High Voltage Direct Current (HVDC) Converter Stations

1. High Voltage Direct Current (HVDC) Converter Stations

2. Contents • INTRODUCTION • LCC HVDC SYSTEM • VSC HVDC SYSTEM • COMPARISON

3. Introduction • First HVDC System Commissioned in 1954, Gotland, Sweden • ±100kV 20MW 97 kilometers of submarine cable • Longest Distance in Operation 1983, DR Congo • ±500kV 560MW 1709 kilometers overhead-line • Longest Submarine Cable 2008, • Norway to Netherlands • ±450kV, 700MW 583 km submarine cable • Connection of asynchronous systems • Highest Voltage in Operation 2010, • Yunnan-Guangdong, China • ±800kV, 5000 MW • First Multi-Terminal HVDC System 1992, • Quebec‐New England • ±450kV 2000MW

4. Basics of HVDC Operation • Taken from a 3-phase AC network • Converted to DC in a converter station • Transmitted by DC line or cable (underground or submarine) • Converted back to AC in another converter station • Injected into AC network

5. Disadvantages of HVAC Systems • Inductive and capacitive elements of overhead lines and cables put limits to the transmission capacity and the transmission distance • Depending on the required transmission capacity the achievable transmission distance for an AC cable will be in the range of 40 to 100 km. It will mainly be limited by the charging current • Direct connection between two AC systems with different frequencies is not possible • Direct connection between two AC systems with the same frequency or a new connection within a meshed grid may be impossible because of system instability, too high short-circuit levels or undesirable power flow scenarios

6. Advantages of HVDC Systems • Major advantage of flexibility in power exchange in comparison with HVAC • Fastcontrol of power flow – practically independently from frequency, voltage or angle at terminal buses • Fast change of direction of transmitted power – due to inherent properties of the electronic equipment in converters • Controllable – power injected where needed, supplemental control, frequency control • Bypass congested circuits – no inadvertent flow • Lower losses • Reactive power demand limited to terminals independent of distances • Narrow Right-of-Way (RoW) – • land coverage and the associated right-of-way cost for an HVDC overhead transmission line is smaller, • reduced visual impact • higher power transmission capacity for sameRoW • no Electromagnetic field (EMF) constraints • Cost Comparison HVDC vs. HVAC • HVDC has a higher installation cost due to the converter stations and filtering requirement • The cost of an HVDC line is less than the cost of an AC line. Long AC lines are more expensive due to shunt and series compensation requirements Comparison 6000MW ‐ HVDC vs. HVAC

7. Introduction HVDC Systems: Current- & Voltage- Link CURRENT LINK VOLTAGE LINK

8. Introduction • three ways of achieving AC/DC/AC conversion in HVDC system: • Natural Commutated Converters: • Most used in the HVDC systems as of today • The component that enables this conversion process is the thyristor, which is a controllable semiconductor • Known as CSC – Classic or LCC – Line Commutated Converters • Producers • SIEMENS– HVDC CSC CLASSIC • ABB – HVDC CSC CLASSIC • ALSTOM – HVDC LCC • Capacitor Commutated Converters (CCC): • Improvement in the thyristor-based commutation • Characterized by the use of commutation capacitors inserted in series between the converter transformers and the thyristor valves • Improve the commutation failure performance of the converters when connected to weak networks

9. Introduction • Forced Commutated Converters: • The valves of these converters are built up with semiconductors with the ability turn-on but also to turn-off. • Two types of semiconductors are normally used GTO (Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor) • Known as VSC – Voltage Source Converters • Introduced a spectrum of advantages, e.g. feed of passive networks (without generation), independent control of active and reactive power, power quality… • Producers: • SIEMENS – HVDC PLUS • ABB – HVDC LIGHT • ALSTOM – HVDC VSC

10. LCC HVDC Systems • AC power is fed to a converter operating as a rectifier. Output of this rectifier is DC power, independent of AC supply frequency and phase. • DC power is transmitted through a conduction medium; overhead line, a cable or a short length of bus bar • Second converter is operated as inverter and allows the DC power to flow into the receiving AC network. • Converter requires alternating AC voltage (Vac) to operate as an inverter. This is why the thyristor-based converter topology used in HVDC is known as a line-commutated converter (LCC).

11. LCC HVDC Systems • Conventional HVDC transmission utilizes line-commutated thyristor technology. • Thyristor - controllable semiconductor that can carry very high currents (4000 A) and is able to block very high voltages (up to 10 kV). • Thyristors used for LCC HVDC valves are amongst the largest semiconductors of any type produced for any industry • By means of connecting the thyristors in series it is possible to build up a thyristor valve, which is able to operate at very high voltages (figure shows 8.5 kV thyristor)

12. LCC HVDC Systems • The required DC system voltages are achieved by a series connection of a sufficient number of thyristors. • A group of four valves in a single vertical stack is known as a “quadrivalve” • Three such quadrivalves being required at each end of each pole • Since the voltage rating of thyristors is several kV, a 500 kV quadrivalve may have hundreds of individual thyristors connected in series groups of valve or thyristor modules 1. Valve branch 2. Double Valve 3. Valve tower – Quadrivalve 4. 6-pulse bridge

13. LCC HVDC Systems • The thyristor valve is operated at net frequency (50 Hz or 60 Hz) • By means of a control angle it is possible to change the DC voltage level of the bridge. • This ability is the way by which the transmitted power is controlled rapidly and efficiently.

14. LCC HVDC Systems • Standard graphical symbols for valves and bridges • 6 pulse convertor • 12 pulse convertor

15. PRINCIPAL SCHEME

16. CONVERTER STATION • Converter station is normally split into two areas: • AC switchyard which incorporates the AC harmonic filters and HF filters • “Converter island” which incorporates • the valve hall(s), control and • services building, converter • transformers and DC switchyard

17. LCC HVDC Systems

18. VALVE HALL • Valves associated with each twelve-pulse bridge are normally contained within a purpose built building • This enclosure provides a clean, controlled environment in which the thyristor valves can safely operate without the risk of exposure to pollution or outdoor conditions. • Within the valve hall, the thyristor valves are typically suspended from the roof of the building • low voltage being closest to the roof • high voltage being at the lowest point • on the valve. • An air gap between the bottom of the • valve and the valve hall floor provides • the high voltage insulation.

19. AC FILTERS • AC side current waveform of a HVDC converter, is highly non-sinusoidal, and, if allowed to flow in the connected AC network, might produce unacceptable levels of distortion • AC side filters are therefore required as part of the converter station in order to reduce the harmonic distortion of the AC side current and voltage to acceptably low levels • Shunt-connected AC filters appear as capacitive sources of reactive power at fundamental frequency, and normally are used to compensate most or all of the reactive consumption of the converter • Design of the AC filters, therefore, normally has to satisfy these two requirements of harmonic filtering and reactive power compensation.

20. AC FILTERS • Design influenced by a number of factors • Specified harmonic limits • AC system voltage conditions • Switched filter size (dictated by voltage step limit, reactive power balance…) • Two main filter types: • Tuned filter or band-pass filter which is sharply tuned to one or several harmonic frequencies (single (e.g. 11th) double (e.g. 11/13th) and triple (e.g. 3/11/13th) tuned types) • Damped filter or high-pass filter offering a low impedance over a broad band of frequencies i.e. designed to damp more than one harmonic. Filter tuned at 24th harmonic will give low impedance for both 23rd and 25th harmonic • Scheme with a 12-pulse converter, the largest characteristic harmonics will be the following: 11th, 13th, 23rd, 25th, 35th, 37th, 47th, and 49th. Level of the 11th and 13th harmonic are generally twice as high as for the rest. Common practice is to provide: • band-pass filters for the 11th and 13th harmonic • high-pass filters for the higher harmonics.

21. AC FILTERS • Possible low-order resonance between the AC network and the filters and shunt banks • When a big HVDC scheme is to be installed in a weak AC system, a low-order harmonic filter (most often tuned to 3rd harmonic) may be also needed. • Each filter branch can have one to three tuning frequencies • AC harmonic filters are typically composed of a high voltage connected capacitor bank in series with a medium voltage circuit comprising air-cored air-insulated reactors, resistors and capacitor banks • Connected directly to the converter bus bar or connected to a “filter bus bar” which, in-turn, is connected to the converter bus bar. • AC harmonic filters are automatically switched-on and off with conventional AC circuit breakers when they are needed to meet harmonic performance and reactive power performance limits.

22. CONVERTER TRANSFORMERS • Interface between the HVDC converter and the AC system and provide several functions • Galvanic isolation between the AC and DC systems • Correct voltage to the converters • Limit effects of steady state AC voltage change on converter operating conditions • Fault-limiting impedance • 30° phase shift required for twelve-pulse operation via star and delta windings • Equipped with on-load tap-changers in order to provide the correct valve voltage • tap changer will adjust to keep the delay angle α at a rectifier at its desired normal operating range • at the inverter, tap changer will adjust to maintain the inverter operation at its desired level of DC voltage or extinction angle γ

23. CONVERTER TRANSFORMERS • CONVERTER TRANSFORMERS • The largest plant item to be shipped to site for an HVDC project • 12-pulse converter requires two 3-phase systems which are spaced apart from each other by 30 or 150 electrical degrees. This is achieved by installing a transformer on each network side in the vector groups Yy0 and Yd5. • Common transformer arrangements in HVDC schemes

24. CONVERTER TRANSFORMERS It is important that the converter transformer be thermally designed to take into consideration both the fundamental frequency load and the AC harmonic currents that will flow from the converter through the converter transformer to the AC harmonic filters.

25. EARTH ELECTRODES • Essential component of the monopolar HVDC transmission system, since they carry the operating current on a continuous basis • Contribute to lower cost costs for the earth electrodes are significantly lower than the costs for a second conductor (with half the nominal voltage) • Earth electrodes are also found in all bipolar HVDC systems • Since the direct currents in the two poles of the HVDC are never absolutely equal, in spite of current balancing control, a differential current flows continuously from the station neutral point to ground. • It is common practice to locate the grounding of the station neutral point at some distance (10 to 50 kilometers) from the HVDC station by means of special earth electrodes.

26. DC SMOOTHING REACTOR • Normally required for power transmission schemes; they are not required for back-to-back schemes • In general it is used to • Reduce the DC current ripple on the overhead transmission line or cable • Limitation of the DC fault currents • Prevention of resonance in the DC circuit • Protect the thyristor valve from fast front transients originating on the DC transmission line (for example a lightning strike) • DC smoothing reactor is normally a large air-cored air-insulated reactor • DC SWITCHGEAR • Switchgear on the DC side of the converter is typically limited to disconnectors-switches and earth switches for scheme reconfiguration and safe maintenance operation

27. DC FILTER • Converter operation results in voltage harmonics being generated at the DC terminals • This AC harmonic component of voltage will result in AC harmonic current flow in the DC circuit • The field generated by this AC harmonic current flow can induce harmonic current flow in open-wire telecommunication systems • In a back-to-back scheme, these harmonics are contained within the valve hall with adequate shielding • With a cable scheme, the cable screen typically provides adequate shielding • With open-wire DC transmission it may be necessary to provide DC filters to limit the amount of harmonic current flowing in the DC line

28. CCC HVDC Systems • CCC is characterized by the use of commutation capacitors in series, between valve bridge and converter transformer • These capacitors provide reactive power proportional to the loading of converter • This eliminates the need for reactive power compensation by shunt capacitors and large filter banks • Commutation capacitors reduce the risk of commutation failures in the converter • Filters still needed to mitigate harmonics, but instead of high MVA filter banks active DC filters and continuously tuned AC filters can be used • Other effects of commutation capacitors • Reduced convertertransformer rating (reactive power flow through transformer minimized) • Reduced required area for the HVDC station due to elimination of switchable filter banks • Reduced valve short circuit currents due to voltage drop across capacitor varistors used to protect capacitors from overvoltages

29. Configurations • Basic HVDC cable transmission scheme is a monopolar installation that uses the earth and sea to return the current. • To avoid potential problems associated with ground return current a monopolar metallic return system is used - return current flows through a conductor in the form of a medium-voltage cable • A further development of the monopolar transmission scheme is the bipolar configuration • Bipolar configuration is actually two monopolar systems combined - one at positive and one at negative polarity with respect to ground

30. Configurations • Monopolar HVDC System with Ground Return • Consists of converter units at each end, a single conductor and return through the earth or sea • At each end of the line, it requires an electrode line and a ground or sea electrode built for continuous operation • It can be a cost-effective solution for a HVDC cable transmission and/or the first stage of a bipolar scheme • Most feasible solution for very long distances and in particular for very long sea cable transmissions.

31. Configurations • Monopolar HVDC System with Metallic Return • Consists of converter units at each end, one high voltage and one medium voltage conductor • Used when construction of electrode lines and ground electrodes results in an uneconomical solution due to a short distance or high value of earth resistivity • In many cases, existing infrastructure or environmental constraints prevent the use of electrodes and metallic return path is used in spite of increased cost and losses

32. Configurations • Bipolar HVDC System • Most commonly used configuration for a bipolar transmission system - high degree of operational flexibility • Operate in monopole configuration as needed • Allows for maintenance or outage of one pole • Up to half of rated capacity • For power flow in the other direction, the two conductors reverse their polarities • Advantage over two monopoles is reduced cost due to one common or no return path and lower losses • Disadvantage is that unavailability of the return path with adjacent components will affect both poles.

33. Configurations • Bipolar HVDC System with Ground Return • ABipolar balanced operation (normal) • BMonopolar ground return operation (converter pole or OHL outage) • Upon a single-pole fault, the current of the sound pole will be taken over by the ground return path and the faulty pole will be isolated. • CMonopolar metallic return operation (converter pole outage) • Following a pole outage caused by the converter, current can be commutated from ground return path into a metallic return path provided by the HVDC conductor of the faulty pole.

34. Configurations • Bipolar HVDC System with Dedicated Metallic Return • Dedicated LVDC metallic return conductor can be considered as an alternative to a ground return path with electrodes • If there are restrictions even to temporary use of electrodes • If the transmission distance is relatively short

35. Configurations • Bipolar HVDC System without Dedicated Metallic Return • Scheme without electrodes or a dedicated metallic return path for monopolar operation will give the lowest initial cost • Monopolar operation is possible by means of bypass switches during a converter pole outage, but not during an HVDC conductor outage. • A short bipolar outage will follow a converter pole outage before the bypass operation can be established