Following topics are covered in this training.
High Voltage DC Transmission
Principle of AC Transmission Schematic of AC system
REASONS FOR AC GENERATION AND TRANSMISSION • Due to ease of transformation of voltage levels (simple transformer action) • Alternating Current is universally utilized.—Both for GENERATION and LOADS and hence for Transmission. • Generators are at remote places, away from the populated areas i.e. the load centers • They are either PIT HEAD THERMAL or HYDEL • Turbines drive synchronous generators giving an output at 15-25 kV. • Voltage is boosted up to 220 or 400 KV by step-up transformers for transmission to LOADS. • To reach the loads at homes/industry at required safe levels, transformers step down voltage
COMPARISION OF HVAC & HVDC SYSTEMS • HVAC transmission is having several limitations like line length , uncontrolled power flow, over/low voltages during lightly / over loaded conditions, stability problems, fault isolation etc • The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. • HVDC transmission allows efficient use of energy sources remote from load centers. Depending on voltage level and construction details, losses are quoted as about 3% per 1,000 km. • In a number of applications HVDC is more effective than AC transmission. Examples include: • Undersea cables, where high capacitance causes additional AC losses. (e.g. 250 km Baltic Cable between Sweden and Germany) . • 600 km NorNed cable between Norway and the Netherlands
COMPARISION OF HVAC & HVDC SYSTEMS • In HVDC Long power transmission without intermediate taps, for example, in remote areas . • Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install • Power transmission and stabilization between unsynchronized AC distribution systems • Connecting a remote generating plant to the distribution grid • Asynchronous operation possible between regions having different electrical parameters . • Facilitate power transmission between different countries that use AC at differing voltages and/or frequencies • Reducing line cost: fewer conductors thinner conductors since HVDC does not suffer from the skin effect
COMPARISION OF HVAC & HVDC SYSTEMS • HVDC Cheaper than HVAC for long distance. Line Cost AC Line Cost DC Terminal Cost DC Terminal Cost AC Break Even Distance COST: HVAC vs. HVDC Transmission
COMPARISION OF HVAC & HVDC SYSTEMS • No restriction on line length as no reactance in dc lines • HVDC can carry more power per conductor because, for a given power rating, the constant voltage in a DC line is lower than the peak voltage in an AC line.
COMPARISION OF HVAC & HVDC SYSTEMS • HVDC uses less current i.e. low losses. • AC current will struggle against inertia in the line (100times/sec)-electrical resistance –inductancereactive power • Direct current : Roll along the line ; opposing force friction (electrical resistance )
COMPARISION OF HVAC & HVDC SYSTEMS • Distance as well as amount of POWER determine the choice of DC over AC
COMPARISION OF HVAC & HVDC SYSTEMS • Direct current conserves forest and saves land • The towers of the dc lines are narrower, simpler and cheaper compared to the towers of the ac lines.
COMPARISION OF HVAC & HVDC SYSTEMS AC Transmission Line Corridor
COMPARISION OF HVAC & HVDC SYSTEMS DC Transmission Line Corridor
COMPARISION OF HVAC & HVDC SYSTEMS DC Transmission Line Corridor
COMPARISION OF HVAC & HVDC SYSTEMS • Lesser Corona Loss than HVAC at same voltage and conductor diameter and less Radio interference. • Direction of power flow can be changed very quickly • HVDC has greater reliability. i.e. bipolar dc is more reliable than 3 phase HVAC • DC requires less insulation. • An optimized DC link has smaller towers than an optimized AC link of equal capacity. • DC line in Parallel with AC link. Corona → (f+25)
BASIC PRINCIPLES OF HVDC TRANSMISSION
HVDC Introduction HVDC technology is used to transmit electricity over long distances by overhead transmission lines or submarine cables. HVDC Principle
Components of HVDC Transmission Systems 1. 2. 3. 4. 5. 6. 7. Converters Smoothing reactors Harmonic filters Reactive power supplies Electrodes DC lines AC circuit breakers Components of HVDC
Components of HVDC Transmission Systems…. Converters • They perform AC/DC and DC/AC conversion • They consist of valve bridges and transformers • Valve bridge consists of high voltage valves connected in a 6-pulse or 12-pulse arrangement • The transformers are ungrounded such that the DC system will be able to establish its own reference to ground Smoothing reactors • • They are high reactors with inductance as high as 1 H in series with each pole They serve the following: • They decrease harmonics in voltages and currents in DC lines • They prevent commutation failures in inverters • Prevent current from being discontinuous for light loads Harmonic filters • Converters generate harmonics in voltages and currents. These harmonics may cause overheating of capacitors and nearby generators and interference with telecommunication systems • Harmonic filters are used to mitigate these harmonics
Components of HVDC Transmission Systems…. Reactive power supplies • Under steady state condition, the reactive power consumed by the converter is about 50% of the active power transferred • Under transient conditions it could be much higher • Reactive power is, therefore, provided near the converters • For a strong AC power system, this reactive power is provided by a shunt capacitor Electrodes • Electrodes are conductors that provide connection to the earth for neutral. They have large surface to minimize current densities and surface voltage gradients DC lines • They may be overhead lines or cables • DC lines are very similar to AC lines AC circuit breakers • They used to clear faults in the transformer and for taking the DC link out of service • They are not used for clearing DC faults • DC faults are cleared by converter control more rapidly
Application based HVDC Transmission Types Upto 600MW 50 Hz 60 Hz HVDC is the unique solution to interconnect Asynchronous systems or grids with different frequencies.
Application based HVDC Transmission Types Upto 3000 MW HVDC represents the most economical solution to transmit electrical energy over distances greater than approx. 600 km
Application based HVDC Transmission Types HVDC is an alternative for submarine transmission. Economical even for shorter distances such as a few 10km/miles
Application based HVDC Transmission Types
HVDC System Configurations HVDC links can be broadly classified into: • • • • Monopolar links Bipolar links Homopolar links Multiterminal links
Monopolar Links • It uses one conductor . • The return path is provided by ground or water. • Use of this system is mainly due to cost considerations. • A metallic return may be used where earth resistivity is too high. • This configuration type is the first step towards a bipolar link.
Bipolar Links • Each terminal has two converters of equal rated voltage, connected in series on the DC side. • The junctions between the converters is grounded. • If one pole is isolated due to fault, the other pole can operate with ground and carry half the rated load (or more using overload capabilities of its converter line).
Homopolar Links • It has two or more conductors all having the same polarity, usually negative. • Since the corona effect in DC transmission lines is less for negative polarity, homopolar link is usually operated with negative polarity. • The return path for such a system is through ground.
Dc as a Means of Transmission DC Transmission has been possible with beginning of • High power/ high current capability thyristor. • Fast acting computerized controls Since our primary source of power is A.C, The three basic steps are 1. Convert AC into DC (rectifier) 2. Transmit DC 3. Convert DC into AC ( inverter)
Conversion Single Phase Half wave Rectifier Frequency Specturm Of Rectified Output 50 45 40 Magnitude Of Each Hormonic 35 30 25 20 15 10 5 0 0 60 120 180 Frequency 240 300 360 420
Single Phase Full wave Rectifier
Single Phase Full wave Rectifier Full wave Rectifier Output Spceturm Magnitude of Harmonics & DC component 70 60 50 40 30 20 10 0 0 60 120 180 Frequency 240 300 360
Six Pulse Rectifier + T1 Va Vc T3 T5 Vo Vb T4 T6 T2 - The operating principle of the circuit is that, the pair of SCR connected between the lines having highest amount of line-to-line voltage will conduct provided that the gate signal is applied to SCRs at that instant. The converters are called Line Commutated converters or current source converter. Every 60º one Thyristor from +ve limb and one Thyristor from –ve limb is triggered
Six Pulse Rectifier Waveforms IG T6 T1 T2 T3 T4 T5 T6 Vo(ωt) Vcb 4,5 Vab Vac 1,6 5,6 Vbc 1,2 Vba Vca 3,4 2,3 4,5 5,6 ωt ia(ωt) α α+π/3 α+2π/3 α+π α+4π/3 α+5π/3 α+2π Id ωt -Id
Operation of Six Pulse Rectifier 0.5 Rectification Region 0 Inversion Region -0.5 -1 0 0.5 1 1.5 (rad) 2 2.5 Normalized Average (DC) Voltage as function of = /6 100 Normalized Harmonic Magnitude Normalized Average Voltage 1 80 60 40 20 0 0 60 120 180 240 300 360 420 480 540 600 660 Frequency Hz Specturm at alpha=pi/6 3
Effect of Control Angel 1 u 3 u u α= firing Angle μ= Commutation Interval C A Vd B 2
DC Terminal Voltage RECTIFICATION 120 º 180 º 240 º 300 º 0 60 º 120 º 180 º 0.866 E . 2 LL E . 2 LL
DC Terminal Voltage INVERSION 0.866 E . 2 LL 120 º 180 º 240 º 300 º 0 60 º 120 º 180 º E . 2 LL
12-Pulse Convertor Bridge Y Give π/6 phase shift with respect to Y Commonly Used in HVDC systems
12-Pulse Convertor Bridge Continuous i + - pow ergui D1 c Three-Phase Transformer (Two Windings) Cu C urrent Measurement b C D5 a B + i - A D3 Scope1 D2 D4 D6 + v - Voltage Measurement Va Vb Scope Vc R D7 D12 b C D10 a B D11 D8 A D9 c Three-Phase Transformer (Two Windings)1 Matlab Model of 12 Pulse Rectifier Specturm Specturm
12-Pulse Convertor Bridge • Commonly adopted in all HVDC applications • Two 6 pulse bridges connected in series • 30º phase shift between Star and Delta windings of the converter transformer • Due to this phase shift, 5th and 7th harmonics are reduced and filtering higher order harmonics is easier • Higher pulse number than 12 is not economical
12-Pulse Convertor Bridge • From Voltage spectrum it can be seen that by using 12 pulse, nearly harmonic free DC output is obtained. Displacement Distortion Power Factor Factor Factor 0.7583 0.9755 0.7397 0.6391 0.9677 0.6185 0.4873 0.9556 0.4656 80 60 40 = /6 100 20 0 0 60 120 180 240 300 360 420 480 540 600 660 Frequency 12 Pulse Rectifier Normalized output Voltage Spectrum Normalized Harmonic Magnitude Normalized Harmonics Magnitude 100 80 60 40 20 0 0 60 120 180 240 300 360 420 480 540 600 660 Frequency Hz 12 Pulse Rectifier Normalized input current Spectrum for =
Control of DC Voltage Rectifier Operation AC System Power Flow Inverter Operation DC System AC System DC System Power Flow Id V1 V3 Id V5 V1 Phase A V3 V5 Phase A Phase B Phase B Ud Phase C Ud Phase C V4 V6 V2 V4 V6 V2 +Ud Rectifier Operation 160 0 5 -Ud 30 60 90 120 150 Inverter Operation 180
Relationship of DC Voltage Ud and Firing Angle α Rect. Limit +Ud Rectifier Operation 160 0 5 30 90 60 120 150 180 Inverter Operation -Ud Ud Inv 60o 30o 0o Limit Ud t 90o Ud 120o 150o t -Ud
. Decrease voltage at station B or increase voltage at station A. power flows from A B Normal direction Decrease voltage at station B or increase voltage at station A. power flows from A B Normal direction
Power reversal is obtained by reversal of polarity of direct voltages at both ends.
Voltage Source Converter 300MW Can generate and absorb reactive power. Power flow is changed by shift voltage waveform ( changing power angle) P VSC Based HVDC U ciU si sin( X li i ci )
Inverter Topologies D1 S1 S3 D3 S4 D4 Vdc LOAD D2 S2 Simple Square-Wave Inverter V0 E1 0 π ωt 2π E-1 Output voltage waveform of SquareWave inverter
Inverter Topologies The harmonic free sinusoidal output is a major area that has been investigated for many years as it is highly desirable in most inverter applications. The techniques regarding harmonic Elimination are • Some switching techniques are utilized for the purpose of enhancing the magnitude of the fundamental component and reducing the harmonics to obtain minimized total harmonic distortion. • Diode Clamped Multilevel Inverter (DCMLI) technique. • Pulse Width Modulation (PWM) technique. • PWM technique in DCMLI.
In these harmonic elimination techniques the lower order harmonics are effectively reduced from output voltage by fundamental switching, so smaller output filters can easily be used to eliminate the remaining higher order harmonics. The topologies are explained in the following sequence: • • • • • • Circuit Diagram. Output Voltage waveform. Fourier Analysis. Switching Angles Calculation. Spectrum of Output Sinusoidal waveform. Calculation of Total Harmonic Distortion
Fundamental idea of harmonic Elimination 6 Fundamental Component 3rd Harmonic 4 2 120 Degree Conduction 210 330 0 30 150 120 Degree Conduction -2 -4 -6 0 50 100 150 200 250 300 Degree Elimination of 3rd Harmonic via Switching 350
Diode Clamped Multilevel Inverter (DCMLI) E4 Sa S'e Vdc Da Sb Db Sc De S'f E3 Df S'g Vdc Dc Sd Dg S'h A LOAD E2 B S'a S'b S'c Se D'a Sf D'b D'c Sg D'e Vdc D'f E1 D'g Vdc S'd Sh 0 Diode Five-Level Bridge Multilevel Inverter E0
Five-level DCMLI voltage levels and their corresponding switch states. V0 E4 E3 E2 E1 E0 E-1 E-2 E-3 E-4 ωt α1 α2 α3 α4 π/2 π Phase voltage waveform of 5-level inverter 2π
Fourier Analysis The Fourier series of the quarter-wave symmetric 5-level DCMLI multilevel waveform Switching Angles Computation The equations used to calculate switching angles are:
5-Level DCMLI Output Voltage Waveform for M=0.82 Voltage Spectrums Normalized to Fundamental Component
Pulse Width Modulated Inverter (PWM) S1 D1 S3 D3 S4 D4 Vdc LOAD D2 S2 Single-phase Full- Bridge PWM inverter V0 E1 0 α1α2 α3 α4 α5 π/2 π E-1 Phase voltage waveform of PWM inverter ωt 2π
Fourier Analysis The Fourier series of the quarter-wave symmetric m-pulse PW waveform is: Switching Angles Computation The equations used to calculate switching angles are:
PWM Inverter Output Voltage Waveform for M=0.82 Voltage Spectrums Normalized to Fundamental Component
Pulse Width Modulated (PWM) multilevel Inverters E1 Sa S'c Vdc Da Sb Dc S'd A LOAD E1 B S'a D'a Sc D'c Vdc S'b Sd 0 E0 Single-phase Full- Bridge PWM inverter 3-level PWM DCMLI voltage levels and corresponding switch states.
. V0 E2 E1 E0 α1α2 α3 α4 α5 π/2 π ωt 2π E-1 E-2 3-level PWM output voltage waveform Fourier Analysis The Fourier series of the quarter-wave symmetric 3-level PWM voltage waveform is: output
Switching Angles Computation The equations used to calculate switching angles are:
3-level PWM Inverter Output Voltage Waveform for M=0.82 Voltage Spectrums Normalized to Fundamental Component
. Conclusion • The PWM inverter though took four switches for implementation (less than other two) but Simulation resulting THD is greater of all. results of • The DCMLI resulting THD is lowest of all but it three took too many devices for implementation. different 1- • The PWM in DCMLI (Combination of PWM φ inverters and DCMLI) have less number of switches were than the DCMLI and low THD than PWM inverter, implying that this technique is presented economically and technically best to implement
. Advantages of Proposed Inverter technique • Improved efficiency due to modified sine wave. • Reduction in power losses, Electromagnetic Interference (EMI). • Well suited for renewable energy systems. • Frequency adaptive inverter-System’s AC output can be easily reconfigured after installation • Cost effective- With simple structure and fewer components. • Requires low output filter values to attenuate the undesired harmonics.
Disadvantages of HVDC Transmission • The disadvantages of HVDC are in conversion, switching and control. • Expensive inverters with limited overload capacity. • Higher losses in static inverters at smaller transmission distances. • The cost of the inverters may not be offset by reductions in line construction cost and lower line loss. • High voltage DC circuit breakers are difficult to build because some mechanism must be included in the circuit breaker to force current to zero, otherwise arcing and contact wear would be too great to allow reliable switching. • HVDC is less reliable and has lower availability than AC systems, mainly due to the extra conversion equipment.
Control of HVDC Systems Content • • Efficient and stable operation. Maximum flexibility of power control without compromising the safety of equipment. • Objectives of Control Principle of operation of various control systems. Implementation and their performance during normal and abnormal system conditions. •
Basic principles of control • Direct current from the rectifier to the inverter Id Vdor cos Vdoi cos Rcr Rl Rci • Power at the rectifier terminal Pdr Vdr I d • Power at the inverter terminal P di Vdi I d P dr RL I d 2 Schematic diagram of control
Basic Means of control • Internal voltages Vdor cos and Vdoi cos can used be controlled to control the voltages at any point on the line and the current flow (power). •This can be accomplished by: •Controlling firing angles of the rectifier and inverter (for fast action). •Changing taps on the transformers on the AC side (slow response). •Power reversal is obtained by reversal of polarity of direct voltages at both ends.
. Control Implementation • Power control To transmit a scheduled power, the corresponding current order is determined by: I ord Po / Vd • Rectifier control and protection Determines firing angles and sets their limits. • Inverter control and protection Determines firing angles and set frequency of resulting AC. • Master Control It coordinates the conversion of current order to a firing angle order, tap changer control and other protection sequences.
Control Implementation 345kV, 50 Hz, 10,000 MVA equivalent 500kV, 60 Hz 5000 MVA equivalent DC line 300 km A A A aA A B B B bB B C C C cC Brect phi = 80 deg. 3rd harm. C + 0.5 H 0.5 H - - Rectifier A Aa A A A B + Bb B B B Cc C C C C Inverter Binv phi = 80 deg. 3rd harm. AC filters 60 Hz 600 Mvar C B A C B A DC Fault AC filters 50 Hz 600 Mvar A-G Fault Master Control Rectifier Control and Protection Master Control Inverter Control and Protection HVDC 12-pulse Transmission System 1000 MW (500kV-2kA) 50/60 Hz A 1000 MW (500 kV, 2 kA) DC interconnection is used to transmit power from a 500 kV, 5000 MVA, 60 Hz system to a 345 kV, 10000 MVA, 50 Hz system. The rectifier and the inverter are 12-pulse converters
Control Implementation • A 1000 MW (500 kV, 2 kA) DC interconnection is used to transmit power from a 500 kV, 5000 MVA, 60 Hz system to a 345 kV, 10000 MVA, 50 Hz system. • The rectifier and the inverter are 12-pulse converters • The converters are interconnected through a 300-km line and 0.5 H smoothing reactors • Frequency adaptive inverter-System’s AC is used. • From the AC point of view, an HVDC converter acts as a source of harmonic currents. From the DC point of view, it is a source of harmonic voltages. • Two circuit breakers are used to apply faults: one on the rectifier DC side and the other on the inverter AC side..
. Conclusion • HVDC is very important issue in transmission energy. • Very large investments in e.g in China and India shows that high-voltage direct current will very important in the future, especially in big, new-industries countries • Recent studies indicate that HVDC systems are very reliable. • The data collected from 31 utilities says that forced unavailability of energy due to the converter station is 1.62%. • The scheduled unavailability of energy is about 5.39%. • HVDC offers powerful alternative to increase stability of a power system as well as to improve system operating flexibility and loss reduction • To keep the losses to a minimum, the control system shall be designed to keep as high voltage as possible.
. Comparison More power can be transmitted per conductor per circuit.