D.N.V.K.BHARGAV13761A0274

EEE DEPARTMENT

HVDC TECHNOLOGY

OVERVIEW• Why HVDC lines?????• History• Basic Structure • Components• Types of HVDC Systems• HVDC Economical Attributes• HVDC Environmental Impacts• HVDC World Scenario• HVDC Indian Population• Emerging Applications• Limitations

WHY HVDC TRANSMISSION

?????

LIMITATIONS OF AC TRANSMISSION SYSTEMS

Inductive and capacitive elements of overhead lines and cables put limits to the transmission capacity and the transmission distance of AC transmission links.

This limitation is of particular significance for cables. Depending on the required transmission capacity, the system frequency and the loss evaluation, the achievable transmission distance for an AC cable will be in the range of 40 to 100 km. It will mainly be limited by the charging current.

Direct connection between two AC systems with different frequencies is not possible.

Direct connection between two AC systems with the same frequency or a new connection within a meshed grid may be impossible because of system instability, too high short-circuit levels or undesirable power flow scenarios.

HISTORY In 1882, a 50-km-long 2-kV DC transmission

line was built between Miesbach and Munich in Germany.

In 1941, the first contract for a commercial HVDC system was signed in Germany: 60 MW were to be supplied to the city of Berlin via an underground cable of 115 km length. The system with ±200 kV and 150 A was ready for energizing in 1945. It was never put into operation.

AC Transmision Line

Converter Station

Inverter Station

AC Transmission Line

DistributionLine

HVDC Transmission Line

TRANSMISSION SCENARIO

HVDC SYSTEM : STRUCTURE

COMPONENTS

• Thyristor Valves• Converter Transformer• Smoothing Reactor• Harmonic Filter• Surge Arrestor• DC Transmission Circuit• Control &Protection

THYRISTOR VALVES SETUP

THYRISTOR VALVES

The thyristor valves make the conversion from AC into DC and thus are the central component of any HVDC converter station.

Monitoring of the thyristor performance is achieved by a simple voltage divider circuit made from standard off-the-shelf resistors and capacitors .

SIX PULSE RECTIFIER

TWELVE PULSE RECTIFIER

CONVERTER TRANSFORMER

• The converter transformers transform the voltage of the AC busbar to the required entry voltage of the converter.

• At the same time, they ensure the voltage insulation necessary in order to make it possible to connect converter bridges in series on the DC side, as is necessary for HVDC technology.

CONVERTER TRANSFORMER ATTRIBUTES

Main components of the Converter Transformer are:

• Core• Tank• Windings • Bushings

HVDC transformers are subject to operating conditions that set them apart from conventional system or power transformers. These conditions include:

• Combined voltage stresses• High harmonics content of

the operating current• DC pre magnetization of the

core

SMOOTHING REACTOR

Functions of Smoothing Reactor:

• Prevention of intermittent current

• Limitation of the DC fault currents

• Prevention of resonance in the DC circuit

• Reducing harmonic currents

SMOOTHING REACTOR ATTRIBUTES

There are basically two types of reactor design:

• Air-insulated dry-type reactors

• Oil-insulated reactors in a tank

The reactor type should be selected taking the following aspects into consideration:

• Inductance• Costs• Maintenance and

location of spare units• Seismic requirements

HARMONIC FILTERS

The filter arrangements of an HVDC converter station have two main duties:

• to absorb harmonic currents generated by the HVDC converter and thus to reduce the impact of the harmonics on the connected AC systems, like AC voltage distortion and telephone interference

• to supply reactive power for compensating the demand of the converter station

DESIGN CRITERIA

Reactive Power RequirementsHarmonic Performance Requirements• (the characteristic harmonics are of the order

n = (12 * k) ± 1 (k = 1,2,3 ...). )Network Impedance

SURGE ARRESTERSSurge arresters are designed

optimally to the following requirements:

• Excellent pollution performance for coastal and desert regions or in areas with extreme industrial air pollution.

• High mechanical stability, e.g. for use in seismic zones.

• Extremely reliable pressure relief behavior for use in areas requiring special protection.

SURGE ARRESTERS CRITERIA The main task of an arrester is to protect the equipment

from the effects of over voltages. During normal operation, it should have no negative effect

on the power system. Moreover, the arrester must be able to withstand typical

surges without incurring any damage. Non-linear resistors with the following properties fulfill

these requirements: • Low resistance during surges so that over voltages are

limited• High resistance during normal operation in order to avoid

negative effects on the power system and• Sufficient energy absorption capability for stable operation

DC TRANSMISSION CIRCUIT

DC transmission lines are mechanically designed as it is practice for normal AC transmission lines; the main differences are:

• The conductor configuration• The electric field requirements• The insulation design

INSULATION ASPECTS

There are 3 different types of insulators applicable for DC transmission lines:

• Cap and pin type• Long-rod porcelain type• Composite long-rod type

Cap and Pin Porcelain Long-Rod Composite Long-Rod

Insulator string length

5270 mm31 insulators

5418 mm4 insulators

4450 mm1 insulator

Creepage per unit 570 mm 4402 mm 17640 mm

Weight of string 332 kg 200 kg 28 kg

Breaking load 160 kN 160 kN 160 kN

DC CONDUCTORS

400 800700600500

500

2000

1000

1500

0

1 43 76

5 64

3

2

542

SYSTEM VOLTAGE (±KV)

ALTITUDE(METER)

DC CABLE

For HVDC submarine cables there are different types available:

1. Mass-Impregnated Cable2. Oil-Filled Cable3. XLPE(Cross-Linked Poly Ethene)4. Lapped Thin Film Insulation

■2 Insulation material■3 Core screen■4 Lead alloy sheath■5 Polyethylene jacket■6 Reinforcement of steel tapes■7 Bedding■8 Armour of steel flat wires

■ 1 Conductor of copper-shaped wires

CONTROL & PROTECTION

Main objectives for the implementation of the HVDC control system are reliable energy transmission which operates highly efficient and flexible energy flow that responds to sudden changes in demand thus contributing to network stability.

The control is divided into the following hierarchical levels:

• Operator control level (WIN CC)• Control and protection level (Simatic TDC)• Field level (I/Os, time tagging, interlocking)

HVDC CONFIGURATIONS

HVDC ATTRIBUTES

ENVIRONMENTAL ISSUES

• An HVDC transmission system can produce effects on environment through several potential impacts occurring during both its construction and working phase

• Some main recurring impacts during the construction phase are related to cable landing, that could generate interferences to marine flora and fauna, due for example to sediment suspension, use of sonar, noise emission and possible collision with operation vessels.

• During the working phase the generation of electric and magnetic fields are predictable; this can annoy the marine fauna and navigation system based on magnetic compasses.

Usually, the main environmental components that could be interfered by an HVDC project are:

• Water (sea bottoms, water streams, quality water)• Biosphere (marine and land flora, fauna and habitat)• Population (interferences with fishing and

navigation, traffic)• Landscape (landscape degradation)• Soil (occupation and consume of valuable soils)• Electric and Magnetic fields• Micro clime (heat generation)• Noise (annoyance to marine fauna and population)

HVDC TRANDMISSION:WORLD PICTURE

HVDC SCENARIO IN INDIA

HVDC LINK CONNECTING REGION

CAPACITY (MW)

Vindyachal North – West 2 x 250

Chandrapur West – South 2 x 500

Vizag – I East – South 500

Sasaram East – North 500

Vizag – II East – South 500

EMERGING APPLICATIONS

HVDC can be effectively used in a number of key areas as follows 

• Power supply to island. • Remote small-scale generation  • Off-shore generation and deep sea crossing  • Multi-terminal systems. • From the technology point of view, wind farm and

off-shore wind farms in particular are well-suited for VSC-HVDC application.

ADVANTAGES OF DC TRANSMISSION SYSTEMS

There are two conductors used in DC transmission while three conductors required in AC transmission.

There are no Inductance in DC transmission. Due to absence of inductance, there are very low voltage

drop in DC transmission lines comparing with AC (if both Load and sending end voltage is same)

There is no concept of Skin effect in DC transmission. Therefore, small cross sectional area conductor required.

A DC System has a less potential stress over AC system for same Voltage level. Therefore, a DC line requires less insulation.

In DC Line, Corona losses are very low.

CONTINUED……………. In High Voltage DC Transmission lines, there are no Dielectric losses. In DC Transmission system, there are no difficulties in synchronizing and

stability problems. DC system is more efficient than AC, therefore, the rate of price of Towers,

Poles, Insulators, and conductor are low so the system is economical. In DC System, the speed control range is greater than AC System. There is low insulation required in DC system (about 70%). The price of DC cables is low (Due to Low insulation) In DC Supply System, the Sheath losses in underground cables are low. DC system is suitable for High Power Transmission based on High Current

transmission. In DC System, The Value of charging current is quite low, therefore, the

length DC Transmission lines is greater than AC lines.

LIMITATIONS OF HVDC SYSTEMS• Due to commutation problem, Electric power can’t be produce at

High (DC) Voltage.• For High Voltage transmission, we can not step the level of DC

Voltage (As Transformer can not work on DC)• There is a limit of DC Switches and Circuit breakers (and costly

too)• Motor generator set is used for step down the level of DC voltage

and the efficiency of Motor-generator set is low than transformer.• So the system makes complex and costly.• The level of DC Voltage can not be change easily. So we can not

get desire voltage for Electrical and electronics appliances (such as 5 Volts, 9 Volts 15 Volts, 20 and 22 Volts etc) directly from Transmission system.

SUMMARY

HVDC systems remain the best economical and environmentally friendly option for the above conventional applications. However, three different dynamics - technology development, deregulation of electricity industry around the world, and a quantum leap in efforts to conserve the environment - are demanding a change in thinking that could make HVDC systems the preferred alternative to high voltage AC systems in many other situations as well.

REFERENCES

• Understanding Facts: Concepts and Technology of Flexible AC Transmission Systems, Narain G. Hingorani, Laszlo Gyugyi

• Flexible AC transmission systems, Song & Johns• www.google.com• Siemens HVDC references

http://www.energy.siemens.com/hq/en/power-transmission/hvdc/hvdc-ultra/references.htmAlstom HVDC resourceshttp://www.alstom.com/