Presentation

HVDC TRANSMISSION

11/05/2010

OUTLINE

2

Introduction.

History of HVDC.

HVAC versus HVDC.

Cost Structure.

Applications.

HVDC Schemes.

SLD of Bipolar Scheme.

HVDC Components and operation.

INTRODUCTION

3

Introduction

A high-voltage, direct current (HVDC) is an electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems.

For long-distance distribution, HVDC systems are less expensive and suffer lower electrical losses.

4

Introduction

Applications

TYPICAL HVDC APPLICATIONS

There are three typical HVDC applications:

Interconnection of non-synchronous AC power systems even at different frequencies.

Power transmission over long undersea cable links.

Point-to-point, long-distance transmission of large blocks of power.

5

1940s

High Voltage DC (HVDC) bulk power transmission was studied in Germany.

1954s

1970s

First commercial application in Sweden: submarine link between mainland and Gotland island (100 kV-20 MW-90 km).

Thyristors (SCRs) took over; today, HVDC operation voltages attain 600 kV, transmitted power over 3000 MW

HVDC has become the dominating technology for long distance transmission of bulk power.

Today ...

History of eventsIntroduction

Applications

History

HVAC Versus HVDC

6

Introduction

Applications

History

AC versus DC

Advantages of AC systems:

It’s possible to generate AC voltages at high voltage levels 11k,22K.etc using Synchronous generators.

The cost of synchronous generator is less than DC Generator, and requires less maintenance.

The AC voltage can be raised or lowered using transformers, it can be raised up to hundreds of kilo volts.

The AC substations are more efficient than DC substations using rectifiers, and also the maintenance of AC station is easier.

Its easy to convert from AC to DC when required.

HVAC Versus HVDC cont’d

7

Disadvantages of AC systems:

The Construction of AC transmission line is more complicated than DC transmission line.

The resistance of AC line is higher due to skin effect. The inductance and capacitance of AC line affects the

voltage profile and causes voltage drop. The copper requirement of AC line is more than DC line The AC line is more sensitive to corona than DC. Synchronization problems of alternators The speed of alternators required to be controlled

within very low limits.

Introduction

Applications

History

AC versus DC

HVAC Versus HVDC cont’d

8

Advantages of DC systems:

The absence of inductance and capacitance, as the frequency of DC is zero. And hence the power loss and voltage drop are much less compared to AC.

Absence of skin effect make use of entire cross section of the conductor.

Insulation required in DC systems is less compared to AC systems, because the stress on this insulation is less.

As stress is less the possibility of using under ground cable is much more possible.

The DC line has reduced interference with radio and communication systems.

Copper requirement is less in DC systems because Two wires are sufficient to transmit power, while we need 3 phases in AC systems

Introduction

Applications

History

AC versus DC

HVAC Versus HVDC cont’d

9

Disadvantages of DC systems:

The power generation is not possible at high voltages due to commutation problems.

The biggest disadvantage of DC systems is that it cannot be stepped up or down as the requirement, and so transmission over high voltage is not possible.

Obtaining AC from DC is not easy in practice.

The limitation of DC switches and circuit breakers causes a problem in DC system.

DC machines requires much more maintenance and there cost is also higher than DC machines

Introduction

Applications

History

AC versus DC

Cost Structure

10

The cost of an HVDC transmission system depends on many factors:

The power capacity to be transmitted.

Distance of transmission.

Type of transmission medium.

Safety and regulatory requirements.

Type of equipment used(filters, Transformers,Towers, conductors, insulators etc.).

Introduction

Applications

History

AC versus DC

Cost Structure

Cost Structure cont’d

11

It is difficult to give a cost figure for an HVDC system. Nevertheless, a typical cost structure for the converter stations could be as follows:

Introduction

Applications

History

AC versus DC

Cost Structure

Cost Structure cont’d

12

Practical Example, shows the price variation for an AC transmission compared with an HVDC transmission for 2000 MW is presented below :

For the AC transmission :

A double circuit line is assumed with a price per km of 250 kUSD/km (each).

AC substations and series compensation (above 600 km) are estimated to 80 MUSD.

For HVDC transmission:

bipolar OH line was assumed with a price per km of 250 kUSD/km.

converter stations are estimated to 250 MUSD.

Introduction

Applications

History

AC versus DC

Cost Structure

Cost Structure cont’d

13

Introduction

Applications

History

AC versus DC

Cost Structure

Cost Structure cont’d

14

Introduction

Applications

History

AC versus DC

Cost Structure

Cost Structure cont’d

15

The plot of the transmission distances versus cost for both HVDC and HVAC.

Break Even Distance

Introduction

Applications

History

AC versus DC

Cost Structure

Block Diagram of HVDC

16

Rectifier Station Inverter Station

resistance

ACsignal

ACsignal

VR

Vi

Id

Filter Filter

Introduction

Applications

History

AC versus DC

Cost Structure

Block Diagram

HVDC Schemes

17

Monopolar HVDC System with Ground Return:

Consists of one or more six-pulse converter units in series or parallel at each end.

a single conductor and return through the earth or sea, as shown below.

At each end of the line, it requires an electrode line and a ground or sea electrode built for continuous operation.

Introduction

Applications

History

AC versus DC

Cost Structure

Block Diagram

HVDC Schemes

HVDC Schemes cont’d

18

Advantages of Monopolar System:

Minimizing cable cost, (attractive).

Minimizing losses.

Simple installation.

Introduction

Applications

History

AC versus DC

Cost Structure

Block Diagram

HVDC Schemes

HVDC Schemes cont’d

19

Disadvantages of Monopolar System:

Sea earth return path, leads to corrosion of other metallic objects(pipelines, cable, sheaths, etc.).

Production of chlorine gas and impact of fish population.

The ground current may also cause interference problems.

The ground current may cause saturation in transformers that are located in the vicinity.

Introduction

Applications

History

AC versus DC

Cost Structure

Block Diagram

HVDC Schemes

HVDC Schemes cont’d

20

A Monopolar HVDC System with Metallic Return:

Advantages: Solve the disadvantages of electrode earth.

Disadvantages: Increases losses due to voltage drop in the metallic

return. Increases the cost.

Introduction

Applications

History

AC versus DC

Cost Structure

Block Diagram

HVDC Schemes

HVDC Schemes cont’d

21

Bipolar HVDC System.

consists of two poles, each of which includes one or

more twelve-pulse converter units, in series or parallel. its a combination of two monopolar schemes with

ground return

Introduction

Applications

History

AC versus DC

Cost Structure

Block Diagram

HVDC Schemes

HVDC Schemes cont’d

22

Advantages of Bipolar HVDC System:

During an outage of one pole, the other could be operated continuously with ground return, or metallic return.

Bidirectional power transfer.

More power handling capability.

Disadvantages of Bipolar HVDC System:

Higher cost.

Introduction

Applications

History

AC versus DC

Cost Structure

Block Diagram

HVDC Schemes

HVDC Schemes cont’d

23

Back to Back HVDC System:

Introduction

Applications

History

AC versus DC

Cost Structure

Block Diagram

HVDC Schemes

HVDC Schemes cont’d

24

500 MW Back-to-Back Converter StationIntroduction

Applications

History

AC versus DC

Cost Structure

Block Diagram

HVDC Schemes

Conclusion

25

Introduction

Applications

History

AC versus DC

Cost Structure

Block Diagram

HVDC Schemes

HVDC Systems are suitable only for Transmission over Distances greater than Break over Distance.

HVDC has become the Dominant technology of Bulk power Transmission.

HVDC components are very large and expensive.

Several Schemes available for HVDC , Bipolar HVDC scheme is the most used.

HVDC is based on the evolution of power electronics.