HVDC Back-to-Back Interconnections Enabling reliable integration of power system Dr Liliana Oprea FICHTNER GmbH&Co KG Swiss Chapter of IEEE PES Baden-Dättwil, 4 September 2013
Table of Contents Need for integration of different power systems Comparison between asynchronous and synchronous interconnection of power systems HVDC technology review System studies for planning a HVDC interconnection Georgia-Turkey HVDC Back-to-Back Interconnection (Black-Sea Transmission Network Project) HVDC reliability enhancer for HVAC system 2
Challenges for energy sector SUSTAINABLE ENERGY for all (SE4all) global initiative launched by the United Nations in 2012 Three interlinked objectives to be achieved by 2030: providing universal access to modern energy services doubling the share of renewable energy in the global energy mix doubling the global rate of improvement in energy efficiency 3
Need for integration of different power system Power system integration is driven by following factors: More efficient use of inter-regional resources Mutually profitable cross-border exchanges Security of supply in countries with scarce primary resources Establishment of power markets There are two technical options for integration: synchronous and asynchronous operation of the power systems 4
Asynchronous vs. synchronous operation Assuming political consensus for efficient use of inter-regional resources and mutually profitable cross-border exchanges World-wide experience for synchronous expansion of a widely spread power system, while observing high standards on: power quality supply reliability asset security and allowing for flexible energy market development, shows that this is a non-feasible undertaking in a short or medium term range 5
Asynchronous vs. synchronous Ooperation Synchronous operation implies system-wide harmonization of the long-term generation and transmission planning Inter-regional energy policy goals prevail over the short- and medium term bilateral exchanges If the synchronous subsystems are very different in size, structure and geographically wide spread: Technical reasons can be the limiting or delaying factor 6
Asynchronous vs. synchronous operation If the energy markets in the participating countries are in different development stages Organizational reasons are the limiting factor (introduce significant delays and uncertainties) 7
Comparison synchronous vs. asynchronous integration CRITERION SYNCHRONOUS ASYNCHRONOUS ENERGY MARKET ADAPTIVITY POOR OUTSTANDING ROBUSTNESS AGAINST GENERATION DEVELOPMENTS UNCERTAINTIES POOR OUTSTANDING IMPLEMENTATION SCHEDULE ESTIMATE 9-10 YEARS *) 3 YEARS EMERGENCY SUPPORT IN CASE OF MAJOR SYSTEM INCIDENT (BLACKOUT START) POOR OUTSTANDING **) *) based on the experince of implementation time for synchronous connection of Turkey to ENTSO-E **) for VSC technology 8
Comparison synchronous vs. asynchronous integration STABILITY (TRANSIENT AND DYNAMIC) CRITERION SYNCHRONOUS ASYNCHRONOUS POOR VERY GOOD NEED FOR HARMONIZING PROTECTION SETTINGS AND UNDER- FREQUENCY LOAD SHEDDING YES NO INITIAL INVESTMENT COSTS LOW HIGH SHORT AND MEDIUM TERM INVESTMENT COSTS MODERATE NONE 9
HVDC Back-to-Back interconnection Alternative to synchronous interconnection normally used in order to create an asynchronous interconnection between two HVAC networks, which could have the same or different frequencies simpler than the construction of two separated converter stations for a HVDC transmission projects HVDC voltage level can be selected without consideration to the optimum values for an overhead line or cable and is therefore normally quite low (150 kv or lower) 10
HVDC Schemes for power systems integration Typical HVDC configurations long-distance HVDC lines or cables with an HVAC/HVDC converter station at each end of the HVDC line Monopolar configuration (with metallic return) AC System 1 AC System 2 11
HVDC Schemes for power systems integration Typical HVDC configurations long-distance HVDC lines or cables with an HVAC/HVDC converter station at each end of the HVDC line Bipolar configuration AC System 1 AC System 2 12
HVDC Schemes for power systems integration Typical HVDC configurations both converters in one location without an HVDC line - HVDC Back-to-Back scheme AC System 1 AC System 2 13
Types of HVDC technologies HVDC Technologies characteristics Line commutated converters (LCC) Active power control Terminals demand reactive power Reactive power balance by shunt bank switching Minimum system short circuit capacity of twice rated power à strong grid required, normally used for remote power supply Courtesy: Siemens Self-commutated converters (Voltage Source Converters VSC) Active and reactive power control Dynamic voltage regulation Modular and expandable Black start capability No short circuit restriction à suitable for weak grids Courtesy: Siemens 14
Comparison between HVDC technologies HVDC Technologies characteristics LCC Converter VSC Converter Power 6400 MW 1200 MW Voltage ±800 kv ±320 kv Short circuit ratio SCR Minimum 2x rated power No particular requirement Active power control Reactive power control Continuous (Min 10% load) Fast continuous Fast continuous 15
Comparison between HVDC technologies HVDC Technologies dependant inherent functionality (CIGRE TB 492) LCC Converter VSC Converter Transient stability improvement Available Available Damping control Active power modulation Active and reactive power control Black start / Island supply Available System losses reduction Possible (by reactive power control) 16
Project development aspects HVDC connection scheme high investment project which needs to be well planned Integrated system studies - several system studies which have to be performed with the HVAC transmission systems including the HVDC link A detailed analysis of the HVAC transmission network is necessary in order to determine: best equipment ratings most suitable connection points operational limits of the HVDC link
Timeline for system studies System studies have to be performed in each phase of the project General timeline
System studies in planning phase HVDC LCC TECHNOLOGY (CLASSICAL HVDC) DOMINANT SIZING ISSUES Redundancy criteria DOMINANT FEASIBILITY ISSUES Short Circuit capability of AC systems (ShC >2 Pnom) Response time for load rejection (AC fault recovery after commutation failures and converter blocking) HVDC link is a STABILITY ENHANCER AND FIREWALL against blackouts
System studies in the planning phase INTEGRATED SYSTEM STUDY Steady state analyses load flow computations and steady state security assessment identification of possible overloads/voltage violations on the AC grids caused by absorption/injection of the DC power reactive power compensation requirements at the converter substations short circuit level (determination of ESCR) Analyses applied to selected extreme network operating conditions
System studies in the planning phase INTEGRATED SYSTEM STUDY Dynamic simulations in order to identify frequency instabilities voltage collapse unacceptable post-contingency operating conditions dedicated defence plans (SPS) applied to extreme contingencies involving the HVDC interconnector The focus of the study is on consequences caused on the two systems of a sudden tripping of the HVDC interconnector in case of very heavy power transfer in order to avoid an uncontrolled cascade of events leading to a possible blackout
System studies in the planning phase Harmonic study Insulation coordination and lightning performance study for the complete HVDC system Sub-synchronous torsional interaction (SSTI) study (alternatively screening or damping study) Study on voltage and power interactions in multi-infeed HVDC systems determination of MIIF (if applicable, a part of Integrated System Study) New CIGRE WG B4.64 - Impact of AC System Characteristics on the Performance of HVDC schemes
System studies in the planning phase Tools Integrated System Study - Power System Analysis Software (eg PSS E, DiGSILENT) HVAC/HVDC Transmission System Model preliminary dynamic model of HVDC system Special studies (specification phase) - PSCAD Model used for harmonics, insulation coordination, SSTI studies
Example of HVDC Back-to-Back interconnection Black Sea Transmission Network - HVDC Interconnection between Georgia and Turkey
Arial view of HVDC Back-to-Back Georgia-Turkey
Simplified configuration of the Back-to-Back HVDC link
Integrated HVAC/HVDC transmission system model 150kV 400kV 500kV Power plants Enguri Georgia Zestafoni Qsani boundary of strong connections to westerm Turkish system Carsamba Kayabasi Borasco Tirebolu Turkey Kalkandere Yusufeli Erzurumi Borcka Derina Achaltsikhe (back-to-back hvdc) Ho rasan Gardabani Georgian-Turkish border Kangal Keban Ozluce Elbistan Karakaya Ataturk
Selection of HVDC converter technology Short circuit analysis major criteria in selection of the HVDC technology Effective Short Circuit Ratio (ESCR) ESCR = S F - Q P d c High ESCR system: ESCR > 2.5 Low ESCR system: 1.5 < ESCR < 2.5 Very low ESCR system: ESCR < 1.5 Method to increase ESCR- installation of synchronous condenser In case of Black Sea B2B 3x60 MVA synchronous condensers installed on Turkish side
Synchronous condenser 60/39 MVA (over- / under-excited)
Validation of dynamic models of integrated system Models have been developed in PSS E and PSCAD in parallel DC Voltage DC Current 3-phase short circuit at 500 kv Georgian side, power from Turkey to Georgia PSS E PSCAD
Validation of dynamic models of integrated system Models have been developed in PSS E and PSCAD in parallel DC Voltage DC Power Firing angle DC Current Comparison of results obtained with PSS E and PSCAD models PSS E PSCAD
Power Oscillation Damping Control Effect of HVDC damping control on system stability 3-phase short circuit on 400 kv transmission line Borcka to Deriner, power from Turkey to Georgia, fault clearing time 0.1 s, reclosing tine 0.65 s
Power Oscillation Damping Control Effect of HVDC damping control on synchronous condensers
Dynamic studies for the HVDC Back-to-Back interconnection HVDC controls used as stability enhancer- power run-back in case of outages of 500 kv transmission line in Georgia, initiated by Special Protection Scheme (event triggered) Enguri- Gardaba ni Run-back 50MW in HVDC (magenta) No mitigation (red) PSS in Enguri & Gardabani (black) Rotor angle [deg] Trip 2 Enguri hydro units Run-back 100MW in HVDC (green) Menji 220kV Voltage [pu] Run-back 50MW in HVDC (magenta) Run-back 100MW in HVDC (green) Trip 2 Enguri hydro units No mitigation (red) PSS in Enguri & Gardabani (black) Gardaba ni 500kV Voltage [pu] Run-back 50MW in HVDC (magenta) Trip 2 Enguri hydro units Run-back 100MW in HVDC (green) No mitigation (red) PSS in Enguri & Gardabani (black) Time [seconds]
Conclusions and outlook HVDC Back-to-Back interconnections- enable power exchange and sharing of reserve power Efficient method for interconnection of systems operating in different synchronous zones HVDC schemes - High investment projects which need to be well planned HVDC Back-to-Back Interconnections Projects Georgia Turkey (commissioning phase) Georgia- Armenia (feasibility study phase) Central Asia Power System Turkmenistan- Afghanistan (project definition phase)
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