SYSTEM IMPEDANCE AND SHORT-CIRCUIT LEVELS BASIC INFORMATION AND TUTORIALS

From the consumer’s point of view another important parameter of the supply system is its impedance as viewed from his terminals. On the one hand, the lower the impedance the greater will be the stress on his switchgear and protective devices, but on the other hand, the higher the impedance the greater will be the risk of annoyance due to distortion caused by either the consumer’s own load or by that of a nearby consumer. 

High network impedances are troublesome to installation designers because they result in low values of fault current, which severely limit the number of series graded protection devices and cause an increase in the I2t energy let through of inverse characteristic devices such as fuses.

The 16th edition of the IEE Wiring Regulations, BS 7671, requires installation designers to have a knowledge of the limits of system impedance to which the supply will be kept in order that they may install the necessary protective devices to an appropriate rating and to operate within the required time.

All supply systems are dynamic and many DNO staff are continually employed laying cables and moving and installing plant in order to ensure that the system configuration meets the demands of the customers. For this reason it is not possible to give an exact impedance figure for any one location, but the appropriate local area office should be able to give installation designers the maximum and minimum likely to be encountered for a particular location. 

A maximum earth loop impedance figure of 0.35W is quoted nationally for l.v., singlephase, PME system supplies of 100 A or less. An appropriate maximum prospective short-circuit current of 16 kA is quoted for many urban supplies; further information may be gained from Engineering Recommendation P25/1.

For many years it has been common practice to express the energy available on short-circuit in terms of ‘short-circuit MVA’. This is simply where V is the normal system voltage between phases and A the symmetrical component of the short-circuit current.

In 1971 the International Electrical Commission (IEC) introduced a standard for switchgear ratings (IEC 56) which specified that the working voltage rating should be expressed in terms of the system maximum, for example 12 kV for an 11kV system, and that short-circuit ratings should be expressed in terms of the maximum symmetrical fault current. A range of ratings was specified, for example 12.5, 16, and 25kA for 12kV gear.

For any three-phase system voltage the short-circuit level and the system impedance are inverse functions of each other, . On l.v. systems the cables will generally be the major contributors to the system impedance as the h.v./l.v. transformers are of low impedance. The governing factor is thus the distance of the consumer from the nearest substation.

On DNO h.v. systems the short-circuit ratings of the switchgear have a considerable economic significance and, therefore, system designers aim at keeping these to as low a figure as practicable. A common method of achieving this is to employ high. From the consumer’s point of view another important parameter of the supply system is its impedance as viewed from his terminals. 

On the one hand, the lowerthe impedance the greater will be the stress on his switchgear and protective devices, but on the other hand, the higher the impedance the greater will be the risk of annoyance due to distortion caused by either the consumer’s own load or by that of a nearby consumer. 

High network impedances are troublesome to installation designers because they result in low values of fault current, which severely limit the number of series graded protection devices and cause an increase in the I2t energy let through of inverse characteristic devices such as fuses. 

The 16th edition of the IEE Wiring Regulations, BS 7671, requires installation designers to have a knowledge of the limits of system impedance to which the supply will be kept in order that they may install the necessary protective devices to an appropriate rating and to operate within the required time.

All supply systems are dynamic and many DNO staff are continually employed laying cables and moving and installing plant in order to ensure that the system configuration meets the demands of the customers. For this reason it is not possible to give an exact impedance figure for any one location, but the appropriate local area office should be able to give installation designers the maximum and minimum likely to be encountered for a particular location. 

A maximum earth loop impedance figure of 0.35W is quoted nationally for l.v., singlephase, PME system supplies of 100 A or less. An appropriate maximum prospective short-circuit current of 16 kA is quoted for many urban supplies; further information may be gained from Engineering Recommendation P25/1.

For many years it has been common practice to express the energy available on short-circuit in terms of ‘short-circuit MVA’. This is simply where V is the normal system voltage between phases and A the symmetrical component of the short-circuit current.

In 1971 the International Electrical Commission (IEC) introduced a standard for switchgear ratings (IEC 56) which specified that the working voltage rating should be expressed in terms of the system maximum, for example 12 kV for an 11kV system, and that short-circuit ratings should be expressed in terms of the maximum symmetrical fault current. A range of ratings was specified, for example 12.5, 16, and 25kA for 12kV gear.

For any three-phase system voltage the short-circuit level and the system impedance are inverse functions of each other, . On l.v. systems the cables will generally be the major contributors to the system impedance as the h.v./l.v. transformers are of low impedance. 

The governing factor is thus the distance of the consumer from the nearest substation. The high impedance is achieved by judicious spacing of the windings and does not increase the transformer losses or costs to any appreciable extent. 

The high impedance does not affect the voltage output as the tap-changer regulates accordingly. At 11kV the impedance of the cables is generally much less significant. Until the publication of IEC 56 many 11kV systems were designed for a maximum level of 250MVA which is equal to 13.1kA. 

The new rating method therefore poses a problem where new or additional switchgear is required on an existing system, since the 16 kA switchgear is more expensive. In general, however, UK manufacturers can supply switchgear tested to 13.1 kA at similar to 12.5kA prices.With the low growth of demand, these 250mVA systems will remain for many years.

PRIMARY AND BACK UP PROTECTION OF TRANSMISSION LINES BASIC AND TUTORIALS

The main protection system for a given zone of protection is called the primary protection system. It operates in the fastest time possible and removes the least amount of equipment from service.

On Extra High Voltage (EHV) systems, i.e., 345kV and above, it is common to use duplicate primary protection systems in case a component in one primary protection chain fails to operate.

This duplication is therefore intended to cover the failure of the relays themselves. One may use relays from a different manufacturer, or relays based on a different principle of operation to avoid common-mode failures.

The operating time and the tripping logic of both the primary and its duplicate system are the same. It is not always practical to duplicate every element of the protection chain.

On High Voltage (HV) and EHV systems, the costs of transducers and circuit breakers are very expensive and the cost of duplicate equipment may not be justified. On lower voltage systems, even the relays themselves may not be duplicated.

In such situations, a backup set of relays will be used. Backup relays are slower than the primary relays and may remove more of the system elements than is necessary to clear the fault.

Remote Backup
These relays are located in a separate location and are completely independent of the relays, transducers, batteries, and circuit breakers that they are backing up. There are no common failures that can affect both sets of relays.

However, complex system configurations may significantly affect the ability of a remote relay to ‘‘see’’ all faults for which backup is desired. In addition, remote backup may remove more sources of the system than can be allowed.

Local Backup
These relays do not suffer from the same difficulties as remote backup, but they are installed in the same substation and use some of the same elements as the primary protection. They may then fail to operate for the same reasons as the primary protection.

CROSSARMS USED IN TRANSMISSION LINES BASIC AND TUTORIALS

Cross arms are now almost limited to carrying polyphase circuits in areas where appearance is not of paramount importance. They are also used where lines cross each other or make abrupt turns at large angles to each other.

They are used as alley or side arms in which the greater part of their length extends on one side of the pole to provide adequate clearances where pole locations may be affected by limited-space rightsof- way. Cross arms are shown in Figure below.


Uses of cross arms: (a) line arm; (b) side arm; (c) buck arm; (d) double arms.


Loadings
The cross arm acts as a beam, supported at the point of attachment to the pole, and must be capable of being subjected to vertical loadings from the weight of the conductors (encased in ice) and a 225-lb worker (specified as an additional safety measure).

It is also subjected to horizontal loadings stemming from winds and from tension in the conductors where the tensions on each side of the pole do not cancel each other; e.g., where spans or conductors are not the same on each side of the pole, at dead ends, bends, or offsets in the line, or where consideration is given to conductor breaking contingencies.

Stresses
The same principles for determining stresses in beams as were applied in the case of poles may also be applied to cross arms; see Figure below.


Bending Moment
The total bending moment M is equal to the sum of all the individual loads multiplied by their distances from the cross section under consideration. Ordinarily, the weakest section should be at the middle of the arm where it is attached to the pole.

At the pin holes, however, the cross section of the cross arm is reduced and may, under unusual circumstances, be the weakest point in the cross arm. The determination can easily be made by computing unit fiber stress at the several points.

Like the pole, the cross arm acts as a beam and the same formula for determining stresses may be employed:

f = M/ lie

where f = maximum unit fiber stress occurring at extreme edges of cross section, lb/in2
M = total bending moment, in•lb
I = moment of inertia of cross section
e = distance from neutral axis to extreme edge, in

The moment of inertia for a rectangular cross section is

I = 1/12 x bd3     and      c = d/2

so that the section modulus

I/c = 1/6 x db3

where the neutral axis is parallel to side d, as shown in Figure below

.