Basic Concept, Operation and Control of HVDC Transmission System

Basic Concept, Operation and Control of HVDC Transmission System 13.00-16.00 hrs. July 29, 2008 Room 2003, T.102, EGAT Head Office Nitus Voraphonpiput, Ph.D. Engineer Level 8 Technical Analysis Foreign Power purchase Agreement Branch Power Purchase Agreement Division Electricity Generating Authority of Thailand

Objective Introducing operation, and control of the High Voltage Direct Current Transmission System. Note: This presentation continues from the morning session. Basic mathematics and electrical engineering knowledge will be useful for attendee. 2

Contents 1. HVAC vs. HVDC 2. HVDC Principle Q&A for 15 minutes Coffee break 10 minutes 3. Control of DC Transmission Q&A for 15 minutes 3

1. HVAC vs. HVDC Why use DC transmission? This question is often asked. One response is that losses are lower, but is it true? Reference [2] has been explained using Insulation ratio and Power capacity in order to proof this statement. 4

1. HVAC vs. HVDC Insulation ratio of HVAC and HVDC (Ref. 1-2) A given insulation length for an overhead line, the ratio of continuous working withstand voltage factor (k) is expressed as, (note 1 k 2 ) k = DC withstand voltage AC withstand voltage(rms) = 1.0 A line has to be insulated for over-voltages expected during faults, switching operations, etc. Normally AC transmission line is insulated against over-voltages of more than 4 times the normal effective (rms) voltage. 5

1. HVAC vs. HVDC This insulation requirement can be met by insulation corresponding to an AC voltage of 2.5-3.0 times the normal rated voltage. k AC Insulation level Rated AC Voltage (phase - ground) 1 = = (rms) 2.5 For suitable converter control the corresponding HVDC transmission ratio is expressed as k DC Insulation level Rated DC Voltage (pole - ground) 2 = = 1.7 6

1. HVAC vs. HVDC Insulation ratio for a DC pole-ground voltage (V d ) and AC phase-ground (V p ) is expressed as insulation ratio( K) = = insulation length required for each AC phase insulation length required for each DC Pole AC insulation level AC withstand voltage DC insulation level DC withstand voltage It can be seen that the actual ratio of insulation levels is a function of AC/DC voltage. Next, determine AC/DC voltage. (rms) = k k k 1 2 V V P d 7

1. HVAC vs. HVDC Determine AC/DC voltage Assumed resistances (R) of the lines are equal in both cases (HVDC and HVAC). AC Loss = 3 x R x I L 2 and DC Loss = 2 x R x I d 2 Let losses in both cases are equal, so that, 3 I d = I L 2 The power of a HVAC system and a bipolar HVDC system are as: AC Power = V P I cos φ DC Power = 2V d I d 3 L 8

1. HVAC vs. HVDC At the same power transfer, So that, AC Power DC Power = 2 1 V p = V d 3 cosφ 3VPI 2V L d cosφ = I d 3 2 V P cosφ = 1 V d Thus, insulation ratio (K) can be written as k 2 1 1.2 K = k 1 k 3 cosφ cosφ 2 It can be seen that HVDC requires insulation ratio at least 20% less that the HVAC which essentially reflects the cost. 9

1. HVAC vs. HVDC Power Capacity Compared a double circuit HVAC line (6 lines) and double circuit DC line of Bipolar HVDC. Power transmitted by HVAC (P ac ) and HVDC (P dc ) are P = ac 6V PI L cosφ P = 6V On the basic of equal current and insulation, I d = I L, K=1: dc d I d P dc = 6 k k k V I k k P 1 1 ac P L = = 2 k2 cos 1.47 P φ cosφ ac 10

1. HVAC vs. HVDC For the same values of k, k 1 and k 2 as above and pf is assumed to 1.0, the power transmitted by overhead lines can be increased to 147%. The percentage line losses, which is inversion of the power transmit, are reduced to 68%. In addition, for underground or submarine cables, power transmitted by HVAC cable can be increase 294 % and line loss reduced to 34%. Note: for cable k equals at least two. 11

1. HVAC vs. HVDC From reference [3], losses are lower is not correct. The level of losses is designed into a transmission system and is regulated by the size of conductor selected. DC and AC conductors, either as overhead transmission lines or submarine cables can have lower losses but at higher expense since the larger cross-sectional area will generally result in lower losses but cost more. The reasons that HVDC have been used are: 1. An overhead d.c. transmission line with its towers can be designed to be less costly per unit of length. 2. It is not practical to consider AC cable systems exceeding 50 km (due to VAR charging of the cable). 3. Some a.c. electric power systems are not synchronized to neighboring networks even though their physical distances between them is quite small. 12

2. HVDC Principle The HVDC valve comprises the thyristors acting as controlled switch. In the OFF state, the thyristor blocks the current to flow, as long as the reverse or forward breakdown voltages is not exceeded. It changes to ON state if it is forward biased (V AK > 0) and has small positive Gate voltage applied between the Gate and the Cathode. Gate (G) Anode (A) Cathode (K) 13

2. HVDC Principle Thyristor switches between conducting state (ON) and nonconducting (OFF) state in response to control signal (firing) as its characteristic. The Gate voltage need not to be present when the thyristor is already in ON state. 14

2. HVDC Principle Anode (A) R d = V AK / I A i A R d V T Anode (A) Cathode (K) R r i r R r = V AK / I A V T P loss-on state = V T.I A(avg.) + R d.i A 2 (rms) Cathode (K) P loss-off state = R r.i r 2 (rms) 15

2. HVDC Principle ON-OFF state - ON state continues until current drops to zero, even reverse bias appears across the thyristor. - The critical time to clear charge carriers in the semi-conductor is referred as the turn-off time t off. If forward bias appears to soon, t < t off, thyristor can not OFF. V AK > 0 and V G >0 OFF V AK > 0 and t < t off ON I A < 0 t > t off OFF 16

2. HVDC Principle ON State OFF State 17

2. HVDC Principle Single Phase Bridge Rectifier I d Th1 Th3 I s Ld V s V d Rd = 10Ω Th4 Th2 U S = 220 V α = o 30 18

2. HVDC Principle V s I s α = 30 Voltage waveform of inductor (L d ), V Ld = Vd R d I d V d Voltage waveform of resistor (R d ), V Rd = R d I d Th 3 Th 1 Th 3 Th 4 Th 2 Th 4 I d 19

2. HVDC Principle I s 150 Hz Harmonics in the voltage and current waveform. 250 Hz 350 Hz DC 100 Hz V d 200 Hz 300 Hz DC I d 100 Hz 50 Hz 20

2. HVDC Principle Even DC side does not have reactive power (Q), the reactive power still presents on the AC side. The reactive power occurrence is caused by the delay angle (α) (or called firing angle) of the current waveform. P = V S I S cos α V s I s Q = V S I S sin α 30 V S α = 30 360 20 ms time I S Phasor of fundamental component 21

2. HVDC Principle V d I s I d 50 Hz 100 Hz 100 Hz 150 Hz 200 Hz 250 Hz 350 Hz 300 Hz Product of V d and I d is (active) power (P). Product of phasor V S and phasor I S is not the apparent power (S). It represents the active power (P) and reactive power (Q). There are harmonic distortion power, which is a new term caused by the higher harmonics (more than 50 Hz). It is represented by D (distortion power). Finally, S 2 = P 2 + Q 2 becames S 2 = P 2 + Q 2 + D 2. 22

2. HVDC Principle V I s s Increasing I d Ith1 L k Ith2 Ith1 Ith2 µ is overlap angle µ I d V d commutation Increasing L k Ld Rd The inductance L k represents reactance on AC side (called commutating reactance). Due to nature of an inductor, The inductor current can not change suddenly. Thus, during turn-off of the Th1 (and Th2) and turn-on of the Th3 (and Th4), both are in conducting state for a short time (overlap time). This phenomena occurs during commutation of the thyristors. It can be seen that if current is high, overlap angel is increased. In addition, if inductance is high, overlap angle is also increased. 23

2. HVDC Principle I s Inductor current can not suddenly be changed, thus there is a slope. V s α = 30 µ cosα + cos( α + µ ) cosφ 2 V d Th 3 Th 1 Th 3 Th 4 Th 2 Th 4 I d 24

2. HVDC Principle The impact of the overlap angle (µ) is the reduction of the average dc voltage (V d ). It decreases the harmonic content of the ac current (I s ) and power factor of the AC side. V d V d V d V T Ideal case V do D R R d V d X = V do = 2 2 π π f X K L K K I d D X X K I d Voltage drop due to commutating reactance is represented as D X I d Overall voltage drop V T and D R are very less compared to D X. Thus, there are usually neglected. I d 25

2. HVDC Principle 3-pulse converter Natural commutation VA Th1 V dα I L L R d d VB Th2 I L t VC Th3 Ld V dα Rd V A = 2 VP sin ωt V B = 2 VP sin ωt-120 V C = 2 VP sin ωt+120 o α = 0 o α = 60 o α = 90 o α =120 V =.17V cosα = V cosα d 1 P d 0 26

2. HVDC Principle Positive average voltage Negative average voltage 1.0 0.5-0.5-1.0 V V d d 0 o α = 60 o 45 Rectifier o 90 V V d d 0 Inverter = cosα o 135 o 180 Rectifier mode can be performed when firing angle is less than 90 degrees. Average voltage is zero when the firing angle is 90 degrees. α Inverter mode can be performed when firing angle is more than 90 degrees. 27

2. HVDC Principle V d α=60 α=30 I d 28

2. HVDC Principle V A, I A 120 V B, I B V C, I C Th 1 Th 2 Th 3 Th 1 Th 2 Th 3 I d 29

2. HVDC Principle V A, I A Reversing phase sequence α=30 α=120 V d Positive voltage Negative voltage I d Inverter mode can be performed as long as the DC current continues flow. 30

2. HVDC Principle V A Lk V dα D X V k V B Lk Id t V A µ µ α α V k IC IA IB IC IA IB V B V d D X = V = d 0 cosα 3 ωl 2π k I d D X 31 t

2. HVDC Principle The commutating reactance (X k ) results in decreasing of DC voltage, but it increases DC voltage in inverter mode. It can also be seen that the overlap time will γ increase when DC current is high and this can cause commutation failure in inverter mode. V d V k α 180 IA µ IB D X γ α 180 IA µ IB t Note: α + µ < 180 The extinction angle (γ) = 180 - α - µ V d D X = V = d 0 cosα + 3 ωl 2π k I d D X 32

2. HVDC Principle 6-pulse converter V d+ α=0 V d = V d+ - V d- V d+ V d V d- V d+ -V d- V d- α=0 The 6-pulse bridge consists of two 3-pulse bridges (positive and negative) connected in parallel. 33

2. HVDC Principle 6-pulse bridge HVDC Smoothing reactor DC line Smoothing reactor power I d power V dr power V di Reactive power I d DC line Reactive power The HVDC comprises two converters connected in anti-parallel through smoothing reactors and DC lines. One converter is operated in rectifier mode to transmit power from the AC network to the other side whereas the other side converter is operated in inverter mode to receive power into the (other side) AC network. 34

2. HVDC Principle Rectifier Operation of the 6-pulse bridge converter Assume α = 15 and µ = 25 cosα + cos( α + µ ) cosφ 2 o o o cos15 + cos(15 + 25 ) 2 o φ 30 I.cosφ V I.sinφ 30 = 0.866 I The converter operates in rectifier mode. It transmits active power while consumes reactive power. 35

2. HVDC Principle Inverter operation of the 6-pulse bridge converter Assume α = 135 and µ = 25 cosα + cos( α + µ ) cosφ 2 o cos135 + cos(135 2 o φ 145 I.cosφ o + 25 o ) = 0.823 V 145 I I.sinφ The converter operates in inverter mode. It receives active power while consumes reactive power. 36

2. HVDC Principle For convenience, the converter operated in inverter mode is often referred to extinction angle (γ). Thus direct voltage in inverter mode (V di ) are expressed as V d D X = V = d 0 cosα + 3 ωl 2π k I d D X, α > 90 o Vd = Vd 0 cosγ DX γ = π α µ Actually, inverter is commonly controlled at constant extinction angle to prevent commutation failure. Therefore, it is not only for convenience, but also for converter control purpose. It is important to note that voltage drop caused by commutating reactance (Dx) is now negative. 37

2. HVDC Principle Voltage vs. current (VI) characteristics at steady state 1.0 V V d d 0 Slope is D X α = 0 1.0 V V d d 0 α = 0 Rectifier Inverter 1.0 Increasing α I I d dn α is the control variable for rectifier and γ is the control variable for inverter. Rectifier Inverter 1.0 Increasing γ Increasing α I I d dn -1.0 α max < 180-1.0 γ = 0 38

2. HVDC Principle 12-pulse bridge HVDC Y V dr I d V di Y Y Y V dry I d V diy Y Y The 12-pulse converter is required to improve harmonic current on AC sides. It comprises two 6-pulse converters connected in series. Harmonic current on AC sides are odd orders starting from 11 th, 13 th. whereas even orders present on the DC side (12 th, 14 th ). To achieve 12-pulse, phase displacement of 30 generated by Star (Y) and Delta ( ) connection of 39the transformers are employed.

2. HVDC Principle V d V dy V d Rectifier operation of the 12-pulse bridge converter Assume α = 15 and µ = 25 I AY I A Y V d I A V d I A I A Y Y V dy I AY 40

2. HVDC Principle power ½R d power Y Y Y power I d V dr V di Y Y Y Reactive power α min < α α min = 5-7 To ensure all thyristor valves are enough forward bias to turn on. voltage ½R d decreasing α I d V dr current V di γ min < γ Reactive power γ min = 15-17 To keep reactive power requirement on inverter side as low as possible. Voltage drop caused by line resistance (R d ) is taken into account and the VI characteristic presents operating point of the HVDC system. 41

2. HVDC Principle Detail Configuration of the HVDC 42

2. HVDC Principle Alternatives for the implementation of a HVDC power transmission system a) Earth Return ii) Bipolar Configuration b) Metallic Return i) Mono-polar Configuration iii) Homo-polar Configuration 43

2. HVDC Principle Alternatives for the implementation of a HVDC power transmission system (continued) 44

3. Control of the DC Transmission Can we use manual control for the rectifier (vary α) and the inverter (vary γ)? If we can not do that, which side should be controlled (rectifier or inverter) or control them both? What is/are the control purpose(s)? 45

3. Control of the DC Transmission Typical control strategies used in a HVDC system consists of: Firing Control {Rectifier} Current Control (CC) {Inverter} Constant Extinction Angle (CEA) Control {Inverter} Current Margin Control (CM) {Inverter} Voltage Control (VC) Voltage Dependent Current Limit (VDCL) Tap change Controls (TCC) Power Reversal 46

3. Control of the DC Transmission Firing Control Function of the firing control is to convert the firing angle order (α*) demanded fed into the valve group control system. There might be voltage distortions due to non-characteristic harmonics, faults and other transient disturbances such as frequency variation. Thus, phase-locked loop (PLL) based firing system is generally applied. v A v B v C Phase Detector v error u A u B u C PI Controller ( 1+ Ts) K Ts sin(.) sin(.) sin(.) v o Voltage Controlled Oscillator - ⅔ π θ α* comparator comparator comparator Gate firing 47

3. Control of the DC Transmission Firing Control (Continued) u A v A 0 time v error 0 2π time θ α* 0 Firing pulse of phase A α time 48

3. Control of the DC Transmission Current Control (CC) The firing angle is controlled with a feedback control system as shown in figure. The dc voltage of the converter increases (by decrease α*) or decreases (by increase α*) to adjust the dc current to its set-point (I d *). ( 1+ Ts) K Ts Y I d V dr v A, v B, v C Y Y i d * - i d + PI α min α max α* Firing Control 6 6 Current measurement 49

3. Control of the DC Transmission Constant Extinction Angle Control (CEA) The firing angle of the inverter is controlled at minimum angle (γ min ) to reduce reactive power requirement. This can be achieved by using Gamma control (γ-control). Current measurement Y V di Y Y Valve voltage γ measurement 6 6 v A, v B, v C Firing Control α* α min α max PI γ - + γ* 50

3. Control of the DC Transmission VI Characteristic of the CC and the CEA voltage voltage V dr α* X V di V di γ* = γ min V dr α*=α min γ* = γ min AC voltage decreasing I d current I d current VI Characteristic The intersection (X) is the operating point of the DC transmission line. If AC voltage on rectifier side decreases, CC decreases α* down to α min to increase DC current (I d ), but there is no operating point (X). This problem can be solved using CMC. 51

3. Control of the DC Transmission Current Margin Control (CMC) A better way is to use the inverter to control current less than of the rectifier by an amount of current margin ( I d ) when the rectifier can not perform CC. Y γ - Control γ* V di Current measurement Y Y 6 v A, v B, v C Firing Control α* Minimum selection α min α max PI + i d = 0.1 to 0.15 i d - + i d * 52

3. Control of the DC Transmission VI Characteristic of CC, CEA and CMC voltage voltage V dr α* X V di CEA γ* = γ min Vdr α*=α min X V di γ* = γ min CMC I d CC AC voltage decreasing Id I d current I d current Combined characteristics of CC, CEA and CMC This method can maintain stable operation when AC voltage of both sides are fluctuated. If AC voltage on rectifier side decreases, CC decreases α* down to α min to increase DC current (I d ), but there is no operating point (X). This 53 problem can be solved by CMC.

3. Control of the DC Transmission What will happen if AC network of the inverter side is too weak! voltage voltage V dr α* CMC X I d More Weak Weak AC V di CEA γ* = γ min V dr α* CMC I d X CEA γ* = γ min V di VC γ* > γ min I d current I d current In this range the intersection is poorly to define and both current controllers will hunt between the operating points. This problem can be solved by adjust VI characteristic of the inverter to voltage control (VC) in order to avoid hunting between two 54 controllers.

3. Control of the DC Transmission Voltage Control (VC) it is very effective when the inverter is connected to a weak AC network. The normal operating point X corresponds to a value of γ higher than the minimum. Thus, the inverter (rectifier as well) consumes more reactive power compared to inverter with CEA. Y Voltage measurement V di 6 Y Y v A, v B, v C 6 Firing Control α* Minimum selection Maximum selection CMC γ -Control α min α max PI v di 55 γ* v di * - +

3. Control of the DC Transmission Voltage Dependent Current Limit (VDCL) Commutation failures can occur during an AC fault on the inverter side. It results in continue conduction of a valve beyond its 120 conduction interval. The CC will regulate the DC current to its rated value, but in the worst case, two inverter valves may form DC short circuit and continue conducting for a long time, which can cause valve damage. To prevent this problem, DC current must be reduced. One possible to detect the AC side fault is the lowering of the DC voltage. This voltage is typically chosen at 40% of the rated voltage. I d 56

3. Control of the DC Transmission Voltage Dependent Current Limit (VDCL) The VDCL is a limitation imposed by the ability of the AC system to sustain the DC power flow when the AC voltage at the rectifier bus is reduced due to some disturbance as well. The VDCL characteristics is presented below. voltage V dr 0.4 VDCL α* CMC I d X VC V di 0.4 voltage V dr α* CMC VDCL I d X VC V di VDCL I d VDCL I d VDCL I dmax I d-min I dmax current I d-min I d current 57

3. Control of the DC Transmission Voltage Dependent Current Limit (VDCL) i d * v d v i i Minimum selection CC V d 1 1+Ts v VDCL v d Voltage measurement 58

3. Control of the DC Transmission Tap Change Control (TCC) When voltage of the AC system of the rectifier and/or of the inverter is fluctuated, transformer taps (both side) can adjust to keep the DC voltage within desired limits or suitable operating point. Generally, the tap will be changed when the firing angle of the rectifier/inverter still reach its more than 10-15 minutes to avoid interaction of other controls. Example: if the firing angle (α) of the rectifier reaches minimum limit (α min ) for long time. It means that the AC voltage of the converter is not appropriate. Thus, AC voltage of the converter must be reduced by tap changing of the converter transformer to free the firing angle of the rectifier. 59

3. Control of the DC Transmission Power Reversal The VI characteristic of power reversion is presented below (VDCL and VC are not included). The station 1 (rectifier) increases firing angle (α) into the inverter region and the station 2 (inverter) decreases its firing angle (α) into rectifier region. This can be performed without altering the direction of current flow. voltage voltage V1 dr α* X γ* = γ min V2 di I d current I d current V1 di V2 dr α* X γ* = γ min 60

3. Control of the DC Transmission Y I d Y V dr V di Y Y Y Y Max. CC Min. α* Firing Control VDCL V d, I d,α, γ Master Control i d * Firing Control VDCL α* Min. CC Max. i d CAE VC TCC γ min V d * p*/v d p o p* p Modulation Signal Power order γ min V d * CAE VC TCC 61

3. Control of the DC Transmission CIGRE s HVDC benchmark was simulated on ATP-EMTP with the typical HVDC control schemes, which the CC mode was employed at rectifier and VC mode was applied at inverter. All simulation results are presented in normalized values. Rectifier Current Control Inverter Voltage Control Start Up HVDC 62

3. Control of the DC Transmission The HVDC started at 0.1 sec. The firing angle of rectifier started at 90 while the extinction angle of inverter started at 90. Firing Angle (α) of Rectifier Firing Angle (α) of Inverter Extinction angle (γ) is also shown Start Up HVDC 63

3. Control of the DC Transmission The HVDC started to reverse power flow direction at 0.5 sec. Firing angle of the rectifier increased (with a ramp rate) into inverter zone while firing angle of the inverter decreased (with a ramp rate) into rectifier zone. Firing Angle (α) of Rectifier and Inverter Power Reversal DC Current 64

3. Control of the DC Transmission The power flow direction of the HVDC reversed at 0.9 sec. Power Reversal 65

3. Control of the DC Transmission VDCL performance during 1-phase fault at AC network of the rectifier station. V a Vb Vc 1 phase Fault at AC network of the rectifier station 66

3. Control of the DC Transmission I REF I d Vdi p.u. Degree Alpha _ r ( α ) Alpha_i( ) r α I d I REF α i i V di α r Fault at AC network of rectifier station 67

3. Control of the DC Transmission VDCL performance during 1-phase fault at AC network of the inverter station. V a Vb Vc 1-phase Fault at AC network of the inverter station 68

3. Control of the DC Transmission p.u. I Degree Alpha _ r ( α r ) Alpha_i( α ) REF I d Vdi i I d V di I REF α i α r Fault at AC network of inverter station 69

3. Control of the DC Transmission Modulation signal is employed when a power system has a special requirement such as frequency control, power oscillation damping, etc. For example, the addition frequency control loop is included into HVDC control system to stabilize frequency of the AC system. 70

3. Control of the DC Transmission Modulation Function of EGAT-TNB HVDC 71

3. Control of the DC Transmission Power Swing Damping (PSD) Function of EGAT-TNB HVDC 72

Thank you very much for your attention

References 1. Ani Gole, HVDC Transmission Lecture Note, University of Manitoba, 2000. 2. Jos Arrilaga, High Voltage Direct Current Transmission, 2 nd, IEE-Press, 1998. 3. Dennis A. Woodford, HVDC Transmission, Manitoba HVDC Research Center, Canada, 1998. 4. Erich Uhlmann, Power Transmission by Direct Current, Springer Verlag, 1975. 5. Vijay K. Sood, HVDC and FACTS Controllers, Kluwer. 2004. 6. Edward Wilson Kimbark, Direct Current Transmission vol.1, Wiley- Interscience, 1971. 7. IEEE Transmission and Distribution Committee, IEEE guide for planning DC links terminating at AC locations having low short-circuit capacities, IEEE, 1997. 8. กฤตยา สมส ย, น ท ศน วรพนพ พ ฒน, ว ทว ส ผ องญาต, การจ าลองระบบส งไฟฟ าแรงส ง กระแสตรงโดย ATP-EMTP, ส มมนาว ชาการระบบส ง กฟผ. 2548. 74