T & D - 37) Design of Overhead Transmission Line Foundation

Design of Overhead Transmission Line Foundation

General

The foundation is the name given to the system which transfers to the ground the various steady state (dead) and variable (live) loads developed by the transmission tower and conductors. Foundations may be variously subjected to compressive or bearing forces, uplift and shear forces, either singly or as a result of any combination of two or three of the forces.

Usually, the limiting design load with transmission line foundations is the uplift load.

In this respect, there is a major difference between the design of foundations for transmission lines compared to the design of foundations for most normal civil engineering structures.

Accordingly, the amount of literature describing design techniques for overhead line foundations is relatively small compared to the literature available for more traditional civil engineering foundation design practice.

The selected foundation design for a particular tower must provide an economical, reliable support for the life of the line. The foundation must be compatible with the soil and must not lose strength with age.

With the progressive increase in transmission system voltages there has been a related increase in foundation sizes and it is worth noting that with a typical quad conductor 500 kV line, single leg uplift and ultimate compression loads of 70 or 80 tonnes are usual for suspension towers.

With tension towers, ultimate loads of 200 or 300 tonnes are often developed.

In ground of poor load-bearing capacity the dimensions of foundations become considerable.

In the past, it was often acceptable to ‘over-design’ foundations to allow for uncertainties in the soil characteristics. With the large sizes of foundations for EHV and UHV transmission it is obvious that significant economies can be made in producing foundation designs to exactly match the soil conditions.

Increasingly, transmission lines are routed through areas of poor ground conditions, often for reasons of amenity. This results in the need for the use of special, generally larger, foundations.

The logistical problems of installing large foundations, often in difficult ground conditions, must be taken into account when considering foundation design.

Types of ground

The ground in which the foundations are installed can vary from igneous, sedimentary or metamorphic rock, noncohesive soils, sand or gravel to cohesive soil, usually clays. Equally, soils with a high organic content, for example peat, can also prevail. Composite soils will also be found, and examples of these are sandy gravels and silty sand or sandy peat.

Fundamental to the proper design of foundations is an accurate series of soil tests to determine the range of soil types for which the foundation designs will be required. It is good practice to carry out soil tests at a rate of 1 in 5 tower sites.

This is generally sufficient to enable an accurate forecast of the range of soil types to be established.

It should be pointed out, however, that with large towers having 15 or 20 m square bases, occasionally each of the four legs of a tower may be founded in four different types of ground.

Types of foundation

There are seven basic types of tower foundations:

1. Steel grillage
2. Concrete spread footing
3. Concrete auger or caisson
4. Pile foundation
5. Rock foundation
6. Raft foundation
7. Novel foundations.

Foundation calculations

There are a number of methods of calculation of foundation uplift and bearing capacity. For the purposes of this article, however, we will confine ourselves to a simple approach which must be treated with care. Nevertheless, the methods indicated will give reasonably accurate results for the relatively shallow foundations which are normally employed with transmission line towers.

A shallow foundation is usually defined as one in which the breadth of the pad is greater than the setting depth.

It is usual to calculate the uplift capacity of a foundation as being equal to the mass of soil contained in the frustum developed between the base of the foundation pad and the soil surface.

The angle of the face of the frustum to the vertical is usually designated @ and will vary from35° to 40° in rock, to 25° in good homogeneous hard clay to zero in saturated noncohesive ground. The soil density will vary from just over 2000 kg/m3 for homogeneous rock to about 1600 kg/m³ for soil with normal moisture content to about 800 or 900 kg/m³ in the case of ground subjected to water uplift.

Methods of calculation of uplift capacity are shown below.

Undercut Pyramid Foundation

Undercut pyramid foundation calculation

Concrete Auger Foundation

Concrete auger foundation calculation

Resource: High voltage engineering and testing – Hugh M. Ryan (Buy this book at Amazon)

T & D - 31) Methods of Controlling Lightning Overvoltages in HV

Hubbell's improved protecta-lite transmission line arrester line has been simplified with fewer parts and less risk of parts wearing out. Several steel plated components were replaced with one copper strap. The strap makes installation easier and reduces the wear points. Vibration and wear are more important on transmission line arresters due to the fact that they are more exposed to wind and vibration.

Introduction

For well shielded transmission lines, the backflashover condition, close to the substation, is of prime concern for determining the location and number of surge arresters required to achieve insulation coordination of the substation for lightning surges.

The risk of a backflashover can be reduced by keeping the tower foot impedances to a minimum, particularly close to the substation (first five to seven towers).

The terminal tower is usually bonded to the substation earth mat and will have a very low grounding impedance (1 ohm).

However, the procedure for ‘gapping’ down on the first three or four towers where line coordinating gaps are reduced in an attempt to reduce incoming voltage surges will increase the risk of a ‘close-in’ backflashover.

Location of Surge Arresters

Considering the system shown in Figure 1, where the transmission line is directly connected to a 420 kV GIS (Gas Insulated Switchgear), a computer model can be created to take into account the parameters previously discussed.

A transient study would reveal the level of lightning stroke current required to cause a backflashover.

Figure 1 - System schematic diagram

Then according to the number of line flashes 100 km/yr calculated for the transmission line and by using the probability curve for lightning current amplitude, a return time for this stroke current can be assessed (Le. 1 in 400 yrs, 1 in 10 yrs, etc.) in, say, the first kilometre of the line.

The voltage then arriving at the substation can be evaluated and compared with the LIWL(Lightning impulse withstand level) for the substation equipment.

The open-circuit-breaker condition must be studied here, since if the line circuit-breaker is open the surge voltage will ‘double-up’ at the open terminal. Various levels of stroke current can be simulated at different tower locations and the resultant substation overvoltages can be assessed.

If it is considered that the LIWL of the substation will be exceeded or that there is insufficient margin between the calculated surge levels and the LIWL to produce an acceptable risk, then surge arrester protection must be applied.

The rating of the MOA (metal oxide surge arresters) will have been assessed from TOV(Temporary overvoltage) requirements, and from the manufacturer’s data a surge arrester model can be included in the system model. Repeating the various studies will reveal the protective level of the arrester and from this the safety factor for this system configuration can be assessed.

IEC 60071 recommends a safety factor of 1.25 for 420 kV equipment (safety factor = LIWL / protective level).

The surge arrester current calculated for this condition should be the ‘worst’ case and can therefore be used to assess the nominal discharge current requirement of the surge arrester (5 kA, 10 kA or 20 kA).

(IEC 60091-1 is the international standard for surge arresters [16], and an accompanying guide isavailable which contains detailed information on the application of surge arresters).

To make full use of the MOA protective level the arrester should be placed as close as possible to the equipment being protected.

In the case of the open line circuit-breaker this may well be 10-20 m distance.

Dependent on the rate of rise of the surge voltage, a voltage greater than the residual voltage at the surge arrester location will be experienced at the terminals of the open-circuit-breaker. This must be taken into account when assessing the substation overvoltage.

Figure 2 illustrates the surge voltage profile of the GIS (Gas Insulated Switchgear) with the line circuit-breaker closed. It shows that additional surge arresters may be required because of the distances involved in the layout of the substation.

Figure 2 - Analysis of lightning surge for gas insulated substation

It then follows that surge arresters have a ‘protective length’  which is sensitive to the rate of rise of the incoming surge voltage, and this must be taken into consideration when assessing the lighting overvoltage on equipment remote from the surge arrester.

T & D - 18) Using High-Speed Grounding Switches

Using High-Speed Grounding Switches

Automatic high-speed grounding switches are applied for protection of power transformers when the cost of supplying other protective equipment is deemed unjustifiable and the amount of system disturbance that the high-speed grounding switch creates is judged acceptable.

The switches are generally actuated by discharging a spring mechanism to provide the ‘‘high-speed’’ operation.

The grounding switch operates to provide a deliberate ground fault on one phase of the high-voltage bus supplying the power transformer, disrupting the normally balanced 120° phase shifted three-phase system by effectively removing one phase and causing the other two phases to become 180º phase shifted relative to each other.

This system imbalance is remotely detected by protective relaying equipment that operates thetransmission line breakers at the remote end of the line supplying the power transformer,tripping the circuit open to clear the fault.

This scheme also imposes a voltage interruption to all other loads connected between the remote circuit breakers and the power transformer as well as a transient spike to the protected power transformer, effectively shortening the transformer’s useful life.

Cleaveland/Price's High speed grounding switch 115 kV - 230 kV, 120 kA momentary / 71kA, 3 sec

Frequently, a system utilizing a high-speed ground switch also includes the use of a motor operated disconnect switch and a relay system to sense bus voltage.

The relay system’s logic allows operation of the motor operated disconnect switch when there is no voltage on the transmission line to provide automatic isolation of the faulted power transformer and to allow reclosing operations of the remote breakers to restore service to the transmission line and to all other loads fed by this line.

The grounding switch scheme is dependent on the ability of the source transmission line relay protection scheme to recognize and clear the fault by opening the remote circuit breaker.

Clearing times are necessarily longer since the fault levels are not normally within the levels appropriate for an instantaneous trip response.

500 kV Motor Operated Disconnect Switch (VIDEO)

Can’t see this video? Click here to watch it on Youtube.

The lengthening of the trip time also imposes additional stress on the equipment being protected and should be considered when selecting this method for power transformer protection.

High-speed grounding switches are usually considered when relative fault levels are low so that the risk of significant damage to the power transformer due to the extended trip times is mitigated.

T & D - 3) Gas-Insulated Transmission Line – GIL

The gas-insulated transmission line (GIL) is a system for the transmission of electricity at high power ratings over long distances. In cases where overhead lines are not possible, the GIL is a viable technical solution to bring the power transmitted by an overhead line underground without a reduction of power transmission capacity.

As a gas-insulated system, the GIL has the advantage of electrical behavior similar to that of an overhead line, which is important to the operation of the complete network. Because of the large cross section of the conductor, the GIL has low electrical losses compared with other transmission systems (overhead lines and cables).

This reduces the operating and transmission costs, and it contributes to reduction of global warming because less power needs to be generated.

Safety of personnel in the vicinity of a GIL is very high because the solid metallic enclosure provides reliable protection. Even in the rare case of an internal failure, the metallic enclosure is strong enough to withstand damage. This allows the use of GILs in street and railway tunnels and under bridges with public traffic. No flammable materials are used to build a GIL. The use of GILs in traffic tunnels makes the tunnels more economical and can solve some environmental problems.

If GIL is added to a traffic tunnel, the cost can be shared between the electric power supply company and the owner of the traffic part (train, vehicles).

The environmental advantage is that no additional overhead line needs to be built parallel to the tunnel. Because of the low capacitive load of the GIL, long lengths of 100 km and more can be built. Where overhead lines are not suitable due to environmental factors or where they would spoil a particular landscape, the GIL is a viable alternative because it is invisible and does not disturb the landscape.

GIL Construction

The GIL consists of three single-phase encapsulated aluminum tubes that can be directly buried in the ground or laid in a tunnel. The outer aluminum enclosure is at ground potential.

Gas-Insulated Transmission Line - Construction

The interior, the annular space between the conductor pipe and the enclosure, is filled with a mixture of gas, mainly nitrogen (80%) with some SF6 (20%) to provide electrical insulation. A reverse current, more than 99% of the conductor current value, is induced in the enclosure. Because of this reverse current, the outer magnetic field is very low.

GIL combines reliability with high transmission capacity, low losses, and low emission of magnetic fields. Because it is laid in the ground, GIL also satisfies the requirements for power transmission lines without any visual impact on the environment or the landscape. Of course, the system can also be used to supply power to meet the high energy demands of conurbations and their surroundings.

The directly buried GIL combines the advantage of underground laying with a transmission capacity equivalent to that of an overhead power line.