Indian Wind Grid Code

Transcription

1 Draft Report On Indian Wind Grid Code Submitted to Centre for Wind Energy Technology Velachery - Tambaram Main Road, Pallikaranai, Chennai , Tamil Nadu, INDIA July 2009 Power Research and Development Consultants Private Limited No 5, 11 th Cross, 2 nd Stage, West of Chord Road, Bangalore , Karnataka, INDIA. Ph: / , FAX: , prdc@vsnl.com Web site:

2 Purchase Order No: C-WET/R&D/Grid Code/PR&DC/ , Dated 03/03/2009 Document ref PRDC/ /C-WET/831 Signature Date Prepared by Mr. Sarasij Das Mr. Ramesh Pampana Reviewed by Approved by Mr. V. Venkata Subba Rao Dr. K. Balaraman Power Research and Development consultants (PRDC) Pvt. Ltd 1

3 Preamble With due consideration of the fact of growing wind energy sector in India, a Technical Working Group was formed by the Ministry of New and Renewable Energy (MNRE), to formulate guidelines in addressing the technical issues/problems of power evacuation and grid synchronization related to wind power projects. The following are the members of the Technical Working Group: i. Shri M.P. Ramesh, Ex-ED (C-WET) - Chairman ii. K.P.Sukumaran, Advisor & Head, WE, MNRE - Member iii. Shri P.S. Jagannatha Gupta, CE (Retd.), KPTCL - Member iv. Shri R.N. Nayak, ED, PGCIL - Member v. Chief Engineer (GM),CEA or his representative - Member vi. Director (Transmission), Ministry of Power - Member vii. Representative from TNEB - Member viii. Representative from IWTMA - Member ix. Shri S.K.Soonee, ED, PGCIL - Member x. Director, MNRE - Member Secretary The Technical Working Group met twice, first on , secondly on with regard to the requirement and development of grid code for wind power generation in India. As a part of addressing the technical issues/problems of power evacuation and grid synchronization related to wind power projects, the Technical Working Group has awarded the task of Developing of grid code for wind power generation in India to M/s Power Research and Development consultants (PRDC) Pvt. Ltd, Bangalore. PRDC has formulated a draft report on the grid code for wind power generation in India named as Indian Wind Grid Code which will be presented before the committee. Power Research and Development consultants (PRDC) Pvt. Ltd 2

4 Chapter 1 INTRODUCTION The Indian Electricity Grid Code (IEGC) provides the technical rules to facilitate the operation, maintenance, development and planning of electricity grid. The objective of IEGC is to maintain safe, reliable and disciplined operation of power system. The IEGC guidelines and standards are to be followed by the various agencies and participants of the power grid. Indian power generation sector is changing its nature like elsewhere in the world with focus on environmental impacts of conventional sources and need to encourage renewable energy. More and more renewable energy sources, mainly wind energy, are being integrated into the grid. Today, wind generation, whose penetration is increasing have significant impact on Indian power grid. The IEGC as well as the state grid codes were originally developed considering the synchronous generators generally used in conventional power plants. Wind turbine generators (WTG) do not have the same characteristics as synchronous generators and hence a modification or change in the grid code is necessary. Indian Wind Grid Code (IWGC) has been developed for the reliable and secure operation of the wind farms and their integration into the Indian electrical system. This grid code can be used in tandem with the IEGC/State Grid code or the IEGC and state grid codes can be amended with the provisions. 1.1 Objective The primary objective of IWGC is to establish the technical rules which all wind farms must comply with in relation to their planning, connection and operation on the Indian grid. 1.2 Scope All grid connected wind farms and those who operate the associated transmission system are required to abide by the principles defined in the IWGC in so far as they apply to them. The IWGC (except sections and 5.10) shall come into effect from dd/mm/yyyy. The timeline for implementing fault ride through capability (section 4.6.6) and wind energy forecasting (section 5.10) shall be specified separately by the concerned authority taking into account the penetration levels of wind energy, cost of implementation and tariff structure and their usefulness in terms of grid management strategies. Power Research and Development consultants (PRDC) Pvt. Ltd 3

5 1.3 Structure of the IWGC IWGC gives guidelines for transmission planning, grid connection and operation of wind farms. The content of IWGC is as follows: i) Role of various organizations and their linkages: This chapter defines the functions of the various organizations as are relevant to IWGC. The organizations and their linkages are defined to facilitate development and smooth operation of regional grids. ii) Planning code for transmission systems evacuating wind power: This chapter provides the policy to be adopted in the planning transmission system for wind power evacuation. The planning code stipulates the various criteria to be adopted during the planning process. iii) Connection code for wind farms: This chapter specifies minimum technical and design criteria to be complied with by wind farms connected to the system or seeking connection to the grid, to maintain uniformity and quality across the power system. iv) Operating code for wind farms: This chapter describes the operational philosophy to maintain efficient, secure and reliable grid operations of power grids having wind farms and conventional power plants. 1.4 Non-compliance In case of a persistent non-compliance of any of the stipulations of the IWGC by a constituent or an agency (other than RPC, RLDC and SLDC), the matter shall be reported by any agency/rldc to the Member Secretary, RPC or the designated agency. The Member Secretary, RPC or the designated agency, shall verify and take up the matter with the defaulting agency for expeditious termination of the noncompliance. In case of inadequate response to the efforts made by the Member Secretary, RPC, the non-compliance shall be reported to CERC/SERC. CERC/SERC, in turn after due process, may order the defaulting agency for compliance, failing which; the CERC/SERC may take appropriate action. RPC or the designated agency shall maintain appropriate records of such violations. In case of a non-compliance of any of the stipulations of the IWGC by RLDC/SLDC or RPC, the matter shall be reported to the CERC / SERC. 1.5 Exemptions Any exemption from provisions of IWGC shall become effective only after approval of the CERC/ SERC, for which the agencies will have to file a petition in advance. Power Research and Development consultants (PRDC) Pvt. Ltd 4

6 1.6 Glossary and definitions Item Definition Act The Electricity Act, 2003 Available Active The amount of active power that the WTG could Power produce based on current wind conditions. BIS The Bureau of Indian Standards Capacity factor The ratio of maximum generation in MW to sum of installed capacity of individual WTGs in the Wind Farm CEA Central Electricity Authority of India CERC The Central Electricity Regulatory Commission referred to in sub-section (1) of Section 76 of the Act CTU Central Transmission Utility means any Government company, which the Central Government may notify under sub-section (1) of Section 38 of the Act. C-WET Centre for Wind Energy Technology Dynamic VAr An electrical facility designed for the purpose of compensation generating or absorbing reactive power. Frequency The automatic adjustment of active power output Response from a WTG in response to frequency changes Grid connection The point where all WTGs of a wind farm are point connected to the grid. Point G in the following figure is referred as Grid connection point. Grid substation Installed capacity IEC IEGC The substation to which the wind farm is connected. The sum of rated generating capacity of each WTG in a wind farm in MW The International Electro technical Commission. Indian Electricity Grid Code Power Research and Development consultants (PRDC) Pvt. Ltd 5

7 Inter State Transmission System (ISTS) IWGC Plant Load Factor Regional Load Dispatch Center (RLDC) Regional Power Committee (RPC) SEB SERC State Load Dispatch Centre (SLDC) State Sub Load Dispatch Centre (SSLDC) State Transmission Utility (STU) TSO Inter-State Transmission System includes i) any system for the conveyance of electricity by means of a main transmission line from the territory of one State to another State ii) The conveyance of energy across the territory of an intervening State as well as conveyance within the State which is incidental to such inter-state transmission of energy iii) The transmission of electricity within the territory of State on a system built, owned, operated, maintained or controlled by CTU. Indian Wind Grid Code Plant load factor is the ratio of the energy actually supplied by a plant (in a year) to the product of the installed capacity and number of hours in a year. Regional Load Dispatch Centre means the Centre established under sub-section (1) of Section 27 of the Act. Regional Power Committee means a Committee established by resolution by the Central Government for a specific region for facilitating the integrated operation of the power systems in that region. State Electricity Board including the State Electricity Department. State Electricity Regulatory Commission. State Load Dispatch Centre is the Centre established under sub-section (1) of section 31 of the Act. State's Sub Load Centre for local control at various places in the state. State Transmission Utility means the Board or the Government Company specified as such by the State Government under sub-section (1) of Section 39 of the Act. Transmission System Operator Power Research and Development consultants (PRDC) Pvt. Ltd 6

8 WTG Wind farm Wind farm operator Wind farm owner Wind turbine Generator A wind farm is a collection of WTGs that are connected to the grid at a common point The operator of the wind farm. Entity having legal right of the wind farm Power Research and Development consultants (PRDC) Pvt. Ltd 7

9 Chapter 2 ROLE OF VARIOUS ORGANIZATIONS AND THEIR LINKAGES Chapter 2 of IEGC shall be followed. Power Research and Development consultants (PRDC) Pvt. Ltd 8

10 Chapter 3 PLANNING CODE FOR TRANSMISSION SYSTEMS EVACUATING WIND POWER This chapter comprises various aspects of transmission system planning for wind power evacuation. Planning policy, planning criteria for transmission lines evacuating wind power are discussed in this chapter. 3.1 Introduction i) The planning code specifies the policy and procedures to be applied in planning of transmission lines for evacuating wind power. ii) Role of various organizations in wind farm planning procedure will be same as planning procedure for conventional generators. iii) The planning procedure shall be governed by IEGC and Electricity Act, Objective The planning code for transmission systems for wind power evacuation shall be part of bigger plan that encompasses overall grid planning. The objectives of the planning code are: i) To specify the principles, procedures and criteria which shall be used in the planning and development of the transmission system evacuating wind power. ii) To promote co-ordination between wind farm developers, system operators and regional constituents in any proposed development of wind farms. iii) To provide methodology and information exchange amongst regional constituents and agencies in planning of transmission system for evacuation of wind power. 3.3 Scope The planning code applies to transmission licensees, wind farms, SEBs, CTU/STUs and Distribution licensees involved in developing the transmission/ evacuation system for wind power evacuation. 3.4 Planning policy CTU/STU/TSO may formulate perspective transmission plan for wind power evacuation in a region. The transmission planning shall consider both short term and long-term expected wind generation in the region. The planning shall fit into Power Research and Development consultants (PRDC) Pvt. Ltd 9

11 National Electricity Plan formulated by Central Government, perspective transmission plan developed by CEA, Electric Power Survey of India published by CEA and policy guide lines (if any) issued by concerned ministry regarding renewable energy development and shall be done taking into account the state transmission plan. 3.5 Planning criterion Study of transmission system for wind power evacuation The transmission system shall be adequate for various wind generation and load scenarios. The transmission system shall operate without violating any system conditions during following scenarios: i) System Peak Load with High Wind Generation {Explanation:- During peak loading condition all the generating units in a region will be running at or near to its maximum capacity. Power flow through the transmission network will be at higher level. Evacuation planning of wind farm shall ensure that power injected by wind farm shall not cause any overloading/ congestion in the network during peak load condition. ii) System Light Load with High Wind Generation {Explanation:- Here, the aim is to ensure that during system light load condition, all the available wind power is evacuated. iii) Local Light Load with High Wind Generation {Explanation:- Sometimes wind farms can have significant local load near the wind farms. Here, the aim is to ensure that during local light load condition, all the available wind power is evacuated to the system. It is to be noted that low local load and low system load may not coincide in many parts of India due to geographical diversity. Power Research and Development consultants (PRDC) Pvt. Ltd 10

12 As the wind farms are distributed over large geographical area, the maximum generation depends on geographical spread. For scenarios mentioned in IWGC section-3.5.1, the High Wind Generation shall correspond to: i) 100% capacity factor for wind farms connected below 66kV. ii) Minimum 90% capacity factor for wind farms connected at 66kV or 110 kv or 132 kv. iii) Minimum 80% capacity factor for wind farms connected above 132 kv. {Explanation:- Normally in India, plant load factors of wind farms would lie in the range of 20-30%.But, capacity factor may go up to 100% in a small wind farm. So, to have economic viability, transmission planning of wind farms should consider capacity factor as a parameter. Wind turbines in a smaller wind farm face similar wind speeds as they are spread over smaller geographical area. Output of these wind farms can reach 100% of installed capacity during high wind season. As the wind farm size grows, capacity factor of wind farm decreases due to large geographical spread. Normally, higher capacity wind farms are connected at higher voltage levels. Here, voltage level at the grid connection point is chosen as criteria because power system behavior can be better categorized with voltage levels than power. 100%, 90%, 80% values are based on consultation experience and also available data from the literature Generally there shall be no restriction on the wind farm size and the voltage level at which it shall be connected to the grid, provided all the requirements in this IWGC are fulfilled. {Explanation:- The relation between evacuating power and voltage level depends on many parameters, such as: - Local network and local load Power Research and Development consultants (PRDC) Pvt. Ltd 11

13 - Transmission conductor characteristics - Availability of substations This relation can vary from one area to other. Providing a definite guideline on evacuating power vs. voltage relationship can restrict setting up of new wind farms in some areas where wind power can be evacuated reliably in spite of violating the evacuating power and voltage guideline Lower ambient temperatures which are generally associated with higher wind velocities may be considered for increasing the loadability of transmission systems planned for evacuating wind power in cases where other alternatives are prohibitively expensive affecting viability of the renewable energy project. IEEE Std IEEE Standard for Calculating the Current-Temperature Relationship of Bare Overhead Conductors Contingency study " shall be followed while calculating line loadability with respect to wind speed. A sample calculation of transmission line loading with respect to wind speed is given in Appendix B. In Appendix B transmission line loading limits with increasing wind speeds are given for Zebra and Panther conductors Contingency study Plant load factors of wind farms are significantly less than the conventional generators. Hence, application of N-1 contingency criteria for planning of transmission line(s) from wind farm to grid substation may not be economically viable. Loss of generation from smaller wind farms may not have significant impact on the grid operation Planning of transmission lines from wind farms connected at 220 kv voltage level and above, to the grid substation shall be based on N-1 contingency criteria. However, wind farms connected below 220 kv voltage level and below 100 MW installed capacity at 220 kv voltage level can be exempted from N-1 planning criteria The upstream network connected from grid substation shall be capable of withstanding and be secured against the following contingency outages without necessitating load shedding or rescheduling of generation during steady state operation as defined in IEGC and State Grid codes a) Outage of a 132 kv D/C line or, Power Research and Development consultants (PRDC) Pvt. Ltd 12

14 b) Outage of a 220 kv D/C line or, c) Outage of a 400 kv S/C line or, d) Outage of single Interconnecting Transformer, or e) Outage of one pole of HVDC bipolar line, or f) Outage of 765 kv S/C line The above contingencies shall be considered assuming a pre-contingency system depletion (planned outage) of another 220 kv D/C line or 400 kv S/C line in another corridor and not emanating from the same substation. All the generating units may operate within their reactive capability curves and the network voltage profile shall also be maintained within voltage limits specified Any one of the events mentioned in the adequacy and contingency study shall not cause: i) Unacceptable high or low voltage ii) Prolonged operation of the system frequency below and above specified limits. iii) System instability iv) Unacceptable overloading of transmission system elements Reactive power compensation Reactive power compensation is important for wind farms to ensure reliable and trouble free grid operation and stable voltage profile. Adequate planning of reactive power compensation can minimize the reactive power loading on the transmission line. Further, there is a close relation exists between voltage instability and reactive power compensation. Hence, the reactive power compensation is to be addressed in the planning exercise and a careful study is required Reactive compensation of wind farms shall be able to maintain power factor between 0.95 lagging and 0.95 leading at grid connection point. As per Indian state grid codes, power factor of conventional generators shall lie between 0.95 leading to 0.85 lagging. Wind grid codes of UK, Germany ask for 0.95 leading to 0.95 lagging power factor. Canadian grid code asks for 0.95 leading to 0.90 lagging power factor. Power Research and Development consultants (PRDC) Pvt. Ltd 13

15 So, it can be seen than grid codes mainly differ on the lagging power factor limit. In India, reactive power injection from wind farms is least expected. So, in IWGC the power factor range is limited between 0.95 leading to 0.95 lagging Planning studies for power evacuation from wind farms through long distance transmission lines shall include voltage stability studies to investigate the requirements of dynamic VAr compensation to prevent voltage collapse during high wind generation. The modeling of WTG shall be based on the actual type planned to be installed in the area by the developer of wind farm. 3.6 Planning data Wind farm owner shall provide planning data to CTU/STU as mentioned in Appendix A Wind power addition plan for every five years issued by the Ministry of New and Renewable Energy shall be considered for the planning of transmission lines of the CTU/STUs. Power Research and Development consultants (PRDC) Pvt. Ltd 14

16 Chapter 4 CONNECTION CODE FOR WIND FARMS This chapter comprises various technical requirements that wind farms have to satisfy for grid connection. These provisions shall apply for wind farms that are connected to the grid from dd/mm/yyyy. 4.1 Introduction The connection code for wind farms specify the minimum technical and design criteria which shall be satisfied by any wind farms seeking connection to ISTSs/STSs/STUs. This shall be pre-requisite for the establishment of an agreed connection. 4.2 Objective The objective of the connection code is to ensure that any new or modified wind farm connections, when established, shall neither suffer unacceptable effects due to its connections to ISTS/STS nor impose unacceptable effects on the system or the grid. 4.3 Scope The connection code applies to all wind farms connected to the grid at any voltage levels. The wind farms shall satisfy all requirements of connection code. 4.4 Procedure for connection The connection procedure of wind farms connected to ISTS shall follow IEGC section-4.4. Wind farms connected to intra state lines shall follow corresponding state grid code for connection procedure. 4.5 Connection agreement The connection agreement of wind farms connected to ISTS shall follow IEGC section-4.5. Wind farms connected to intra state lines shall follow corresponding state grid code for connection agreement. 4.6 Technical requirements to be met at grid connection point of wind farms The entire grid connected wind farms shall satisfy technical requirements at the grid connection point of the wind farm as mentioned in the following subsections. Power Research and Development consultants (PRDC) Pvt. Ltd 15

17 4.6.1 Transmission system voltage requirements Transmission system voltage range The wind farms shall be able to deliver available or rated power when the voltage at the grid connection point remains within following range: Table 4.1: Voltage withstand limits for wind farms Voltage (kv) Nominal % Limit of variation Maximum Minimum % to -10% % to -9% % to -9% % to -12.5% % to -9% % to -10% The minimum and maximum voltages for 400, 220 and 132 kv buses are taken from IEGC. The minimum and maximum voltages for 110, 66 and 33 kv buses are taken from the planning criteria of Revised TNEGC(page 24) The permissible voltage at the point of commencement of supply during the steady state operation is +5% / -10% for system upto 33 kv voltage level Resonance Wind farms shall avoid introducing undue resonance leading to over voltage at grid connection point. Of particular concerns are torsional interaction, self excitation of induction machines, transformer ferro-resonance, and the resonant effects of capacitor additions. Wind farms connected to the grid through series compensated transmission lines shall investigate the possibility of subsynchronous resonance due to torsional interactions Voltage unbalance Voltage unbalance is defined as the deviation between the highest and lowest line voltage divided by the average line voltage of three phases. Power Research and Development consultants (PRDC) Pvt. Ltd 16

18 Connection of a WTG to an unbalanced system will cause negative phase sequence current to flow in the rotor of the machine. Wind farms shall be able to withstand voltage unbalance limits specified in following Table 4.2: Table 4.2: Voltage unbalance limits Voltage level (kv) Unbalance (%) <220 3 CEA grid standard is followed Reactive power capability of wind farms The reactive compensation system of wind farms shall be able to attain following characteristics: i) Wind farms connected at 66 kv and below shall maintain power factor between 0.95 lagging and 0.95 leading at grid connection point. A generating unit operating at leading power factor absorbs reactive power from the transmission system. ii) Above 66 kv, wind farms shall be able to operate in voltage-power factor operating region shown in Figure 4.1. Power Research and Development consultants (PRDC) Pvt. Ltd 17

19 Voltage (kv) leading (absorption) lagging (injection) Figure 4.1: Voltage vs. power factor characteristics of wind farms connected above 66 kv Power Factor The voltage vs. power characteristic is based on the principle that wind farms should not draw/inject large reactive power at lower/higher system voltages. In general, the allowable power factor range of wind farms is 0.95 lagging to leading. But, a comparatively higher leading power factor requirement is placed when the system voltage is lower. Similarly, a lower lagging power factor is required when the system voltage is higher. In other words, reactive power drawl/injection shall be minimized at lower/higher voltages. This is depicted in Figure 4.1. The Voltage vs. power factor characteristics in IWGC is derived from German grid code of VE-T. Power Research and Development consultants (PRDC) Pvt. Ltd 18

20 4.6.3 Frequency tolerance range i) Wind farms shall be capable of operating continuously for system frequency range of 47.5 to 51.5 Hz. ii) Above 51.5 Hz and below 47.5 Hz, allowable frequency tolerance range of wind farms will be according to wind turbine specifications. iii) Wind farms shall remain connected to the grid when rate of change of frequency is within 0.5 Hz/sec. The frequency range of 47.5 to 51.5 Hz is also proposed in the draft IS standard on Wind turbines - Design requirements, which is under finalization by BIS - Wind turbine Sectional Committee ET 42. Hence it is suggested that the upper limit may be restricted to Hz Active power control Wind farms with connected at 66 kv and above shall have the ability to limit the active power output at grid connection point as per system operator s request. The request from grid cooperator shall be under the conditions elaborated in IWGC section and During system operations, grid operator in extreme conditions may ask the wind farms to limit the power injection into the grid. The request from grid cooperator shall be under the conditions elaborated in IWGC section and Wind farms connected at 66 kv and above shall be able to respond to system operator s request Active power output of wind farms shall vary with respect to frequency as shown in Figure 4.2. Power Research and Development consultants (PRDC) Pvt. Ltd 19

21 Percentage of Available Power 100% 50% 0% Hz Frequency Figure 4.2: Variation of active power output of wind farms with respect to frequency of wind farm The objective is to utilize wind power at its maximum. This characteristic is derived from Irish grid code. The upper limit of 50.3 is derived from recent amendment of IEGC in Situations where wind turbines can be disconnected from grid The wind farms shall be equipped with voltage and frequency relays for disconnection of the wind farm at abnormal voltage and frequencies. The relay settings shall be outside the operating range of voltage and frequency mentioned in IWGC section and section Wind farms connected below 66 kv can get disconnected from the grid during system faults (fault ride through capability is not mandatory). Power Research and Development consultants (PRDC) Pvt. Ltd 20

22 Normally, wind farms connected below 66 kv are smaller in size. The requirement of fault ride through can affect the economics of smaller wind farms at present scenario Situations where wind turbines must remain connected to the grid Wind farms shall remain connected to the grid during normal system operation Wind farms connected at 66 kv and above shall remain connected to the grid during system fault. Reactive power compensation equipment must also remain connected during system fault Fault ride through requirements Fault ride through requirements shall be applicable to all new wind farms planned or commissioned after the date specified by concerned authority with due consideration of penetration level, cost and tariff. Wind farms connected at 66 kv and above shall have the operating region as shown in Figure 4.3 during system faults. Wind farms can be disconnected if the operating point falls below the line in Figure 4.3. During fault ride through, the WTGs in the wind farm shall have the capability to meet the following requirements: a) Shall minimize the reactive power drawl from the grid. b) The wind turbine generators shall provide active power in proportion to retained grid voltage as soon as the fault is cleared. Power Research and Development consultants (PRDC) Pvt. Ltd 21

23 V pf Voltage (kv) V f Must not trip 0 T 3000 Time (ms) Figure 4.3: Fault ride through characteristics Where, V f = 15% of Nominal System voltage V pf = Minimum voltages mentioned in IWGC section The fault clearing time for various system nominal voltage levels is given in the following Table 4.3: Table 4.3: Fault clearing time and voltage limits Nominal system Fault clearing V pf (kv) V f (kv) voltage (kv) time, T(ms) Higher fault clearance times for the wind farms may be agreed to with the SEBs/STUs. In such case, the SEBs/STUs shall specify to the wind farm operators the required opening times of circuit breakers at various locations. Power Research and Development consultants (PRDC) Pvt. Ltd 22

24 With increasing penetration, wind farms will have major impact in Indian power system. So, the behavior of wind farms should tend to be same as conventional power plants. Staying connected during system fault is a step towards that direction. Today s wind turbine technology has matured enough to provide this requirement. All the international grid codes demand this criterion from wind farms. The Fault ride through requirements in IWGC resembles to international practice. The fault clearing time is taken from IEGC, state grid codes. However, it may be advised that this facility may be provided for the future wind farms development with due consideration of the cost impact Ability to withstand repetitive faults Wind farms connected below 66 kv shall be capable of withstanding repetitive faults in the grid as the fault occurrence in these systems is frequent. In India, the occurrence of faults in sub-transmission system is frequent. Hence, the wind turbines may be thermally stressed. In such cases the machines shall withstand the repetitive faults or shall disconnect from the grid. Similar requirement is presented in Danish grid code Protection All the grid connected wind farms must have protection systems to protect the wind farm equipment as well as the grid, such that no part system shall remain unprotected during faults The protection schemes for the wind farms shall be developed in coordination with the grid protection schemes and this shall be carried out keeping in mind the wind turbine manufacturing capabilities such as fault ride through capability, voltage and frequency operational ranges etc The faults on the wind farm and/or its apparatus shall be cleared without any time delay and in any event shall be cleared within in the fault clearing times mentioned in IWGC section with reliability, selectivity and sensitivity. Power Research and Development consultants (PRDC) Pvt. Ltd 23

25 The protection co-ordination for the wind farms shall be monitored by the SEBs/STUs/ISTSs The following are the minimum protection schemes that shall be installed for wind farm protection: i) under/over voltage protection ii) under/over frequency protection iii) over current and earth fault protection iv) load unbalance (negative sequence) protection v) differential protection for the grid connecting transformer vi) capacitor bank protection vii) tele-protection channels (for use with distance protection) between the grid connection point circuit breaker and user connection point circuit breaker Back-up protection shall be provided for required isolation/protection in the event of failure of the primary protection systems provided to meet the fault clearance time requirements The protection requirements for the wind farm substation and for the transmission system evacuating the wind power shall be as per the specifications of STUs/SEBs/ISTSs Signals and data communication requirements Wind farms connected at 66 kv and above shall have communication channel which is continuously available to system operator. The communication facility shall be provided at substation level of wind farms Signals from wind farm to system operator Wind farm operator shall send following signals to the system operator: i) Meteorological data ii) Active/reactive power output iii) On-off status iv) Voltage regulation set point (if any) Signals from system operator to wind farms System operator shall send following signals to the wind farm operator: Power Research and Development consultants (PRDC) Pvt. Ltd 24

26 i) Active power curtailment based on merit order ii) Voltage regulation set point iii) Start/stop instructions Data to be submitted to system operator The wind farm operator shall update the following information to the system operator in case of any changes made: i) Single line diagram of the wind farm ii) Site common drawings System recording instruments A wind farm shall have data acquisition system/disturbance recorder/fault locator for monitoring/ recording wind farm performance Wind farm equipment This section discusses the standard requirements of wind farm equipments Lightning protection Lightening protection of WTG system shall be according to IEC TR Wind turbine generator systems Part 24: Lightning protection Earthing Wind turbine grounding systems shall follow the recommendations of IEC TR (section 9) Equipment standard i) All sub-station equipments of wind farm shall comply with BIS/IEC or prevailing code of practice. ii) All equipment shall be designed, manufactured and tested and certified in accordance with the quality assurance requirements as per IEC/BIS. iii) Wind turbine shall have a valid type certificate issued by an accredited certification body including CWET. If the type certificate is issued by an agency other than CWET, it shall be on the quarterly list of models and manufacturers issued by CWET/MNRE. Power Research and Development consultants (PRDC) Pvt. Ltd 25

27 Ground clearance Energized parts shall be maintained at safe vertical and horizontal clearances as dictated by Indian Electricity Rules/Central Board of Irrigation and Power standard adopted for conventional generators and its associated sub-stations Grid connecting transformer configuration for wind farms The grid connecting transformer configuration shall be designed to provide: i) A favorable circuit to block the transmission of harmonic currents. ii) Isolation of transmission system side and wind farm side ground fault current contributions The preferred configuration of the grid connecting transformer is delta connection on the wind farm side and grounded wye connection on the transmission system (grid) side. Delta connection on the high voltage side of the grid connecting transformer is not permitted. Alternate transformer configuration including wye-wye or wye-wye with a delta connected tertiary is also acceptable for the grid connecting transformer. If the wind farm is directly getting connected to the existing utility substation, the standard practice of utility shall be followed. The purpose of prohibiting delta connection on the high voltage side of the grid connecting transformer is to block the harmonics current and to detect the earth faults on the grid side Auxiliary supply Voltage and frequency excursions within the specified requirements in the auxiliary power supply shall not trip the wind farm. The auxiliary supply of reactive power compensating equipment shall be as per IEC Wind Turbine Safety and Design Revenue metering Revenue metering shall be in accordance with the Central Electricity Authority (Installation and Operation of Meters) Regulations. Power Research and Development consultants (PRDC) Pvt. Ltd 26

28 Procedure for site access, operational activities and maintenance The connection agreement shall indicate procedures necessary for site access, site operational activities and maintenance standard for equipment of the CTU/STU at wind farm owner premises and vice-versa Responsibilities for operational safety Wind farm operator shall be responsible for operational safety in the wind farm. Wind farm owner shall be responsible for safeguarding all the equipments in the wind farm against manufacturing defects, improper installation and due to external impacts in connection with the following: i) short circuit and earth currents ii) recovery voltage during clearing of grid short circuits and earth faults iii) rise in voltage on fault-free phases in the event of single phase earth faults iv) phase failure v) out-of-phase reclosing and other impacts that occur during abnormal operating conditions Power Research and Development consultants (PRDC) Pvt. Ltd 27

29 Chapter 5 OPERATING CODE FOR WIND FARMS The operating code specifies the operating conditions that the wind farms shall comply with for safety and reliable operation of the grid and shall be applicable to the wind farms connected to the grid, and the SEBs/STUs/SSLDCs/SLDCs/RLDCs. 5.1 Operating policy i) The wind farms connected to the grid shall comply with this operating code and shall operate as an integrated system with the grid. ii) Control centers of the grid connected wind farms shall be manned round the clock by qualified and adequately trained personnel. 5.2 Wind farm security and operating aspects The operating margins for the wind farms during the normal and the constrained operation shall be as below Operating margins Voltage at the grid connection point The wind farm operator shall operate the wind farm continuously for the voltage ranges mentioned in following Table 5.1 during the steady state operation. Table 5.1: Operating voltage limits for wind farms Voltage (kv) Nominal % Limit of variation Maximum Minimum % to -10% % to -9% % to -9% % to -12.5% % to -9% % to -10% Power Research and Development consultants (PRDC) Pvt. Ltd 28

30 Frequency of operation for wind farms For the operating range of frequencies between 47.5 Hz to 51.5 Hz, the WTGs shall operate according to the frequency response curve specified in IWGC section No WTGs shall be started if the frequency is above 51.5 Hz Active power and power factor The grid connected wind farm shall be capable of supplying the active power between the limits of 0.95 power factor lagging to 0.95 power factor leading at the grid connection point Reactive power and voltage control The wind farms shall have provision for VAr compensation/support such that they do not draw reactive power from the grid. VAr exchanges with the grid shall be priced as follows: a) The wind farm owner pays for VAr drawl from grid when voltage at the grid connection point is below 97% b) The wind farm owner gets paid for VAr given to the grid when voltage is below 97% c) The wind farm owner gets paid for VAr drawl when voltage is above 103% d) The wind farm owner pays for VAr given to the grid when voltage is above 103% This requirement is as mentioned in IEGC. This requirement can be met by all the WTGs The wind farm operator shall endeavor to minimize the VAr drawl from the grid when the voltage at the grid connection point is below 95% of rated, and shall not supply to the grid VAr when the voltage is above 105%. As such to control the VAr exchange, the wind farm operator shall provide the VAr compensation or request SLDC/RLDC to change the taps of the grid connecting transformer. The WTG manufacture provides VAr compensation facility for each WTG (switch able capacitor banks). So using this facility the wind farm operator can control the VAr exchange depending on the grid connection point voltage. Power Research and Development consultants (PRDC) Pvt. Ltd 29

31 Varying the grid connecting transformer taps also controls the VAr exchange with the grid The charge/payment for VAr exchange from the grid shall be at nominal paise per kvarh as specified by CERC/SERC and the transaction will be between the wind farm owner and the state utility. Although not uniform, all the SERCs have fixed a nominal charge for the VAR drawl by the WTG from the grid. For e.g., in Tamil Nadu, Rs 0.25/kVArh if the ratio of kvarh drawn to KWh exported is upto 10%and Rs0.50/ kvarh for more than 10% Not withstanding the above, SSLDC/SLDC/RLDC may direct a wind farm to curtail its VAr drawl/injection in case the security of grid or safety of any equipment is endangered Ramp rate limits All the grid connected wind farms with installed capacity 50 MW and above shall have the ramp up/down capability. Ramp control facility regulates the active power generated from the WTG and also minimizes the variations in the generated power that may arise because of wind variations. The performance is similar to that of governor control in case of synchronous machines The system operator may ask the wind farm operator to curtail the generated power due to increasing wind speed, turbines returning to service after some outage or to increase the generated power due to increase in system demand etc The WTGs shall have two ramp rates a) 10 minute maximum ramp rate b) 1 minute maximum ramp rate Power Research and Development consultants (PRDC) Pvt. Ltd 30

32 The ramp rate limits shall be applicable for all ranges of operation including start up, normal operation and shut down of the WTGs. An exception to this can be situations where there is fall in wind speeds The wind farm operator shall inform SLDC/RLDC, the maximum and minimum ramp rates of the WTGs at the time of commissioning. Any changes made to the ramp rate limits shall be informed to the SLDC/RLDC and shall be done as per their instructions Ramp limits for wind farms at the grid connection point shall be as given below (Table 5.2) depending on the wind farm installed capacity. Table 5.2: Ramp rate limits for wind farms Wind Farm Installed 10 min Maximum 1 min Maximum Capacity (MW) Ramp(MW) Ramp(MW) Installed Capacity/1.5 Installed Capacity/5 > This is in line with international practice. As per Irish wind grid code grid the ramp rate averaged over 1 minute should not exceed 3 times the average ramp rate over 10 minutes The ramping up/down of the wind power generation shall be done by the wind farm operator as instructed by the system operator. On case to case basis, the maximum ramp limits mentioned in IWGC section may be changed on the mutual consent between the system operator and the wind farm operator provided the WTGs ramp limits are not exceeded Power quality All the wind farms connected to the grid shall endeavour to maintain the voltage wave-form quality at the grid connection point. The wind farms shall comply with the IEC : Wind Turbine Generator Systems, Part 21: Measurement and Assessment of Power Quality Characteristics of Grid Connected Wind Turbines standard. Power Research and Development consultants (PRDC) Pvt. Ltd 31

33 Power quality in relation to a wind turbine describes the influence of a wind turbine on the power and voltage quality of the grid. The main influences of wind turbines on the grid concerning power quality are the voltage flicker, harmonics (for wind turbines with power electronic equipment), voltage changes & fluctuations and the in-rush currents Voltage flicker The IEC (IEC, 1997) and IEC (IEC, 2003) standards shall be followed with respect to voltage flicker limits and measurement techniques. Flicker means the flickering of light caused by fluctuations of the mains voltage, which can cause distortions or inconvenience to people as well as other electrical consumers. The flicker measurement is based on measurements of three instantaneous phase voltages and currents, which are followed by an analytical determination of P st (short-term flicker disturbance factor) for different grid impedance angles Harmonics Harmonics measurements shall be taken in accordance with methodologies of IEC or IEEE STD The harmonic content at the grid connection point shall be as follows: According to the guidelines (IEC ), harmonic measurements are not required for fixed-speed wind turbines (Type A), where the induction generator is directly connected to the grid. Harmonic measurements are required only for variable-speed turbines with electronic power converters (Types C and D). a) Harmonic content of the supply voltage is indicated by the following index: Total harmonic distortion of voltage = V THD (expressed as percentage) Power Research and Development consultants (PRDC) Pvt. Ltd 32

34 V THD V = 100 n= 40 2 n 2 n= 2 V1 Where V n : n th harmonic of voltage V 1 : fundamental frequency (50 Hz) voltage The maximum limits of V THD shall be as per the following Table 5.3: Table 5.3: Voltage harmonic limits System Voltage (kv) Total Harmonic Distortion (%) Individual Harmonic of any Particular frequency (%) b) Harmonic content of the supply current is indicated by the following index: Total Harmonic Distortion of current = I THD (expressed as percentage) I THD I = 100 I 2 n 2 1 Where I n : n th harmonic of current I 1 : fundamental frequency (50 Hz) current The maximum limits of I THD shall be as per the following Table 5.4: Table 5.4: Current harmonic limits Voltage level <69 kv >69 kv I THD The limits for V THD are taken from CEA standards (Grid standards) Regulations-2006 and the limits for I THD are taken from IEEE STD-519, Power Research and Development consultants (PRDC) Pvt. Ltd 33

35 Voltage fluctuations The wind farm operation shall comply with the following permissible voltage fluctuation limits at the grid connection point. a) Voltage fluctuation limit for step changes which may occur repetitively is 1%. b) Voltage fluctuation limit for occasional fluctuations other than step changes is 2%. The voltage fluctuations in a wind farm can occur because of the switching operations (capacitor banks, WTG start/stop), inrush currents during WTG starting etc. Such voltage fluctuations shall be limited to the values mentioned in the above section Start and stop criteria All the WTGs in a wind farm shall have the capability to receive the start/stop signal from the wind farm operator and shall respond to the signal without any time delay. This is to necessitate the wind farm owner s control over the WTG operation. The system operator may request the wind farm operator to start/stop the WTGs as the situation demands. So, the WTGs shall respond to the start/stop command send by wind farm operator without any time delay During the wind generator start-up, the wind farm operator shall ensure that the reactive power drawl (inrush currents incase of induction generators) shall not affect the grid performance. Fixed speed WTGs directly connected to the grid directly draws huge inrush current during starting. This may cause voltage fluctuations and flickering at the grid connection point. Power Research and Development consultants (PRDC) Pvt. Ltd 34

36 The wind farm operator has to ensure that the start up and stopping of the WTGs comply with the voltage quality requirements. Because, the switching operations and the inrush currents may cause harmonics, voltage flicker and voltage fluctuations It is recommended that all WTGs in the wind farm shall not start and /or stop simultaneously owing to high windy conditions. Simultaneous starting/stopping of the WTGs can cause power quality problems. Also, it can cause large changes in the power injected into the grid Operation during transmission congestion During network congestion the wind farm operator shall act according to the instructions given by system operators. System operator (SSLDC/ SLDC/ RLDC) shall make reasonable effort to evacuate the available wind power. System operator shall instruct wind farm operator to back down wind generation only as a last resort, in view of the fact that the variable cost for wind generation is all most equal to zero (just like overflowing reservoir mention in Merit Order Dispatch). Taking into consideration the zero fuel costs and environmental issues, it is recommended to evacuate all the available wind generated power to the grid. During transmission congestion, the conventional generation shall be backed down. Under extreme conditions, when the wind power generated exceeds the system demand and when the local voltage limits are violated, it shall be the responsibility of the wind farm operator to back down sufficient amount of the wind generation, to maintain system security. This shall be done as per the system operator s instructions. Power Research and Development consultants (PRDC) Pvt. Ltd 35