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FURTHER ELECTRICAL POWER ASSIGNMENT 1 THE MODERN POWER SYSTEM-PART1-1 JOHN O’CONNOR 16/10/09

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Page 1: Further Electrical Power

FURTHER ELECTRICAL POWER

ASSIGNMENT 1

THE MODERN POWER SYSTEM-PART1-1

JOHN O’CONNOR 16/10/09

Page 2: Further Electrical Power

CONTENTS

Summary………………………………………………………….2 Introduction……………………………………………………….3 Report & Analysis………………………………………………..6 Conclusion………………………………………………………..18 Discussion………………………………………………………..18 References………………………………………………………..20

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SUMMARY

This report entails information about the modern power system, specifically the construction and properties of overhead transmission lines, and my findings are: Considerations for the construction of steel lattice towers are

• Compressive forces on the steel members • Bolted connections • Bracing • Forces acting on the tower, including foundations, horizontal wind load,

weight of conductors and fittings, longitudinal loads from conductor tension, wind effects on the conductors (Aeolian vibrations)

• Environmental conditions • Conductor types and fittings • International design standards

Protection for high voltage overhead lines:

• Distance relay operation involving zones and measurement of the impedance of the line to determine the fault.

• Typical faults include lightning strikes and objects coming into contact with the conductors.

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INTRODUCTION

This report is a study of the modern power system, specifically overhead lines and cables. The following information is provided:

The diagram above shows a typical supply system. The 275kV/400kV section is highlighted by the arrow. The purpose of this report is to produce a technical manual that describes the construction and properties of the 275kV/400kV overhead line connecting T/X1 and T/X2.

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The following information is also provided:

Figure 1.3 shows a simplified force diagram of a line tower with forces A to E representing the following forces: A – Horizontal conductor wind load B – Horizontal structure wind load C – Component of wind loading in respect of wind direction and structure area D – Vertical conductor weight span times conductor weight, incl. insulators and fixings E – Longitudinal loads due to conductor tension The purpose of this report is also to describe:

1. How the design of a 275kV/400kV tower ensures mechanical stability against the above forces. Draw a force diagram that shows the above forces and the forces acting upon the foundations.

2. The design standards (IEC, EN, BS) and how they apply to lattice steel towers.

3. The factors influencing conductor selection for 275kV/400kV lattice

steel towers, e.g. environmental conditions, types of conductors, etc. Also explain what ACSR conductors are and how they are classified.

4. The commonly used overhead line fittings for 275kV/400kV

transmission systems.

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The following information is also provided:

The diagram above shows a transmission system. The lightening bolt shows a fault between bus 3 and bus 4. The nominal voltage is 138kV and the power transfer is 27MW. The final purpose of this report is to explain what faults typically occur on transmission lines like this and state the means of localising faults.

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REPORT AND ANALYSIS

The overhead line section from T/X1 to T/X2 is supported using steel lattice towers. These towers are used at higher voltage levels and can support heavier conductors along with longer conductor spans in hazardous conditions, such as high wind areas. Wooden poles or lighter steel poles would not perform as well.

(Diagram taken from Bayliss.C 1999 Transmission and Distribution Electrical Engineering. Newnes)

The diagram above shows some typical tower outlines.

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Towers can be categorized into the following groups:

Suspension towers Straight line and deviation angles up to about 2°

10° angle or section tower Angles of deviation up to 2° or at section positions also for heavy weight spans or with unequal

effective negative weight spans 30° angle Deviation angles up to 30° 60° angle Deviation angles up to 60° 90° angle Deviation angles up to 90°

Terminal tower Terminal tower loading taking full line tension on one side of tower and

none or slack span on other- usually substation entry

There are different versions of description for these towers. The description is abbreviated as shown: A double circuit 30° angle tower for twin conductor use could be described as D30T. It can also be known as D30. Task 1. MECHANICAL STABILITY Steel lattice transmission towers are subjected to many forces, so it is vital that they are able to withstand these and maintain stability. It is important that each part of the tower is correct for the designed loadings. Slenderness ratios (l/r where l is length and r is radius of gyration) are used to determine how long a steel member can be before its load carrying ability is compromised. The tower designers refer to BSEN 10025 for structural steel standards and BS5950 for the slenderness ratios. The tower design is based on compression formula for different sections of the steel work. The design is usually carried out be specialist structural engineers.

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The table below shows the compression formula for leg members with different length/radius of gyration values.

(Table taken from Bayliss.C 1999 Transmission and Distribution Electrical Engineering. Newnes)

Bending forces, compression, tension forces, flexural buckling and lateral buckling are all factors that need to be addressed by the structural engineers when they are designing a steel lattice tower. The type of bolts used is also important. Because of the forces acting on the steel towers, there is a very slight bending, so the bolts used take this into consideration. Friction grip bolts, which are commonly used for rigid structures that are exposed to high shear loads, are not suitable.

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FORCE DIAGRAM Showing forces acting upon a steel lattice tower. Horizontal structure wind load

• Horizontal structure wind load • Horizontal conductor wind load • Longitudinal conductor load • Wind load (vibrations) • Weight of conductors and fittings • Uplift and compression on foundations due to wind loads

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TASK 2 The design standards that apply to 275kV/400kV steel towers. BS5950 covers the design comprehensively. The tables for l/r slenderness ratios are derived from this standard. EN50341 covers the design of steel structures. BSEN10025 is another hot rolled steel structural standard. EN24016 is the standard for ISO metric black hex bolts and precision bolts. IEC 60815-1 and -2 is the guide for the selection of insulators in respect of polluted conditions. IEC 60826 is the guide for design criteria for overhead lines EN50341 covers loadings for overhead lines in excess of AC45kV. BS8100 covers loading and strength of overhead transmission lines. TASK 3 Factors that influence conductor selection for 275kV/400kV steel lattice towers. Overhead lines operate in a vast variety of climatic conditions, from desert to frozen steppe. The environmental conditions have to be considered: (Taken from Bayliss.C 1999 Transmission and Distribution Electrical Engineering. Newnes) Temperature. The max, min and average temperature can affect conductor current rating and sag. For temperate conditions typically 20°C with 55°C temperature rise. For tropical conditions 35°C or 40°C with 40°C or 35°C temperature rise. Maximum conductor operating temperature should not exceed 75°C for bare conductors to prevent annealing of aluminium. Conductor temperatures up to 210°C are possible with ‘GAP’ conductors. Wind Velocity Required for structure and conductor design. Electrical conductor ratings may be based on cross wind speeds of 0.5 m/s or longitudinal wind speeds of 1 m/s.

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Solar Radiation Required for conductor ratings but also for fittings such as composite insulators that may be affected by exposure to high thermal and ultraviolet (UV) radiation. Typical values of 850W/m^2 and 1200W/m^2 may be assumed for temperate and tropical conditions respectively. Rainfall Important in relation to flooding (necessity for extension legs on towers), corona discharge and associated electromagnetic interference, natural washing and insulator performance. Humidity Effect on insulator design. Altitude Effect on insulator and conductor voltage gradient. Ice and Snow Required for design of conductor sags and tensions. Build up can also affect insulation as well as conductor aerodynamic stability. Atmospheric Pollution Effect on insulation and choice of conductor. Soil Characteristics Electrically affecting grounding requirements (soil resistivity) and structurally the foundation design (weights, cohesion and angle of repose) Lightning Effect on insulation levels and also earth wire screening arrangements necessary to provide satisfactory outage performance. Seismic Factor Effect on tower and foundation design. General Loadings Refer also to IEC 60826 (Design criteria for overhead lines), EN50341 (Overhead lines exceeding AC45kV- supersedes 60826 for European use) and BS8100 (Loading and strength of overhead transmission lines).

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CONDUCTOR SELECTION. The selection of the conductors for 275kV/400kV overhead lines must take various factors into account. The conductors for 36kV and above both aluminium conductor steel reinforced (ACSR) and all aluminium alloy conductor (AAAC) can be used. ACSR conductors are used both because they are less costly than the other options and they have good mechanical strength properties. They also have widespread manufacturing capacity. Copper is a fantastic conductor material, having very high corrosion resistance and able to be used in conditions prone to sand blasting (desert). The downside to copper is the cost. It is more costly than aluminium. ACSR conductors have better resistance to long term creep or relaxation that can occur with AAAC conductors. This is due to the steel core in ACSR conductors. There are problems that can arise with the ACSR conductors. One such problem is bulge corrosion. This is corrosion that increases the conductor diameter by way of corrosion deposits that are a byproduct of bi-metallic corrosion. Because of the capacity for steel to corrode the corrosion of the steel core in ACSR conductors has been abated by using high temperature greases. The greases prevent the onset of galvanic corrosion between the galvanized steel core and the outer aluminium wires. The table below shows the typical properties of some ACSR conductors.

(Table taken from Bayliss.C 1999 Transmission and Distribution Electrical Engineering. Newnes)

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Diagram of conductor arrangements for ACSR conductors.

(Diagram taken from Bayliss.C 1999 Transmission and Distribution Electrical Engineering. Newnes)

TASK 4 Overhead line fittings. There are many fittings used on 275kV/400kV overhead lines. The list below covers the important ones.

• Joints. Joints on overhead lines must be capable of withstanding the full line current, as well as the occasional fault current, in the ambient temperature range where the overhead line is located. Joints on ACSR conductors are constructed of two parts, a galvanized or stainless steel inner cylindrical sleeve and a surrounding outer cylindrical aluminium sleeve. Both sleeves are compressed together using a hydraulic press. Compression grease is used to coat the aluminium and steel to prevent corrosion.

Page 15: Further Electrical Power

• Stockbridge damper

(Taken from www.pfisterer.com)

The Stockbridge damper is designed to control Aeolian vibrations. The damper controls a vibration effect that is at a high frequency. The effect of Aeolian vibration can be damage to the conductor and eventual possible failure.

• Spacer damper

(Taken from www.pfisterer.com)

The spacer damper is used to maintain conductor separation. It also has a role in combating vibration.

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• Armour rods

(Taken from www.pfisterer.com)

Armour rods are used to protect the conductor from compression, bending, abrasion and flash over. They are also used to repair damaged aluminium based conductors and restore mechanical strength and conductivity.

• Suspension clamps

(Taken from Overhead Line Fittings Equipment Ltd)

Suspension clamps are used to suspend the conductor.

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• Grip suspension clamps.

(Taken from www.pfisterer.com)

Armour grip suspension clamps are recognized as the best transmission suspension system since their introduction in 1951.

TASK 5 Faults on transmission systems

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The typical faults that can occur on the transmission line shown previously are lightning strikes or items coming into contact with the line, phase to phase from damage to conductor fittings or from wind/environmental damage. When faults do occur there are systems that detect, locate and initiate the removal of the faulted equipment from the system in the shortest desirable time. Relay protection is employed to carry out these tasks. For overhead line networks distance relays are more commonly used, as these are faster acting compared with current operated relays. Distance relays measure the impedance of the line they are protecting. If a fault occurs the ratio of applied voltage to current changes and the impedance decreases as the current increases. The relay senses this change and trips the appropriate breakers. The impedance of an overhead line is proportional to its length. Distance relay protection schemes protect the line in three stages. The first zone covers from the relaying point to 80% of the way to the next substation. If a fault occurs in this zone the protection operates at the first substation. The next zone covers from the 80% point, past the next substation to the third substation. A fault in this zone causes the protection to operate at the first substation if zone 2s time limit provided the protection at substation 2 has not operated. The zone three covers an area a little way past the third substation. It has a longer delay. The measuring elements of each of the zones have settings related to the line impedance. Zone 1 is set to 80% of the line impedance. Zone 2 is set to 120% and zone three is set to between 200% to 300%.

(Taken from Reeves.E.A & Heathcote.M.J 2003. Electrical Pocket Book.)

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If a fault occurs in the zone1, polarized relay operates to cut off the supply at the first substation. If its distance is within the zone 2, the timer times out and trips the circuit breaker. If the fault is within the zone 3, the circuit breaker trips after the timer has timed out.

CONCLUSION

The first task looked at the mechanical stability of steel lattice towers. Attention was paid to the properties of the tower parts and the forces acting upon them. This gives an insight into the considerations that the designers of overhead line steel lattice towers have to deal with. There are many design standards that must be adhered to also. The main ones are outlined in the report and analysis task 2. Along with the physical parts of the tower, research was carried out as to the environmental conditions such as temperature, humidity, altitude etc. The types of conductor was researched especially the most common type- ACSR conductors along with the different types of fittings found on high voltage overhead lines. Research was carried out into the protection of overhead lines involving the typical faults, distance relays and how they operate with zones of protection.

DISCUSSION

Aeolian vibrations are wind induced mechanical vibrations that act upon the overhead line conductors and shield wires. It is important to control these vibrations as they can cause damage that can result in reduced reliability and serviceability of the lines. Lines damaged by vibration often have to be de-rated or even taken out of service whilst repairs are made. The cause of the Aeolian vibration is small vortices or eddies that are formed when a smooth stream of air passes across a cylindrical shape. The vortices produce alternating pressures that produce movement at 90° to the direction of air flow. The type of air flow is critical in this scenario. Air that contains turbulence will not produce these vortices, only smooth air flow. The Aeolian vibrations will only generally become a problem in areas with wind speed below 15 miles per hour. Turbulence in air is brought about through wind speed and the terrain over which the wind passes.

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The frequencies at which the vortices alternate is inversely proportional to the diameter of the conductor. The equation shown below shows the relationship between this: Vortex frequency = 3.26V/d Where: V is the wind velocity component normal to the conductor in miles per hour. d is the conductor diameter in inches. 3.26 is an empirical constant. The vortex frequency of a 795 kcmil ACSR conductor under the influence of an 8mph wind is 23.5 Hz. Abrasion damage is caused by the vibration if the magnitude of the vibration is sufficient. Over time the surface of the conductor becomes worn away and can cause fatigue failure. The photos below shows abrasion damage:

(Taken from www.pfisterer.com)

(Taken from www.pfisterer.com)

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Dampers are used to reduce the level of Aeolian vibrations. Many designs have been tried since the early 1900s, with the most common type being the Stockbridge type damper, invented by G.H Stockbridge in 1924. This design works by having weights suspended from the ends of specially designed and manufactured steel strand, which is secured to the conductor with a clamp. When attached to a vibrating conductor the movement of the weights will produce bending of the steel strand. The individual wires in the strand rub together and thus dissipate the energy.

REFERENCES

• Reeves.E.A & Heathcote.M.J 2003. Electrical Pocket Book Newes

• Bayliss.C 1999 Transmission and Distribution Electrical Engineering. Newnes.

• www.pfisterer.com

• Overhead Line Fittings Equipment LTD