BSTP EN HVDC PSSE Training, Day 1 - THEORY.pptx

BSTP EN HVDC PSSE Training, Day 1 - THEORY.

1. ENERGY TECHNOLOGY AND GOVERNANCE PROGRAM Black Sea Transmission System Planning Project (BSTP) Workshop on Modeling High Voltage Direct Current (HVDC) Converter Stations This presentation is made possible by the support of the American people through the United States Agency for International Development (USAID). The contents are the responsibility of the United States Energy Association and do not necessarily reflect the views of USAID or the United States Government. March 2 1 -2 2 , 2017 Tbilisi, Georgia

2. INTRODUCTION LCC HVDC SYSTEM VSC HVDC SYSTEM COMPAR ISON CONTROL OF HVDC SYTEMS REACTIVE POWER BALANCE SHORT CIRCUIT RATIO ADDITIONAL BENEFITS OF HVDC SYSTEMS ENTSO-E HVDC NETWORKC CODE Contents

3. INTRODUCTION LCC HVDC SYSTEM TECHNOLOGY VSC HVDC SYSTEM TECHNOLOGY TECHNOLOGIES COMPARED CONTROL OF HVDC SYTEMS REACTIVE POWER BALANCE SHORT CIRCUIT RATIO ADDITIONAL BENEFITS OF HVDC SYSTEMS ENTSO-E HVDC NETWORK CODE

4. 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 Introduction

5. 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 Basics of HVDC Operation

6. 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 Disadvantages of HV A C Systems 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

7. Major advantage of flexibility in power exchange in comparison with HVAC Fast control 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 , reduce d visual impact higher power transmission capacity for same RoW 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 Advantages of HVDC Systems Comparison 6000MW ‐ HVDC vs. HV AC

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

9. 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 ALST O M – 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 Introduction

10. 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 ALST O M – HVDC VSC Introduction

11. INTRODUCTION LCC HVDC SYSTEM VSC HVDC SYSTEM COMPAR ISON CONTROL OF HVDC SYTEMS REACTIVE POWER BALANCE SHORT CIRCUIT RATIO ADDITIONAL BENEFITS OF HVDC SYSTEMS ENTSO-E HVDC NETWORK CODE

12. 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). LCC HVDC Systems

13. 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) LCC HVDC Systems

14. 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 LCC HVDC Systems 1. Valve branch 2. Double Valve 3. Valve tower – Quadrivalve 4. 6-pulse bridge

15. The thyristor valve is operated at net frequency (50 H z or 60 H z) 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. LCC HVDC Systems

16. Standard graphical symbols for valves and bridges 6 p ulse convertor 12 p ulse convertor LCC HVDC Systems

17. PRINCIPAL SCHEME

18. 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 CONVERTER STATION

19. LCC HVDC Systems

20. 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. VALVE HALL

21. 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. AC FILTERS

22. 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 49 th . 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. AC FILTERS

23. 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. AC FILTERS

24. 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 CONVERTER TRANSFORMERS

25. 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 CONVERTER TRANSFORMERS

26. 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. CONVERTER TRANSFORMERS

27. 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. EARTH ELECTRODES

28. 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 DC SMOOTHING REACTOR

29. 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 DC FILTER

30. 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 CCC HVDC Systems 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 converter transformer 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

31. 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 Configurations

32. 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. Configurations

33. 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 Configurations

34. 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. Configurations

35. Bipolar HVDC System with Ground Return A Bipolar balanced operation (normal) B Monopolar 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. C Monopolar 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. Configurations

36. 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 Configurations

37. 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 Configurations

38. INTRODUCTION LCC HVDC SYSTEM VSC HVDC SYSTEM COMPAR ISON CONTROL OF HVDC SYTEMS REACTIVE POWER BALANCE SHORT CIRCUIT RATIO ADDITIONAL BENEFITS OF HVDC SYSTEMS ENTSO-E HVDC NETWORK CODE

39. LCC HVDC control system only has one degree of freedom – when to turn on the thyristor . Thyristors can only be turned on (not off) by control action, and rely on the external AC system to effect the turn-off process. This limits the usefulness of LCC HVDC in some circumstances. AC system to which the LCC HVDC converter is connected must always contain synchronous machines in order to provide the commutating voltage. LCC HVDC converter cannot feed power into a passive system. With some other types of semiconductor device such as the insulated-gate bipolar transistor (IGBT), both turn-on and turn-off can be controlled, giving a second degree of freedom. VSC HVDC Systems

40. As a result of turn-on and turn-off capability, IGBTs can be used to make self-commutated converters. In such converters, the polarity of DC voltage is usually fixed and being smoothed by a large capacitance, can be considered constant. For this reason, an HVDC converter using IGBTs is usually referred to as a Voltage Source Converter VSC. The development of Insulated Gate Bipolar Transistors (IGBT) with high voltage ratings have accelerated the development of voltage sourced converters for HVDC applications. VSC HVDC Systems

41. The operation of the converter is achieved by Pulse Width Modulation (PWM). With PWM it is possible to create any phase3 angle and/or amplitude (up to a certain limit) by changing the PWM pattern, which can be done almost instantaneously. PWM offers the possibility to control both active and reactive power independently. From a transmission network viewpoint, it acts as a motor or generator without mass that can control active and reactive power almost instantaneously. VSC HVDC Systems

42. IGBT cells have a small size (around 1 cm2). Many IGBT cells are connected in parallel in IGBT chips and then in modules capable to handle current up to 2.4 kA with blocking voltage up to 6.5 kV. As for the thyristors , many modules are connected in series into valves to withstand to the high voltage levels. So, a valve may comprise up to 20 billions of IGBT cells. VSC HVDC Systems

43. PRINCIPAL SCHEME

44. VSC converter consists of two level or multilevel converter, phase-reactors and AC filters. VSC normally use the 6-pulse connection because the converter produces much less harmonic distortion than LCC. 12-pulse connection is unnecessary. CONVERTER STATION

45. The converter reactor is one of the key components in VSC. It permits continuous and independent control of active and reactive power. The main purposes of the converter reactor are to limit the short circuit current at the IGBT valves and to provide a low-pass filter of the PWM pattern The harmonic currents related to the switching frequency and generated by the converter are blocked by the converter reactor. There is one converter reactor per phase. CONVERTER REACTOR

46. Up to now, implemented VSC converters have been based on two or three-level technology which enables switching two or three different voltage levels to the AC terminal of the converter. Converter voltage created by PWM is far from the desired voltage. It needs AC filters to achieve an acceptable waveform. PWM MODULATION Achieved voltage Desired voltage

47. Both, the size of voltage steps and the related voltage gradients can be reduced or minimized if the AC voltage generated by the converter can be selected in smaller increments than at two or three levels only. The more steps that are used, the smaller is the proportion of harmonics and the lower is the high-frequency noise. Converters with high number of steps are termed multilevel converters. A new and different approach is Modular Multilevel Converter (MMC) technology. MMC MODULATION

48. MMC consists of six converter arms. Each of them comprises a high number of power modules (PM) and one converter reactor connected in series. The power modules contain: IGBT half bridge as a switching element DC capacitor unit for energy storage MMC MODULATION

49. It is possible to separately and selectively control each of the individual power modules in all phase units. Two converter arms of each phase unit represent a controllable voltage source. The total voltage of the two converter arms in each phase unit equals the DC voltage. By adjusting the ratio of the converter arm voltages in one phase unit, the desired sinusoidal voltage at the AC terminal is achieved. MMC MODULATION

50. Special cases of monopolar HVDC interconnections Back-to-back indicates there is no DC transmission line and both converters are located at the same site i.e. station Valves for both converters may be located in one valve hall DC filters are not required, nor are electrodes or electrode lines, the neutral connection being made within the valve hall. Back-to-Back HVDC Systems

51. Mainly used as interconnections between adjacent asynchronous networks which can not be synchronized They can also be used within a meshed grid in order to achieve a defined power flow Used in Japan for interconnections between power system networks of different frequencies (50 and 60 Hz) Back-to-Back HVDC Systems

52. Three or more HVDC substations geographically separated with interconnecting transmission lines or cables Parallel multi - terminal DC If all substations are connected to the Monopolar configuration/Bipolar configuration same voltage Series multi - terminal DC If one or more converter bridges are added in series in one or both poles A combination of parallel and series connections of converter bridges is a hybrid multi - terminal system Multiterminal HVDC Systems

53. INTRODUCTION LCC HVDC SYSTEM VSC HVDC SYSTEM COMPAR ISON CONTROL OF HVDC SY S TEMS REACTIVE POWER BALANCE SHORT CIRCUIT RATIO ADDITIONAL BENEFITS OF HVDC SYSTEMS ENTSO-E HVDC NETWORK CODE

54. Layout and Footprint of the converter station. VSC converters are also considerably more compact than line-commutated converters (mainly because much less harmonic filtering is needed) e.g. 600MW LCC converter station requires about 14000 m2 whereas a VSC HVDC needs only 3000m2. This requirement is very important on offshore platforms. LCC vs VSC Transformers The VSC controller allows the use of standard two-winding transformers This gives more flexibility to build and design the offshore station Harmonics LCC require harmonic filters, VSC only simple high‐pass filter for high order harmonics

55. VSC converters are self-commutating, not requiring an external voltage source for its operation i.e. do not rely on synchronous machines in the AC system for its operation. Therefore the Possibility of the converters starting with a dead grid, not needing any start-up mechanism (“Black-start” capability). They can feed power to an AC network consisting only of passive loads, something which is impossible with LCC HVDC. In contrast to LCC HVDC converters, VSC converters maintain a constant polarity of DC voltage and power reversal is achieved instead by reversing the direction of current. This makes voltage-source converters much easier to connect into a Multi-terminal HVDC system. For the same reason XLPE cables cannot be used with HVDC LCC . LCC vs VSC

56. Reactive power control and stability Im p roved voltage stability VSC controller can control the reactive power and the voltage. Reactive power flow can be independently controlled at each AC network and the reactive power control is independent of active power control This gives a serious advantage to the VSC technology in fault through capability and black start capability. The reactive control for the classical technology is done by capacitor bank (slow switching scheme), thus the flexibility is not good and a continuous control can not be done. Voltage stability problems may also be experienced at the terminals of HVDC links used for either long distance or back-to-back applications. They are usually associated with the unfavorable reactive power “load” characteristics of the converters. Transmission capacity HVDC LCC up to 6400 MW and Udc =±800kV= HVDC VSC up to 1100 MW and Udc =±300kV= LCC vs VSC

57. HVDC Converter Development LCC vs VSC

58.   LCC HVDC VSC HVDC Size single range converter 150 - 1500MW 50 – 1100MW Semiconductor technology Thyristor IGBT DC voltage ±800kV ± 32 0kV Converter technology Line commutated Self commutated Control of reactive power No, only switching regulation yes, continuous control Voltage control Limited Extensive Fault ride through No Yes Black start capability No Yes Power reversal without interruption No Yes Minimum ESCR 2 No required Minimum DC power flow 5-10% of rated power No minimum required Typical losses per convertor 0,80 % 2% Operating experience >20 years 8 years Operating experience offshore No Yes Construction time 3 (2)* years 1 year LCC vs VSC

59.   Long distance transmission over land/sea Interconnections of asynchronous networks WPP connection to network Feed of isolated loads LCC or CCC HVDC with OHL/Cables √ √     CCC Converters in Back-to-Back   √     VSC converters in Back-to-Back   √ √   VSC Converters with land/sea cables √ √ √ √ LCC vs VSC

60. INTRODUCTION LCC HVDC SYSTEM VSC HVDC SYSTEM COMPAR ISON CONTROL OF HVDC SY S TEMS REACTIVE POWER BALANCE SHORT CIRCUIT RATIO ADDITIONAL BENEFITS OF HVDC SYSTEMS ENTSO-E HVDC NETWORK CODE

61. Rectifier uses a current control (CC) and a α -limit control, which includes minimum α -limit Goal of minimum firing angle limit is to make sure there is enough voltage across valves before the firing takes place, otherwise the commutation will fail In CC mode, firing angle is controlled and thereby DC voltage to keep the DC current at the desired value If the measured DC current is smaller than a reference value, firing angle delay will be decreased and vice versa Tap changer control of the converter transformer is used to keep α within allowed range Control of LCC HVDC

62. Inverter is provided with a constant extinction angle control (CEA) and a current control. In CEA mode a compromise is made between a low risk of commutation failures (if gamma is to large) and a low reactive consumption (if gamma is small) which result in value of gamma of around 15 In normal operation, rectifier is in current control mode and inverter in CEA mode. If the AC voltage decreases, firing angle in rectifier will also decrease to keep the DC voltage up. When α hits minimum limit, rectifier will switch to α -limit control and inverter will take over current control Control of LCC HVDC

63. Id c =( Vrectifier-Vinverter )/Rd c Under steady-state operation, the inverter control system is normally arranged to maintain the DC voltage at a certain point on the HVDC link (known as the “compounding point”) at a target value (typically 1.0 pu ) The “compounding point” is usually at the rectifier DC terminal and hence the inverter must calculate this voltage based on the DC voltage at the inverter terminals, the DC current and the known resistance of the transmission circuit The rectifier normally controls the DC current flowing in the circuit and does this by adjusting its output DC voltage to give a current flow as described by the above equation Control of LCC HVDC

64. POWER CONTROL Power transferred between sending and receiving end of the HVDC link is controlled to meet an operator-set value at some point, known as the compounding point. Typically compounding point is at the rectifier DC terminal It can also be at the inverter DC terminal, the mid-point of the DC transmission conductors (e.g. border between two countries), the inverter AC terminal or the rectifier AC terminal If the power demand is changed then the power order will ramp to the new power transfer level at a rate of change (“ramp rate”) pre-selected by the operator. Control of LCC HVDC

65. FREQUENCY CONTROL All HVDC lines have the ability to regulate the power by means of frequency feed back loop acting on the HVDC line control system HVDC scheme can control the AC frequency of an AC system by automatically adjusting the power being delivered into that AC system in order to balance the load with the supply. Fast power control by HVDC reduces the under-frequency or over-frequency which can result from a changing load in a small system For example receiving end AC system could have an upper frequency limit to automatically stop further increases in the power being delivered by the HVDC scheme. Equally, the receiving AC system can have a lower frequency limit which, if reached, automatically increases the power being delivered into the receiving AC system. This can normally be overridden by sending end minimum frequency limit, sending end system will help out the receiving end AC system as much as possible without risking a cascade failure. Control of LCC HVDC

66. POWER-FREQUENCY CONTROL Usually, a combination of control modes is used. Normally, the control signal that acts on the power controller of the DC link is a scheduled delivered power by the DC link . That signal remains the same as long as the scheduled predetermined frequency remains within the limits (dead band) . In case of violating the limits, the frequency control system of the DC link will take over to support the system frequency by mod ifing its power output as needed (droop control) . If the maximum capacity of DC line is reached, then frequency control system turns to be out of action. Control of LCC HVDC

67. POWER MODULATION CONTROL Power being transferred through a HVDC link can be automatically modulated to provide damping to low-frequency power oscillations within either, or both, interconnected AC systems This is determined by studies during the design phase of the HVDC scheme POWER DEMAND OVERRIDE In response to certain events, such as loss of an AC transmission line, loss of an AC generator or loss of a major load, the HVDC interconnection can be programmed to respond in a pre-defined manner. Example : if the loss of a line may result in instability within the AC system, the HVDC interconnection can be preprogrammed to reduce the power transfer at a pre-determined ramp rate to a safe value as established by studies. Control of LCC HVDC

68. DC Voltage control makes sure of the power balance between sending and receiving ends Active Power can be controlled by changing the phase angle of the converter ac voltage with respect to the filter bus voltage Reactive power can be controlled by changing the magnitude of the fundamental component of the converter ac voltage with respect to the filter bus voltage Control of VSC HVDC

69. By controlling these two aspects of the converter voltage, operation in all four quadrants is possible. This means that the converter can be operated in the middle of its reactive power range near unity power factor to maintain dynamic reactive power reserve for contingency voltage support similar to a SVC It also means that P transfer can be changed rapidly without altering Q exchange with ac network or waiting for switching of shunt compensation. Reactive power control on both sides is responsible for the transformer secondary winding AC voltage magnitude Control of VSC HVDC

70. VSC HVDC uses Pulse Width Modulation (PWM) control to give the desired fundamental frequency voltage Control signal is needed to achieve the control of the switching of the IGBT valves. Sinusoidal control signal Vcontrol is compared with a triangular signal Vtri to decide which valve should be conducting. Control of VSC HVDC

71. The triangular wave form Vtri is at a switching frequency fs. The control signal Vcontrol is used to modulate the switch duty ratio at a frequency f1(modulating frequency). In a 3-phase transmission, each “leg” of the converter is controlled separately from the 2 others Control of VSC HVDC

72. Output voltages are similar than with one leg Due to 120 phase shift, line to line rms voltage at fundamental frequency is: Control of VSC HVDC

73. INTRODUCTION LCC HVDC SYSTEM VSC HVDC SYSTEM COMPAR ISON CONTROL OF HVDC SY S TEMS REACTIVE POWER BALANCE SHORT CIRCUIT RATIO ADDITIONAL BENEFITS OF HVDC SYSTEMS ENTSO-E HVDC NETWORK CODE

74. LCC HVDC converters consume substantial reactive power Large proportion of reactive power must normally be supplied locally within the converter station Important design consideration of LCC HVDC scheme relates to reactive power loading that a converter imposes on the network Main sources of capacitive (positive) reactive power in a HVDC station are the AC harmonic filters reduce harmonics injected into AC system generate reactive power AC filter is composed of capacitances, inductances and resistances At fundamental frequency the HV-connected capacitor is main contributor to reactive power generated. Reactive power balance

75. The reactive power consumption of an HVDC converter depends on the active power, the transformer reactance and the control angle. The reactive power consumption increases with increasing active power. A common requirement to a converter station is full compensation or overcompensation at rated load. In addition, a reactive band for the load and voltage range and the permitted voltage step during bank switching must be determined. These factors will determine the size and number of filter and shunt capacitor banks. Reactive power balance

76. Converter operating power factor can be approximately calculated from the overlap angle and the converter firing angle: Rectifier: cos φ= ½ x [cos( α)+ cos( α +µ)] Inverter: cos φ= ½ x [cos( γ)+ cos( γ+µ)] Power factor should be high as possible To avoid high reactive power consumption in the converters Keep rated power of the converter as high as possible Minimize losses In order to keep power factor high, α for rectifier and γ for inverter must be kept low (however, not too low) α 15° - 20° γ minimum value 15° Reactive power balance

77. Based on power factor reactive power absorption is approximately Qdc = tan [ arc cos (φ)] x Pdc This gives reactive power as a function of Load i.e. changing active power P of an HVDC station Reactive power demand of a converter is presented under three different control methods Ud c = const , α/γ = const Ud c = const , Uv = const Id c = const Reactive power balance

78. In order to meet the AC harmonic performance and exchange of reactive power with AC system at desirable level, each filter has to be switched in at a certain DC power transmission level Reactive power balance

79.   Q Id 1. .13 .5 Filter Unbalance Converter Reactive power balance Reactive power balance at the connection point

80.     Reactive power balance Another requirement imposed on reactive power control is that of not exceeding a specified AC voltage step change as a consequence of switching a filter bank (or any reactive power element) Magnitude of a voltage step change as a consequence of switching a filter can be approximated as

81. INTRODUCTION LCC HVDC SYSTEM VSC HVDC SYSTEM COMPAR ISON CONTROL OF HVDC SY S TEMS REACTIVE POWER BALANCE SHORT CIRCUIT RATIO ADDITIONAL BENEFITS OF HVDC SYSTEMS ENTSO-E HVDC NETWORK CODE

82. The AC/DC system interactions can be extremely impacted by the strength of the AC network relative to the HVDC link capacity. The weakness of AC system can be due to its high impedance or its low inertia. The strength of the AC/DC system can be measured by its short circuit ratio (SCR) which is the ratio between the short circuit MVA of the AC system ( Ssc ) compared to the DC converter MW rating ( Pdc ) The SCR presents inherently the strength of the AC system. Short Circuit Ratio  

83. SCR gives just the AC system strength taking into account the DC transmission Index called effective short circuit ratio (ESCR) is introduced to measure the AC system strength taking into account the effects of the HVDC equipment connected to the AC side ESCR is ratio between the short circuit level reduced by the reactive power of the shunt capacitor banks and AC filters connected to the AC bus at 1.0 per-unit voltage and the rated DC power   Short Circuit Ratio

84. Lower ESCR means more pronounced interaction between the HVDC substation and the AC network. AC networks can be classified in the following categories according to strength: strong systems with high ESCR: ESCR > 3.0 systems of low ESCR: 3.0 > ESCR > 2.0 weak systems with very low ESCR: ESCR < 2.0 In the case of high ESCR systems, changes in the active/reactive power from the HVDC substation lead to small or moderate AC voltage changes. Therefore, the additional transient voltage control at the bus bar is not normally required. Short Circuit Ratio

85. In the case of low and very low ESCR systems, the changes in the AC network or in the HVDC transmission power could lead to voltage oscillations and a need for special control strategies Dynamic reactive power control at the AC bus at or near the HVDC substation by some form of power electronic reactive power controller SVC or STATCOM may be necessary Connecting the HVDC to the weak AC systems will cause the following problems in AC system High dynamic over-voltages that come from the excessive reactive power at the HVDC terminals after the DC power being interrupted followed by zero absorption of reactive power. Voltage instability which is associated to the loading sensitivities of the HVDC link. Harmonic resonance due to the parallel resonance between AC capacitor filters and the AC system at lower harmonic. Voltage flickers due to the continuous switching of shunt capacitors and reactors causing unacceptable transient voltage flickers. Short Circuit Ratio

86. INTRODUCTION LCC HVDC SYSTEM VSC HVDC SYSTEM COMPAR ISON CONTROL OF HVDC SY S TEMS REACTIVE POWER BALANCE SHORT CIRCUIT RATIO ADDITIONAL BENEFITS OF HVDC SYSTEMS ENTSO-E HVDC NETWORK CODE

87. Static VA R Compensators (SVCs), Static Compensators (STATCOMs) or synchronous compensators, may also be used to ensure that the desired reactive balance is maintained within specified limits under defined operational conditions. More advanced solution consists of a STATCOM STATCOM provides Necessary commutation voltage to the HVDC converter Continuous AC voltage control Fast reactive power compensation to the network under transient conditions Removal of possible non - characteristic harmonic interactions ADVANCED REACTIVE POWER CONTROL

88. In CCC scheme reactive power is compensated by the series capacitors installed in series between the converter valves and the converter transformer. Elimination of switched reactive power compensation simplify the AC switchyard and reduces area required for CCC HVDC station. With VSC converters there is no need to compensate any reactive power consumed by the converter itself and the current harmonics on the AC side are related directly to the PWM frequency. Therefore the amount of filters in this type of converters is reduced dramatically compared with natural commutated converters. Reactive power at CCC HVDC

89. Objective to establish the necessary sub-bank rating and switching sequence to meet the reactive power control requirements of the scheme. Establish HVDC converter absorption under all extremes of operating condition tolerances From this converter absorption, the total reactive power required, allowing for the appropriate tolerance conditions, is established. Established switch points which keep the net reactive power interchange of the converter plus AC reactive power banks with the AC systems within the established limits. Reactive power study

90. Sub-synchronous oscillation damping. A steam turbine and electric generator can have mechanical sub-synchronous oscillation modes between the various turbine stages and the generator. If such a generator feeds into the rectifier of a DC link, supplementary control may be required on the DC link to ensure the sub-synchronous oscillation modes of concern are positively damped to limit torsional stresses on the turbine shaft. Sub-Synchronous Torsional Interaction (SSTI) factor

91. In view of the semiconductor and microprocessor based control systems included, modern HVDC links can be operated remotely. Moreover, modern HVDC systems are designed to operate unmanned. There are some existing installations in operation completely unmanned. This feature is particularly important in situations or countries where skilled people are few, and these few people can operate several HVDC links from one central location. In recent years VSC HVDC transmission has been introduced. Whilst sharing some commonality with LCC HVDC in terms of the asynchronous nature of the interconnection and benefits it can bring to AC system, technologies differ in several ways. Overview HVDC System Benefits

92. Direction of power flow can be changed very quickly (bi-directionality). HVDC link doesn´t increase the short-circuit power in the connecting point ( no need to uprate existing switching equipment ) . The need for RoW is much smaller for HVDC than for HVAC, for the same transmitted power. The environmental impact is smaller. VSC technology allows controlling active and reactive power independently without needs for extra compensating equipment. VSC technology gives a good opportunity to alternative energy sources to be economically and technically efficient. HVDC transmissions have a high availability and reliability rate, shown by more than 30 years of operation. Overview HVDC System Benefits No limits in transmitted distance, for OH lines, sea or underground cables. HVDC can carry more power for a given size of conductor. Very fast control of power flow, which implies stability improvements.

93. HV D C has a lot of other value added compared to conventional HV AC transmission. Interconnecting two AC systems using AC tie lines require automatic generation controllers of both systems to be coordinated using the tie line power and frequency. However, the interconnected AC systems with control coordination are still subjected to some operational problems such as Large oscillations which may lead to equipment’s tripping. Faults level problem (High short circuit levels). Transmission of disturbances from one system to the other. Using the DC line as a tie line would eliminate most of the mentioned problems. DC line is insensitive to the frequency and it would connect two asynchronous systems and isolate the system disturbances (“firewall”). Overview HVDC System Benefits

94. Strong control system for HVDC lines will enhance the dynamic performance of the AC systems in several aspects: Damping electromechanical AC system oscillations To enhance the transient stability in the AC system Control of frequency and reactive power oscillation. STABILITY IMPROVEMENT Control system provides accurate and fast control of the active power flow - to increase the transient stability of the AC system. After a specific disturbance, HVDC link can be controlled in a manner such that the DC power can be ramped up and down quickly to restore the balance between generation and load in both sides of the AC system. In some situation ramping up the power is necessary to assist system stability and this can be done by means of the short term overloading capabilities of the HVDC link. Controlling the HVDC converters so as to provide reactive power and voltage support, can be useful to augment transient stability. Overview HVDC System Benefits

95. DYNAMIC STABILIZATION OF AC SYSTEMS A power system is stable if and after any disturbance it returns to condition of equilibrium. HVDC link will dynamically support the system by means of alleviating instability problems such as power swinging after the disturbances. HVDC will contribute in damping process of the system during and after any disturbance by a small signal modulation of its transmitted active power. This DC power modulation is proportional to the frequency difference between the inter-tied systems. Fast modulation of DC transmission power can be used to damp power oscillations in an AC grid and thus improve the system stability. Overview HVDC System Benefits

96. INTRODUCTION LCC HVDC SYSTEM VSC HVDC SYSTEM COMPAR ISON CONTROL OF HVDC SY S TEMS REACTIVE POWER BALANCE SHORT CIRCUIT RATIO ADDITIONAL BENEFITS OF HVDC SYSTEMS ENTSO-E HVDC NETWORK CODE

97. The Network Code on High Voltage Direct Current Connections (NC HVDC) specifies requirements for long distance DC connections, links between different synchronous areas and DC-connected Power Park Modules, such as offshore wind farms, which are becoming increasingly prominent in the European electricity system. This is a relatively new area in which fewer standards or grid codes exist, making a pan-European approach particularly beneficial. Following on from the Network Code on Requirements for Generators and the Demand Connection Code, the NC HVDC is build on the same foundations, to create a consistent and complete set of connection codes. Latest Status Update – EC Regulation 2016/1447 (EU) 2016/1447 https://www.entsoe.eu/major-projects/network-code-development/high-voltage-direct-current/Pages/default.aspx ENTSO-E HVDC NETWORK CODE

98. Sets requirements for HVDC connections and offshore DC connected generation (PPM – Power Park Modules ) Set of coherent requirements for generators (of all sizes) in order to meet the future power system challenges Set requirements for new demand users and DSO connections and to outline demand side response requirements related to system frequency HVDC and DC connected PPM

99. Frequency ranges stay connected to the Network and remain operable within the Frequency ranges and time periods (minutes in table) capable of automatic disconnection at specified frequencies Rate-of-change-of-Frequency withstand capability up to a 2.5 Hz/s Active power controllability ( control range and ramping rate ) adjust the transmitted active power within the HVDC System maximum output following an Instruction from the Relevant TSO modifying the transmitted active power in accordance with pre-defined regulation sequences in case of Disturbance in one of the connecting AC Networks fast active power reversal from the Maximum Capacity in one direction to the Maximum Capacity in the other provide FCR -Frequency Containment Reserve and FRR -Frequency Restoration Reserve linking various Control Areas or Synchronous Areas adjust the ramping rate of active power variations in accordance with instructions sent by the Relevant TSO take automatic remedial actions (stopping the ramping, blocking FSM, LFSM-O, LFSM-U or Frequency control) ACTIVE POWER CONTROL Frequency Range Time period for operation 47.0 Hz – 47.5 Hz 30 minutes 47.5 Hz – 51.5 Hz Unlimited 51.5 Hz – 52.0 Hz 30 minutes

100. Synthetic inertia rapidly adjusting the active power injected to or withdrawn from the AC network in order to limit the rate of change of Frequency emulating Synchronous Generator Performance Frequency Sensitive Mode (FSM) respond to Frequency deviations in each connected AC network by adjusting the active power transmission (diagram, ranges) The Frequency Response Dead - band of Frequency deviation and Droop are selected by the Relevant TSO AUTOMATIC REMEDIAL ACTIONS Parameters Ranges Frequency response deadband 0-±500 mHz Droop s1 (upward regulation) Minimum 0,1 % Droop s2 (downward regulation) Minimum 0,1 % Frequency response insensitivity Maximum 30 mHz

101. Limited Frequency Sensitive Mode Over - frequency (LFSM-O) adjust Active Power transmission to the AC Network(s) according to diagram at a Frequency threshold 50.2 - 50.5 Hz with a Droop having a minimum value of 0.1 % At over - frequencies where ∆f is above ∆f1 the HVDC System has to reduce Active Power according to the Droop setting Limited Frequency Sensitive Mode Under - frequency (LFSM-U) adjust the Active Power Frequency Response to the AC Network(s) according to diagram at a Frequency threshold 49.8 Hz - 49.5 Hz with a Droop having a minimum value of 0.1 %. At under - frequencies where ∆f is below ∆f1 the HVDC System has to increase Active Power output according to the Droop S AUTOMATIC REMEDIAL ACTIONS

102. Voltage ranges stay connected to the Network and capable of operating at the maximum output of HVDC Converter Station within the ranges of the Network Voltage at the Connection Point Reactive power exchanged with the Network ensure that the reactive power of its HVDC Converter Station exchanged with the Network at the Connection Point is limited to values defined Reactive Power variation shall not result in a Voltage step exceeding the allowed value at the Connection Point - Maximum Voltage step shall be specified by the Relevant TSO ) Short circuit contribution during faults requirements Reactive Short Circuit Current contribution at the Connection Point s Provide at least 2/3 of the rated reactive current REACTIVE POWER CONTROL Voltage Range Time period for operation 0.85 pu – 1.118 pu Unlimited 1.118 pu – 1.15 pu To be established by each relevant system operator, in coordination with the relevant TSO but not less than 20 minutes <300 kV Voltage Range Time period for operation 0.85 pu – 1.05 pu Unlimited 1.05 pu – 1.0875 pu To be specified by each TSO, but not less than 60 minutes 1.0 875 pu – 1. 10 pu 60 minutes 300-400 kV

103. Reactive power capability Reactive Power capability requirements in the context of varying Voltage: U-Q/ Pmax -profile, within the boundary of which the HVDC Converter Station shall be capable of providing Reactive Power at its Maximum Capacity Reactive power control mode as a minimum operate in any of the following reactive-power-control modes at perform respective control utilizing its capability: - Voltage Control mode - Reactive-Power Control mode - Power-Factor Control mode REACTIVE POWER CONTROL

104. Voltage against-time-profile at the Connection Point(s) fault conditions under which the HVDC Converter Station shall stay connected to the Network and continue stable operation after the power system has recovered following fault Clearance Uret - retained Voltage at the Connection Point(s) during a fault Tclear - duration of the fault, Urec , and trec specify a point of lower limits of Voltage recovery following fault clearance. Ublc - blocking Voltage at the connection point at time Tblc , Tblc - instant when the HVDC System un blocks F RT – Fault R ide Through

105. Auto- reclosure Transient faults on HVAC lines in the Network adjacent or close to HVDC Systems shall not cause any of the equipment in the HVDC System to disconnect from the Network due to auto - reclosure of lines in the Network HVDC Systems with overhead lines shall be capable of auto - reclosing for transient faults within the HVDC System Converter energization and synchronization shall smooth any voltage transients to a steady-state level not exceeding 3% of the pre-synchronization AC Voltage Power oscillation damping capability Sub-synchronous torsional interaction damping capability contribute to electrical damping of torsional frequencies TSO shall define the necessary extent of SSTI studies Harmonics HVDC System robustness Reconnection - reconnect after an incidental disconnection due to a Network disturbance, the Relevant TSO shall define the conditions under which an HVDC System shall be capable of reconnecting to the Network Black start Capability Isolated network operation OTHER REQUIREMENTS

106. Models are needed for Interconnection study Grid planning strudy Operations security assessment Future system assessment Power flow studies (Steady State Studies) Active/Reactive power capability Voltage control settings Collector system Short circuit Studies Connection point adequacy evaluation Sizing of equipment and configuration Dynamic Studies Transient stability analyses (reactive power and voltage control) Frequency stability analyses (power-frequency control) Voltage stability analyses (reactive power and voltage control) Small signal Stability analyses (parameter settings and power oscillation damping) Other studies (cannot or partialy performed using PSS/E) Harmonics analyses SSTI – Sub-synchronous Interaction ( SSTI studies MODEL REQUIREMENTS

107. Questions and Discussion