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

:.or faults outside its capability. The reason for earth fault protection on these circuits is that

,, h fault loop impedance may restrict the fault

aft

.r .-. 2n t to less than 1000 A, resulting in very slow

.,,,Iranee on circuits with large fuses. This results in ised damage to plant and danger to personnel.

'Th. is discussed further in Section 12.9.5 of this The protection circuit is shown in Fig 11.44.

The thermal overload relays are either directly coninto the supply lines to the machine or via :-tent transformers, depending on the rating of the CEGB practice is that an overload relay can directly connected on circuits having full load curup to and including 60 A where the wiring is Where relays are mounted on hinged panels,

•H.:% may be directly connected up to and including \ o nly. For circuits above these ratings, protective

are current transformer operated.

On electrically-held contactors, the protection relay

.cmacts interrupt the AC supply to the contactor coil. He circuit is shown in Fig 11.45 (a). For latched con- , .: ,: tors, the protection relay contacts close to activate DC-operated trip coil. The circuit is shown in Fig

45 (b).

THERMAL OVERLOAD SINGLE

PHASING PHASE UNBALENCE ETC

Fin. 11.44 Protection circuit for motors of 50 kW and above for contactor circuits with 250 A fuses (or greater). The earth fault element is time-delayed to allow the instantaneous overcurrent relay to operate first

12.5.2 Motor circuits at 11 kV and 3.3 kV

The thermal overload protection on these circuits is essentially the same as that supplied on the 415 V motor circuits which use current transformers. Phase to phase and phase to earth fault protection is added in the form of instantaneous relays. It is essential to have this form of protection even on 3.3 kV motor

switching devices, as the earth fault currents are limited to 1000 A. The fuses fitted as standard in these devices are 400 A, resulting in very slow clearance on earth faults. Additionally, the switching device operated from the relay will clear the fault without blowing the fuse. This also applies on phase to phase faults to a limited extent at low fault infeeds which can be cleared by the circuit-breaker in the motor switching device arrangement.

12.5.3 Thermal overload relay

Referring to Fig 11.46, the thermal overload relay consists of three bimetal strips, one in each phase of the motor, which become heated as a result of the motor current flowing through them, causing bending or rotational movement which closes a pair of contacts to initiate a trip or alarm signal. The precise physical movement of the bimetallic strip is a function of the relay setting current.

The bimetallic strips are helically wound and, when they carry the motor line current, the heat generated causes the assembly to rotate about a common axis, so that it closes the trip or alarm contact on the relay if the motor rating is exceeded. One important feature which this relay embodies is temperature compensation to accommodate for any changes in ambient temperature, so that the actuating helix will either 'wind up' or 'wind down' depending on whether there is a rise or fall in temperature.

The contact arrangement is shown in Fig 11.46. The three thermal elements are mounted axially inline and are insulated from the case and from one another. The two outer elements are connected together to form one pole of the trip circuit, whilst the centre element forms the other. A radial arm on the centre element carries a lattice frame fitted with five contacts, one of which is the overload contact.

Each of the arms on the outer movements carries two contacts which normally float between a pair of contacts on the centre frame and protect against phase unbalance or open-circuit. Under normal operating conditions, all three contact-carrying arms deflect through the same angle so that the two contacts on each of the two outer arms remain midway between the two pairs of contacts on the centre contact frame.

The thermal characteristic required varies with the run-up time and the heating-up and cooling-down time of the motor. The thermal overload relay used by the CEGB has three characteristics of which the 20-minute type is most commonly used (Fig 11.47). This gives an operating time for motor starting currents (5 to 8 ti mes relay rating) of 40 seconds, approximately, and suits most motors used by the CEGB on their auxiliary systems. Applying one characteristic to all motors cannot give close protection for all motors, but has proved very reliable and gives effective protection.

Ideally, the relay should match closely the thermal characteristic of the motor both heating-up and

cooling-down and ensure that its operation at all times stays within the safe withstand capability of the m a - chine. With this kind of relay, a compromise is reached where, in some areas, the motor is overprotected.

The instantaneous earth fault relay uses a stabilising resistance in series with a low impedance current operated relay. The principle adopted has already been explained in Section 6.3 of this chapter and it is cm. nected (as explained in Section 12.3.1) to the auxiliary transformers. The stabilising resistance prevents operation due to current transformer saturation caused by motor starting currents. The relay must also have a low transient overreach to allow a low setting without picking up on the first peak of the starting current.

For certain applications, the provision of adequate stalling protection can sometimes be a problem. Thi s is particularly so where the safe stall time for th e motor is close to or longer than its run-up time, as in submerged cooling water pumps, or where the motor is close to its stall capability when it is operating at high load conditions. Under these circumstances, a special stalling relay is required in order to provide close relay settings between tripping time and motor stall withstand time. Referring to Fig 11.48, the current transformer/control contactor operates at values of current above three times rated relay current to switch in the thermal element. If the current falls below three times, as in a healthy motor start, the thermal unit is disconnected.

In addition to the electromechanical thermal overload relays discussed, electronic relays are now being produced. The CEGB have not to date (1988) approved the use of the electronic relay, mainly because of its inability to provide fast phase to phase and phase to earth fault protection. It is proposed to use electromechanical protection, as provided in the past, in combination with an electronic relay for thermal overload protection. The advantage of the electronic relay over the electromechanical as a thermal overload device is that the relay thermal characteristic matches the mathematical thermal model of the motors more closely.

Owing to modern advances in insulation technology, the quality of the insulation nowadays is such that less is required than was previously necessary for the same motor design, with a consequent reduction in frame size for the same rating. This results in a smaller heating time constant with which the electromechanical relay is unable to cope for all the requirements, and it is becoming increasingly unsuitable for close protection of motors of present day design. The requirements are:

•To fit the relay 'cold' operating curve below the motor 'cold' thermal withstand curve.

•To fit the relay 'hot' operating curve below the motor 'hot' thermal withstand curve.

•To fit the relay 'hot' operating curve above the motor 100% volts and 80% volts starting current/ time characteristic.

a LOAD CONTACT

PRESET

E LOAD POINTER

R

1 0

RUNNING LOAD

I NDICATING POINTER

HEATER CALIBRATING

ADJUSTMENT

NNG ,HOTI

FROM

_CAD

'25”, SETTING

MULTIPLES OF RATED CURRENT

FIG. 11.47 Motor thermal relay — 20 minute characteristic

:tronic circuits whose behaviour is determined mathematical model that describes the thermal

•„ ::enstics of the motor. The relay can be either

or digital in operation, the digital relay being

• •iexible. Further, quantities measured by the re- be displayed at the relay by means of a liquid

crystal device, or similar. At 415 V, such sophisticated protection might not be considered necessary. However, there are essential motors connected at 415 V which are required to run following a reactor trip at a nuclear power station to maintain reactor safety, and unnecessary tripping of one of these motors by its thermal relay renders it unavailable until the relay resets to allow a motor start. Digital relays can be programmed to trip the motor, for example, on voltage disturbance, before the motor reaches a temperature which would preclude an immediate restart.

Digital type protection relays, using microprocessors, therefore have very wide ranging capabilities. Most relays employing digital techniques will memorise the

Protection

SEPES PLAG ;NOICATOR

FIG. 11.48 Stalling relay schematic diagram

values of circuit parameters, e.g., load current in each phase, thermal overload capacity actually used up and also the amount remaining before reaching trip state, percentage unbalance currents, earth fault and shortcircuit currents. This information can be easily accessed at the relay to give running values of the various parameters as well as preand post-fault trip data. Other useful features include a pre-trip alarm and trip inhibit facility.

12.6 Cables

So far, the protection requirements of motors, transformers, and generators have been discussed: these schemes usually also protect their cables which are connected to them. In these instances, faults in the cable are cleared by the main protection. Should these faults persist undetected by the main protection, the back-up protection will clear the fault, its current and type setting determining the short-circuit rating of the cable.

For cables not connected to the items of plant referred to above, e.g., interconnectors, the same philosophy of main and back-up protection is adopted. The main protection is circulating current high impedance as described in Section 6.3 of this chapter. Figure 11.49 shows the 11 kV and 3.3 IcV Unit Board cable interconnector, where it can be seen that the main protection consists of a circulating current scheme. Back-up protection is provided by a combined overcurrent and earth fault relay which is located at each end of the interconnector. The relay used for this purpose consists of three elements, two used for overcurrent protection and the third for earth fault protection.

At 415 V, the protection requirements are a little different, in that the circulating current protection is phase to earth only and the back-up protection is pro-

Chapter

vided by the overcurrent relay on the HV side of t he transformer. This operates in two stages; the first stag e trips the interconnector, the second stage the transformer HV and LV circuit-breakers.

12.7 Busbar protection

Whilst the busbar protection schemes available ar e perfectly adequate to cater for phase faults and earth faults, their use within the CEGB for power station systems has always been the subject of debate.

Experience indicates that busbar faults are very i n _ frequent. The busbars are usually air insulated, which itself reduces significantly the possibility of busbar faults. This supports the view that a dedicated busbar protection scheme is unnecessary.

The CEGB has not used busbar protection on any 660 MW unit power station other than Drax where, having been installed in the late 1960s on the first half, it was repeated for consistency on the second half. It has been considered for many years that to introduce a separate form of protection specifically to cover busbar faults reduces the overall reliability of the electrical system, since it complicates the protection scheme and increases the risk of malfunction. This would result in shutting down the complete switchboard, causing severe operational inconvenience and would be costly in lost production. This is particularly so for unit boards, which supply all the motors essential for the running of the main unit. At 11 kV for example, it would possibly result in the loss of a boiler feed pump, cooling water pumps, induced draught and forced draught fan motors and would inevitably result in the tripping of the main unit. The risk of losing such strategic items of plant, with the consequential loss of generation, due to the possible malfunction of a busbar protection scheme is unacceptable. For this reason, busbar protection is no longer fitted to auxiliary switchboards in this way.

There is, however, still a need to guard against any faults likely to affect the busbars. The method adopted to cover for busbar faults uses the protection arrangements already discussed under transformer protection, i.e., the standby earth fault protection will operate for any busbar fault involving earth, and the back-up overcurrent protection fitted to the HV side of the transformer will operate for any busbar phase to phase faults. At 415 V, where busbar faults have been caused by human error when using multirange instruments, the back-up protection is set down to 150 ms. This can only be achieved by eliminating all forms of overcurrent protection using inverse time relays on the 415 V supply system. This method of protection at 415 V is helped considerably by the design of that system. This is explained in Section 12.9 of this chapter.

12.8 High breaking capacity (HBC) fuses

The fuse has shown itself to be an extremely reliable and safe form of protection and has been used in this

Am, over a long period of time with great success. whilst the principal function of the high breaking ,, pae ity (NBC) fuse is to provide short-circuit pro-

colon, it can also provide an adequate degree of ,a erload and earth fault protection for smaller circuits. It must be realised that, when considering the earth

:ault protection properties of the fuse, a significant amount of earth fault current must flow before the Jo ice operates, so an adequate earth path must be provided back to the star point of the transformer.

Most 415 V circuits are fuse protected and fuses are used to a significant extent for 3.3 kV circuits. CEGB practice on the use of fuses as protection and thrair co-ordination with other protection is explained

he following section.

12.9Protection co-ordination

12.9.1Characteristics of 415 V fuses

!fl order to maintain a degree of interchangeability,

'uNes are specified which fall into the characteristic hands of 1EC 269 (BS 88). Figure 11.50 shows the characteristic bands for 10 A, 20 A, 40 A, 80 A, 160 A,

A fuses. The lower curve of each band

:N the pre-arcing ti me of the fuse in the size stated and he upper curve is the total operating time of that size.

characteristics conforming to this standard these bands. This includes the ± 10%

loltance allowed to the manufacturer on his published

.haracteristic. This ensures that when replacing fuses, grading between them will always be maintained. For example, a 315 A and a 630 A fuse of any manufacturer

hich conforms to the upper and lower limit of the iLlie bands of BS88 will always grade with one another.

These curves indicate that grading between fuses will

always be maintained if a 2:1 ratio in fuse sizes is used. Experience shows this is to be so, although at very high fault currents which are operating both fuses in less than 5 ms, some grading may be lost with a 2:1 ratio.

12.9.2 Characteristics of inverse time relays

There are three types of recognised inverse time overcurrent relay and the characteristics are specified in EEC 255 - 4 (BS142), they are:

•Ordinary inverse.

•Very inverse.

•Extremely inverse.

Typical characteristics for overcurrent relays are shown in Fig 11.51. The characteristic equation, as specified in BS142, is

t

(G/Gb)a — 1

where t = theoretical operating Lime, s k = constant for that relay

G= current in relay, A

Gb = relay setting current, A

a = index characterising the algebraic function

The ratio G/Gb is therefore the plug setting multiplier. By careful choice of the values k and a, the three inverse curves at a time multiplier of unity can be produced. Suggested values of k and a for the four types

of relay are given below:

Since the characteristic curves are based on the time multiplier (1.0) then, when co-ordinating relays at other ti me multipliers, a check must be made using the actual curves specified by the manufacturer and the setting adjusted (if necessary) to obtain the required grading margin. This is particularly important when using very and extremely inverse relays, where the time of operation is not always proportional to time multiplier setting. These comments do not apply to electronic relays the characteristic values for which are shown above and for which the time multiplier is directly proportional.

The setting accuracy is also to much closer tolerances

± 5 070.

Relay errors must be within those quoted in BSI42. In that standard the allowable error at twice the setting is more than twice the allowable error at 20 times the setting. This does not usually affect co-ordination margins between relays, where the most critical margins more often than not occur at the higher current multiples. It can affect the grading between a fuse and a relay.

12.9.3 Characteristics of definite time relays

The characteristic of a definite time relay with 2, 4

and 8 s settings is shown in Fig 11.51. The operating time is independent of the current input which means that the grading interval can be reduced as CT errors do not have to be included. This is offset to some extent by the fact that the allowable relay errors are ±10o instead of ±7.5 070. From a practical point of view, the inverse and the definite time relays could be treated in the same way as far as grading margins are concerned.

Section 12.9.6 of this chapter gives the equation for the grading interval. In order to simplify the grading process, the CEGB co-ordinates the system using inverse time relays and reserves the application of definite ti me relays to specific cases, as follows:

(a)For fast clearance over a wide range of fault levels. When grading with a fuse this can sometimes be

better achieved using a definite time relay than with an extremely inverse time relay. At the lower plug setting and time multiplier setting of the extremely inverse relay, co-ordination between fuse and relay is lost at the lower fault levels. Faults between phases generally produce higher fault current than for earth faults at 415 V, where the earth fault

VERY INVERSE 1=13 5 .11.11

EXTREMELY INVERSE t=so , iz

I C

CURRENT IN MULTIPLES OF SETTING

FIG. 11.51 Characteristics of an overcurrent relay with various time settings

loop impedance can be high. It can also be used successfully where it is difficult to clear low level earth faults on interconnector circuits and busbars. At 415 V, it would be grading with a fuse, so that its current setting must be high enough to co-ordi- nate with the largest fuse on the outgoing circuits.

(b)They are ideal for use throughout the supply system as back-up protection, such as on radial feeders at the same voltage level. Radial feeders are rarely used on CEGB auxiliary systems other than 415 V where fuses are the only protection.

(c)Where above a fixed current setting, operation is required in a definite time.

They cannot provide close grading with a mixture of fuses and inverse relays unless the definite time relay is the first relay in the grading chain. An inverse relay will always grade much closer to another inverse relay than a definite time relay.

It is practice in modern relays to incorporate both inverse time and definite time characteristics in one relay.

12.9.4 Characteristics of thermal relays

The characteristics of thermal relays are designed to

Auxiliaries systems

follow the thermal capability of the protected plant and, in particular, of induction motors (see typical characteristics on Fig 11.47 and compare with those on Fig 11.51). They have longer operating times than the inverse relays. The relay has other protection functions such as stall, single phasing, negative phase sequence and earth fault protection. It is only necessary for co-ordination purposes to know the characteristics of the instantaneous and thermal relay and, if working in conjunction with a fuse, the fuse characteristic. The process will be explained later to show how the combined characteristic made up of a fuse, an instantaneous relay and a thermal relay reduces clearance times of back-up protection further up the system and, in the case of large motors connected at 11 kV, determines whether differential protection is required.

12.9.5 Calculations

The data required for a co-ordination study are:

•A single line diagram of the supply network, as shown in Fig 11.52.

•The impedances of all power transformers, rotating machines and feeder circuits.

The impedances are first converted to the per unit (p.u.) value by using p.u. value = actual value/base value, where the base value chosen depends on the actual value. If actual value is:

(a)Volts The base value is the nominal phase to neutral voltage of the relaying point.

(b)Amps The base value is preferably the full load current of the circuit protected. This is not essential but is a convenient value to choose.

(c) Ohms The base value in ohms is then (a) divided by (b). The per unit system can be completely defined by specifying (a) and root 3 times the product of (a) and (b), i.e., a base MVA value. If the base value in ohms is required, it is given by:

(phase to phase voltage in kV) 2 /MVA base

Note: Any two base quantities will fix the third but

(a) must be one of them.

The maximum and minimum values of short-circuit current that are expected to flow through the relay during normal and transient conditions are next calculated. Generally, when considering the settings of overcurrent relays, the minimum short-circuit is not required since the current setting is dependent on the maximum continuous loading of the circuit, the relay resetting current ratio and the primary current setting of the relays in front. The recommendation is to use a factor of 1.3 times the largest of these three values. However, for the instantaneous relays, as with unit protection relays, it is important to check that at the minimum fault level the current is at least twice the

setting current and the current transformer knee-point is not reached before twice the relay setting current. This will ensure positive operation of the instantaneous relay.

This can be checked simply by the following method:

(i)Calculate the relay circuit impedance at twice the setting. This is the sum of the relay, plus the leads, plus the CT, plus any other relays in the circuit.

(ii)Either from the accuracy limit factor or from the CT magnetising characteristic, derive the CT knee point voltage. The accuracy limit factor is defined as the primary current up to which the trans-

former maintains its accuracy within its prescribed limits of error, expressed as a multiple of its rate current.

(iii)Divide (ii) by (i), giving the knee-point voltage: this should be greater than twice the setting.

Suppose the current transformer is 10 VA with an accuracy limit factor of 15 and a rated secondary current of 1 A. Then, if the secondary burden is 1 11, the current transformer can deliver 15 x 10 volts (knee point voltage) into 111 which is 150 A. A setting of 75 times is therefore possible. Fifteen to 20 times is more likely to be the required setting on high set instantaneous relays.

will be 0.25t, where t is the operating time of .the relay in front. This does not apply to current independent relays, since they are independent of current transformer errors. The total grading interval is (0.25t +

0.25).

The resulting ti me of operation of the next relay for

I DMT relays is the operating time of the relay in front

(t)plus the grading interval (At + B), giving t + (At

+B) = (1.25)t + 0.25; and between definite time relays, which are independent of current transformer errors above their setting and meeting class EIO (i.e., A

0.2), giving t + 0.2t + 0.25 = (1.2)t + 0.25.

Time grading — relay to fuse

As far as the practice of grading inverse time relays with fuses is concerned, providing that the relay is not closer in time than 150 ms at the maximum fault level, the time grading is achieved. It was mentioned in the

section on current grading that at low time multiplier setting, the fuse and relay may not grade at the low fault level. This is shown in Fig 11.55 region 'X', where relay 2 (a) meets the requirement at the higher fault levels but crosses the fuse characteristic at the lower fault level. This is easily checked by plotting the grading curves. The crossing of the fuse curve at the low fault level can sometimes be allowed on the basis that phase to phase faults at a low level are unusual and, by the nature of the transformer phasor relationship, grading is achieved for earth faults. To achieve fast operating times and good co-ordination a relay with an extremely inverse characteristic should be used.

Transient overreach

For a given RMS value of operating current, a relay will usually operate at a lower value due to the offset of the initial peaks of fault current. The degree of offset