Grid requirements to connect DPGS based on RES

Grid requirements to connect DPGS based on RES
Marco Liserre Marco Liserre

Introduction Grid requirements for DPGS are stringent and subject to changes They are different for different renewable energy sources IEEE made an attempt, with IEEE 1547 series, to have a common approach for all DPGS below 10 MW In fact power level is maybe more important than source type Grid operators consider low power DPGS as a kind of “disturbance” or “negative” load Higher power DPGS are starting to be consider a resource for grid stability IGCT will be more important for high power Probably Silicon carbide will oust the Silicon device Marco Liserre

Introduction Safety issues are also important due to the higher penetration of DPGS at low voltage level Power quality and EMC are stringent too In the following the grid requirements are reviewed with focus on: Photovoltaic systems Wind systems However considerations on the influence of the power level will be made IGCT will be more important for high power Probably Silicon carbide will oust the Silicon device Marco Liserre

Photovoltaic systems Marco Liserre

Outline International Regulations Public Voltage Quality
Response to abnormal grid conditions Power Quality Anti-islanding requirements References Conclusion Marco Liserre

International Regulations
Grid connection requirements IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems IEEE Standard for Conformance Tests Procedures for Equipment Interconnecting Distributed Resources with Electric Power Systems IEEE , Recommended Practice for Utility Interface of Photovoltaic (PV) Systems – incorporated in IEEE 1547 UL 1741, Standard for Inverters, Converters, and Controllers for Use in Independent Power Systems - elaborated by Underwriters Laboratories Inc. – compatibilzed with IEEE 1547 IEC61727 [6] Photovoltaic (PV) systems - Characteristics of the utility interface - December 2004 IEC Ed : Testing procedure of islanding prevention measures for utility interactive photovoltaic inverter (describes the tests for IEC 61727) – approved in 2007 VDE Automatic disconnection device between a generator and the public low-voltage grid” – Safety issues- applied on German Market EMC IEC , Ed. 3.0 – “Electromagnetic compatibility (EMC) –Part 3-2: Limits –Limits for harmonic current emissions (equipment input current ≤16 A per phase)”, ISBN , November 2005 EN , Ed. 1.2 —“Electromagnetic compatibility (EMC) –Part 3-3: Limits – Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems, for equipment with rated current ≤16 A per phase and not subject to conditional connection”, ISBN , November 2005 IEC , Ed. 1 – “Electromagnetic compatibility (EMC) –Part 3-12:Limits – Limits for harmonic currents produced by equipment connected to public low-voltage systems with input current >16 A and ≤75 A per phase” , November 2004 IEC , Ed. 1 —“ Electromagnetic compatibility (EMC) – Part 3-11: Limits – Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems – Equipment with rated current ≤75 A and subject to conditional connection” , August 2000Standard EN – “Voltage Characteristics of Public Distribution System”, CENELEC: European Committee for Electrotechnical Standardization, Brussels, Belgium, November 1999 Utility Voltage Quality Standard EN – “Voltage Characteristics of Public Distribution System”, CENELEC: European Committee for Electrotechnical Standardization, Brussels, Belgium, November Marco Liserre

Public Voltage Quality – EN 50160
voltage unbalance for three phase inverters. Max unbalance is 3% voltage amplitude variations: max +/-10% frequency variations: max +/-1% voltage dips: duration < 1 sec, deep < 60% voltage harmonic levels. Max voltage THD is 8% Odd harmonics Even harmonics Not multiple of 3 Multiple of 3 Order h Relative voltage (%) 5 6 3 2 7 9 1.5 4 1 11 3.5 15 0.5 6..24 13 21 17 19 23 25 Marco Liserre

Response to abnormal grid conditions
IEEE 1547 IEC61727 VDE Voltage range (%) Disconnection time (s) V < 50 0.16 0.10 110 ≤ V < 85 0.2 50 ≤ V < 88 2.00 50 ≤ V < 85 110 < V < 120 1.00 110 < V < 135 V ≥ 120 V ≥ 135 0.05 Voltage deviations Obs. The purpose of the allowed time delay is to ride through short-term disturbances to avoid excessive nuisance tripping IEEE 1547 IEC61727 VDE Frequency range (Hz) Disconnection time (s) 59.3 < f < 60.5* 0.16 fn-1 < f < fn+1 0.2 47.5 < f < 50.2 Frequency deviations Obs. The VDE allow much lower frequency limit and thus frequency adaptive synchronization is required. IEEE 1547 IEC61727 VDE 88 < V < 110 [%] AND 59.3 < f < 60.5 [Hz] 85 < V < 110 [%] fn-1 < f < fn+1 [Hz] Min. delay of 3 minutes N/A Reconnection after trip Obs. The time delay in IEC61727 is an extra measure to ensure resynchronization before reconnection in order to avoid possible damage Marco Liserre

Power Quality DC Current Injection Current harmonics
IEEE 1574 IEC61727 VDE Idc < 0.5 [%] of the rated RMS current Idc < 1 [%] Idc < 1A Max Trip Time 0.2 s Obs. For IEEE 1574 and IEC61727 the dc component of the current should be measured by using harmonic analysis (FFT) and there is no maximum trip time condition IEEE 1547 and IEC 61727 Individual harmonic order (odd)* h < 11 11 ≤ h < 17 17 ≤ h < 23 23 ≤ h < 35 35 ≤ h Total harmonic distortion THD (%) (%) 4.0 2.0 1.5 0.6 0.3 5.0 Current harmonics Obs. The test voltage for IEEE1574/IEC61727 should be produced by an electronic power source with a voltage THD < 2.5% (typically ideal sources) Odd harmonics Even harmonics Order h Current (A) 3 2.30 2 1.08 5 1.14 4 0.43 7 0.77 6 0.30 9 0.40 8 ≤ h ≤ 40 0.23 x 8/h 11 0.33 13 0.21 13 ≤ h ≤ 39 0.15 x 15/h if IEC is not considered, the practice is that the harmonic limits are set by the IEC for class A equipments Obs. The current limits in IEC are given in amperes and are in general higher than the ones in IEC For equipments with a higher current than 16 A but lower than 75A another similar standard IEEE applies Marco Liserre

Power Quality Average Power Factor
Only in IEC61727 it is stated that the PV inverter shall have an average lagging power factor greater than 0,9 when the output is greater than 50%. Most PV inverters designed for utility-interconnected service operate close to unity power factor. In IEEE1574 as this a general standard that should allow also distributed generation of reactive power there is no requirement for the power factor No power factor requirements are mentioned in VDE Obs. Usually the power factor requirement for PV inverters should be interpreted now as a requirement to operate at quasi-unity power factor without the possibility of regulating the voltage by exchanging reactive power with the grid. For high power PV installations connected directly to the distribution level local grid requirements apply as they may participate in the grid control. For low power installations it is also expected that in the near future the utilities will allow them to exchange reactive power but new regulations are still expected. Marco Liserre

Anti-islanding Requirements
What is Islanding? Islanding for grid connected PV systems takes place when the PV inverter does not disconnect very short time after the grid is tripped, i.e. it is continuing to operate with local load. In the typical case of residential electrical system co-supplied by a roof-top PV system, the grid disconnection can occur as a result of a local equipment failure detected by the ground fault protection, or of an intentional disconnection of the line for servicing. In both situations if the PV inverter does not disconnect the following consequences can occur: Retripping the line or connected equipment damaging due to of out-of-phase closure Safety hazard for utility line workers that assume de-energized lines during islanding In order to avoid these serious consequences safety measures called anti-islanding (AI) requirements have been issued and embodied in standards Marco Liserre

Anti-islanding Requirements – IEEE 1574
In IEEE 1574 the requirement is that after an unintentional islanding where the distributed resources (DR) continues to energize a portion of the power system (island) through the PCC, the DR shall detect the islanding and cease to energize the area within 2 seconds. Adjustable RLC load should be connected in parallel between the PV inverter and the grid. The resonant LC circuit should be adjusted to resonate at the rated grid frequency and to have a quality factor of 1 or in other words the reactive power generated by [VAR] should equal the reactive power absorbed by [VAR] and should equal the power dissipated in [W] The parameters of the RLC load should be fine tuned until the grid current through S3 should be lower than 2% of the rated value on a steady-state base. In this balanced condition, the S3 should be open and the time before disconnection should be measured and should be lower than 2 seconds. The UL 1741 standard in US has been harmonized with the anti-islanding requirements stated in IEEE 1547 Marco Liserre

Anti-islanding Requirements – IEC62116
In IEC similar AI requirements as the IEEE1547 is proposed. The test can also be utilized by other inverter interconnected DER. In the normative reference IEC the ratings of the system valid in this standard has a rating of 10 kVA or less, the standard is though subject to revision. The test circuit is the same as in the IEEE test power balance is required before the island detection test. The requirement for passing the test contains more test cases but the conditions for confirming island detection do not have a significant deviation compared to the IEEE test. The inverter is tested at three levels of output power (A %, B 50-66% and C 25-33% of inverters output power). Case A is tested under maximum allowable inverter input power, case C at minimum allowable inverter output power if > 33 %. The voltage at the input of the inverter also has specific conditions. All conditions are to be tested at no deviation in real and reactive load power consumption then for condition A in a step of 5% both real and reactive power iterated deviation from -10% to 10% from operating output power of inverter. Condition B and C are evaluated by deviate the reactive load in an interval of ±5 % in a step of 1 % of inverter output power. The maximum trip time is the same as in IEEE standards 2 s. In IEC61727, there is no specific description of the anti-islanding requirements. Instead reference to IEC62116 is done. Marco Liserre

Anti-islanding Requirements – VDE-0126-1-1
The VDE allows the compliance with one of the following anti-islanding methods: A. Impedance measurement B. Disconnection detection with RLC resonant load The test circuit is the same of the one reported in IEEE and the test conditions are that the RLC resonant circuit parameters should be calculated for a quality factor bigger than 2 With balanced power the inverter should disconnect after the disconnection of S2 in maximum 5 seconds for the following power levels: 25%, 50% and 100%. For three-phase PV inverters a passive anti-islanding method is accepted by monitoring all three phases voltage with respect to the neutral. This method is conditioned by having individual current control in each of the three phases. Finding a software based anti-islanding method has been a very challenging task resulting in a large number of research work and publications. Marco Liserre

Conclusions An overview of the most relevant standards related to the grid connection requirements of PV inverters is given. High efforts are done by the international standard bodies in order to “harmonize” the grid requirements for PV inverters worldwide. The IEEE1574 standard has done a big step in the direction of issuing a standard that includes grid requirements not only for PV inverters but for all distributed resources under 10 MVA. Underwriters Laboratories in US has revised this year the UL 1471 by accepting the grid requirements of IEEE1574 and also IEC62116 was revised to harmonize with the requirements of IEEE1574 in the anti-islanding requirements. Even the very specific German standard VDE was revised in 2006 where the grid impedance measurement has become optional and an alternative requirement very similar to IEEE1574 was included. All these positive actions needs to be followed by adoption in different countries that still use their own local regulations. Marco Liserre

References Marco Liserre liserre@ieee.org
[1] Dugan, R.C.; Key, T.S.; Ball, G.J., "Distributed resources standards," Industry Applications Magazine, IEEE , vol.12, no.1, pp , Jan.-Feb. 2006 [2] IEEE Std – “IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems,", ISBN SH94811, April 2000. [3] UL standard 1741, “Inverters, Converters, and controllers for Use in Independent Power Systems”, Underwriters Laboratories Inc. US, 2001 [4] IEEE Std – “Standard for Interconnecting Distributed Resources with Electric Power Systems," ISBN SH95144, IEEE, June 2003 [5] IEEE Std – “Standard Conformance Test Procedures for Equipment Interconnecting Distributed Resources with Electric Power Systems” ISBN SH95346, IEEE, July 2005 [6] IEC Ed.2 – “Photovoltaic (PV) Systems - Characteristics of the Utility Interface”, December, 2004 [7] IEC CDV Ed. 1 – “Test procedure of islanding prevention measures for utility-interconnected photovoltaic inverters”, IEC 82/402/CD:2005 [8] VDE V “Automatic disconnection device between a generator and the public low-voltage grid”,VDE Verlag, Doc nr , 2006 [9] IEC , Ed. 3.0 – “Electromagnetic compatibility (EMC) –Part 3-2: Limits –Limits for harmonic current emissions (equipment input current ≤16 A per phase)”, ISBN , November 2005 [10] EN , Ed. 1.2 —“Electromagnetic compatibility (EMC) –Part 3-3: Limits – Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems, for equipment with rated current ≤16 A per phase and not subject to conditional connection”, ISBN , November 2005 [11] Standard EN – “Voltage Characteristics of Public Distribution System”, CENELEC: European Committee for Electrotechnical Standardization, Brussels, Belgium, November [12] IEC , Ed. 1 – “Electromagnetic compatibility (EMC) –Part 3-12:Limits – Limits for harmonic currents produced by equipment connected to public low-voltage systems with input current >16 A and ≤75 A per phase” , November 2004 [13] IEC , Ed. 1 —“ Electromagnetic compatibility (EMC) – Part 3-11: Limits – Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems – Equipment with rated current ≤75 A and subject to conditional connection” , August 2000 Marco Liserre

Wind systems Marco Liserre

Outline Grid codes, description and purpose
Transmission system operator demands Active power control, frequency control Reactive power control, voltage control Ride-Through Capabilities Conclusion Marco Liserre

Grid Codes description and purpose
Operate a wind farm/wind turbine like a power station/plant Grid Code: Technical document containing the rules governing the operation, maintenance, & development of the system defined at the Point of Common Coupling – PCC (not turbine specific) Steady state Frequency /Power control Low/high frequency support Voltage support/reactive power compensation Power Quality, flicker, harmonics Transient /dynamic state Fault ride through, to stay connected during low voltage on the grid Ramp rate Communication /power dispatch Reliable communication Wind forecasting Participate power market Marco Liserre

Recent Grid Codes Europe: The grid codes of Europe are affected by the fact that the grid has traditionally been strong and stable – but the fact that the wind power penetration has been increasing - LVRT (Low Voltage Ride Through) has entered the scene and most grid codes at least specifies LVRT requirements as defined by the German E.ON. In Spain, Scotland and Ireland the grid codes exceeds the “standard” requirements. Australia & New Zealand: Are characterised by a weak and unstable grid with frequency variations from -10 % to +6 % (in extreme) and -6 % to +4 % (more common). Voltage control and site dependent requirements are standard North America: Characterised by a large number of “smaller” power systems requiring local control capabilities such as voltage control. The PF range is more standardised as 0.9c to 0.9i. Marco Liserre

Grid Codes Trends Voltage control: Future demands is going towards operation in a voltage set point control mode; with a continuously-variable, continuously-acting, closed loop control voltage regulation system, acting like a synchronous generator, where reactive power changes are based on measured voltage. Power control: The trend in power control is fast ramp rates – both up and down, in order to support the frequency of the grid. The latest comments for GB grid codes for power recovery after grid faults states power restoration of 90 % within 1 s. Further frequency control is required in some countries, both under frequency and over frequency support. Plant control: Having wind power plants tending the capabilities of primary control units, traditional power system control features are indisputable. As the need for more dynamical response will increase, the needs for fast and reliable control-infrastructure between the turbines in a park facility are increasing. Marco Liserre

Grid Codes Trends Low voltage ride-through: Is becoming standard for all grid codes! In addition to symmetrical faults, which are three-phased, new trend setting grid requirements will be covering single and two-phase faults ride-through capability. Voltage support during grid disturbances is becoming a common requirement. An increase in the low voltage duration is foreseen – today GB codes mention 3 min. at 85 % voltage. Simulation models: Validated park control models with full disclosure are already defined for various grid code drafts. Park simulation models are an integrated part of the tender phase in more and more projects. Most connection agreements are decided on background of simulation studies. Also non-confidential block diagrams are required, mainly Australia and New Zeeland, but US are also requiring open-source models. PSCAD, DigSilent and PSS/E are preferred tools. Marco Liserre

Different National Grid Codes
[AESO] Canada Wind Power Facility - Technical Requirements (Draft proposal) [CER] Ireland Wind farm Transmission Grid Code Provisions - A Direction by the Commission for Energy Regulation [Eltra] Denmark Vindmølleparker tilsluttet net med spændinger over 100 kV [E.ON] Netz Germany Netzanschlussregeln- Hoch- und Höchstspannung [ESB] National Grid Ireland [REE] Spain Wind Code Changes - Distribution Code Modification Proposal Form Operation procedures for the electrical system. PO 12.1, 12.2 and 12.3 [NECA] Australia National Electricity Code - Version 1.0 Amendment 8.6 [NGC] National Grid The Grid Code - Issue 2, revision 16 [Vattenfall] Germany Netzanschluss- und Netznutzungsregeln der Vattenfall Europe Transmission GmbH [VDN] Germany Transmission Code Netz- und Systemregeln der deutschen Übertragungsnetzbetreiber [Western Power] Australia Technical Code Version 1 Marco Liserre

Voltage and Frequency limits
Eltra – Denmark. Voltages and frequencies used for design of a wind turbine with voltages below 100 kV Marco Liserre

E-On – Germany. Voltage and frequency range for generating units in the E-On grid. Marco Liserre

Great Britain Voltages and frequencies in GB grid Marco Liserre

Transmission System Operator demands
Primary control: Maintain the balance between generation and demand in the network using turbine speed regulators Automatic control to stabilize the grid frequency in seconds Secondary control: Secure import/export balancing with neighbouring areas with reserve generating capacities. Control within minutes In case of a steady major deviation in the control area, to restore the frequency and to free capacity for the primary control Can be manual or automatic Tertiary control: As automatic or manual change in the working points of generators in order to restore adequate secondary control reserve at the right time Marco Liserre

NORDEL UCTE Marco Liserre

UCTE: Primary control Insensitive dead band 10 mHz Frequency deviation of 200 mHz it must be possible to activate the total primary control power range required by the power plant in 30 sec and to supply it for at least 15 min. Primary control must be again available after 15 min of activation. Dispatch 2-8% of rated capacity for primary frequency control. Secondary control It restores the frequency to its rated value and releases engaged primary reserves Start within 30 sec. Fully activated within 15 min. N-1 network security Ability to re-establish supply after black out Marco Liserre

NORDEL: Decoupling of power plants Emergency power by HVDC connections Lowering generation Frequency control/primary control Load shedding, diconnection of connection lines Disconnection of large combined power plants The reserve is activated Primary control 0 MW in frequency control reserve (50,1-49,9 Hz) 192 MW in momentarily disturbance reserve (49,9-49,5 Hz) 50% (5sek),100 % (30 sek), HVDC emergency power, Re-established within 15 min. Secondary control Fast reserve 600 MW within 15 min Marco Liserre

Power control Production limit control both on transmission and distribution Reduction below 20% of maximum power in less than 2s on transmission level Automatic power control after faults up to full power reduction or increase within 30s on transmission level Distribution level decrease and increase in power from % of rated power per minute Marco Liserre

Regulation functions for active power
System protection Protection function that shall be able to perform automatic down-regulation of the power production to an acceptable level for electrical network. In order to avoid system collapse it should act fast. Frequency control All production units shall contribute to the frequency control. Automatic control of power production based on frequency measurement to re-establish the rated frequency. Stop control Wind farm shall keep the production on the actual level even if it is an increase in the wind speed Marco Liserre

Balance control The power production shall be adjusted downwards or upwards in steps at constant levels. Production rate Sets how fast the power production can be adjusted upwards or downwards Absolute production limit Limit the maximum production level in the PCC in order to avoid the overloading of the system. Marco Liserre

Delta control The wind farm shall operate with a certain constant reserve capacity in relation to its momentary possible power production capacity. Horns Reef offshore windfarm 10*8*2MW=160MW: Operates with 10% Delta Control Marco Liserre

Horns Reef Marco Liserre

Frequency control Eltra – Denmark Requirements for wind turbines connected to grids with voltages below 100 kV Marco Liserre

Frequency control E-On: power reduction at over frequencies
Marco Liserre

Frequency control Ireland: Frequency control characteristic
Marco Liserre

Reactive power control
Eltra – Denmark The reactive power flow between the wind turbine including the transformer and the electrical network must be calculated as an average value over 5 min within the control band Marco Liserre

E-On – Germany Every generating units shall provide in the connection point the range of reactive power provision shown in the figure without limiting delivered active power Type of regulation Power factor Mvar regulation Voltage regulation Marco Liserre

Great Britain Every generating unit other than synchronous one with a completion date after 1 January 2006 should be able to support an active reactive power flow shown in the figure. Marco Liserre

Voltage quality Eltra – Denmark. Requirements for wind turbines to grids with voltages below 100 kV Marco Liserre

Ride-through capability
Eltra – Denmark. Requirements for wind turbines to grids with voltages below 100 kV The wind turbine shall be disconnected from the electrical grid according to the figure. Under some special situations a WT shall not be disconnected from the electrical network Marco Liserre

Eltra – Denmark Requirements for wind turbines to grids with voltages below 100 kV The wind turbine shall stay connected in the following cases. 3-phase short-circuit for 100 msec; 2-phase short-circuit with or without ground for 100 msec followed after msec by a new short-circuit of 100 msec duration. Sequences in which WT should keep connected: At least two 2-phases short-circuits within 2 min interval; At least two 3-phases short-circuit within 2 min interval. Energy reserve to remain connected when: At least six 2-phases short-circuits with 5 min interval; At least six 3-phases short-circuit with 5 min interval. Marco Liserre

E-On - Germany Three-phase short-circuits or fault related symmetrical voltage dips must not lead to instability above the red line Between lines red and blue: All generating plants should experience the fault without disconnection from the grid. If, due to the grid connection concept, a generating plant cannot fulfill this requirement, it is permitted with agreement from E-On to shift the limit line while at the same time reducing the resynchronisation time and ensuring a minimum reactive power injection during the fault If, when experiencing the fault, the individual generators becomes unstable or the generator protection responds, a brief disconnection of the generating plant from the grid is allowed by agreement with E-On. At the start of a brief disconnection resynchronisation of the generating plant shall take place within 2 seconds at the latest. The active power infeed must be increased to the original value with a gradient of at least 10% of the rated generator power per second. The highest value of the 3-phase line-to-line grid voltage is considered in this figure Marco Liserre

E-On - Germany The generating plants shall support the grid voltage with additional reactive current during a voltage dip. The voltage control shall act within 20 msec after fault recognition. The generator unit shall provide a reactive current on the low voltage side of the transformer equal to at least 2% from the rated current for each percent of the voltage dip. If necessary the generating unit shall be able to provide full rated reactive current. Marco Liserre

REE – Spain The wind turbines shall remain connected during three-phase, two-phase or single-phase to ground faults with a voltage profile as shown in this figure. In the case of isolated two-phase faults the valley of the voltage profile is set to 60%. Marco Liserre

REE – Spain. No active/reactive power will be consumed at the PCC neither during the fault period nor during the grid voltage recovery period after the fault clearance. The wind turbine should inject maximum reactive current both during the fault and after the fault is cleared and the grid voltage is in the recovering process with maximum delay of 150ms. Marco Liserre

Comparison of different national voltage profiles for fault ride-through capability Marco Liserre

Resume of several national grid codes
Marco Liserre

Conclusions Few European countries have dedicated grid codes for interconnection requirements of RES and in most of the cases these requirements reflects the penetration of renewable sources into the electrical network. Many different grid codes around the world focusing on: Frequency /Power control Voltage support/reactive power compensation Power Quality, flicker, harmonics Fault ride through All considered grid codes requires fault ride-through capabilities. Voltage profiles are given by the depth and the clearance time of the voltage dip. In some of the grid codes the calculation of the voltage during all types of unsymmetrical faults is very well defined e.g Ireland, while others does not define clearly this procedure. On the other hand Germany and Spain requires grid support during faults by reactive current injection up to 100% from the rated current. This demand is relative difficult to meet by some type of wind turbines. Marco Liserre

References Marco Liserre liserre@ieee.org
S. Heier, Grid Integration of Wind Energy Conversion Systems. John Wiley & Sons, 1998. T. Ackermann, Wind Power in Power Systems. John Wiley & Sons, Ltd., 2005, iSBN: L. H. Hansen, L. Helle, F. Blaabjerg, E. Ritchie, S. Munk-Nielsen, H. Bindner, P. Sørensen and B. Bak-Jensen, “Conceptual survey of Generators and Power Electronics for Wind Turbines” Risø National Laboratory, December 2001, 106 p., ISBN IEEE15471, “IEEE standard for interconnecting distributed resources with electric power systems,” July 2003. Eltra and Elkraft, “Wind turbines connected to grids with voltage below 100 kV,” E.ON-Netz, “Grid code – high and extra high voltage,” E.ON Netz GmbH, Tech. Rep., [Online]. Available: eng.jsp S. M. Bolik, “Grid requirements challenges for wind turbines” Fourth International Workshop on Large Scale Integration of Wind Power and Transmission Networks for Offshore Wind Farms, Oct Geza Joos, “Review of grid codes” First International Conference on the integration of RE and DER, 1-3 december, Brussel, Belgium. P.B. Eriksen, T. Ackermann, H. Abildgaard, P. Smith, W. Winter, J.R. Garcia, “System operation with high wind penetration” IEEE Power and Energy Magazine, Vol. 3, No. 6, Nov.-Dec pp I. Erlich, U. Bachmann, ”Grid Code Requirements Concerning Connection and Operation of Wind Turbines in Germany” IEEE Power Engineering Society General Meeting, June 12-16, 2005 pp Marco Liserre

Conclusions: the role of the grid converter
Grid requirements constraints on the grid converter: Harmonic limits -> hardware (dc voltage rating) and control Dc current and leakage current -> hardware (converter structure, transformer, filter) and control (modulation) Islanding detection -> control Operation within a frequency range -> control (PLL) Operation under over/voltage condition -> hardware (dc voltage and semiconductor rating) Reactive power injection (set/point, voltage control or power factor control) -> hardware (dc voltage rating, filter design) and control Frequency control -> control (PLL) Fault Ride-through capability -> hardware (semiconductor rating) and control (estimation and control of inverse sequence) Marco Liserre

Acknowledgment Part of the material is or was included in the present and/or past editions of the “Industrial/Ph.D. Course in Power Electronics for Renewable Energy Systems – in theory and practice” Speakers: R. Teodorescu, P. Rodriguez, M. Liserre, J. M. Guerrero, Place: Aalborg University, Denmark The course is held twice (May and November) every year Marco Liserre 54

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