Self-Commutating Conversion

Self-Commutating ConversionFlexible Power Transmission-The HVDC Option1IntroductionLine Commutated Convertors ShortcomingsLarge reactive power requirement (both during rectification and inversion),Injection of low-order harmonic currentsRisk of inverter commutation failuresDependence on reasonably strong AC systems to provide the commutating voltages.These problems can be eliminated in self-commutated conversion by the use of more advanced switching devices with turn-on and turn-off capability (IGBT)2Voltage Source Converters (VSC)Power transmission system requires precise control of active and reactive power flow to maintain the system voltage stability. This is achieved through an electronic converter and its ability of converting electrical energy from AC to DC or vice versa. Voltage Source Converters operating with the specified vector control strategy can perform independent control of active/reactive power at both ends. This ability of VSC makes it suitable for connection to weak AC networks, i.e. without local voltage sources. For power reversal, the DC voltage polarity remains the same for VSC based transmission system and the power transfer depends only on the direction of the DC current.3LCC - VSC ComparisonLCC HVDCMature TechnologyRequires strong ac systemLower losses 0.8% per converterRequires 60% reactive powerAC-DC system interactionsHarmonicsCommutations failureSpecial TransformersMulti terminal operation possible but hardControlled DC Current to zero (Idref=0)Dc voltage + to - as alpha changesrectifier to inverterVSC HVDCRapid growthHelps ac systemControl real and reactive powerindependentlyLosses reducing 1.1- 1.2% per converterNo Commutation failureLess Special TransformersFlexible DispatchHarmonics with MMC no issueDC Grid (multi-terminal) possibleDC voltage is a constant polarityDC Line faults are problematic4Line-Commutated Converters (LCC)Depends on the line voltage of the connected AC System for commutationUse uncontrolled or semi-controlled switching devices (diodes & thyristors)Fixed DC current directionLarge inductance enables it to be almost constantDirection of power flow controlled through reversal of the polarity of DC voltageOn the AC side, behaves approximately as a current source, injecting both grid-frequency and harmonic currents6-pulse and 12-pulse configuration

5Voltage Source Convertorself-commutated convertersUse IGBT with turn-on and turn-off controlFixed polarity of DC voltage Constant, smoothed by a large capacitancePower reversal by reversing current directionEasier to connect into aMulti-terminalHVDC system or DC GridMore compact than LCC as mainly much less harmonic filtering is requiredVSC HVDC systems normally use the six-pulse connection Less harmonic distortion as compared to LCCSimplified construction of the converter transformerBasically a VSC generates ac voltage from a dc voltage. It is, for historical reasons, often referred to as an inverter, even though it has the capability to transfer power in either direction6Two-level VSC Operating PrinciplesTwo-level VSC Operating PrinciplesThe converter valves are assumed losslessDC capacitors ripple negligibleThe converter transformer lossless

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Two-level VSC Operating Principles

Two-level single phase VSCAC terminals are switched in a bipolar way between two discrete voltage levels (+Vd & Vd)Conducting period is 180o, square wave switching schemeSince the conduction in switch is unidirectional, anti parallel diode (free-wheeling) is connected across to ensure the bridge voltage has one polarity & current can flow in both directionsCapacitor mid point is reference for AC output voltage

8Two-level VSC Operating PrinciplesS1 is ON,+ve current will flow through S1S2 is ON, -ve current will flow through S2Both switches OFF, AC presentDiode forms uncontrolled rectifireCharge Capacitors-ve current will flow through D1+ve current will flow through D2

9Two-level VSC Operating PrinciplesOnly one switch at a time is ONSimultaneous ON will result short circuit Blanking period-diode conductsS1 ONS1OFF- Inductance maintains present current, D2 conduct, voltage reverse polarityS2 ON, current reversalS2 OFF, D1 conductsOperates in four quadrants

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Single-phase full wave bridgeDC capacitor -stiff voltageTwo AC connection points1-4 valves sequence of turn-off & on1-2 ON, a+ & b- , vab=+Vd3-4 ON, a- & b+, vab= -VdVab is independent of ac current flowThe ac current is the result of interaction of the converter generated ac voltage with the ac system voltage and impedanceT1-t2- 1-2 ON, vab+, iab-, dc to ac, INVT2-t3, 1-2 ON, vab+, iab+, ac t- dc, RECTT3-t4, 3-4 ON, vab-, iab+, ,dc to ac, INVT4-t5, 3-4 ON, vab-, iab-, ac to dc, RECTId-cuurent flow in dc bus+ ac to dc & -dc to acPower flow from ac to dc with lagging power factor

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Single-phase full wave bridge12

Single-phase full wave bridgeDC capacitor -stiff voltageTwo AC connection points1-2 ON, a+ & b- , vab=+Vd3-4 ON, a- & b+, vab= -VdVab is independent of ac current flowThe ac current is the result of interaction of the converter generated ac voltage with the ac system voltage and impedanceT1-t2- 1-2 ON, vab+, iab-, dc to ac, INVT2-t3, 1-2 ON, vab+, iab+, ac t- dc, RECTT3-t4, 3-4 ON, vab-, iab+, ,dc to ac, INVT4-t5, 3-4 ON, vab-, iab-, ac to dc, RECTId-cuurent flow in dc bus+ ac to dc & -dc to acPower flow from ac to dc with lagging power factor

13ac volt.ac currentdc currentVabiabidPower Transfer ControlPassive load on AC SidePower flow is unidirectional from DC input side to passive AC loadActive load on AC sidePower flow is bi-directional in either directionPhase angle of AC o/p voltage +ve or ve w.r.t AC system voltage

14Inverter Operation-P & QActive and Reactive Power exchange depends on phase angle and magnitude of ends voltagesFor reactive power compensation no DC source required

15Inverter Operation-different conditionsIf >0 & V2>V1, P is transferred from VSC to AC system, else reverseIf =0, V1=V2, No P & Q is exchangedV2>V1, I/90o capacitive action (generate Q)V2 vCR ic = i3 builds up at the expense of i1 At all times i1+i3= Id, If the commutation reactances are balanced, the rates of change of i3 and i1 are equal;therefore, the voltage drops across XCR and XCY are the sameduring the overlap period, the direct voltage vd is the mean value of vCY and vCR. 65

The commutation process66

Three-phase full wave six-pulse diode converterAt t3, Valve 3 becomes FB & starts to conduct with valve 1 carrying the full de current. The conducting of both valves 1 & 3 gives a short circuit between phases a and b with the short-circuit current rising from phase b through valve 3 into phase a through valve 1. Once the short-circuit current equals the de current through valve 1, its net current reaches zero, valve 1 stops conducting and the commutation is complete. Period of commutation, angle o, 67

THYRISTOR-BASED CONVERTERAt68

Three-phase full wave six-pulse diode converterThe short-circuit current between the two phases for this period of commutationAssuming that when is = id, wt = oOutput voltage is reduced comparedDuring commutation, output follows the mean of the two short-circuited voltages. The lost voltage corresponds to the shaded area every 60 degrees and is given byDC voltage drop in the converter due to the commutation of dc current69

Three-phase full wave six-pulse diode converterDC voltage drop is directly proportional to Id the current is shifted to the right by the commutation process. the ac side power factor is reduced from unity to a lower value (lag), that some reactive power is consumed.This power factor reduction corresponds to the reduction in the dc voltage. Equating the dc and ac power

Combibe with Vo=1.35E and I1=0.78Id

For practical estimation, power factor angle may be taken as

THYRISTOR-BASED CONVERTERif the devices had a turn-on control, the start of each commutation could be delayed and hence the output voltage reduced or even reversed at will.

No Delay in firing of valve72Delaying the firing instants of the valves with respect to the voltage crossings, The commencement of the natural commutations

Effect of firing delay on voltage waveforms:73

Delaying the firing, the commencement of the natural commutations can be delayed by a definite time intervalVoltage area, and the mean dc are reduced proportionally to the magnitude of the delayLarge smoothing reactor on the DC side, the voltage waveform of will produce constant DCTHYRISTOR-BASED CONVERTER