HVDC transmission

Electrical and Electronics Engineering (B.E./B.Tech) · Power systems · free study notes for exams, placements and interviews

In short

HVDC transmission converts AC to DC at a sending converter station, transmits bulk power over a DC line or cable, and inverts it back to AC at the receiving end. It is used where AC transmission is uneconomic or impossible: very long overhead lines, long submarine or underground cables, and asynchronous interconnections between grids or systems of different frequency. In India, POWERGRID operates several ±500 kV and ±800 kV bipoles (Rihand–Dadri, Talcher–Kolar, Ballia–Bhiwadi, Biswanath Chariali–Agra, Champa–Kurukshetra, Raigarh–Pugalur) plus back-to-back stations at Vindhyachal, Chandrapur, Vizag and Sasaram that tie the regional grids. The topic carries regular GATE marks (converter equations, firing angle, bridge waveforms, control modes) and appears in interviews for POWERGRID, NTPC, state transcos, Siemens Energy, Hitachi Energy, GE Vernova, BHEL and CPRI/ERDA-type research roles.

Explained

Why DC, and where it wins

No reactance: a DC line carries no steady-state inductive voltage drop and needs no reactive charging current, so there is no stability-limited length and no Ferranti effect. Cable length is unlimited (AC cables are limited to roughly 50–80 km by charging current).
No synchronism needed: the two AC systems need not be in phase or even at the same frequency — hence back-to-back ties and 50/60 Hz links.
Fully controllable power: converter firing angle sets power flow in magnitude and direction within milliseconds; the DC link does not take part in AC power swings, so faults do not propagate and short-circuit level is not increased.
Less conductor and right-of-way: a bipole with two conductors carries the power of a three-conductor AC circuit at the same conductor voltage rating; corona and radio interference are lower; no skin effect, so the full conductor cross-section is used.
Costs: converter terminals are expensive, the line is cheap; AC terminals are cheap, the line is dearer. The curves cross at the break-even distance — roughly 600–800 km overhead and 30–50 km for cables (figures vary with voltage, terrain and equipment prices).
Limitations: no DC circuit breaker in general service for large currents (HVDC CBs exist but are new and costly), so tapping a line is hard; converters generate harmonics and consume reactive power; no simple DC transformer; converter stations need large filter and smoothing equipment, plus skilled maintenance.

Link configurations and components

Monopolar: one conductor (usually negative polarity, which reduces corona and radio noise) with ground/sea return; cheapest, but earth-return current corrodes buried metal and disturbs compasses. Metallic return is used where ground return is not permitted.
Bipolar: two conductors of opposite polarity, neutral earthed at both ends; normal operation carries no earth current, and on loss of one pole the other can run monopolar at about 50% (more with overload rating). This is the standard Indian configuration.
Homopolar: two or more conductors of the same polarity with ground return; rarely built.
Back-to-back: rectifier and inverter in the same station with no DC line; used purely for asynchronous interconnection (Vindhyachal 2 × 250 MW was India's first, 1989).
Multi-terminal (MTDC): three or more converters on one DC network, in parallel (common voltage) or series (common current). The ±800 kV, 6,000 MW North-East–Agra link is a multi-terminal scheme with converters at Biswanath Chariali, Alipurduar and Agra.
Station equipment: converter transformer (one Y and one Δ secondary to form a 12-pulse group; on-load tap changer for control range), thyristor valves in valve halls with water cooling, smoothing reactor (limits DC ripple, limits fault current rate of rise, prevents commutation failure from current discontinuity), AC harmonic filters, DC filters, surge arresters, electrode line and earth electrode, and the control and protection system.

Converter theory: the 6-pulse and 12-pulse bridge

The basic unit is the three-phase full-wave (Graetz) bridge: six valves, each conducting 120°, two in series at any instant, giving 6 pulses of DC ripple per cycle.
Firing (delay) angle α is measured from the instant the valve would start conducting as a diode. With α < 90° the bridge rectifies; with α > 90° (and a DC source forcing current) it inverts.
Commutation (overlap) angle μ is the period when two valves of the same row conduct together as current transfers; commutation is driven by the transformer leakage reactance, which causes a current-dependent voltage drop. Extinction angle γ = 180° − α − μ: the margin left for the outgoing valve to regain blocking ability, typically held at 15–18° minimum.
Two 6-pulse bridges in series, fed by Y-Y and Y-Δ transformers (30° phase shift), make a 12-pulse converter: the lowest AC-side harmonics become the 11th and 13th (order 12k ± 1) and DC ripple becomes 12k, so filters are smaller. Every modern LCC scheme is 12-pulse.
A line-commutated converter (LCC) always draws lagging current: the rectifier behaves like a load consuming reactive power, and the inverter also consumes reactive power from the receiving AC bus — roughly 50–60% of the MW rating in total, supplied by the AC filters, capacitor banks, synchronous condensers or SVC/STATCOM.
Inverter risks: commutation failure (the outgoing valve fails to turn off and the current returns to it) from AC voltage dips, waveform distortion or too small γ; voltage-dependent current order limit (VDCOL) reduces current order when AC voltage collapses to avoid repeated failures.

LCC versus VSC

LCC (thyristor, "classic"): current-source converter; DC current direction is fixed, power reversal is by reversing DC voltage polarity (α swapped between ends). Needs a strong AC system for commutation; cannot start into a dead network (no black start); highest ratings and lowest losses (about 0.7–0.8% per station).
VSC (IGBT, PWM or modular multilevel converter): voltage-source converter; DC voltage polarity is fixed and power reverses by reversing DC current, which makes DC grids and cable (XLPE) use practical. Independent control of P and Q at each terminal, black-start and islanded supply capability, small filters, compact footprint; slightly higher losses (about 1% per station for MMC, higher in earlier two-level designs) and lower ratings than LCC, and DC-side faults are harder to handle in two-level designs.
Typical use: LCC for bulk long-distance and back-to-back; VSC for offshore wind, city in-feeds, weak-grid connections and multi-terminal DC.

Control of an HVDC link

Power flows because of a small voltage difference: Id = (Vd,rectifier − Vd,inverter)/R. Since R is small, both terminals must be controlled closely or current would swing wildly.
Normal strategy: the rectifier is on constant current (CC) control with a minimum-α limit (about 5°, to keep enough voltage across the valve for firing); the inverter is on constant extinction angle (CEA) or constant DC voltage control, which fixes the link voltage. The inverter also has a current controller with a current margin (typically 10–15% below the rectifier's order).
If the rectifier AC voltage falls and it hits its α-min limit, it can no longer hold current; control shifts to the inverter, which then regulates current at (Iorder − margin) and the rectifier holds α-min — this is the classic "mode shift" on the Vd–Id characteristic.
Tap changers slowly bring α back into its efficient working band (about 12–18° rectifier) after a control excursion; VDCOL protects against weak-AC conditions; current/power order comes from the master control, and modulation can be added for damping or frequency support.
Other functions: bypass/blocking of valve groups, forced retard for emergency power reduction, power reversal, and fault clearing by control — a DC line fault is cleared by forcing the rectifier to α ≈ 90–150° so the current goes to zero, then restarting after a de-ionisation pause.

Harmonics, insulation and Indian practice

Characteristic AC harmonics are of order pk ± 1 (p = pulse number) and DC-side ripple of order pk; non-characteristic harmonics come from unbalance and firing errors. Filters are tuned single/double-tuned shunt branches plus a high-pass; active DC filters are used on long lines.
Standards: converter station equipment to the IEC 60700 (thyristor valves), IEC 61803 (losses) and IEC 60919 (performance) families; Indian lines and clearances follow CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 (which replaced the 2010 regulations) and relevant IS codes; CEA Technical Standards for Connectivity to the Grid and the Indian Electricity Grid Code (IEGC) 2023 govern operation; planning and tariff approval sit with CEA, CERC and POWERGRID as CTU.
Indian landmarks: Vindhyachal back-to-back (1989); Rihand–Dadri ±500 kV, 1,500 MW (1991, India's first long-distance bipole); Chandrapur–Padghe, Talcher–Kolar (2,000 MW), Ballia–Bhiwadi (2,500 MW); ±800 kV UHVDC at NE–Agra (6,000 MW) and Champa–Kurukshetra (3,000 MW per pole-pair, 6,000 MW total); Raigarh–Pugalur ±800 kV with a VSC in-feed into Trichur (Kerala); Pugalur–Thrissur is India's first VSC HVDC. Several new HVDC links are planned for renewable evacuation (Pang (Ladakh)–Kaithal ±350 kV HVDC for Ladakh green power, and offshore/desert RE corridors) — scheme details and dates change, so verify current status against CEA/POWERGRID documents.

Key points

HVDC - AC→DC→AC transmission with controllable power flow; converter terminals costly, line cheap.
Break-even distance - about 600–800 km overhead, 30–50 km cable, beyond which HVDC is cheaper.
LCC - thyristor, current-source, needs strong AC, reverses power by reversing DC voltage polarity.
VSC - IGBT/MMC, voltage-source, independent P and Q, black-start, reverses power by reversing current.
Graetz bridge - 6 valves, each conducting 120°, 6-pulse ripple; 12-pulse is two bridges with a 30° shift.
α (firing angle) - delay from natural commutation; α < 90° rectifier, α > 90° inverter; α-min ≈ 5°.
γ (extinction angle) - 180° − α − μ; held ≥ 15–18° to avoid commutation failure.
Overlap angle μ - commutation period caused by transformer leakage reactance; gives a current-dependent voltage drop.
Control pair - rectifier on constant current, inverter on constant extinction angle (or constant Vd), with a current margin.
Current margin - 10–15%, prevents both ends fighting for current control; mode shift when rectifier hits α-min.
Bipolar - standard configuration; one pole lost → the other runs monopolar with ground return at about 50% power.
Back-to-back - no DC line; asynchronous tie only (Vindhyachal, Chandrapur, Vizag, Sasaram in India).
Harmonics - AC side pk ± 1, DC side pk; 12-pulse → 11th, 13th lowest; filters mandatory.
Reactive power - an LCC consumes about 50–60% of its MW rating in MVAr at both ends; filters and capacitors supply it.
Smoothing reactor - limits ripple and di/dt on faults, keeps current continuous.
No DC breaker in wide service - DC faults are cleared by converter control (forced retard), which makes tapping difficult.
Indian UHVDC - ±800 kV NE–Agra (6,000 MW, multi-terminal) and Champa–Kurukshetra; Pugalur–Thrissur is the first VSC link.
Ground electrode - needed for monopolar/emergency return; causes corrosion and must be sited away from buried metal.

Formulas and examples

No-load (ideal) DC voltage of a 6-pulse bridge: Vd0 = (3√2/π)·VLL ≈ 1.35·VLL
where VLL = line-to-line rms secondary voltage of the converter transformer (V), Vd0 in V

Rectifier DC voltage with overlap: Vd = Vd0·cos α − (3/π)·Xc·Id = 1.35·VLL·cos α − 3Xc·Id/π
α = firing angle (deg), Xc = commutating (leakage) reactance per phase (Ω), Id = DC current (A)

Inverter DC voltage: Vd = Vd0·cos γ − (3/π)·Xc·Id (magnitude form; γ = extinction angle)
Commutation relation: cos γ = −cos(α + μ); γ = 180° − α − μ

DC current and power: Id = (Vdr − Vdi)/Rline ; Pd = Vd·Id ; for a bipole Pd = 2·Vpole·Id
Rline = loop resistance of the DC line (Ω)

Overlap angle: cos α − cos(α + μ) = 2·Xc·Id/(√2·VLL)

Converter power factor (approx.): cos φ ≈ Vd/Vd0 = cos α − 3Xc·Id/(π·Vd0); Q = P·tan φ
AC line current (fundamental, 6-pulse): I1 = (√6/π)·Id ≈ 0.78·Id ; rms of ideal square-wave current Iac = √(2/3)·Id ≈ 0.816·Id

Harmonic orders: AC side h = pk ± 1, DC side h = pk (p = pulse number, k = 1, 2, 3…); Ih ≈ I1/h for an ideal rectifier

12-pulse bridge (two series bridges): Vd = 2[Vd0·cos α − 3Xc·Id/π]

Example 1 — DC voltage, overlap and power factor of a 6-pulse bridge. Converter transformer secondary VLL = 345 kV, Xc = 15 Ω/phase, α = 15°, Id = 2,000 A.

Vd0 = 1.35 × 345 = 465.8 kV.
Drop = 3·Xc·Id/π = 3 × 15 × 2,000/3.1416 = 28,648 V ≈ 28.6 kV.
Vd = 465.8 × cos 15° − 28.6 = 465.8 × 0.9659 − 28.6 = 449.9 − 28.6 = 421.3 kV.
Pd = 421.3 kV × 2 kA = 842.6 MW.
cos φ ≈ Vd/Vd0 = 421.3/465.8 = 0.904 → φ = 25.3°, Q = 842.6 × tan 25.3° = 398 MVAr (≈ 47% of P).
Overlap: cos α − cos(α + μ) = 2 × 15 × 2,000/(1.4142 × 345,000) = 60,000/487,900 = 0.1230 → cos(α+μ) = 0.9659 − 0.1230 = 0.8429 → α + μ = 32.6° → μ = 17.6°.

Example 2 — Bipole power and loss. A ±500 kV bipole carries 1,800 A per pole over 800 km; conductor resistance 0.015 Ω/km per pole.

Power per pole = 500 kV × 1.8 kA = 900 MW → bipole = 1,800 MW.
Resistance per pole = 0.015 × 800 = 12 Ω → loss per pole = I²R = 1,800² × 12 = 38.88 MW → both poles = 77.8 MW, i.e. 4.3% of 1,800 MW.
Receiving-end voltage per pole = 500 − (1.8 × 12) = 500 − 21.6 = 478.4 kV; received power = 478.4 × 1.8 × 2 = 1,722 MW.

Example 3 — Inverter firing angle for a required γ. Inverter with Vd0 = 500 kV per bridge-group, Xc = 12 Ω, Id = 2,500 A, required γ = 18°.

Drop = 3 × 12 × 2,500/π = 28,648 V ≈ 28.6 kV.
Vd(inv) = 500 × cos 18° − 28.6 = 500 × 0.9511 − 28.6 = 475.5 − 28.6 = 446.9 kV.
Overlap: cos γ − cos(γ + μ′) = 2XcId/(√2·VLL), with VLL = 500/1.35 = 370.4 kV → RHS = 2 × 12 × 2,500/(1.4142 × 370,400) = 60,000/523,800 = 0.1146.
cos(γ + μ) = 0.9511 − 0.1146 = 0.8365 → γ + μ = 33.2° → μ = 15.2°, so α(inv) = 180° − γ − μ = 180 − 18 − 15.2 = 146.8°.

Example 4 — Link current and control mode. Rectifier Vd0r = 420 kV at α = 10°, Xc = 10 Ω; inverter holds Vdi = 390 kV constant; DC line loop resistance 8 Ω.

Vdr = 420 cos 10° − 3 × 10 × Id/π = 413.6 kV − 9.549·Id(kA) kV.
Id = (Vdr − Vdi)/8 → 8·Id = 413.6 − 9.549·Id − 390 → 17.549·Id = 23.6 → Id = 1.345 kA.
Vdr = 413.6 − 9.549 × 1.345 = 400.8 kV; power at rectifier = 400.8 × 1.345 = 539 MW; at inverter = 390 × 1.345 = 524.6 MW; line loss = 1.345² × 8 = 14.5 MW (checks: 539 − 524.6 = 14.4 MW).

Example 5 — Break-even distance. AC option: terminals ₹400 crore, line ₹4.25 crore/km. HVDC option: terminals ₹2,600 crore, line ₹0.75 crore/km.

400 + 4.25L = 2,600 + 0.75L → 3.5L = 2,200 → L ≈ 629 km. Beyond about 630 km HVDC is cheaper on first cost; including the lower line losses and the absence of reactive compensation shifts the cross-over to a shorter distance. (Illustrative figures — real project costs differ.)

Feature	AC transmission	HVDC (LCC)	HVDC (VSC)
Conductors for same power	3	2 (bipole)	2
Line reactance / charging	Limits length	None	None
Needs synchronism	Yes	No	No
Power control speed	Slow (via FACTS/generation)	Milliseconds	Milliseconds
Reactive power at terminals	Supplies/absorbs	Consumes ~50–60% of MW	Independently controllable
Converter losses per station	—	~0.7–0.8%	~1%
Black start into dead grid	Yes	No	Yes
Fault level contribution	Increases	None	None
Typical max rating	—	Up to ~8,000 MW (±800 kV)	A few thousand MW

Parameter	Rectifier	Inverter
Angle	α ≈ 12–18° normal, 5° minimum	γ ≈ 15–18° minimum, α ≈ 140–150°
Normal control	Constant current (CC)	Constant extinction angle / constant Vd
On AC voltage dip	Hits α-min, loses current control	Takes over current control at I − margin
Vd polarity for power reversal	Becomes negative	Becomes positive (roles swap)

Where it is used

National bulk transfer: POWERGRID's ±500 kV and ±800 kV bipoles move surplus Eastern/North-Eastern generation to the Northern and Southern regions; back-to-back stations provide controlled inter-regional exchange.
Renewable evacuation and remote generation: solar/wind corridors from Rajasthan, Gujarat and Ladakh, Himalayan hydro, and offshore wind (VSC) connections.
Submarine and underground links: island and city supplies where AC cables are infeasible; VSC in-feeds into congested urban load centres.
Equipment and EPC employers: Hitachi Energy (Vadodara/Maneja), Siemens Energy, GE Vernova, BHEL, Toshiba Transmission, L&T, Kalpataru and KEC for station and line works; CPRI and ERDA for testing.
Job roles: HVDC station O&M and control-protection engineer, converter design and valve engineer, system studies engineer (PSCAD/EMTDC, PSS/E, ETAP, DIgSILENT), load-despatch engineer at NLDC/RLDC, protection and relay commissioning.
Adjacent uses: HVDC-style converters in railway traction feeds, large industrial DC drives, and in grid interconnection studies for India–Sri Lanka and India–Nepal/Bhutan proposals.

Interview questions

Q: Why is HVDC preferred for long submarine cables? A: A DC cable draws no charging current, so length is not limited by capacitance; an AC cable beyond roughly 50–80 km is consumed by its own charging current and needs compensation that is impossible mid-sea.
Q: What is the break-even distance and what sets it? A: The distance at which the higher converter-station cost of HVDC is offset by its cheaper line and lower losses — roughly 600–800 km overhead, 30–50 km for cables, depending on voltage, terrain and equipment prices.
Q: Why is the rectifier put on current control and the inverter on extinction-angle control? A: The DC line resistance is small, so current is very sensitive to a voltage difference; one end must fix the voltage (inverter at minimum safe γ, which is also the most efficient) while the other regulates current.
Q: What is the current margin and why is it needed? A: The inverter's current order is set 10–15% below the rectifier's, so only one end controls current at a time; if the rectifier loses control at α-min, the inverter takes over smoothly at the reduced current.
Q: How is power reversed in an LCC link and in a VSC link? A: In LCC the DC current direction is fixed, so the voltage polarity is reversed by swapping α between the stations; in VSC the DC voltage polarity is fixed and the current direction reverses.
Q: What is commutation failure and what causes it? A: The outgoing valve of the inverter fails to recover its blocking ability and the current returns to it, short-circuiting the bridge; caused by AC voltage dips, distortion, excessive Id or too small γ.
Q: Why is a 12-pulse converter preferred over 6-pulse? A: The 30° phase shift between Y and Δ transformer windings cancels the 5th and 7th harmonics, so the lowest AC harmonics are the 11th and 13th and DC ripple is 12-pulse — far smaller filters and smoother DC.
Q: Does an HVDC converter generate or consume reactive power? A: An LCC consumes lagging reactive power at both terminals, about 50–60% of the MW rating, supplied by AC filters, shunt capacitors or SVC/STATCOM; a VSC can do either on command.
Q: What is the function of the smoothing reactor? A: It limits DC ripple and harmonic current, keeps the DC current continuous at light load, and limits the rate of rise of fault current so the control can retard the rectifier in time.
Q: How is a DC line fault cleared without a DC breaker? A: Converter control forces the rectifier to a large firing angle (forced retard, α ≈ 90–150°), driving the current to zero; after a de-ionisation delay the link is restarted, often at reduced voltage.
Q: Why is minimum α about 5° at the rectifier? A: Enough forward voltage must exist across the valve at firing for reliable turn-on and for commutation margin; firing at α = 0 leaves no control reserve for a rise in DC voltage order.
Q: Why can an LCC not energise a dead AC network? A: Thyristors are line-commutated and need an existing AC voltage for commutation; a VSC with self-commutated IGBTs can impose its own voltage and frequency, so it can black-start.
Q: Does HVDC increase the short-circuit level of the receiving system? A: No — the converter limits its current by control, so HVDC adds load/generation without raising fault level, unlike an AC tie.

Common mistakes

Using 1.35·VLL for a 12-pulse converter without doubling it, or mixing phase and line voltage in Vd0.
Forgetting the commutation drop (3XcId/π) and quoting Vd0·cos α as the actual DC voltage.
Measuring α from the voltage zero crossing instead of from the natural commutation instant.
Saying power is reversed in an LCC link by reversing the current — the current direction is fixed by the thyristors; the voltage polarity reverses.
Claiming converters supply reactive power; an LCC always consumes it, at both the rectifier and the inverter.
Forgetting that in a bipole the power is 2 × Vpole × Id, and that loss must be counted in both poles.
Treating "no DC breaker" as meaning no fault protection — faults are cleared by converter control; also confusing HVDC with FACTS devices, which control an AC line rather than replace it.