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1 Abstract—This paper provides an overview of grid codes for the connection of wind farms to electric power systems. Furthermore the requirements of frequency control, voltage control, frequency range and ride-through capability are compared between different countries. The grid codes studied are developed by Transmission System Operators of countries with high wind penetration. The increasing development of wind power generation requires
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   1    Abstract —This paper provides an overview of grid codes for the connection of wind farms to electric power systems. Furthermore the requirements of frequency control, voltage control, frequency range and ride-through capability are compared between different countries. The grid codes studied are developed by Transmission System Operators of countries with high wind penetration. The increasing development of wind power generation requires standardized operating requirements for all countries.  Index Terms — network reliability, power system faults, wind power generation, grid connection requirements, voltage ride through capability. I. I  NTRODUCTION  HE expansion of wind power is a fact in many countries. The importance of renewable energy by 2050 in the global energy mix is reflected in the World Energy Council. According to an objective of the European Union by 2010 22% of electricity generation will be from renewable sources. The European Wind Energy Association (EWEA) predicts a wind generation that will cover 12% of the electricity consumption by 2020 and a 20% by 2030 [1], [2], [3], [4]. The increasing penetration of wind generation has an impact on the stability and reliability of the power system. In order to maintain continuity and security of the electric supply some countries have developed specific grid codes for the connection of wind turbines and wind farms to the power grid. Moreover, some countries require network support during disturbance conditions. However, others still demand wind generators to disconnect when the network is not generating under normal conditions. The most proactive countries that established grid codes were the pioneers in wind power. This paper is an overview of the aspects dealt with in the different codes. Grid codes are set by the electrical stakeholders (mainly Transmission System Operators,TSO) * N. Sangroniz is with Arteche Centro de Tecnología, Derio Bidea 28. 48100 Mungia, Spain (e-mail: ns@arteche.es). + J. Mora is with Inelap, Calle 2 #7 Fracc. Industrial Alce Blanco.  Naucalpan de Juárez, 53370 Estado de México (e-mail:  jmora@arteche.com.mx). ^ M. Duarte Teixeira is with Arteche EDC, Rua Juscelino K. de Oliveira, 11400 CIC, Curitiba-PR 81450-900 (e-mail: mdt@arteche.com.br ). and normally there is harmonization on national codes but not on international level. A comparison is made of the main requirements between different countries in order to show the lack of a harmonized set of grid code requirements and the convenience of the countries with no grid codes to join proactive group, as is the case of Brazil. A wind energy auction in Brazil is expected to be held in late 2009, and it is expected to call for up to 1000MW in generating capacity. Projects for more than 4500MW are qualified for the auction. II. COUNTRIES AND REQUIREMENTS  The grid codes considered in this comparative study are: ã   Canada (Alberta) [5], ã   Denmark [6], [7], ã   Germany [8], ã   Spain [9], [10] and ã   United States [11]. The requirements under consideration are extracted from the grid codes of the countries analyzed: frequency control, voltage control, frequency range and ride-through capability. Active power control is required by several grid codes to support the frequency of the system. Reactive power compensation can be done by controlling the system voltage. Security of supply and overloading prevention are ensured  by means of the frequency control within acceptable limits. Ride-through requirements force the wind farm to support the network during and after a fault and to be protected against damage from faults. Other requirements of grid codes related to voltage quality (flicker, harmonics,…), wind farm modeling or communication control are not considered in the present analysis although are included in codes of some countries. III. COMPARATIVE STUDY OF GRID CONNECTION REQUIREMENTS    A. Frequency control Some grid codes require the capability to vary active power output in response to changes in system frequency. Ramp rates are defined in order to limit active power output. Review of International Grid Codes for Wind Generation  Natalia Sangroniz*, José Arturo Mora+, Mateus Duarte Teixeira^ T   2 1) Canada: Frequency control requirement is subject to review pending the outcome of further studies associated with wind power variability.   2)  Denmark: Voltage above 100kV: It shall be possible to limit the production of a wind farm to a random set-point value in the range of 20 to 100% of rated power. It shall be possible to set the regulation speed at upward and downward regulation in the interval 10 to 100% of rated power per minute. Voltage below 100kV: It must be possible to limit a wind turbine’s production to any power set-point in the range 20-100% of its rated  power. It must also be possible to control the regulating speed for both limiting and delimiting production from outside and to select a limit in the range 10 to 100% of the rated power per minute. 3) Germany: It must be possible to reduce the power output in any operating condition and from any operating point to a maximum power value. The reduction of the power output to the signaled value must take place with at least 10% of the grid connection capacity per minute, without the wind farm being disconnected from the grid. 4) Spain: Frequency control is not defined. 5) United States: Frequency control is not defined. TABLE   I FREQUENCY CONTROL REQUIREMENTS BY COUNTRY   Country Frequency control Canada Under study  Denmark from 10 to 100% of rated  power per minute Germany at least 10% reduction ramp  per minute Spain  Not defined United States  Not defined  B. Voltage control Wind turbines must operate at normal voltage rates and stay connected during voltage changes within the voltage ranges determined. The wind turbine capability of supplying reactive power contributes to the voltage control. 1) Canada: The AESO (Alberta Electric System Operator) will specify a transmission system voltage operating range (minimum through maximum) at the point of connection that the WPF (Wind Power Facilities) shall be able to operate within. A WPF reactive capability shall meet or exceed 0.9 lagging  power factor to 0.95 leading power factor. 2)  Denmark: Voltage above 100kV: The wind farm shall be equipped with reactive power compensation ensuring that the reactive power as a mean value over 10 seconds is kept within the control band, as shown in Figure 1. This applies in the connection point at all production levels in the stated full-load range for the voltage shown in Figure 2. A wind farm shall be dimensioned to produce at voltage and frequencies that deviate from rated values in the minimum hours indicated in Figure 2. Voltages and frequencies for which the figure states time-limited operation will occur in less than ten hours per year. Abnormal voltages and frequencies shall not result in a  production decrease larger than the one indicated in Figure 2. The full-load range indicates the voltage range within which the wind farm shall be able to supply its nominal  power. Voltage below 100kV: Averaged over 5 minutes, the reactive power that a wind turbine (including wind turbine transformer) exchanges with a grid must lie within the control band shown in Figure 1 unless the exchanged power is less than 25kVAr. Fig. 1. Requirements concerning a wind turbine's exchange of reactive power with a grid in Danish code.  Nominal voltage, U  N  Lower Voltage limit, U L  Lower voltage limit for full-load range, U LF  Upper voltage limit for full-load range, U HF  Upper Voltage limit, U H  400 KV 320 KV 360 KV 420 KV 440 KV 150 KV 135 KV 146 KV 170 KV 180 KV 132 KV 119 KV 125 KV 145 KV 155 KV Fig. 2. Dimensioning voltage and frequency in Danish grid code.   3  3) Germany: When active power is taken from the E.ON grid, the connect must maintain, as standard, a power factor between 0.95 (inductive) and 0.925 capacitive at the grid connection  point. A further exchange of reactive power is only  permissible if this has been separately contractually agreed. 4) Spain: Voltage control is not defined. 5) United States: A wind generating plant shall maintain a power factor within the range of 0.95 leading to 0.95 lagging, measured at the Point of Interconnection. TABLE   II FREQUENCY CONTROL REQUIREMENTS BY COUNTRY   Country Power factor Canada 0.9 lagging to 0.95 leading  Denmark See Figure 1 Germany 0.95 lagging to 0.925 leading Spain  Not defined United States 0.95 lagging to 0.95 leading C. Frequency range As a general rule grid codes establish a frequency range for continuous operation mode and a wider limited range during some period of time. 1) Canada: WPFs that need to protect equipment for off-nominal frequency operation shall ensure that protective relaying accommodates operation for the specified time frames. The trip settings of the relays shall not be less than the minimum time prescribed in Table III with respect to the frequency setting. TABLE III UNDER/OVER FREQUENCY LIMITS IN ALBERTA Frequency (HZ) Minimum Time Delay >61.7 Hz 0 seconds 61.6 Hz to 61.7 Hz 30 seconds 60.6 Hz to 61.6 Hz 3 minutes >59.4 Hz to 60.6 Hz Continuos Operation >58.4 Hz to 59.4 Hz 3 minutes >57.8 Hz to 58.4 Hz 30 seconds >57.3 Hz to 57.8 Hz 7.5 seconds >57.0 Hz to 57.3 Hz 45 cicles 57 Hz or Less 0 seconds 2)  Denmark: Voltage above 100kV: Refer to Figure 2, there is not restrictions for frequency in the range between 49.5 and 50.5 Hz. Voltage below 100kV:  Normal operation must be between 49 and 51Hz. 3) Germany: There is no restriction when the frequency is in the range  between 47.5 and 50.2Hz. The wind farm units must, when operated at a frequency of more than 50.2Hz, reduce the current active power with a gradient of 40% of the  presently available power of the generator. When the frequency returns to a value of 50.05Hz, the active power may be increased again. Finally, when the frequency is less than 47.5Hz or more than 51.5 the generator must disconnect. 4) Spain: The wind generators must disconnect if the frequency is  below 48Hz during more than 3 seconds or above 51Hz. 5) United States: Frequency range is not defined. TABLE   IV FREQUENCY CONTROL REQUIREMENTS BY COUNTRY   Country Frequency range Canada See table III  Denmark See Figure 1 Germany 47.5Hz to 50.2Hz –> No restrictions < 47.5Hz or >51.5Hz -> Disconnection Spain < 48Hz during more than 3s or > 51Hz -> Disconnection United States  Not defined  D. Ride-through capability The TSOs requirements some years ago were to oblige to wind generator to disconnect during fault conditions. In this situation the generators connected to the faulty line are expected to trip but if the generation in the adjacent healthy lines remains connected during and after the fault, the power system would not be expose to an additional loss of generation and consequently to a drop of the system frequency. 1) Canada: Voltage Ride Through requirements are applicable to all transmission generating facilities where WPF Aggregated MW Capacity is greater than 5 MW, a WPF shall be capable of continuous operation between 90% to 110% of rated voltage. Voltage described in Figure 3 is based on the rated voltage level at point of connection. 2)  Denmark: Voltage above 100kV: The wind farm shall remain connected after the faults described in Table V in the transmission grid. A wind turbine must have sufficient capacity to meet the foregoing requirements in the event of two independent sequences of at least two 1-phase or two 2-phase or two 3-phase short circuits within 2 minutes. A wind turbine should have   4 sufficient reserves to withstand two independent sequences of at least six 1-phase or six 2-phase or six 3-phase short circuits at 5 minutes intervals. TABLE   V VOLTAGE RIDE - THROUGH CAPABILITIES ABOVE 100 K  V  IN DANISH CODE   Three-phase short circuit Short circuit in 100ms Two-phase short circuit with/without earth contact Short circuit in 100ms followed by a new short circuit 300…500ms later, also with a duration of 100ms Single-phase short circuit to earth Single-phase earth fault 300…500ms later, also with a duration of 100ms 0 0,625 3 t (s) 1 0,15 0,9 U (pu) 1,1 WPF generation may trip above HV Requirement WPF generation may trip below LV Requirement Fig. 3. Voltage ride through requirements in Alberta grid code. Voltage below 100kV: A wind turbine must remain connected after the faults in the transmission grid listed in Table VI. A wind turbine must have sufficient capacity to meet the foregoing requirements in the event of two independent sequences of at least two 2-phase or two 3-phase short circuits within 2 minutes. A wind turbine should have sufficient reserves to withstand two independent sequences of at least six 2- phase or six 3-phase short circuits at 5 minutes intervals. TABLE   VI VOLTAGE RIDE - THROUGH CAPABILITIES BELOW 100 K  V  IN DANISH CODE   Three-phase short circuit Short circuit in 100ms Two-phase short circuit with/without earth contact Short circuit in 100ms followed by a new short circuit 300…500ms later, also with a duration of 100ms 3) Germany: For synchronous generators, three-phase short circuits must not cause instability or a disconnection from the grid when for fault-clearing times up to 150ms in the entire operating range of the generating plant. Figure 4 shows the limit curves for the voltage pattern at the grid connection for asynchronous generators. Three- phase short circuits or fault-related symmetrical voltage dips must not lead to instability above the Limit Line 1 or to disconnection of the generating plant from the grid. For all generating plants that do not disconnect from the grid during the fault, the active power output must be continued immediately after clearance and increased to the srcinal value with a gradient of at least 20% of the rated  power per second. 0 0,7 1.5 3 t (s) 1 0,15 0,7 U (pu) 0,45 0.15 Limit Line 1 Limit Line 2 Fig. 4. Limit curve for the voltage at the grid connection in the event of a fault in German grid code. 4) Spain: Wind turbines should not disconnect in case of three-phase, two phase-to-ground or one-phase short circuit represented  by Figure 5. For two phase ungrounded short circuits the lower voltage limit must be 0.6 instead of 0.2 in Figure 2. After fault-clearing the time necessary to recover nominal values depends on the percentage of the wind generation  penetration related to the short circuit power. 0 0,5 1 15 t (s) 1 0,2 0,8 0,95 pu U (pu) Fig. 5. Limit curve for the voltage at the grid connection in the event of a fault in Spanish grid code. 5) United States: Wind farms should be able to remain connected during voltage disturbances described by Figure 6. At time 0.0 s, the voltage drops. If the voltage remains at a level greater than 15 percent of the rated voltage for less than 0.625 s, the wind farm shall stay connected. Further, if the voltage returns to 90 percent of the rated voltage within 3s of the  beginning of the voltage drop, the wind farm shall be still connected.
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