330 V respectively.

Ensuring that the voltage at the input terminals remains within these limits at 415 V is not just a single cable consideration. Voltage drop occurs across each series cable in a supply system, and therefore the voltage drop across any single cable must be some lower value in order that the total voltage drop remains

6% TO

6-.virCH B O A RD

3 35V,8 15V TRANSFORMER

STEADY STATE FULL LOAD .6% TO - I% MOTOR TARTINO .6% TO - 11%

No. 6.34 Cable voltage drop

AV = [I(R cos0 + X sin4s) + V5(1 — cos)]

where AV = voltage drop, V

I = current, I

= line angle

STEADY STATE FULL LOAD • 6% TO - 3 5%

MOTOR STARTING r 64 TO - 14%

• 6' TO - 10.5 TO -20%

Plc. 6.33 Voltage regulation profile

Vs = supply voltage, V

R = AC resistance of conductor, ft

coscr3 = the power factor

X = conductor equivalent star reactance at 50 Hz,

The term V5(1 — coso) is very small and for simplification can be safely ignored. Therefore:

AV = ER cosi:// + IX sing)

and 070 R = 1001 (R cos 0 + X sinci5)/V5

The maximum cable route length for a given size of cable and voltage regulation limit may be determined by expressing R and X in per unit length values and re-arranging the voltage regulation formula:

L m =

. .43 ( 100 1 (RL coo + XL Sin)

TABLE 6.20

protection requirements and for the next two larger cable sizes. These are tabulated for each feeder circuit fuse and motor circuit size for a range of voltage regulation values.

A typical example is shown in Appendix H of this chapter.

4.6 Cable system design

Having discussed the main technical requirements it is now time to see how these, along with certain additional requirements associated with maximum cable route length, are applied. To assist, circuits are separated into circuit types, i.e., feeder or motor circuits, and these are then grouped according to the type of fault current breaking device employed. Under each of the headings, the applicable requirements are given and the basis for determining cable size is described. First, the cable size for normal full load operation is determined after the application of any rating factors. This cable size is then used to determine compliance with the remaining requirements. If necessary, the cable size is increased until a size in the range is found which complies with all the aPplicable technical requirements.

Since cables are only available in a range of conductor sizes it is important to be clear about the term 'cable size'. This means the selection, from this range, of the cable with the smallest conductor size which meets the applicable technical requirements.

There is an additional requirement for motor circuit cable sizing so far not discussed. This is that the cable must be able to withstand a short-circuit fault directly following the second hot start, i.e., the maximum short-circuit conductor temperature must not be exceeded under these circumstances. It is assumed that the time intervals between the successive starts and between the second start and the short-circuit fault are too short to allow conductor cooling. The temperature rise for this situation is calculated in a series of steps.

To a reasonable approximation, the conductor temperature at normal full-load running is given by:

Oft = OA + (OM — OA) (I FLIIC)2

'after two consecutive hot starts is determined from:

The final conductor temperature following a shortcircuit fault is obtained from:

2 2

KS

=logn Resc + 0)/( 0 2sT +

If the final conductor temperature exceeds the insulation maximum short-circuit conductor temperature then the next larger cable size is tried.

4.6.1 Feeder circuits

Air break circuit-breaker (415 V, 3.3 kV and 11 kV)

(a) Continuous operation CEGB policy for large feeder circuits is to size the cable to the current rating of the circuit-breaker rather than the circuit design full load current. This approach affords the maximum allowance for design uncertainty and possible future development.

(b) Fault conditions Short-circuit minimum crosssectional area is determined for the back-up protection fault clearance time. This needs to be determined for each individual case.

(c) Voltage regulation The cable size for steady state

voltage regulation is checked for 415 V and 3.3 kV circuits.

(d) Single core cables Sheath voltages, plus current sharing if more than one cable per phase is used, are checked.

3.3 kV fused switching device

(a) Continuous operation The circuit full load current is used to size the cable for continuous operation.

(b) Fault conditions A fuse to BS2692: Part 1 [6] of a rating which provides take-over from the switching device (see Chapter 5) is fitted to all feeder circuit switching devices. Short-circuit currents above a take-over current are cleared by the fuse, and below this take-over current are cleared by the switching device on operation of the circuit high set instantaneous overcurrent relay. If the fuse and relay protection characteristic curves are plotted together with the cable l 2 t value adiabatic line, in a similar manner to Fig 6.36, it can then be seen whether the cable is adequately protected.

(c)Voltage regulation The cable size for steady state

voltage regulation is checked.

415 V fuse

(a) Continuous operation The first of the summarised circuit protection requirements given in Section 4.3.3 of this chapter may be expressed for a

are therefore sized to the circuit fuse rating. For design purposes, the full load current is normally taken to be equal to the fuse rating.

(b) Fault conditions For economic reasons, local earth bonding is not normally applied to final 415 V feeder circuits at the plant equipment end (earthing is provided via the cable armour). In the event of an earth fault at the plant or equipment, there will be a local rise in potential with respect to any extraneous conductive parts which are separately earthed. This is shown in Fig 6.35.

JNTERNAL FAULT

RESULTS IN LOCAL

RISE IN EARTH POTENTIAL

FIG. 66..35 Local rise in earth potential

As this may give rise to transfer potentials in excess of 55 volts, it is necessary to ensure that the protection operates in a time not greater than 460 ms (see Section 11.2.2 of this chapter). The minimum fault current to achieve this clearance ti me can readily be determined from the fuse time versus current characteristic. The earth fault current is determined by the earth loop impedance which, in turn, is a function of cable route length and therefore for a given cable size there is a safe maximum route length.

To simplify the calculation of this length, the source impedance and cable reactance (the cable sizes involved are normally less than 35 mm 2 ) are assumed to be negligible. Expressing resistance in terms of per metre length gives:

V

P

'EF

L(R., + Ra)

240

therefore L,-,-, a, -

where Lmax = maximum route length, m

In this instance R, and R a are calculated for the full load current concerned. The temperature rise due to the earth fault current is assumed to be negligible. Cable armour resistances are given in Appendix B of this chapter.

If the maximum route length to operate the protection in 460 ms is found to be shorter than the circuit route length, then the next larger size cable is tried, until a suitable cable size is found.

If the earth fault had occurred on energising the circuit, the cable conductor and armour would initially be at ambient temperature. Consequently, the resistance values would be lower than for the full-load operating condition, shortening the fuse operating time.

(c)Voltage regulation The cable size for steady state

and motor starting voltage regulation is determined as described in Section 4A of this chapter.

4.6.2 Motor circuits

Air break circuit- breaker (3.3 kV and 11 kV)

(a) Continuous operation The motor rated full load current is used to size the cable for continuous operation.

(b) Fault conditions This requirement normally dic- tates the circuit cable size, as clearance of shortcircuit current is based on the back-up protection operating time.

Typically, back-up protection on a 3.3 kV switchboard is provided by the incoming transformer feeder high set instantaneous overcurrent relay on the 11 kV switchboard which, to allow for grading with the largest 3.3 kV outgoing circuit, would operate in about 0.6 s.

(c) Voltage regulation Motor starting and steady state voltage regulation are checked at 3.3 kV.

(d) Single- core cables Sheath voltages, plus current sharing if more than one cable per phase is used, are checked.

3.3 kV fused switching device

(a) Continuous operation The motor rated current determines the cable size for continuous operation.

(b)Fault conditions A motor starting fuse to BS5907

[7]is fitted to all motor circuit fused switching devices. Short-circuit currents above the switching device take-over current are cleared by the fuse, while those below this value are cleared by the switching device as explained for 3.3 kV FSD feeder circuits. Back-up protection in the zone covered

by the circuit high set instantaneous overcurrent relay is provided by the fuse and the incoming feeder circuit or interconnector high set instantan-

eous overcurrent relay also as previously described. The cable 1 2 t value is determined after two consecutive hot starts for a temperature rise up to the maximum short-circuit conductor temperature. As before, the cable I 2 t value adiabatic line is superimposed on the fuse and protection coordination curves to determine whether cable shortcircuit protection is provided. This is shown in Fig 6.36.

(b) Fault conditions Motor circuit contactors are to BS5424 1161 Category AC-3 which have a breaking capacity of 8-times rated operational current

cable armour. The maximum route length can therefore be determined using the same expression as used for feeder cable maximum route length, i.e.,

where L max = maximum route length, m

The same assumptions regarding source impedance and cable reactance are made as before.

(c) Voltage regulation Voltage regulation requirements for steady state and motor starting conditions are checked as described in Section 4.4 of this chapter.

4.7 Practical examples

To illustrate the steps followed during power cable system design, a worked example is given for a feeder and motor circuit at each system voltage.

4.7.1 Feeder circuits

1/ kV interconnector

An 11 kV interconnector circuit is required to carry a full-load current of 2200 A, The rating of the circuitbreaker at both ends is 2400 A. The back-up protection clearance time is 1.0 s. The cables are to be installed in air at an ambient temperature of 35 ° C with a route length of 80 m. Access to CURE103 (see Section 4.2.6 of this chapter) is available. Determine a suitable cable arrangement.

(a) Continuous operation The continuous current ratings of 11 kV single-core cables laid in air at an ambient temperature of 25 ° C are given in Appendix C as:

Single-core 300 mm 2 — 675 A

Single-core 500 mm 2 — 900 A

F tiSt `.7...IRACTV1ISr.0 ,ZONE!

PELAY COLa

RELAY HOT

(b)Fault conditions The minimum cable cross-sec- tional area required under short-circuit conditions

with a back-up protection clearance time of 1.0 s is given in Table 6.14 as 417 mm 2 . Short-circuit requirements are therefore met.

(c)Voltage regulation As the cable route length is relatively short it is most unlikely that voltage regulation would be a concern. The voltage drop calculated using CURB03 is 38 V corresponding to a voltage regulation of 0.63/4.

(d)Sheath voltage The standing sheath voltage is given by CURB03 as 20 V which is well within the 55 V acceptance limit. To determine the sheath voltage occurring due to a through fault, the formula given in Section 4.2.6 of this chapter is used, i.e.,

Vsc = V s X ISC/IFL

39 400

= 20 x

2 400

=328 V which is less than the 2 kV acceptance limit

In conclusion, three single-core 500 mm 2 cables per phase are required.

ti

Fiu. 6.38 Minimum earth fault current to interrupt fuse

The rating factor for an ambient temperature of 35°C is given in Appendix E as 0.91. Applying this rating factor gives:

Me cables are NiZeCi to the rating of the circuit- breaker (see Section 4.5.1 of this chapter) and by electing the larger cable size, three cables per phase

appear to be suitable, i.e., 3 x 819 A = 2457 A, provided current sharing is satisfactory.

Inputting cable details, co-ordinates of the route , ectior.ts with cable transposition, and the current

into CURB03 produces the following output:

From examination, the currents in each cable are all less than 819 A.

3.3 kV/0.415 kV transformer

A 2 MVA 3.3 kV/0.415 kV AN transformer is to be supplied from a 3.3 kV switchboard using a circuitbreaker with a rating of 800 A. The back-up protection clearance time is 1.2 s. The cable route length is 500 m, part of which is in air at an ambient temperature of 25 ° C and part buried direct in the ground having a thermal resistivity of 1.5 K.m/W and a temperature of 15 ° C. The ground in one section has recently been made-up but is otherwise of average consistency. The depth of burial is 0.8 m except in the made-up ground where this is increased to 1.25 m. The maximum duct length when passing through walls is 300 mm. Determine the cable size required:

(a)Continuous operation The continuous current ratings of a 3.3 kV single core 400 mm 2 cable laid in air at 25 ° C and in the ground at 15 ° C taken from Appendix C are:

Account does not need to be taken of the short length of cable passing through ducts (see Section 4.2.4 of this chapter) but rating factors need to be applied for the ground thermal resistivity and for the increased depth of burial.

The soil thermal resistivity is 1.5 K.m/W and from Appendix E the rating factor for this is 0.91.

The rating factor for a depth of burial of 1.25 m is given in Appendix E as 0.95. Applying these factors to the rating of the cable buried direct in the ground gives:

575 A x 0.91 x 0.95 = 497 A

The cable rating in ground is lower than that in air and is therefore the limiting factor. Since the cables are sized to the rating of the circuit-breaker, 2 x 400 mm 2 cables per phase are required. Current sharing for the proposed installation arrangement is satisfactory if installed as shown in Fig 6.25. For the purpose of this example, it is assumed that the spacing between the two sets of cables for the route sections buried in ground is sufficient to avoid group rating.

(b)Fault conditions The minimum cable cross-sec- tional area required for a short-circuit fault with

a back-up protection clearance time of 1.2 s is given in Table 6.14 as 507 mm 2 . Short-circuit

requirements are therefore satisfactory with 2 x 400 mm 2 cables in parallel.

(c)Voltage regulation This is checked using CURB03 and for this example is taken to be satisfactory.

(d)Sheath voltage The sheath voltages are checked in the same way as shown in the previous example and, for the purpose of this example, are assumed to be satisfactory.

In summary, two single-core 400 mm 2 cables per phase are required.

415 V three-phase and neutral (TPN) distribution board feeder

A supply to a 415 V TPN distribution board is protected by a 63 A fuse. The cable is laid in air at an ambient temperature of 35 ° C with a route length of 85 m. The power factor is to be taken as 0.85 and the maximum allowable voltage regulation is no. Determine a suitable cable size.

(a)Continuous operation The continuous current ratings for 4-core 16 mm 2 and 35 mm 2 cable sizes laid in air at an ambient temperature of 25 ° C are given in Appendix D as:

4-core 16 mm 2 — 65 A

4-core 35'mm 2 — 104 A

(b)Fault conditions The maximum allowable route length under fault conditions is given by:

L max = 240/IEF (Rc + Ra)

From the 63 A fuse characteristic in BS88: Part 2, to interrupt the fuse in 460 ms requires a fault current IF of 640 A.

To obtain the conductor and armour resistances we must first calculate their operating temperatures using the formula given in Section 4.6 of this chapter:

Conductor temp OFL = OA + (Om — 0 A)( 1 Ftilc) 2 35 + (70 — 35)(63/92) 2

=35 + 16

=51 ° C

The armour temperature rise is taken to be half the conductor temperature rise:

°FLA = OA + 16/2

= 35 + 8 = 43 ° C

For multicore cables up to 300 mm 2 , skin and proximity effects (see Section 4.2.3 of this chapter) are negligible and therefore the AC resistance is taken to be the same as the DC resistance. From Appendix B:

Conductor resistance Re

=Rao [I + Ct20 (0 FL — 20)1

=868 [1 + 0.00403 (51 — 20)]

=9764/m

Armour resistance R a

Ran [1 + cr2o (0 FLA — 20)1

=960 [1 + 0.00403 (43 — 20)]

=l049 2/m

With a 35 ° C ambient temperature, the rating factor from Appendix E is 0.88, giving:

On the basis of a full load current of 63 A, the 4- core 35 mm 2 cable size is initially selected.

This value is greater than the actual route length of 85 m.

(c) Voltage regulation The maximum cable route length for a given voltage regulation is given by: