Dr Liliana Oprea

FICHTNER GmbH&Co KG

Swiss Chapter of IEEE PESBaden-Dättwil, 4 September 2013

HVDC Back-to-Back Interconnections –Enabling reliable integration of power

system

Table of Contents

•Need for integration of different power systems

•Comparison between asynchronous and synchronous interconnection of powersystems

• HVDC technology review

•System studies for planning a HVDC interconnection

•Georgia-Turkey HVDC Back-to-Back Interconnection (Black-Sea TransmissionNetwork 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

World-wide experience for synchronous expansion of a widelyspread power system, while observing high standards on:

•power quality•supply reliability•asset security

and allowing for flexible energy market development, shows thatthis is a non-feasible undertaking in a short or medium term range

Asynchronous vs. synchronous operation

Assuming political consensus for efficient use of inter-regional

resources and mutually profitable cross-border exchanges

5

If the synchronous subsystems are very different in size,

structure and geographically wide – spread:

Technical reasons can be the limiting or delaying factor

Asynchronous vs. synchronous Ooperation

Synchronous operation implies system-wideharmonization of the long-term generation andtransmission planning

Inter-regional energy policy goals prevail over the short- andmedium term bilateral exchanges

6

If the energy markets in the participating countries are in

different development stages

Organizational reasons are the limiting factor

(introduce significant delays and uncertainties)

Asynchronous vs. synchronous operation

7

Comparison synchronous vs. asynchronous integration

CRITERION SYNCHRONOUS ASYNCHRONOUS

ENERGY MARKET ADAPTIVITY POOR OUTSTANDING

ROBUSTNESS AGAINST GENERATIONDEVELOPMENTS UNCERTAINTIES

POOR OUTSTANDING

IMPLEMENTATION SCHEDULE ESTIMATE 9-10 YEARS *) 3 YEARS

EMERGENCY SUPPORT IN CASE OFMAJOR SYSTEM INCIDENT (BLACKOUTSTART)

POOR OUTSTANDING**)

8

*) based on the experince of implementation time for synchronous connection of Turkeyto ENTSO-E**) for VSC technology

Comparison synchronous vs. asynchronous integration

CRITERION SYNCHRONOUS ASYNCHRONOUS

STABILITY (TRANSIENT ANDDYNAMIC)

POOR VERY GOOD

NEED FOR HARMONIZINGPROTECTION SETTINGS AND UNDER-FREQUENCY LOAD SHEDDING

YES NO

INITIAL INVESTMENT COSTS LOW HIGH

SHORT AND MEDIUM TERMINVESTMENT COSTS

MODERATE NONE

9

HVDC Back-to-Back interconnection

Alternative to synchronous interconnection

•normally used in order to create an asynchronous interconnection between two HVACnetworks, which could have the same or different frequencies

•simpler than the construction of two separated converter stations for a HVDCtransmission projects

•HVDC voltage level can be selected without consideration to the optimum values for anoverhead 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 eachend of the HVDC line•Monopolar configuration (with metallic return)

11

ACSystem 1

ACSystem 2

HVDC Schemes for power systems integration

12

Typical HVDC configurations

•long-distance HVDC lines or cables with an HVAC/HVDC converter station at eachend of the HVDC line•Bipolar configuration

ACSystem 1

ACSystem 2

HVDC Schemes for power systems integration

13

Typical HVDC configurations

• both converters in one location without an HVDC line - HVDC Back-to-Back scheme

ACSystem 1

ACSystem 2

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

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

14Courtesy: Siemens

Courtesy: Siemens

Comparison between HVDC technologies

HVDC Technologies characteristics

15

LCC Converter VSC Converter

Power 6400 MW 1200 MW

Voltage ±800 kV ±320 kV

Short circuit ratioSCR

Minimum 2x rated power No particular requirement

Active power control Continuous(Min 10% load)

Fast continuous

Reactive power control Fast continuous

Comparison between HVDC technologies

HVDC Technologies dependant inherent functionality (CIGRE TB 492)

16

LCC Converter VSC Converter

Transient stabilityimprovement

Available Available

Damping control Active power modulation Active and reactive powercontrol

Black start /Island supply

Available

System losses reduction Possible (by reactivepower control)

Project development aspects

HVDC connection scheme – high investment project which needs to bewell planned

Integrated system studies - several system studies which have to beperformed with the HVAC transmission systems including the HVDC link

A detailed analysis of the HVAC transmission network is necessary inorder 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 commutationfailures 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 byabsorption/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 theHVDC interconnector

The focus of the study is on consequences caused on the two systems of a suddentripping of the HVDC interconnector in case of very heavy power transfer in order toavoid 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 HVDCsystem

•Sub-synchronous torsional interaction (SSTI) study (alternatively screening ordamping 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 thePerformance of HVDC schemes

System studies in the planning phaseTools

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 interconnectionBlack Sea Transmission Network - HVDC Interconnection between Georgiaand 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

Georgian-Turkish border

Borcka

Carsamba

Kayabasi

Derina

Yusufeli

Erzurumi

Borasco

KalkandereTirebolu

OzluceKeban

Karakaya

Kangal

Zestafoni

Qsani

150kV400kV500kVPower plants

Horasan

Ataturk

Elbistan

boundary of strongconnections towesterm Turkishsystem

Georgia

Turkey

Enguri

GardabaniAchaltsikhe(back-to-back hvdc)

Selection of HVDC converter technologyShort circuit analysis – major criteria in selection of the HVDC technology

Effective Short Circuit Ratio (ESCR)

d

cF

PQS

ESCR-

=

High ESCR system: ESCR > 2.5Low ESCR system: 1.5 < ESCR < 2.5Very low ESCR system: ESCR < 1.5

Method to increase ESCR- installation of synchronous condenser

In case of Black Sea B2B – 3x60 MVA synchronous condensersinstalled on Turkish side

Synchronous condenser

60/39 MVA (over- / under-excited)

Validation of dynamic models of integrated systemModels have been developed in PSS®E and PSCAD in parallel

3-phase short circuit at 500 kV Georgian side, power from Turkey to Georgia

DC Voltage

DC Current

PSS®E PSCAD

Validation of dynamic models of integrated systemModels have been developed in PSS®E and PSCAD in parallel

Comparison of results obtained with PSS®E and PSCAD models

DC Voltage

DC Current

PSS®E PSCAD

DC Power

Firing angle

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

Time [seconds]

Menji220kV

Voltage

[pu]

Gardabani 500kVVoltage

[pu]

Enguri-Gardaba

ni

Rotorangle[deg]

Trip 2 Enguri hydro units

No mitigation (red)PSS in Enguri & Gardabani (black)

Run-back 100MW in HVDC (green)Run-back 50MW in HVDC (magenta)

Trip 2 Enguri hydro units

No mitigation (red)PSS in Enguri & Gardabani (black)

Run-back 100MW in HVDC (green)Run-back 50MW in HVDC (magenta)

Trip 2 Enguri hydro units

No mitigation (red)PSS in Enguri & Gardabani (black)

Run-back 100MW in HVDC (green)

Run-back 50MW in HVDC (magenta)

Dynamic studies for the HVDC Back-to-Back interconnectionHVDC 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)

Conclusions and outlook

HVDC Back-to-Back interconnections- enable power exchange and sharingof reserve power

Efficient method for interconnection of systems operating in differentsynchronous zones

HVDC schemes - High investment projects which need to be well planned

HVDC Back-to-Back Interconnections ProjectsGeorgia –Turkey (commissioning phase)Georgia- Armenia (feasibility study phase)Central Asia Power System – Turkmenistan- Afghanistan (project definitionphase)

Thank you for your attention