CHAPTER 8 QUESTIONS AND ANSWERS

8.1       QUESTIONS

1.            What are the principal voltages used in England and Wales for the bulk transportation of power?  Which are operated by the CEGB and which by the Area Electricity Boards?

2.            At what voltages and frequency do large and small consumers in the UK receive their power?

3.            At what voltage do the large power station generators operate?

4.                What do you understand by ‘unit construction’ of a generation system?

5.                A transmission line is to be 90 miles long. What would be the optimum operating voltage? What standard voltage would probably be chosen?

6.                How is domestic voltage obtained at ‘street’ level (a) in the UK, (b) in the USA, (c) on an offshore installation?

7.                How is the national frequency level in the UK controlled?

8.            How are voltage levels in UK networks controlled?

9.            What is the statutory limit on voltage variation of power supplied to consumers?  Whose responsibility is it?

10.          What can an industrial consumer do to regulate his voltage?

11.          What do you understand by the term ‘infinite busbar’?

12.          How is the infinite busbar concept used?

13.          What effect would increasing load have on an 11kV distribution line?  How can it be regulated?

14.          What effect would increasing load have on a 400kV transmission line?  Why the difference from Q13?  What action can the Control Engineer take?

15.          Where does a typical onshore installation draw its power from?  How is it supplied, and at what voltage?

16.          Who owns the installation’s main supply transformers?

17.             If there is more than one supply source, what steps are taken to keep them separate?

18.          What type of switchgear is usually provided for the high-voltage incomers to the onshore installation and for the high-voltage feeders?

19.          What steps are taken to ensure the maximum reliability of supplies to the LV distribution systems?

20.           If all external power supplies to an onshore installation are lost, what steps are taken to ensure continuation of the minimum vital supplies?

21.          Why are tariffs in two parts?

22.          If an industrial consumer’s maximum kVA demand is higher than it need be, what can he do to reduce it?

8.2       ANSWERS

(Figures in brackets after each answer refer to the relevant chapter and paragraph in the text.)

1.           

Transmission voltages:               400kV	CEGB
275kV
Distribution voltages:                   132kV (formerly transmission)	Area
33kV	Electricity
11kV	Boards
	(3.2)

2.            Large consumers 11kV (a few at 33kV) at 50Hz.

               Small consumers 415V (4-wire) (giving 240V, single-phase) at 50Hz.                (3.2)

3.            Previously between 11kV and 22kV, now more commonly 25kV.                      (3.2)

4.            Unit construction is where the generator terminals are solidly connected to the primary terminals of the step-up line transformer, with no intervening switchgear.  The prime mover, generator and transformer form a single electrical unit.  The first switching control is the circuit-breaker on the HV side of the step-up transformer.

                                                                                                                                             (3.2)

5.            At 1.2kV per mile, the optimum operating voltage would be about 90 x 1.2 = 108kV.  The nearest standard 132kV would probably be chosen.                                                                        (3.2)

6.            (a)    Street distribution in the UK is at 415V 50Hz 3-phase 4-wire from street pillars.  Domestic supplies would be taken off line-to-neutral at 240V, single-phase.

(b)    In the USA street distribution is at 440V 60Hz 3-phase 3-wire.  Domestic supplies are transformed from this to 1 17V single-phase.

(c)    On offshore installations LV distribution is at 440V 60Hz 3-phase 4-wire.  Domestic supplies are taken off line-to-neutral at 254V single-phase.                                                       (3.2)

7.            National frequency is closely monitored, and the gain or loss is indicated by gain or loss of cycles compared with a standard.  If there is a continuing loss, more energy is being taken out of the national network than is being put in.  The National Control Centre orders certain running stations to take on more load, or orders standby stations to start up and take on load, until balance is restored and the lost cycles recovered.                                                                                                             (3.3)

8.            Voltage levels at various points of their networks are monitored by the CEGB Regional Control Centres, and correcting action is taken.  This may be by AVR adjustment, by transformer on-load tap changing or by use of reactors

                                                                                                                                    (3.3 & 4.3)

9.            ±6%.  Maintenance of these limits is the responsibility of the Area Electricity Boards in England and Wales, and of the NOSHEB or SSEB in Scotland.                                                        (3.3)

10.          Very little, unless he has on-load tap changing. If his supply is outside the ±6% limit and he cannot get his Area Electricity Board to correct it, he can only change his transformer taps which, being probably of the off-load type, would entail shutting down while the tap is changed.                     (3.3)

11.          It represents a source of supply of such a large capacity that it has no limit. Also its source impedance is regarded as zero, which means that there is no frequency regulation and no voltage regulation as load changes. It thus represents an infinitely large generator with perfect governing and perfect voltage regulation.                                                                                                              (4.2)

12.          Where the National Grid supplies a consumer installation of limited size, the massive backing of the grid system can be regarded as an ‘infinite busbar’ relative to the limited consumer network. (4.2)

13.          An 11kV distribution line, having both resistance and inductance, will cause an increasing voltage drop as load increases, especially if that load contains a reactive part.  Such voltage drops can be compensated for by on-load tap changing within the distribution system.                                  (4.3)

14.          A 400kV line presents a large fixed capacitance to the power source, and the capacitive current which consequently flows causes a voltage rise in the line unless offset by a voltage drop due to other causes.  Such a cause would be normal loading, where the reactive component causes a voltage drop.  Thus for low loading there would be a net voltage rise, but as the loading increased the amount of rise would become less until, at some point, there was a balance and no voltage rise or fall at all.  A further increase of loading would produce a net voltage fall.

The Control Engineer can compensate for voltage rise (which occurs during periods of low power such as at night or weekends) by on-load tap changing, by AVR control of generators or by switching-in reactors to absorb the leading megavars.                                                                               (4.3)

15.          From two or more bulk supplies provided by the Supply Authority.  They are supplied at extra-high voltage (at least 132kV) and transformed down to 33kV or 11kV at the installation’s bulk supply point.    (5.1)

16.          The EHV grid transformers, and their HV switchgear, remain the property of the Supply Authority.                                                                                                                              (5.1)

17.          The incoming supplies, which are usually derived from different sources, are separated by a section breaker at the HV switchboard.  This section breaker is normally kept open, and each supply source feeds separate parts of the installation.  Interlocks prevent both incomers and the section breaker being closed together, so avoiding paralleling the two sources.  Similar interlocks on the LV switchboards prevent paralleling through the transformers.

If one supply source fails, its incomer breaker is locked open and the section breaker automatically closes, so restoring supply to the failed half of the board.

                                                                                                                           (5.2.1)

18.          Oil circuit-breakers.                                                                                            (5.2.1)

19.          Every LV switchboard has two alternative incomers from separate transformers fed from the two sides of the HV switchboard; these are fed from different external supply sources.  Thus the failure of an external source, of a transformer or of a cable does not deprive an LV board of power.  The alternative is available and takes over the whole board by automatic closing of the section breaker, as in Answer 17 above.  Each of the alternative transformers and cable systems is designed to carry the full load on its own.   (5.2.4)

20.          In the unlikely event of all external supplies being lost, an emergency diesel-driven generator automatically starts and takes over a limited amount of the LV load.  This is done through ‘secure power’ switchboards to which the most vital consumers are connected.                                               (5.2.4)

21.          Tariffs have a variable part to recover cost of energy actually consumed - and hence of fuel burnt - and other running costs.  They also have a fixed part, independent of energy consumed, to recover a contribution to the capital cost of the whole power installation. This fixed part is usually related to a consumer’s maximum demand, averaged over successive 30-minute periods, during any one accounting period.                                                                                                                   (6.2)

22.       Since kVA is the vector sum of kW and kvar, and since kW represents the rate of energy consumed and is therefore all needed, an industrial consumer can only reduce the kvar.  This term represents the magnetising current of his machinery, and this can be offset by a corresponding amount of capacitive current such as would be drawn by capacitors.  He must therefore consider power-factor correction if he wishes to reduce his tariff costs.  (See also the manual ‘Electric Motors’.)