A Seminar Report on
HVDC TRANSMISSION TECHNOLOGY
Supervised by: Dr. NITIN GUPTA Assistant Professor
Submitted by: SAMEEKSHA GUPTA (2010UEE192)
DEPARTMENT OF ELECTRICAL ENGINEERING MALAVIYA NATIONAL INSTITUTE OF TECHNOLOGY JAIPUR
JANUARY 2014
1
CANDIDATES’ DECLARATION I
hereby
declare
that
the
seminar
report
entitled,
“HVDC
transmission technology” has been prepared by me under the guidance and supervision of Dr. Nitin Gupta during the Academic Session 2013 2014.
SAMEEKSHA GUPTA ID: 2010UEE192 Final yr B.Tech. Electrical Engineering
DEPARTMENT OF ELECTRICAL ENGINEERING MALAVIYA NATIONAL INSTITUTE OF TECHNOLOGY JAIPUR (INDIA)
2
CERTIFICATE This is to certify that the seminar report entitled,
“HVDC
Transmission technology” has been completed and submitted by the Final yr B. Tech. student Ms. Sameeksha Gupta (2010UEE192) under my guidance. The report was prepared during the academic Session 20132014. The report is found to be satisfactory and approved for submission.
Dr. Nitin Gupta Assistant Professor Department of Electrical Engineering
3
ACKNOWLEDGEMENT I express my deep sense of gratitude to my respected guide Dr.Nitin Gupta Assistant Professor Department of Electrical Engineering, Malaviya National Institute of Technology, Jaipur for his valuable guidance, continuous encouragement and supervision at every level of preparation of this seminar report. I am thankful to my seminar coordinators Mr. Ashok Kumar Agrawal, Associate Professor and Mr. Vipin Kumar Jain, Associate Professor, Department of Electrical Engineering for permitting me to present this seminar. I am also thankful to my friends for their valuable support and inspiration that helped me to prepare this report in a better way
4
CONTENTS Page No. CANDIDATE’S DECLARATION
2.
CERTIFICATE
3.
ACKNOWLEDGEMENT
4.
LIST OF FIGURES
7.
LIST OF TABLES
8.
LIST OF ABBREVIATIONS
9.
CHAPTER-1
10-15
INTRODUCTION 1.1 Definition 1.2 Need of HVDC Systems 1.3 Brief History 1.4 Use of HVDC Technology Around The Globe
CHAPTER-2 SYSTEM
WORKING OF HVDC TRANSMISSION
16-22
2.1 HVDC Transmission System 2.2Principles of AC/DC Conversion 2.3Transmission Modes 2.3.1Monopolar Link 2.3.2Bipolar Link 2.3.3Homopolar Link 2.4Principles of HVDC Control
CHAPTER-3
ADVANTAGES OF HVDC SYSTEM
23-25
3.1Interconnection of Power Networks 3.2Economics 3.3Long Distance Bulk Power Delivery 3.4Environmental Benefits
5
CHAPTER-4
26-27
DISADVANTAGES OF HVDC SYSTEM 4.1Cost 4.2Harmonics 4.3Integration of HVDC System into AC Netwroks 4.4Stability of Networks
CHAPTER-5
HVDC APPLICATION: Rural electrification using 28-30
overhead HVDC transmission lines 6.1Introduction 6.2Method 6.3Result 6.4Conclusion CHAPTER-6
HVDC PROJECTS IN INDIA
31-33
6.1HVDC Links in India 6.2HVDC Back to Back Projects 6.3HVDC Project Development Issues 6.4Future Prospect CHAPTER-7
OTHER APPLICATIONS 7.1Hybrid HVDC 7.2HVDC Light 7.3WAMS Enabled VSC-HVDC Control 7.4 WAMS Enabled Control for Oscillation Damping 7.5 WAMS Enabled Control for Maximum Power Transfer
CHAPTER-8
OPPORTUNITIES AND CHALLENGES 8.1LCC HVDC 8.1.1Advantages 8.1.2Disadvantages 8.2VSC Transmission 8.2.1Characteristics 8.3HVDC System Challenges 8.3.1Power Loss 8.3.2Dispatch And Control 8.3.3Integration of HVDC Network in AC network 8.3.4Harmonics 8.3.5Operation of HVDC with Ground fault
CHAPTER-9
CONCLUSION
34-36
37-41
42
6
REFERENCES
LIST of FIGURES FIGURE NO. 3.1 4.1 5.1 5.2 5.3 5.4 5.5 7.1 7.2 8.1 8.2
TITLE Investment Cost vs. Transmission Distance Cost Structure for HVDC Transmission Construction Main components of a HVDC transmission a typical arrangement Monopolar Line Bipolar Line Homopolar Line Converter Stations The Gotland HVDC Light Converter Station Application Control Functions of VSC-HVDC Circuit Diagram for LCC HVDC Circuit Diagram for VSC Transmission
PAGE NO. 19 21 25 26 27 27 28 34 34 37 38
7
LIST of TABLES
8
LIST of ABBREVIATIONS
9
CHAPTER 1 INTRODUCTION
1.1 DEFINITION A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current (AC) systems.
1.2 NEED FOR HVDC SYSTEMS For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance each cycle. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be warranted, due to other benefits of direct current links.
HVDC allows power transmission between unsynchronized AC transmission systems. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. HVDC also allows transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid, by allowing exchange of power between incompatible networks.
1.3 BRIEF HISTORY HVDC technology first made its mark in the early under-sea cable interconnections 10
of Gotland (1954) and Sardinia (1967), and then in long distance transmission with the Pacific Intertie (1970) and Nelson River (1973) schemes using mercury-arc valves. A significant milestone occurred in 1972 with the first Back to Back (BB) asynchronous interconnection at Eel River between Quebec and New Brunswick; this installation also marked the introduction of thyristor valves to the technology and replaced the earlier mercury-arc valves. The first 25 years of HVDC transmission were sustained by converters having mercury arc valves till the mid-1970s. The next 25 years till the year 2000 were sustained by line-commutated converters using thyristor valves. It is predicted that the next 25 years will be dominated by force-commutated converters . Initially, this new force-commutated era has commenced with Capacitor Commutated Converters (CCC) eventually to be replaced by self-commutated converters due to the economic availability of high power switching devices with their superior characteristics.
The first commercially used HVDC link in the world was built in 1954 between the mainland of Sweden and island of Gotland.
Since the technique of power
transmission by HVDC has been continuously developed.
In India, the first HVDC line in Rihand-Delhi in 1991 i.e. I 500 KV, 800 Mkl, 1000 KM. In Maharashtra in between Chandrapur & Padaghe at 1500 KV & 1000 MV. Global HVDC transmission capacity has increase from 20 MW in 1954 to 17.9 GW in 1984. Now the growth of DC transmission capacity has reached an average of 2500 MW/year.
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1.4 USE OF HVDC TECHNOLOGY AROUND THE GLOBE Here is a list of HVDC installations around the globe:
12
13
Table(contd). : Listing of HVDC installations
14
Table(contd). : Listing of HVDC installations
15
Table(contd). : Listing of HVDC installations
Notes for table:
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CHAPTER 2 WORKING OF HVDC TRANSMISSION SYSTEM
2.1 HVDC TRANSMISSION SYSTEM In case of HVDC transmission, following systems are used (Ref):(i) Two pole one wire. (ii) Two pole two wire. (iii) Three pole two wire. (iv) Three pole three wire.
The standard voltages used are :100 , 200, 300, 400, 600 & 800 KV. 17
The HVDC system is accepted for transmission of power for following reasons : (i) For long distance high power transmission. (ii) For interconnection between two a.c. systems having their own load frequency control. (iii) For back to back a synchronous tie substations. (iv) For under-ground or submarine cable transmission over long distance at high voltage.
At present, HVDC links have been installed in the world upto the year 2001, 100 links are expected with a total transfer capacity of 75000 MW. The choice between 400 KV A.C 705 KV AC, 1100 KV AC and HVDC transmission alternatives is made on the basis technical and economic studies for each particular line and associated A.C. system although, alternating current system continuous to be used for generation, transmission, distribution & utilization of electrical energy.
2.2 PRINCIPLES AC/DC CONVERSION HVDC transmission consists of two converter stations which are connected to each other by a DC cable or DC line. A typical arrangement of main components of an HVDC transmission is shown in fig.
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Two series connected 6 pulse converters (12-pulse bridge) consisting of valves & converters transformer are used. The valves convert AC to DC, and the transformer provide a suitable voltage ratio to achieve the desired direct voltage and galvanic separation of the AC & DC systems. A smoothing reactor in the DC ckt reduces the harmonic currents in the DC line, & possible transient over currents. Filters are used to take care of harmonics generated at the conversion. Thus we see that in an HVDC in an HVDC transmission, power is taken from one point in an AC network, where it is converted to DC in a converter station ( rectifier ), transmitted to another converter station (inverter) via line or cable and injected into an ac system.
By varying the firing angle & ( point on the voltage wave when the gating pulse is applied & conduction starts ) the DC output voltage can be controlled between two limits, +ve and negative. When a is varied, we get, maximum DC voltage when = 00. Rectifier operation when 0< < 900 Inverter operation When 900< < 1800 While discussion inverter operation, it is common to define extinction angle = 1800 .
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Fig.2.1: Main components of a HVDC transmission a typical arrangement
2.3 TRANSMISSION MODES
TYPES OF HVDC SYSTEMS Three types of dc links are considered in HVDC applications.
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2.3.1 Monopolar Link
A monopolar link has one conductor and uses either ground and/or sea return. A metallic return can also be used where concerns for harmonic interference and/or corrosion exist. In applications with dc cables (i.e. HVDC Light), a cable return is used. Since the corona effects in a dc line are substantially less with negative polarity of the conductor as compared to the positive polarity, a monopolar link is normally operated with negative polarity.
Fig.2.2:Monopolar Line
2.3.2 Bipolar Link
A bipolar link has two conductors, one positive and the other negative. Each terminal has two sets of converters of equal rating, in series on the dc side. The junction between the two sets of converters is grounded at one or both ends by the use of a short electrode line. Since both poles operate with equal currents under normal operation, there is zero ground current flowing under these conditions. Monopolar operation can also be used in the first stages of the development of a bipolar link. Alternatively, under faulty converter conditions, one dc line may be temporarily used as a metallic return with the use of suitable switching. 21
Fig.2.3:Bipolar Line
2.3.3 Homopolar Link
In this type of link two conductors having the same polarity (usually negative) can be operated with ground or metallic return. Due to the undesirability of operating a dc link with ground return, bipolar links are mostly used. A homopolar link has the advantage of reduced insulation costs, but the disadvantages of earth return outweigh the advantages.
22
Fig.2.4:Homopolar Line
2.4 PRINCIPLES OF HVDC CONTROL
One of the most important aspects or HVDC systems is its fast and stable controllability. In DC transmission, the transmitted power can be rapidly controlled by changing the DC voltages. The current in the system can only flow in one direction for a given setting power is transported from rectifies to inverter and by altering voltages, the power flow direction is reversed.
Fig.2.5:Converter Stations In HVDC transmission, one of the converter stations, generally the inverter station, is so controlled that the direct voltage of the system is fixed & has rigid relation to the 23
voltage on the AC side. Tap changers take care of the slow variations on the AC side the other terminal station (rectifier) adjust the direct voltage on its terminal so that the current is controlled to the desired transmitted power.
In fig.
( L – 1) where R is the Resistance of link & includes loop transmission resistance (if any), and resistance smoothing reactors and converter valves the power received is, therefore, given as
( L – 2)
The rectifier and inverter voltages are given by
( L – 3)
( L – 4)
Where, :- number of series connected bridges. :- line to line AC Voltages at the rectifier and inverter bridges, respectively. :- Commutation reactance at the rectifier and inverter, respectively.
24
From equation ( L-2). It is clear that the DC power per pole is controlled by relative control of DC terminal voltages, the converter control angles
control on DC voltage is exercised by as given by
Eqs ( L – 3) and ( L – 6 ). Normal operating range of control angles is :
The prime considerations in HVDC transmission are to minimise reactive power requirement at the terminals and to reduce the system losses. For this DC voltage should be as high as possible and
should be as low as possible.
25
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CHAPTER 3 ADVANTAGES OF HVDC SYSTEM
3.1 Interconnection of power networks
The significant advantage of HVDC systems is that it could be used as a tie line to interconnect separate AC networks. When two separately asynchronous ac systems, for example where one operates at a frequency of 50Hz and the other at 60Hz or where the two systems are operated at the same frequency but different phase angles, using DC link to connect the two ac systems is the only practical method. DC power is independent of the frequency and relative phase of the power systems. The HVDC interconnection between two ac systems will not suffer from power swings and risk of tripping arising from overload. HVDC interconnection’s performance is much better than ac interconnection.
HVDC asynchronous interconnection also has very good protection effect about outages transmit through power networks. August 14 2003, the blackout in Northeast America gives an example of protection effect from HVDC link . HVDC link prevented the outage developing past the asynchronous interconnection interface with Quebec when outage propagated through Qntario and New York.
A HVDC interconnection between power networks enhances power systems in capacity, controllability and improves
power delivery rate. With HVDC
interconnections, transmitting additional power through the AC systems can be achieved, which produces a mean to improve systems capacity. Based on a constant power transfer, it is easy to control active power in HVDC link. 27
3.2 Economics
For the same transmission capacity, HVDC transmission lines cost less than HVAC transmission lines in the same length. Fig shows the investment costs for and overhead line transmission with AC and HVDC.
Fig.3.1: Investment Cost vs. Distance Plot
As can be seen from Figure above a certain distance, the break-even-distance, the costs of HVDC transmission line are much smaller than AC transmission line. A bipolar system only has two lines compared to three lines in an AC system which results in a smaller cost in tower design and construct for delivering the same capacity power. The Three Gorges Project in China would require 5 x 500kV ac lines 28
compared to the 2 x ±500kV, 3000MW bipolar HVDC lines used. Savings also could be found in control and maintenance devices costs.
3.3 Long distance bulk power delivery
HVDC has a good performance in long distance bulk power delivery with underground and submarine cables. It can transfer more power in fewer lines than in AC system under the same situation. In an ac system the reactive power flow which caused by the cable resistance limits the transmission distance and adds costs. Furthermore, reactive power compensation is needed in ac transmission system for long distance power delivery. Unlike the case of ac transmission, HVDC system performs better. Lower line losses and economic benefits make HVDC a better alternative for long-distance power delivery. Using underground and submarine cables, there is no distance limitation for power delivery and about a half the line losses of comparable ac system. HVDC transmission system is considered to be a better choice for connecting offshore wind farms to grid or delivering power from remote resources to large Urban areas.
3.4 Environmental benefits
When we connect different ac systems by HVDC links, it effectively means there is no need to build new power stations additionally near to the demand locations. Reference highlighted that there is no induction or alternating electro-magnetic fields form HVDC transmission. No skin effects, effective cable transmission and lower losses ensure that there are less environmental impacts. 29
CHAPTER 4 DISADVANTAGES OF HVDC SYSTEM
4.1 Cost
As can be seen from the figure below, the highest cost in constructing HVDC 30
transmission system is spent on power electronics and converter transformers.
Fig.4.1 Cost Structure for HVDC Transmission Construction To build a converter station is much more expensive than an ordinary ac substation of similar rating because a better technical performance of a HVDC system needs many more components.
4.2 Harmonics
All electronic converters produce harmonics during the conversion process. In modern HVDC systems, the number of connected converters increased, the harmonics are also increased. Harmonics will affect power quality, electronic devices and even lead to system oscillation. The harmonics are recognized as one of the biggest problems in HVDC systems.
31
4.3 Integration of HVDC system into ac networks
Connecting a HVDC system to an ac system is a challenging project. In HVDC systems, the large ac harmonic filters can cause significant over-voltages during fault recovery . However, HVDC system gives good performance of fault protection in power networks.
4.4 Stability of the networks
In the future, power grids are expected to have more and more HVDC interconnections. The interactions between these multiple HVDC schemes will become more important. Communication failures between these HVDC schemes may result in system instability .
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CHAPTER 5 HVDC APPLICATION: Rural electrification using overhead HVDC transmission lines
5.1 Introduction
One of mankind’s greatest modern challenges is poverty alleviation. The provision of electricity can greatly assist in this regard. The tapping of small amounts of power from an HVDC transmission line represents a solution to this problem especially in rural areas. This paper analyses the dynamic characteristics of a parallel-cascaded tapping station. The results obtained clearly indicate that the parallel-cascaded tapping station proves to be a viable solution to tapping small amounts of power from an HVDC transmission line.
Orthodox methods for the provision of electricity supplies, such as a central power station with a transmission and distribution network, may not be the most economical means of providing electricity supplies in developing countries, particularly in rural 33
areas where the demand per customer is only a small fraction of a kW . Mobilising of capital and developing of new technologies is necessary in supplying power to these rural areas.
Other than various advantages of, HVDC transmission,it does suffer a significant disadvantage compared to high voltage alternating current (HVAC) transmission, with regard to tapping off power from transmission lines. It has not been proven to be economically and technically feasible to tap off small amounts of power from HVDC transmission lines. This is a substantial drawback considering that most HVDC transmission lines pass over many rural communities that have little or no access to electricity.
The parallel-cascaded tapping station proves to be a viable solution to tapping small amounts of power from an HVDC transmission line. But the main reasons for the non-application of this concept are that the rural villages, into which the power will be tapped, usually have weak AC systems, which have few or no rotating machine loads. Also the issue currently at hand is whether the tapping station should be connected in series or in parallel to the HVDC transmission line.
Although research has shown satisfactory results for one series tap connected at the middle of the HVDC transmission line, taking Example of the African context, HVDC transmission line transporting power from Central Africa to Southern Africa will be at least 3000 km long. Therefore, it is very likely that the HVDC transmission line will pass more than two (maybe more than 10) rural communities, spaced along the HVDC transmission line. Hence, it would not be economically feasible to have one series tap at the middle of the HVDC transmission line supplying power to all these communities. Further, a series tap causes a volt drop on the HVDC transmission line, which increases the main rectifier and inverter thyristor valve losses and stresses.
34
There is therefore a need to devise a method for multiple power tap offs from HVDC transmission lines for rural applications.
5.2 Method:
Firstly, a novel DC-to-DC converter was designed for connection in parallel with the HVDC transmission line and step down the high DC voltage to a lower DC voltage. Secondly, a voltage source inverter was used to invert the lower DC voltage into a three-phase voltage. Voltage source converters (VSC) feeds power to AC systems with low short circuit ratio or even passive networks with no local power generation. To compensate for the converter transformer, the load was connected in a delta configuration, which was the same way the winding on the converter side of the transformer was connected. The function of the delta configuration in this application was to eliminate the DC component of the phase voltage. To reduce the voltage stress on the VSI IGBT valves, a novel DC-to-DC converter was explored to step down the high transmission line DC voltage down to a lower voltage. A buck, step-down, convertor produces a lower average output DC voltage than the applied DC input voltage. The output voltage fluctuations are diminished by using a low- pass filter, consisting of an inductor and capacitor. The corner frequency fc of the lowpass filter is selected to be much lower than the switching frequency, thus essentially eliminating the switching frequency ripple in the output voltage.
35
5.3 Results:
The HVDC system characteristics during a three-phase faultfault is solidly grounded. The HVDC system takes approximately 0.6 s to stabilise after the clearance of the fault. A load change in the rural AC system has an unnoticeable effect on the HVDC system.
5.4 Conclusions:
The parallel-cascaded tapping station demonstrated that it has a negligible effect on the dynamic performance of the main HVDC link. The results obtained clearly indicate that the parallel- cascaded tapping station proves to be a viable solution to tapping small amounts of power from an HVDC transmission line. Therefore, HVDC transmission need not suffer a significant disadvantage compared to high voltage alternating current (HVAC) transmission, since power can now be tapped off from HVDC transmission lines.
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CHAPTER 6 HVDC PROJECTS IN INDIA
6.1 HVDC links in India The first HVDC link to be commissioned in the country was Rihand-Dadri in 1991 connecting. Thermal power plant in Rihand, Uttar Pradesh (Eastern Part of Northern Grid) with Dadri (Western Part of Northern Grid). It has a line length of about 816 km. It was built by ABB and is currently owned by PGCIL. Each Pole has a continuous power carrying capacity of 750 MW with about 10% two hours overload and 33% five seconds overload of 6x315 MVA at Rihand Terminal and 6x305 MVA at Dadri Terminal. The next project, Chandrapur-Padge HVDC link connecting Chandrapur (Central India) and Padge (Mumbai) in 1999. It transmits 1500 MW power over 752 km and helps in stabilizing the Maharashtra grid by increasing power flow on the existing 400 KV lines and minimizing total line losses. The Talcher-Kolar link connecting Talcher, (Odisha) with Kolar, (Karnataka) was completed in June 2003, designed for transmission of 2000 MW continuous rating with inherent short term overload capacity over 1369 km, making it the longest HVDC link with a converter transformer rating of 6x398 MVA. The 780 km HVDC link connecting Ballia, Uttar Pradesh and Bhiwadi, Rajasthan in monopolar mode in March 2010 and was furthered to operate in bipolar mode in March 2011. During inclement weather conditions it operates at 70-80% DC voltage owing to reverse power flow capability with a converter transformer rating of 8x498 MVA on both side. The MundraMohindergarh link has been the most recently commissioned HVDC link connecting the Western region to the Northern region for over 986 km operating at 1500 MW. It is the first link to be commissioned by a private firm (The Adani Group).
37
6.2 HVDC Back to Back Projects
The first commercial Back to back HVDC project Vindyanchal (commissioned in April 1989) distributes power of 2x250 MW and connects Vindhyanchal Super Thermal Power Station to Singrauli Super Thermal Power Station. It has the advantage of bidirectional power flow. The plant achieves the load diversity of Northern and Western region of the Indian Grid using a Converter Transformer of 8x156 MVA. Chandrapur back to back was the second such project, commissioned in 1993 connecting Chandrapur Thermal Power Station to Ramagundum Thermal Power Station. Coupled with the bidirectional power flow capability, it achieves load diversity of Western and Southern Region of the Indian Grid with a Converter Transformer of 12x234 MVA. Sasaram Back to back was commissioned in September 2002 delivering 500 MW having a Converter Transformer rating of 6x234 MVA. It connects Pusali (Eastern Region) to Sasaram (Eastern part of Northern grid). The Block 1 of the Gazuwaka back to back HVDC Project was commissioned in 1999 and Block 2 in March 2005. It connects Jeypore to Gazuwaka Thermal Station with a converter transformer rating of 6x234 MVA for block 1 and 6x201.2 MVA for block 2. It meets the high demand of southern region using the surplus power available.
6.3 HVDC PROJECT DEVELOPMENT ISSUES
There are various concerns regarding the above mentioned system which include creation of high capacity long distance transmission corridors to enable minimum cost per MW transfer, the complexity involved in realizing and extending present systems to Multi-Terminal systems, limited overload capacity of the static inverters coupled with the difficulty in installation. The high cost of installation of the plant due to the umpteen number of protection equipment required to eliminate the harmonics have been some of the issues faced in the development of existing HVDC systems. It has 38
also been observed that implementation on DC circuit breakers is a complex task owing to the requirement of current being made zero forcefully which helps prevents arcing and contact wear and hence reliable switching. And the project so developed should also have minimal effect on the environment. Thus, to account for the ever increasing demand of power, strong, lossless transmission methods need to be developed between the generating stations and the bulk power consumers.
6.4 FUTURE PROSPECT Various projects are being planned which include the introduction of 800 KV, 3000 MW upgradable to 6000 MW Multiterminal systems, in order to facilitate the transfer of power from generating stations to bulk load centers. The proposed site for rectifier station is in Bishwanath Chariali and Alipurduar handling 3000 MW and the Inverter station at Agra handling 6000 MW power. This system is proposed to originate from Assam and pass through West Bengal, Bihar and terminate in Uttar Pradesh with an approximate length of 1728 km. It will be the highest capacity HVDC project of the world considering the continuous 33% overload feature. Each pole of the multiterminal shall been designed for 2000 MW which are the highest capacity poles in the world. The Earth Electrode shall be designed for 5000 Ampere DC continuous current which shall be the first of its kind in the world. This project is expected to commission by 2015. It also includes the extension of the Mundra- Mohindergarh HVDC link currently operating at 1500 MW to its full installed capacity of 2500 MW. The proposed HVDC link project by PGCIL between India and Sri Lanka connecting Madurai (Southern India) and Anuradhapura (Central Sri Lanka) would be of 285km length including 50km of submarine cables. The project would take the final form in two phases, first would enable the transfer of 500 MW and 1000 MW,the target capacity in the second phase in near future. Such a connection would enable the two countries to sell excess energy thus saving resources. Another proposed HVDC link connecting Behrampur (India) with Bheramara (Bangladesh) is announced by Power Grid Corporation of India Limited (PGCIL) and Bangladesh Power Development Board (BPDB).The line will have initial transfer capacity of 500 MW, which will later 39
be increased to 1000 MW. The 125 Km line will cover 40 km of its length in Bangladesh and rest in India. Bangladesh is supposed to start spelling 250 MW Power by the end of 2012. Further, research has been going on in the field of implementation of Adaptive Neuro-Fuzzy logic for the fault identification of the present HVDC systems. The ANFIS system has an advantage over normal controllers in the fact that they do not require mathematical modeling i.e. absolute data to work. In the present installations, 70% of the data would be provided to the ANFIS system and the rest 30% would be left for testing and validation. The circuit shall be enriched with a conventional PI controller to help store the results. Another advantage of using this technique would be in terms of the delay angle. Earlier in fault identification systems, the entire working of the circuit depended on the correct choice of the delay angle which had an upper limit usually of 60°. However, no such limitation exists in this system.
CHAPTER 7 OTHER APPLICATIONS
7.1 Hybrid HVDC Hybrid HVDC combines the advantages of conventional HVDC and VSC-HVDC.
Characteristics of Hybrid HVDC: 1. The power transmitted in the Hybrid HVDC varies fiom a few MW to hundreds of MW. 2. VSC on the receiving side has the turn-off capability and can work as a passive inverter. 3. VSC on the receiving side can maintain voltage and frequency stability in independence on the AC system. 4. The VSC not only requires no reactive power &om AC system but also can 40
operate as STATCOM to compensate reactive power dynamically. 5. No short circuit capacity increase takes place in the AC system since the AC current of VSC is controllable. 6. The sending converter adopts conventional HVDC rectifier system with perfect technology and low price.
7.2. HVDC LIGHT
HVDC Light is a balanced converter technology, which makes it natural to operate in a bipolar mode. The converter control is based on the Pulse Width Modulation (PWM) concept, which enables flexible controllability of active and reactive power. Advantages of HVDC Light cables: 1. The new HVDC Light cables have insulation of extruded polymer. The robustness of the cable opens the way for new cable applications i.g. direct ploughing of underground cables, insulated aerial cables and submarine cables for particularlysevere conditions. 2. As the polymeric DC insulation is thinner than for an extruded AC cable of the same voltage, the HVDC Light will have a more dense power capacity. 3. Overcoming limitations due to voltage stability. 4. Direct access to load centers.
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Fig.7.1: The Gotland HVDC Light Converter Station
7.3 WAMS Enabled VSC-HVDC Control
Fig.7.2: Application control functions of VSC-HVDC
A broad range of application control functions can be implemented in VSC-HVDC 42
systems for enhancement of ac network steady-state and dynamic performance.
Wide area measurement systems could enhance the performance of VSC-HVDC systems by providing the necessary remote measurements to initiate effective control for transfer capability improvement and against disturbances such as power oscillations.
7.4 WAMS Enabled Control for Oscillation Damping
VSC-HVDC system could superimpose modulated active power to damp oscillations in the ac system. A feedback signal such as from active power flow measurement could be used to drive a supplementary damping control scheme.
7.5 WAMS Enabled Control for Maximum Power Transfer A system with voltage stability limits along a transmission corridor experience congestion due to accompanying transmission constraint. Embedded VSC-HVDC provides counter measures for both transient and longer term voltage instability mechanisms. Fast modulation of its reactive power could provide the VAR requirements for the transient problem.
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CHAPTER 8 OPPORTUNITIES AND CHALLENGES
Here are the technical issues faced by users of HVDC transmission and how HVDC could be made more generally acceptable as a transmission solution are discussed.
HVDC transmission is available in two different technologies, i.e. line-commutated current-sourced converter (LCC HVDC) and self-commutated voltage sourced converters (VSC Transmission). Both technologies convert ac to dc and vice versa, and use direct current for transmission between terminals. This means that power transmission can be performed between asynchronous networks. There is no reactive power flow on the dc line, therefore, there is no technical limit to the transmission distance.
The limit to distance is economic, since the power loss in the transmission line may eventually become unacceptably high, when practical conductor diameters are used. The practical transmission distance increases with the voltage.
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8.1 LCC HVDC
Mono-polar LCC HVDC scheme, which has one converter at each end and provides a Single transmission block. It is generally considered equivalent to a single-circuit ac transmission link. Rectifier
Inverter
Fig.8.1: Circuit Diagram for LCC HVDC 8.1.1 ADVANTAGE:
The rectifier takes power from its ac network and the inverter injects power into its ac network. Control systems control the two converters such that the desired active power is transmitted between the two. One terminal controls the de voltage, and the other the direct current. The active power between the converters is fully controlled and does not depend on the magnitude, phase angle or frequency of the ac voltage at either end of the HVDC scheme. The ability to rapidly control the active power can be very beneficial.
8.1.2 DISADVANTAGE:
HVDC converter station is many times (>10 times) larger than an equivalently rated ac substation. Because of their capacitance the ac harmonic filters reactive power banks can result in large ac over-voltages during load rejection and dynamic 45
conditions, e.g. during fault recovery
8.2 VSC Transmission
The scheme has one converter at each end and is a single transmission block. The Voltage Sourced Converter (VSC) creates an ac voltage by switching the ac terminals between the dc terminals The IGBT can withstand voltage and conduct current in one direction only, and use a diode connected in anti parallel, to enable the converter to conduct direct current in both directions.
8.2.1 CHARACTERISTICS : Filters are required only for higher frequency harmonics, and can be much lower rating than those used for LCC HVDC schemes.
Sending end
VSCA
Rdc I dc
VSC E.
Receiving end
Fig.8.2: Circuit Diagram for VSC Transmission
The reactive power exchange can be controlled independently at the two converters, 46
and independently of the active power transmission. The ability to control the reactive power at the ac terminals is one of the most significant differences between a VSC Transmission scheme and a LCC HVDC scheme.
VSC Transmission scheme generates its own ac voltage from the dc capacitor, which means that it can operate as a power supply to a passive ac network. VSC Transmission scheme using the latest technology will have an efficiency at full load
of >96.5%, excluding the power loss in the transmission line.
8.3 HVDC System challenges
8.3.1 POWER LOSS
The power loss in a HVDC converter station is higher than that in an ac substation, because of the conversion between ac and dc and the harmonics produced by this process. However, the power loss in a HVDC transmission line can be 50 to 70% of that in an equivalent HVAC transmission line. Thus for large distances, an HVDC solution may have lower loss.
Significant reduction in the power loss of a HVDC can be achieved by use of Silicon Carbide, diamond or other materials.
8.3.2 DISPATCH AND CONTROL
A LCC HVDC scheme can change its power factor by the switching of ac harmonic 47
filters and shunt capacitors/reactors. The resulting control of reactive power/ac voltage is in steps, which is generally acceptable to the ac network, particularly if the ac network is relatively strong. Smooth control of the reactive power by aLeC HVDC scheme could be achieved by the addition of a SVC at the ac terminals. In principle, the reactive power could also be controlled by the insertion of a TCSC in series with the converter transformer impedance. The reactive power could also be controlled by the converter firing angle, and the steady state impact at the other terminal could be eliminated through converter transformer tap changer.
One of the great benefits of any type of HVDC scheme is that its active power can be controlled irrespective of the ac voltage phase angle or angle at its terminals. Grid codes typically stipulate that a generator has to be able to operate with a controllable power factor, and that the reactive power capability has to be available throughout most of its operating range. Typically, ac voltage controllability is also required. The ability of a VSC Transmission scheme to control the reactive power at its two terminals independently of each other and independently of the active power.
8.3.3 INTEGRATION OF HVDC NETWORK IN AC NETWORK
Integration of a HVDC terminal into an ac system requires some specialist engineering. The large ac harmonic filters, particularly for LCC HVDC, can cause significant overvoltages during fault recovery, if the ac network strength is relatively weak. Development in HVDC control has resulted in improved performance during and after faults in the ac network, and the perfonmance can be optimised to suit particular network requirements. Nonetheless, the performance is different from that of an ac connection, and network planners have a natural tendency to use the more familiar ac options, even though the system performance could, in some cases, be improved with an HVDC scheme. The dynamic and transient performance of an HVDC scheme can be improved by the incorporation of dynamic reactive power control capability. 48
8.3.4 Harmonics
All power electronic converters produce harmonics as a byproduct of the conversion process. In order to prevent these harmonics spreading into the ac network, where they could cause problems, ac harmonic filters are used at the ac terminals of the HVDC scheme. Since LCC HVDC produces harmonics at relatively low frequencies (primarily 550Hz and above), the problem is worse for this type of HVDC than it is for VSC Transmission (usually > 1kHz). Another issue is that the ac harmonic filters and any shunt capacitor banks used for reactive power compensation can actually cause magnification of the distortion caused by other remote harmonic sources.
8.3.5 Operation of HVDC Scheme with Ground return The cost of an HVDC system can be significantly if it is permissible to operate with a single/HV metallic conductor. Furthermore, the power loss in the transmission line duri ng earth return operation is almost half of that applicable to operation with a LV metallic return conductor. Early HVDC schemes routinely used earth or sea electrodes for the neutral return current, when operating in mono-polar mode. Care must be taken in the design and location of electrodes, since the direct current flowing between them could result in corrosion of metallic structures.
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CHAPTER 9 CONCLUSION
In this report, an overview of HVDC transmission systems has been presented. HVDC was first introduced in the 1950s. It produced many advantages including the interconnection of asynchronous networks, economic benefits, long-distance bulk power delivery and environmental benefits. Recently there have been world’s first ±800kV HVDC project in South China and an appeared HVDC transmission project in Indian. The growth in offshore wind farms and other renewable power stations in Europe in the future will lead to a new power grid and this is expected to be HVDC.
Both Advantages and disadvantages have been analysed and comparision of the various controls of HVDC technology have been carried out,which have great potential in transmitting power to offshore industry and will undoubtedly provide useful solutions in many fields in the future. Besides, the development of power electric devices will also promote HVDC technology advance significantly and HVDC systems have great prospects in the future.
The growth in environmental opposition and the need for energy diversity will result in a dramatic growth in the application of HVDC schemes, as a solution to future power transmission challenges. To enable the full potential for HVDC schemes to be exploited, it is necessary to take into account the issues which have been highlighted. Some aspects requires education of the public, some training of planners and the advisors of investors, and some requires R&D, primarily by the HVDC manufacturers.
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REFERENCES
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