High Voltage Direct Current Transmission System HVDC Seminar

Electrical and Electronics department

SEMINAR REPORT 2013

Seminar Report On

HIGH VOLTAGE DIRECT CURRENT TRANSMISSION SYSTEM

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING 2013

1


SEMINAR REPORT 2013

ACKNOWLEDGEMENT First of all I bow my head before God Almighty for the blessing he had showered on me for the success completion of this work. I would like to take this opportunity to extend my sincere gratitude to Prof. X, Head of Department, Electrical & Electronics Engineering, for extending every facility to complete my seminar work successfully. I would like to express my sincere indebtedness to Prof. X, Department of Electrical & Electronics Engineering, for her valuable guidance, wholehearted co-operation and duly approving the topic as staff in charge. I also extend my gratitude towards the staffs, students and parents for their sincere support and motivation.

.

2


SEMINAR REPORT 2013

ABSTRACT

High

Voltage

Direct

Current

(HVDC)

technology

has

characteristics which make it especially attractive in certain transmission applications. The number of HVDC projects committed or under consideration globally has increased in recent years reflecting a renewed interest in this field proven technology. New HVDC converter designs and improvements in conventional HVDC design have contributed to this trend. This paper provides an overview of the rationale for selection of HVDC technology and describes some of the latest technical developments

3


SEMINAR REPORT 2013

CONTENTS

INTRODUCTION……………………………………………………….4 WHY WE USE DC TRANSMISSION………………………………….7 HVDC CONVERTER ARRANGEMENT……………………………...7 APPLICATION OF HVDC CONVERTER……………………………..9 ENVIORNMENTEL CONSIDERATIONS……………………………..10 HVDC CONTROL AND OPERATION………………………………...14 COMPARISON OF AC AND DC TRANSMISSION…………………..14 INHERENT PROBLEMS ASSOCIATED WITH HVDC………………16 CONCLUSION…………………………………………………………..17 REFERENCE ………………………………………………………..…..18

4


SEMINAR REPORT 2013

INTRODUCTION Electric power transmission was originally developed with direct current. The availability of transformers and the development and improvement of induction motors at the beginning of 20th century, led to greater appeal and use of AC transmission. Through research and development in Sweden at Allmana Svenska Electriska Aktieboglet (ASEA), an improved multi-electrode grid controlled mercury arc valve of high powers and voltages was developed from 1929. Experimental plants were setup in 1930‟s. DC transmission now become practical when long distances were to be covered or where cables were required. The increase in need for electricity after the 2nd world war stimulated research. In 1950, a 116km experimental transmission line was commissioned from Moscow to Kasira at 200kv. The first commercial HVDC line built in 1954 was 98km submarine cable with ground return between the island of Gotland and the Swedish mainland. Thyristors were applied to dc transmission in the late 1960s and solid state values become a reality the highest functions dc voltage for dc transmissions is +/- 600kv for the 785km transmission line in brazil. Dc transmission is now an integral part of the delivery of electricity in many countries throughout the world.

5


SEMINAR REPORT 2013

WHY WE USE DC TRANSMISSION? The question is often asked, “Why we use DC transmission?” One response is that losses are lower. But this is not correct the level of losses is designed in to a transmission system and is regulated by the size of the conductor selected. DC and AC conductors either as over head transmission lines or submarine cables can have lower losses but at higher expense since the larger cross-sectional are will generally results in lower but cost more. When converters are used for DC transmission, it is generally by economic choice driven by one of the following reasons. 1. An overhead DC transmission line with its towers can be designed to be less costly per unit of length than an equivalent AC line designed to transmit the same level of electric power. However the DC converter stations at each end are more costly than the terminating station of an AC line and so there is a breakeven distance above which the total costs of DC transmission is less than its AC transmission alternative. The DC transmission has lower visual profile than an equivalent AC line and so contributes to a lower environmental impact. There are other environmental advantages to a DC transmission line through the electric magnetic fields being DC instead of AC. 2. If transmission is by submarine or underground cable, the breakeven distance is much less than overhead transmission. It is not practical to consider AC cable systems exceeding 50km but DC cable system are in service whose length is in the hundreds of kilometers and even distances of 600km or greater have been considered feasible. 3. Some AC power systems are not synchronized to the neighboring networks even though their physical distances between them os quite small. Thais occur in Japan where half the country is a 60hz network and other is 50hz system. It is physically impossible to connect the two

6


SEMINAR REPORT 2013

together by direct AC methods in order to exchange electric power between them. However if a DC converter station is located vthe required power flow even though the AC systems so connected remain asynchronous.

7


SEMINAR REPORT 2013

HVDC CONVERTER ARRANGMENT

HVDC converter bridges and lines or cables can be arranged into a number of configurations for effective utilization. Converter bridges may be arranged either monopolar or bipolar as shown in 12 pulse arrangement. Various ways HVDC transmission is used are shown in simplified form and include the following. BACK-to-BACK: there are some applications where two AC systems to be inter connected are physically in the same location or substation. No transmission line or cable is required between the converter bridges in this case and the connection may be monopolar or bipolar. Back-to-Back links are in Japan for interconnections between power system networks of different frequencies they are also used as interconnections between adjacent asynchronous networks. Transmission between Two Substations: When it is economical to transfer electric power through DC transmission or cables from one geographical location to another, a two-terminal or point-to-point HVDC transmission is used. In other words, DC power from a DC from a DC rectifier terminal is dedicated to one other terminal operating as an inverter. This is typical of most HVDC transmission system. Multiterminal HVDC Transmission System: When three or more HVDC substations are geographically separated with interconnecting transmission lines or cables, the HVDC transmission system is a multiterminal. If the entire substations are connected to the same voltage then the system is parallel multiterminal DC. Parallel multiterminal DC transmission has been applied when the substation capacity exceeds 10% of the total rectifier substation capacity. A combination of parallel and series connections of converter bridges is a hybrid multiterminal system.

8


SEMINAR REPORT 2013

Unit Connection: When DC transmission is applied right at the point of generation, it is possible to connect the converter transformer of the rectifier directly to the generator terminals so the generated power feeds in to the DC transmission lines. This might be applied with hydro and wind turbine driven generators so that maximum efficiency of the turbine can be achieved with speed control. Regardless of the turbine speed, the power is delivered through the inverter terminal to the AC receiving system at its fundamental frequency of 50 or 60 Hz. Diode Rectifier: It has been proposed that in some applications where DC power transmission is in one direction only, the valves in the rectifier converter bridges can be constructed from diodes instead of thyristors. Power flow control would be achieved at the inverter and in the case where the unit connection is used; AC voltage control by generator field exciter could be applied to regulate DC power. This connection may require high speed AC circuit breakers between the generators and the rectifier converter bridges to protect the diodes from over currents resulting from a sustained DC transmission line short circuit.

MONOPOLAR CONFIGURATION

BIPOLAR CONFIGURATION

9


SEMINAR REPORT 2013

APPLICATIONS OF HVDC CONVERTERS

The first application for HVDC converters was to provide point to point electric power interconnections between asynchronous AC power networks. There are other applications which can be met by HVDC converter transmission which include: Interconnection between asynchronous systems: Some continental electric power system consists of asynchronous networks such as East, West, Texas and Quebac networks in North America and island loads such as the island of Gotland in the Baltic Sea make good use of HVDC interconnections. Deliver energy from remote energy sources: Where generation has been developed at remote sites of available energy, HVDC transmission has been an economical means to bring the electricity to load centers. Import electric energy into congested load areas. In areas where new generation is impossible to bring into new service to meet load growth or replace inefficient or decommissioned plant, underground DC cable transmission is available means to import electricity. Increasing the capacity of existing AC transmission by conversion to DC transmission: New transmission rights of way may be possible to obtain. Existing overhead transmission lines if upgraded to or overbuilt with DC transmission can substantially increase the power transfer capability on the existing right of way. Power flow control: AC networks do not easily accommodate desired power flow control. Power marketers and system may require the power flow control capability provided by HVDC transmission. Stabilization of electric power networks: Some wide spread AC power system networks operate at stability limits well below the thermal capacity of their transmission conductors.

10


SEMINAR REPORT 2013

ENVIORMENTAL CONSIDERTAIONS The electrical environmental effects from HVDC transmission lines can be characterized by field and ion effects as well as corona effects (4), (5). The electric field arises from both the electrical charge on the conductor and for a HVDC overhead transmission line from charges on air ions and aerosols surrounding the conductor. These gives rise to DC as well as due to ion current density flowing through the air. A DC magnetic field is produced by DC current flowing through the conductors. Air ions produced by HVDC lines from clouds which drift away from the line when blown by the wind and may come in contact with humans, animals and plants outside the transmission lines right-of –way or corridor. The corona effects may produce low levels of radio interference, audible noise and ozone generation. Field and corona effects The field and corona effects of transmission lines largely favor DC transmission over AC transmission. The significant considerations are a follows 1. For a given power transfer requiring extra high voltage transmission, the DC transmission line will have a smaller tower profile than the equivalent AC transmission carrying the same level of power, this can also lead to less width of right- of-way for DC transmission option. 2. The steady and direct magnetic field of DC transmission line near at the edge of transmission right-of-way will be about the same value in magnitude as the earth‟s naturally occurring magnetic field. For this reason alone, it seems unlikely that this small contribution by HVDC transmission lines to the background geometric field would be the basis of concern. 3. The static and steady electric field from DC transmission at the levels experienced beneath lines or edges of the right-of –way have no known adverse biological effects. There is no theory or mechanism to explain

11


SEMINAR REPORT 2013

how a static electric field at the levels produced by DC transmission lines could effect human health. Electric fields from ac transmission lines have been under more intense scrutiny than fields generated from dc transmission lines. 4. The ion and corona effects of dc transmission line lead to a small contribution of ozone production to higher naturally occurring background concentrations. Exacting long term measurements are required to detect such concentrations. 5. If ground return is used with monopolar operation, the resulting dc magnetic field can cause error in magnetic compass readings taken in the vicinity of the DC line or cable. This impact is minimized by providing a conductor or cable return path in close proximity to the main conductor or cable for magnetic cancellation. Another concern with continuous ground current is that some of return current may flow in metallic structures and intensify corrosion if cathodic protection is not provided.

12


SEMINAR REPORT 2013

HVDC CONTROL & OPERATING PRINCIPLES

Conventional HVDC For conventional HVDC transmission one line sets the while the other terminal regulates the DC current by controlling its output voltage relative to that maintained by the voltage setting terminal. Since the DC line resistance is low large changes in current and hence power can be made with relatively small changes in firing angle „α‟. Two independent methods exist for controlling the converter DC output voltage. These are 1) By changing the ratio between direct voltage and AC voltage by varying delay angle or 2) By changing the converter AC voltage via load tap changers (LTC) on the converter transformer. Whereas the former method is rapid the later method is slow due to the limited speed of response of the LTC. Use of high delay angles to achieve the larger dynamic range, however increases the converter reactive power consumption. To minimize the reactive power demand while still providing adequate dynamic control range and commutation margin, the LTC is used at the rectifier terminal to give the delay angle within its desired steady state range .Example: 13-18 degrees and at the inverter to keep the extinction angle within its desired range, E.g.: 17-20 degrees if the angle is used for DC voltage control or to maintain rated DC voltage if operating in minimum commutation margin control mode. VSC-Based HVDC Power can be controlled by changing the phase angle of the converter AC voltage with respect to the filter bus voltage. Whereas the reactive power can be controlled by changing the magnitude of the fundamental component of the converter AC voltage with respect to the filter bus voltage. By controlling these two aspects of the converter voltage, operation in all four quadrants is possible. This means that the converter can be operated in the

13


SEMINAR REPORT 2013

middle of its reactive power range near unity power factor to maintain dynamic reactive power reserve for contingency voltage support similar to a static var compensator. It also means that the real power transfer can be changed rapidly without altering the reactive power exchange with the AC network or waiting for switching of shunt compensation.

14


SEMINAR REPORT 2013

COMPARISON OF AC AND DC TRANSMISSION Advantages of DC: 1. More power can be per conductor per circuit: The capabilities of power transmission of an AC link and DC link are different. For the same insulation, the direct voltage is equal to the peak value of the alternating voltage. For the same conductor size, the current can transmitted with both AC and DC, if skin effect is not considered. In practice, AC transmission is carried out using either single circuit or double circuit 3 phase transmission using 3 or 6 conductors. For DC only one-half the amount of copper is required for the same power transmission. 2. Use of Ground Return possible: In the case of HVDC transmission, ground return may be used, as in the case of a monopolar DC link. Also the single circuit bipolar DC link is more reliable, than the corresponding AC link, as in the event of a fault on one conductor the other conductor can continue to operate at the reduced power with ground return. For the same length of transmission, the impedance of the ground path is much less for DC than for the corresponding AC because DC spreads over a much larger width and depth. In fact, in the case of DC the ground path resistance is almost entirely depending on the earth electrode resistance at the two ends of the line, rather than on the line length. 3. Smaller Tower Size: The DC insulation level for the same power transmission is likely to be lower than the corresponding AC level. Also the DC line will only need two conductors whereas three conductors are required for AC. Thus both electrical and mechanical considerations dictate a smaller tower. 4. Higher Capacity Available for Cables: In contrast to the overhead line, in cable break down occurs by puncture and not by external flashover. Mainly due to the absence of ionic motion, the working stress DC insulation may be 3 to 4 times higher than under AC.

15

of

the


SEMINAR REPORT 2013

5. No Skin Effect: Under AC conditions, the current is not uniformly distributed over the cross section of the conductor. The current density is higher in the outer region and result in under utiliasation of the conductor cross section. Skin effect under conditions of smooth DC is completely absent and hence there is a uniform current in the conductor, and the conductor metal is better utilized. 6. Less Corona and Radio Interference: Since corona loss increases with the frequency, for given conductor diameter and applied voltage, there is much lower corona loss and hence more importantly less radio interference with DC. Due to this bundle conductors become unnecessary and hence give a substantial saving in line costs. 7. No Stability Problem: The DC link is an asynchronous link and hence any AC supplied through converters or DC generations do not have to be synchronized with the link. Hence the length of the DC link is not governed by stability. In AC links the phase angle between sending end and receiving end should not exceed 30 at full-load for transient stability. 8. Asynchronous

Interconnection

Possible:

With

AC

links,

interconnections between power systems must asynchronous. Thus different frequency systems cannot be interconnected. Such systems can be easily interconnected through HVDC links. 9. Lower Short Circuit Fault Levels: When an AC transmission system is extended, the fault level of whole system goes up, sometimes necessitating the expensive replacement of circuit breakers with those of higher fault levels. This problem can be overcome with HVDC as it does not contribute current to the AC short circuit beyond its rated current. 10.Tie Line power is easily Controlled: In the case of an AC tie line, the power cannot be easily controlled between the two systems. With DC tie lines, the control is easily accomplished through grid control. the reversal of the power flow is easy.

16


SEMINAR REPORT 2013

INHERENT PROBLEMS ASSOCIATED WITH HVDC

1. Expensive Convertors: Expensive convertor stations are required at each end of a DC transmission link, whereas only transformer stations are required in an AC link.

2. Reactive Power Requirement: Convertors require much reactive power, both in rectification as well as in inversion. At each convertor the reactive power consumed may be as much at 50% of the active power rating of the DC link. The Reactive power requirement is partly supplied by the filter capacitance, and partly by synchronous or static capacitors that need to be installed for the purpose.

3. Generation of Harmonics: Convertors generates a lot of harmonics both on the DC side and the AC side. Filters are used on the AC side to reduce the amount of harmonics transferred to the AC systems. on the DC system smoothing reactors are used. These components add to the cost of convertors.

4. Difficulty of Circuit Breaking: Due to the absence of a natural current zero with DC, circuit breaking is difficult. This is not a major problem in single HVDC link systems, as circuit Breaking can be accomplished by a very rapid absorbing of the energy back into the AC system.

5. Difficulty of Voltage Transformation: Power is generally used at low voltage, but for reasons of efficiency must be transmitted at high voltage. Absence of the equivalent of DC transformers makes it necessary for voltage transformation to carried out on the AC side of the system and prevents a purely DC system being used.

6. Difficulty of High Power generation: Due to the problems of commutation with DC machines, voltage, speed and size are limited. Thus comparatively lower power can be generated with DC.

17


SEMINAR REPORT 2013

CONCLUSION

HVDC transmission system is a very superior type of transmission system topology, which serves for power transmission and thus contributes the advantage like Use of ground return possible, Skin effect, Tower size etc. Although HVDC posses some disadvantages. The extent of advantages makes it a very suitable one for the transmission. For long distance transmission of electricity HVDC transmission is the best one than Extra High Voltage AC transmission.

18


SEMINAR REPORT 2013

REFERENCES

1. “A Refined HVDC Control System”- Ekstrom. A and Liss. G (IEEE) 2. “Rapid

City

Tie-New

Technology

Tames

The

East,

West

Interconnection”- M. Bahrman, D. Dickson, P. Fisher, M. Stolz.

3. “HVDC With Voltage Source Converter And Extruded Cables For Up to +/-300kv and 1000MW”- B.Jacobson, V. Jiang-Hafner, Rey, G. Asplund

4. “Multiterminal Integration of the Nicolet Convertor Station into the Quebec-New England phase II Transmission System”-D. McCallum, G. Moreau, J. Primeau.

19

High Voltage Direct Current Transmission System HVDC Seminar

Recommend Documents