Download - High Voltage Transmission System
HIGH VOLTAGE TRANSMISSION SYSTEM BEEDEE 709/ MPSDEE 709
VII SEMESTER B.TECH./ M.TECH. (Integrated)
SASTRA UNIVERSITY SCHOOL OF ELECTRICAL & ELECTRONICS ENGINEERING
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
COURSE INSTRUCTOR: Dr. S. VENKATESH/ SAP-SEEE
1
COURSE OBJECTIVES • Provide insights into modelling and design of EHV lines which cater to the following:
Computational methods to obtain the line inductance and capacitance for various configurations
Various factors that affect the design of line parameters
• Present an overview of HVDC converter operation and strategies for control of power flow in DC lines
Generic Converter Configuration
Graetz Bridge Converter Operation (Rectifier and Inverter)
Detailed Analysis Graetz Bridge Converter (With and Without Overlap Angle)
Control Aspects and Control Characteristics
De-energization and Energization of DC LInks
• Provide an overview of the various types of overvoltages
Ferroresonance
Switching
Lightning
• Deliberate on the Methods of protection of HVAC and HVDC transmission systems
2
COURSE CONTENT
• UNIT- I: Introduction (10 Periods)
EHVAC and HVDC transmission - Comparison between HVAC and HVDC overhead and underground transmission scheme - Standard transmission voltages - Factors concerning choice of HVAC and HVDC transmission - Block diagram of HVAC and HVDC transmission schemes – Modern trends in EHVAC and HVDC transmission systems.
• UNIT- II: EHVAC Transmission and Corona (18 Periods)
Problems of EHVAC transmission at power frequency - Generalized constants - Power circle diagram and its use - Voltage control using compensators - Properties of bundled conductors - Inductance and capacitance of EHV line - Surface voltage gradient on single, double and more than three conductor bundles - Design aspects of EHV Lines.
Corona effects - Power loss - Increase in radius of conductors - Qualitative study of corona pulses - Corona pulse generation and properties.
• UNIT- III: HVDC Converters and control (15 Periods)
Converter configurations for HVDC system - Three-phase fully controlled Graetz Bridge converters for HVDC system - Operation as rectifiers and line commutated inverters - Analysis of Bridge Converters (Without and With Overlap – Two & Three Valve Conduction mode)- Converter equivalent circuits.
Basic means of control - Desired features of control - Control characteristics - Power reversal - Constant current control - Constant extinction angle control- Energization and de-energization of DC links.
• UNIT- IV: Overvoltage in EHV Systems (12 Periods)
Origin and types - Ferroresonance overvoltage - Switching surges, reduction of switching surges on EHV systems - Introduction to EHV cable transmission, electrical characteristics of EHV cables, properties of cable insulation materials - EHV insulators - Characteristics and pollution performance - Protection of HVAC and HVDC systems.
3
REFERENCES • Rakosh Das Begamudre, “EHV AC Transmission Engineering”, Wiley
Eastern Limited, 2006.
• E.W.Kimbark, “Direct Current Transmission, Volume- I”, Wiley Interscience , 1971
• Prabha Kundur, “Power System Stability and Control”, 2nd Reprint Edition, Tata McGraw Hill (P) Limited, New Delhi, 2006.
• K. R. Padiyar, “HVDC Power Transmission Systems: Technology and System Interactions”, New Age International Pvt. Ltd, First Edition 1990, Reprint 2005.
• J. Arillaga, “High Voltage Direct Current Transmission”, Peter Pregrinus, London, 1983.
• Dr. S. N. Singh, “High Voltage DC Transmission”, National Programme on Technology Enhanced Learning (NPTEL) Web Course Series
• IEC 60038: 2002-07, “IEC Standard Voltages”, Edition 6.2, 2002 • www.sari-energy.org/PageFiles/What_We_Do/activities/HVDC_Training
/Presentations/Day_1/1_HVDC_SYSTEMS_IN_INDIA.pdf, “HVDC Systems in India”, Powergrid Report
4
EVOLUTION OF EHVAC AND HVDC TRANSMISSION SYSTEM
• The first public power station
Holborn , London – 1882
Produced direct current (DC) at low-voltage
Service limited to localized areas and used mainly for electric lighting
• The first major a.c. power station
• Deptford, London - 1890
• Supplying power to central London
• Distance - 28 miles
• Operating Voltage- 10 kV
• EHVAC transmission has seen its development since the end of the Second World War (1945) Installation of 345 kV in North America
– 400 kV in Europe
• In 70 years, the highest commercial voltage has increased globally substantially due to the raising demand
1200 kV – Ultra High Voltage AC Transmission (UHVAC)
800kV – Ultra High Voltage DC Transmission (UHVDC)
• India has embarked on setting up 1200kV a.c. transmission system
The first step towards this is the implementation of 1200kV D/C/ Bina Test Station)
• India is also implementing power transmission projects to transmit HVDC power at 800kV transmission Including the first 800kV Multi Terminal HVDC Project in the world: Biswanath- Agra
5
MAJOR HVAC TRANSMISSION SYSTEMS IN CRONOLOGICAL ORDER
Year Power Transmission
Voltage Level (kV)
Location and Country
1890 10kV Deptford, London
1907 50kV Munich, Germany
1912 110kV Lauchhammer, Germany
1926 220kV North Pennsylvania, U.S.
1936 287kV Boulder Dam, Arizon- Nevada, U.S.
1952 380kV Harspränget − Hallsberg, Sweden
1959 525kV Russia, Formerly USSR
1965 735kV Manicouagan- Montreal, Canada
1967
1969
765kV Russia, Formerly USSR
Ohio-Kentucky, AEP, U.S.
1985 1150kV Russia, Formerly USSR
1999 1000kV Kita- Iwaki Powerline, Japan
2009 1000kV China
2013 1200kV Bina Test Station, Madha Pradesh, India (Results will be
utilized for Wardha- Aurangabad 1200kV Project-
Ongoing)
6
MAJOR HVDC TRANSMISSION SYSTEMS IN CRONOLOGICAL ORDER
Year Power Transmission
Voltage Level (kV)
Location and Country
1954 100kV, 96km, 20MW Gottland, Sweden
1961 100kV, 64km, 160MW England- France
1970 400kV, 1362km, 1440MW Pacific Intertie, U.S.
1972 450kV, 892km, 1620MW Nelson River 1, Canada
1978 533kV, 1414km, 1920MW Cabora Bassa, South Africa
1978
1985
250kV, 930km, 900MW
500kV, 892km, 1800MW
Nelson River 2, Canada
1984
1986
300kV, 785km, 200MW
600kV, 892km, 2383MW
Itaipu 1, Brazil
1986 500kV, 784km, 1920MW Intermountain, U.S.
1987 600kV, 805km, 3150MW Itaipu 2, Brazil
1990 500kV, 1000km, 1200MW Gezhouba– Shanghai, China
1991 500kV, 910km, 1500MW Rihand-Delhi, India
2002 500kV, 960km, 3000MW Three Gorges, China
2003 500kV, 1369km, 2000MW Talcher- Kolar, India
Ongoing 800kV, 1728km, 6000MW Biswanath- Agra, India (First Multi-
terminal HVDC Project)
7
NEED FOR UHV/ EHV AC & DC TRANSMISSION FOR BULK POWER TRANSFER
• High Power Transfer/ Evacuation Requirements:
Electric power (P) transmitted on an overhead a.c. line increases approximately with the surge
impedance loading or the square of the system’s operating voltage ( P= V2/ Zs with Zs 250Ω)
V (kV) 400 700 1000 1200 1500
P (MW) 640 2000 4000 5800 9000
• Aspects related to Right of Way (RoW): • Erecting power transmission lines involves obtaining the extremely elusive RoW • The difference in RoW requirement for a 400kV line and a 1,200kV is not extremely
significant.
8 Source: Technical Report, “HVDC Systems in India”, Powergrid, India
NEED FOR UHV/ EHV AC & DC TRANSMISSION FOR BULK POWER TRANSFER
• Power Demand-Supply Situation: (In the Indian Context) Power generation centres are typically in eastern and north-east regions while
consumption centres are spread across rest of India – North, West and South
Generation hubs are limited consumers, warranting the need for carrying power across long distances
Provides possibility to import power from hydropower-rich neighbours (Bhutan
9
Source: Technical Report, “HVDC Systems in India”, Powergrid, India
INCREASE IN TRANSMISSION VOLTAGE IN INDIA
10 Source: Technical Report, “HVDC Systems in India”, Powergrid, India
MAJOR MILESTONES IN HVDC TRANSMISSION SYSTEMS IN INDIA
Power Transmission
Voltage Level (kV)
Location
2 x 250MW, 70kV, Back-to-Back Link Vindhyachal
500kV, 814km, 1500MW, Bipolar Link Rihand- Dadri (Northern Region)
2 x 250MW, 140kV, Back-to-Back Link Chandrapur (Southern & Western Region)
500kV, 752km, 1500MW, Bipolar Link Chandrapur-Padghe (Western Region)
500MW, 140kV, Back-to-Back Link Vishakapatnam (Southern- Eastern Region)
500MW, 140kV, Back-to-Back Link Sasaram (Northern- Eastern Region)
2 x 500MW, 140kV, back-to-Back Link Gazuwaka (Eastern- Northeastern)
500kV, 1376km, 2000MW, Bipolar Link Talcher- Kolar (Eastern- Southern Region)
500kV, 780km, 2500MW, Bipolar Link Ballia- Bhiwadi
800kV, 1728km, 3000MW/ 6000MW,
Multi-terminal HVDC Link
Biswanath- Agra (North Eastern i.e. Assam-
West Bengal- Bihar- Uttar Pradesh)- Ongoing
1 x 500MW Interconnector Project India- Bangladesh Grid – Under Consideration
± 400 kV, 334km, 4 x 250 MW, Bipolar Link
with Submarine Cable ( app 90 Km)
Indo-Sri Lanka Inteconnector Link (Madurai- Sri Anuradhapura)- Under Consideration
11
SOME IMPORTANT ISSUES RELATED TO UTILITY OF EHV/ UHV
• Complexities related to bundled conductors Need for detailed studies related to spacers, spacer span calculation, Phase Pull
Force Calculation etc
• High surface voltage gradient on conductors: Inhomogeneous/ Non-Uniform Electric Field
• Effect of Corona Discharges Audible Noise, Radio Interference, Corona Energy Loss, Carrier Interference and TV
Interference.
• High electrostatic field under the line- environmental challenges: – Effect of Electrostatic field on human, animal, plants
• Switching Surge Overvoltages which cause more devastation to air-gap insulation (than lightning or power frequency voltages)
• Increased Short-Circuit currents and possibility of ferro-resonance • Use of gapless metal-oxide arresters replacing the conventional gap-type
Silicon Carbide arresters-lightning and switching-surge duty – Limitations related to Energy Capability – Complexities related to overlapping requirements of Lightning and Switching
overvoltages
• Insulation coordination based on switching impulse levels
12
STANDARD TRANSMISSION VOLTAGES
• Voltages adopted for transmission of bulk power have to conform to standard specifications formulated internationally. (IEC 60038)
A.C. three-phase systems having a nominal voltage above 35 kV and not exceeding 1200 kV
13 Source: IEC 60038: 2002-07, “IEC Standard Voltages”, Edition 6.2, 2002
COMPARISON OF HVAC AND HVDC TRANSMISSION SYSTEMS
• Major factors considered by a system planner of Power System for the choice
Choice of Power Transmission (HVAC/ HVDC)
Economics of Transmission
Investment Cost
ROW
Transmission Towers
Conductors
Insulators
Terminal Equipment
Operational Cost
Line Losses
Dielectric Power Losses
Corona Losses
Skin Effect
Technical Performance
Aspects related to Power Electronics Devices and
Converters
Stability Limits
Voltage Control
Line Charging Current
Line Compensation
Problems of Interconnections
Reliability
Energy Availability
Transient Reliability
MTTF
MTTR
14
ECONOMICS OF POWER TRANSMISSION- INVESTMENT COST
• Investment Cost:
Right of Way (RoW)
Lesser in HVDC than HVAC Systems
15 Source: IS 5613: 1989 (Part 3/ Sec 2), “Code of Practice for Design, Installation and Maintenance of Overhead Power Lines- Part 3: 400kV Lines”, Reaffirmed 2004
• Transmission Towers Simpler in Construction and Cheaper for HVDC
Associated number of conductors are reduced (2 as compared to 3 for a S/C Transmission Line)
Number of Insulator Strings are reduced (2 sets as compared to 3 for a S/C Transmission Line)
ECONOMICS OF POWER TRANSMISSION- INVESTMENT COST
16 Source: Technical Report, “High Voltage Direct Current Transmission- Proven Technology for Power Exchange”, Siemens, Germany
CONFIGURATION OF TYPICAL TRANSMISSION TOWERS
17 Source: Robert D Castro, “Overview of the Transmission Line Design Process”, Electrical Power Systems Research , 35, pp. 109-118, 1995
ECONOMICS OF POWER TRANSMISSION- INVESTMENT COST
• Number of Conductors: Reduced in the case of HVDC ( 2 instead of 3 for S/C Configuration)
• Power handling Capability: (assuming similar Insulator Characteristics- Insulation Level )
Can carry as much power as AC does ( 2 instead of 3 for S/C Configuration)
• Insulators/ Insulation: Reduced requirements in the case of HVDC
• Terminal Equipment: Cost increases in HVDC transmission system due to the following:
Metal Oxide Surge Arresters for DC Applications
Conversion Equipment- Increased Ratings of Thyristor Valves
Filters (DC and AC) for suppression of Harmonics
Non- availability of Voltage transformation equipment (transformers) in DC
Operational Cost: Line Losses: [2I2R= 3I2R]
About 67% that of AC system (assuming same current carrying capacity)
18
ECONOMICS OF POWER TRANSMISSION- OPERATIONAL COST
• Dielectric Power Losses: Reduced in the case of HVDC ( more-so with DC power cables)
P= (V2/ R)
From Phasor Diagram: tan δ = (V/R)/VωC;
V/R= V ωC tan δ;
Therefore P = V2 ωC tan δ = V2 2f C tan δ
19
ECONOMICS OF POWER TRANSMISSION- OPERATIONAL COST
• Corona Losses: Peek’s Empirical Formula:
20
phkmkWd
rVV
fxP
op//
225510214
where ‘f’ is the supply frequency
Vp is the operating voltage in kV
V ois the critical disruptive discharge voltage in kV
δ is the air density correction factor
‘r’ is the radius of the conductor
‘d’ is the spacing between conductors
Corona Losses due to DC are far lesser than AC (due to the term f in the equation above
• Skin Effect:
Absence of skin effect in DC reduces marginally the power losses
ECONOMICS OF POWER TRANSMISSION- COMPARISON OF COST
21
Source: Technical Report, “High Voltage Direct Current Transmission- Proven Technology for Power Exchange”, Siemens, Germany
TYPES OF HVDC LINKS
22
Monopolar Link Bipolar Link
Homopolar Link Back-to- Back Link
TECHNICAL PERFORMANCE- ASPECTS RELATED TO POWER ELECTRONICS DEVICES AND CONVERTERS
• Fast Controllability of Power Transmitted HVDC transmission system provides better avenues due to the advent of developments in
fast switching high power rated power electronic devices (SCR, GTO, MCT)
• Full Control over the of Power Transmitted HVDC transmission system offers facility to control the firing angle (α) and extinction angle
(γ) of the converters which regulates the power flow magnitude and direction
23
TECHNICAL PERFORMANCE- ASPECTS RELATED TO POWER ELECTRONICS DEVICES AND CONVERTERS
24
ciLcr
doidor
dRRR
CosVCosVI
TECHNICAL PERFORMANCE- STABILITY LIMITS
• Stability Limit: Ability of an ac system to operate with all synchronous machines in synchronism
• If a long line is loaded to a certain values (steady state stability limit) the synchronous machine accelerates and goes out of synchronism with those in the receiving end.
• This slipping out of the electro-dynamic system results in failing to transmit power
leads to objectionable fluctuation in the voltage
• Even if a line is operated below steady-state limit, the machine at sending end and receiving end may lose synchronism after a large disturbance (short circuit) unless the line is operated below its transient stability limit (lower than the steady state stability limit)
• Practically, for small disturbances the transient stability limit becomes the measure of the steady state stability limit
25
SinX
VVP
Rs
• In DC transmission link there is no direct influence of stability problems due to the following aspects: Two separate as systems interconnected by a dc link need not necessarily
operate in synchronism (even if their nominal frequencies are equal)
No influence of ‘X’
Each of the separate as systems may have its own internal stability issues
Sustained interruptions of the power on the dc line constitutes a mild threat to stability (caused by loss of a large load in the sending end system/ loss of a generator in the receiving end system)
26
TECHNICAL PERFORMANCE- STABILITY LIMITS
• Assuming that transmission line is lossless, Reactive power absorbed by the line Q L = I2ωL Reactive power supplied by the line Q C = V2ωC
• When reactive power supplied and absorbed by the line are equal the resultant leads to the concept of SURGE IMPEDANCE LOADING (Zs) V2ωC = I2ωL
27
TECHNICAL PERFORMANCE- VOLTAGE CONTROL
sZ
CL
I
V
At SIL: voltage throughout the length of the line is the same transmission line is terminated by a load corresponding to its surge impedance
with the voltage at both ends being constant
• In practice the load given to a overhead line is larger than SIL • The net reactive power absorbed by the line must be provided from one/ both
ends of the line and from intermediate series capacitors When I2ωL > V2ωC – VOLTAGE SAG When I2ωL < V2ωC – VOLTAGE RISE
• Maintenance of constant voltage requires REACTIVE POWER CONTROL IN AC SYSTEMS
• HVDC transmission system does not require reactive power control • However, converters at both ends of the DC line require reactive power from the
ac systems
28
TECHNICAL PERFORMANCE- VOLTAGE CONTROL
TECHNICAL PERFORMANCE- LINE CHARGING CURRENTS
29
• Line charging current in AC more over poses serious problems in cables
As length increases (for a lightly loaded system) the capacitance increases
Receiving end voltage becomes more than sending end voltage (Ferranti Effect in the case of Overhead lightly loaded/ no load transmission line)
• Line Compensation
AC lines require reactive power compensation systems to overcome issues related to line charging currents and stability limitations
• Series Capacitors and Shunt Inductors
• Issues related to AC Interconnection
Two power systems connected with an AC tie (synchronous interconnection), Automatic Generation Control of both systems have to be coordinated
• Using Tie- line power and frequency signals
Problems arise due to:
Presence of large power oscillations
Increase in fault level
disturbance from one system to the other
Two separate as systems interconnected by a dc link NEED NOT necessarily operate in synchronism (even if their nominal frequencies are equal)
30
TECHNICAL PERFORMANCE- AC INTERCONNECTION
• Role of Ground Impedance in AC transmission
Presence of zero-sequence currents cannot be permitted due to high ground impedance
• Leads to poor efficiency of power transfer
• Increase in RI
Role of Ground Impedance in DC transmission
Negligible in DC currents
DC links can operate using one conductor with ground return (Monopolar Mode)
In Monopolar Mode:
AC network feeding the DC converter station operates with balanced voltages and current (single pole operation of DC is possible for extended periods)
Objectionable only when buried metallic structures (pipes) are present
Leading to corrosion due to DC current flow
31
TECHNICAL PERFORMANCE- GROUND RETURN
• Applications
Long distance bulk power transmission
Underground or underwater cables
Asynchronous interconnection of AC systems operating at different frequencies
Control and stabilization of power flows in AC ties
Limitations
Difficulty in breaking DC currents
Inability to use transformers to transform voltage levels (transformer)
High cost of conversion equipment
Generation of harmonics which necessitate AC and DC filters are costly
Complexities of control
32
APPLICATIONS & LIMITATIONS OF HVDC TRANSMISSION
RELIABILITY
• Energy Availability
33
%1100
timetotal
timeoutageequivalenttyAvailabiliEnergy
Equivalent time is product of outage time fraction of system capacity lost due to outage
• Transient Reliability
faultsACrecordableofNo
designedasperformedsystemHVDCtimesofNoliabilityTransient
.
.100Re
Recordable AC faults cause one or more AC bus phase voltage to drop below 90% of voltage prior to fault
• Energy Availability and Transient Reliability of existing HVDC systems • > 95%
• Developments in technology have ensured improved reliability • Control and Protection • LTT has lead to elimination of high voltage pulse transformer and auxiliary supplies
FACTORS INFLUENCING THE CHOICE OF TRANSMISSION
• CHOICE BASED ON VOLTAGE LEVEL
• CHOICE BASED ON INSULATION RATIO
• CHOICE BASED ON POWER TRANSFER CAPABILITY
• Choice Based on Voltage Level:
• Objective: Choice made such that cost for transmission of power (P) is minimized
• The following are the costs:
Cost due to Investment (C1)
Cost due to Losses (C2)
Cost due to Investment (C1):
Proportional to Voltage Level (V)
Proportional to number of conductors (n) and area of cross-section (q)
Overhead charges (A0)
34
C1= A0 + A1 nV + A2 nq
FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- VOLTAGE LEVEL
Cost due to Losses (C2):
Loss/ length of transmission
Time of operation of the transmission system (T)
Load Loss Factor (L)
Cost/ Energy (p)
35
Losses = n I2R
nVP
V
n
P
condI
/
nqV
P
qnV
PnR
nV
PnlengthLoss
222
/
C2 = Loss x Time of Operation x Loss Load Factor x Cost/ energy
FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- VOLTAGE LEVEL
Total Cost (C) = C1 + C2
To optimize the transmission system cost the material cost (nq) should be minimized
36
LTpAwherenqV
PAC
3
2
32
nqV
PAnqAnVAAC
2
3210
0
)(2
2
32
nqV
PAA
nqd
dC
2
2
32nqV
PAA
2
32
2
V
PAAnq
2
3
A
A
V
Pnq
FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- VOLTAGE LEVEL
37
2
3
2
3
2
3
210
A
A
V
PV
PA
A
A
V
PAnVAAC
V
PAAnVAAC
3210min2
FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- INSULATION RATIO
38
Factors that affect insulation of overhead transmission lines: Operating Voltage Switching Surge Overvoltage Lightning Overvoltage Operating Voltage influences the Leakage Distance Switching and lightning over-voltages influence the required insulator chain length and
striking distance
FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- INSULATION RATIO
39
• Withstand Voltage Factor (K Factor):
Ratio of d.c. voltage and a.c. voltage with respect to ground
Typical values of K:
K=√2 – Indoor Porcelain
K=1 – Outdoor Porcelain (implies poor wet flashover performance)
K- 2 to 6- Power Cables
O/H lines are insulated for over-voltages expected during faults, switching operations etc
AC transmission lines are normally insulated against over-voltages > 4 times rated voltage
AC insulation Factor (K1):
K = d.c. withstand voltage level/ a.c. (rms) withstand voltage level
K 1= a.c. insulation level/ rated a.c. voltage (rms) level
K 1= a.c. insulation level/ Ep 2.5
FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- INSULATION RATIO
40
For d.c. lines,
DC Insulation Factor (K2):
Insulation Ratio:
K 2= d.c. insulation level/ rated d.c. voltage
K 2= d.c. insulation level/ Vd 1.7
Insulation Ratio = Insulation required by 1 AC phase/ Insulation required by 1 DC pole
voltagewithstadcdlevelinsulationcd
voltagewithstadcalevelinsulationca
RatioInsulation
....
....
FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- INSULATION RATIO
41
voltagewithstadcdlevelinsulationcd
voltagewithstadcalevelinsulationca
RatioInsulation
....
....
voltagewithsandcd
voltageratedcd
voltageratedcd
levelinsulationcd
voltagewithsandca
voltageratedca
voltageratedca
levelinsulationca
RatioInsulation
..
..
..
..
..
..
..
..
voltagewithsandcd
VK
voltagewithsandca
VK
RatioInsulationd
p
..
..
2
1
FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- INSULATION RATIO
42
voltagewithsandcd
voltagewithsandcd
VK
VKRatioInsulation
d
p
..
..
2
1
d
p
VK
VKKRatioInsulation
2
1
• Assuming same Power Transmitted (P) equal losses in both transmission
systems
Power Loss in a.c. system (Lac):
Power Loss in d.c system (Ldc):
Assuming equal losses:
RILpac
23
RILddc
22
RIRIpd
2232
pdII
2
3
FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- INSULATION RATIO
43
Power Transmitted by a.c. (Pac):
Power Transmitted by d.c . (Pdc):
Insulation Ratio:
• If K=1, K1=2.5 and K2=1.7
• Insulation Ratio = 1.2
• INSULATON REQUIRED FOR AC IS MORE THAN THAT FOR DC SYSTEM
ppacIVP 3
dddcIVP 2
ddppIVIV 23
p
p
pp
d
pp
dV
I
IV
I
IVV
2
3
32
23
2
3
d
p
VK
VKKRatioInsulation
2
1
3
2
2
1
K
KKRatioInsulation
FACTORS INFLUENCING CHOICE OF TRANSMISSION- -POWER HANDLING CAPABILITY
44
Conversion of Double Circuit 3 Phase AC Line to 3 DC Circuits of 2 Conductors:
Power Transmitted by a.c . (Pac):
• Power Transmitted by a.c . (Pdc):
Considering that equal current and insulation are considered:
IL = Id
Power Ratio:
If =1, K1=2.5 and K2=1.7
Power transmitted with DC can be increased to 147% (47% > AC)
LpacIVP 6
dddcIVP 6
d
p
VK
VKKRatioInsulation
2
1
pdV
K
KKV
2
1
2
1
K
KK
V
V
P
P
p
dc
ac
dc
BLOCK DIAGRAM OF HVAC AND HVDC TRANSMISSION SYSTEMS
45
COMPONENTS OF HVDC LINK
46 Source: S. Kamakshaiah, V. Kamaraju, 'HVDC Transmission', 1st Edition, Tata McGraw Hill, 2011
COMPONENTS OF HVDC LINK
47
COMPONENTS OF HVDC LINK • Converter (Bridge) Unit:
Located in “VALVE HALL” Single (or) Multi- Bridge (Series/ Parallel) Valve Configuration- Single/ Double/ Quadri Cooling Arrangement- Air/ Oil/ Water/ Freon/ De-ionized Water Valve Firing Strategy- Light guide system using Optic fiber Protection- Snubber Circuits, Surge Arresters
Converter Transformer: Winding- 3φ, 2 winding/ 1φ, 3 winding/ 1φ, 2 winding Vector Group- Star- Star and Star- Delta
Valve side of the transformer- Neutral point is ungrounded AC side of transformer- grounded
Leakage reactance of transformer chosen to limit S/C current DC magnetization & Core Saturation Designed to withstand DC voltage stresses K factor based Transformer- accommodate harmonic currents generated
by non-linear loads K factor is a weighting factor of the harmonic currents in the load according to
their effects on transformer heating (ANSI- IEEE C57.110)
48
COMPONENTS OF HVDC LINK
49 Source: Valve Hall in Chandrapur, India, ABB constructed Chandrapur- Padghe HVDC Link
• Smoothing Reactor
Reduce incidence of commutation failure in inverter (due to dip in AC voltage)
Smooth ripples in DC current (during lightly loaded condition)
– Limit the value of peak current in rectifier (due to S/C on DC line)
Limit current in valves during bypass pair operation (due to discharge of shunt capacitance of DC line)
Prevent commutation failures in inverter (reducing rate of rise of DC in bridges when direct voltage of another series connected bride collapses)
50
COMPONENTS OF HVDC LINK
Source: Songo- Mozambique Converter Station, Hidroeléctrica De Cahora Bassa (HCB) in Mozambique and Eskom in South Africa, 2013
• FILTERS
Provide a path of low impedance to AC harmonics
Tuned filters- Single & Double Tuned
Damped Filters
Connected between converter transformer and AC station
Suppress HF currents
Filtering oscillations and ripples in DC
REACTIVE POWER SOURCE
Huge reactive power requirements at the converter terminal
Reactive power requirement of about 50- 60% of active power
Provided by AC Filters (partly)
Shunt Capacitors (switched)
Synchronous Condensers
Static VAR Systems 51
COMPONENTS OF HVDC LINK
• DC CIRCUIT BREAKER
No natural current zero in the case of DC circuit
Can be brought to zero ONLY by applying a counter voltage higher than system voltage
Need for dissipation of large energy stored in the inductance of the DC circuit
52
COMPONENTS OF HVDC LINK
RECENT TRENDS IN HVDC TRANSMISSION SYSTEM
• Developments in major aspects: Power Semiconductor Devices Digital Electronics & Control Protection Equipment
Power Semiconductor Devices: New devices- GTO: 8kV, 4kA; IGBT: 6.5kV, 1kA; Thyristor: 6kV Size of device- >100mm (diameter) leading to reduced no. of
parallel connections Increase in current rating (higher overload capacity) Increase in voltage rating Manufacturing Process- Silicion cost reduced by 15-20% using
magnetic CZ (czochralski) method instead of conventional method Development of LTT (improved reliability of converter operation) ZnO arresters Cooling Methods (Deionized water cooling- Two phase flow using
forced vaporization)
53
RECENT TRENDS IN HVDC TRANSMISSION SYSTEM
• Development of HVDC- VSC Stations: Interconnecting weak AC systems
Connecting large-scale wind power to the grid
HVDC interconnections to be expanded to become Multi-terminal link 54
RECENT TRENDS IN HVDC TRANSMISSION SYSTEM
• Suspended Valves: Quadri-valve structure leads to compact
assembly BUT they are tall (about 16m) In regions where seismic effects are observed
appropriate to suspend the valves from ceiling Comprises spring and damper arrangement for
mechanical isolation of valve (from building due to vibrations)
Connection to wall bushings and between walls Flexible Bus
• Static Induction Thyristor: Thyristor with a buried gate structure (gate
electrodes are placed in n-base region) Normally ON-state Gate electrodes must be negatively biased to
hold off-state Rating: 4000V, 400A (developed in Japan as an
alternative to GTO)
55
RECENT TRENDS IN HVDC TRANSMISSION SYSTEM
• Assymetrical Thyristor: Blocking voltage for symmetrical thyristors < 4 to
5kV
Series Connection of Asymmetrical Thyristors (ASCR) and a diode
Possibility of adjusting the turn –OFF time
Not used in HVDC (under experimentation)
• Light Triggered Thyristor: Infinite gate isolation
Total noise immunity (control circuit)
Fast Turn-On time
Elimination of HV Pulse Transformer and Power Auxiliaries
Light Sources- Gallium Arsanide LED, Laser Diodes
56
COMPARISON OF DEVELOPMENTS IN RATINGS OF POWER ELECTRONIC DEVICES
57
RECENT TRENDS IN HVDC TRANSMISSION SYSTEM
• Converter Control: Development of Micro-computer based Converter Control Equipment
Redundant Converter Control
Possibility to perform scheduled preventive maintenance on stand-by system (due to reduced outage rate)
Possibility for adaptive control algorithms/ expert systems for fault diagnosis and protection
Conversion of Existing AC Lines
Constraints of RoW in AC
Issues related to Electromagnetic Induction in AC Lines
58
RECENT TRENDS IN HVAC TRANSMISSION SYSTEM- FACTS
• Issues related to Reactive Power Flow in AC Transmission Systems: Consumer loads require reactive power (continuously varying and
increases the transmission losses and affect voltage in the tranmission network)
Slow mechanically switched components which are used for reactive power compensation leads to less precise and less efficient control of transmission characteristics (use of passive elements such as reactors and capacitors)
Leads to limitations in power transfer, steady state and dynamic stability limits
• Aspects related to Long Distance High Voltage large Power Transfer Capacity • Offshore wind farms also have very long transmission lines (can be “tens
to hundreds of miles”) 59
RECENT TRENDS IN HVAC TRANSMISSION SYSTEM- FACTS
• FACTS (Flexible Alternationg Current Transmission System) Developed in 1986 by EPRI, USA
Slatt Substation in Northern region. 500 kV, 3-phase 60 Hz substation, developed by EPRI (Bonneville Power Administration and General Electric Company)
• "A power electronic based system and other static equipment that provide control of one or more AC tranmission system parameters to enhance controllability and increase in power transfer capability“
• Merits of FACTS: Reactive Power Control
Power Oscillation Damping
Improved Power Transfer Capability
Enhanced Steady State, Transient and Dynamic Stability
Improved Voltage Quality due to enhanced Voltage Control
60
RECENT TRENDS IN HVAC TRANSMISSION- IMPROVEMENTS IN POWER TRANSFER CAPABILITY
61
In the case of a no-loss line, voltage magnitude at the receiving end is the same as voltage magnitude at the sending end: Vs = Vr=V. Transmission results in a phase lag δ that depends on line reactance X.
Active power P is the same at any point of line
Reactive power at sending end is the opposite of reactive power at receiving end
Active power mainly depends on δ Reactive power mainly depends on voltage magnitude
RECENT TRENDS IN HVAC TRANSMISSION- IMPROVEMENTS IN POWER TRANSFER CAPABILITY
(SERIES COMPENSATION)
62
FACTS for series compensation modify line impedance: • X is decreased so as to increase the transmittable active power. However, more reactive power must be provided.
RECENT TRENDS IN HVAC TRANSMISSION- IMPROVEMENTS IN POWER TRANSFER CAPABILITY
(SHUNT COMPENSATION)
63
• Reactive current is injected into the line to maintain voltage magnitude. • Transmittable active power is increased but more reactive power is to be provided.
TYPES OF FACTS CONTROLLERS
• Basic Types of FACTS Controllers
Series Controllers
Shunt Controllers
Combined Series- Series Controllers
Combined Series- Shunt Controllers
Series Controllers
Series controller could be a variable impedance or a variable source both are power electronics based.
In principle, all series controllers inject voltage in series with the line.
Shunt Controllers
Shunt controllers may be variable impedance connected to the line voltage causes a variable current flow
Represents injection of current into the line
64
TYPES OF SERIES CONTROLLERS
• Types of Series Controllers Static Synchronous
Series Compensator (SSSC)
Thyristor Controlled Series Capacitor (TCSC)
Thyristor Switched Series Capacitor (TSSC)
Thyristor Controlled Series Reactor (TCSR)
Thyristor Switched Series Reactor (TSSR)
65
Series capacitor bank is shunted by a thyristor controlled reactor
Series reactor bank is shunted by a thyristor controlled reactor
TYPES OF SHUNT CONTROLLERS • Types of Shunt Controllers
Static synchronous compensator (STATCOM) Static VAR compensator (SVC)
Thyristor‐controlled reactor (TCR) • Reactor is connected in series with
a bidirectional thyristor valve. Thyristor valve is phase-controlled. Equivalent reactance is varied continuously
Thyristor‐switched reactor (TSR) • Thyristor is either in zero- or full-
conduction. Equivalent reactance is varied in stepwise manner.
Thyristor‐switched capacitor (TSC) • Capacitor is connected in series
with a bidirectional thyristor valve. Thyristor is either in zero- or full- conduction. Equivalent reactance is varied in stepwise manner.
Mechanically‐switched capacitor (MSC)
66
ENHANCEMENT OF STABILITY IN POWER SYSTEM
• Transient stability analysis based on “Equal Area Criterion”
• If A2 >=A1, system is stable; otherwise, system is unstable
67