Overall objective

Means

• Subtask 10: Review of Previously Installed PV Experiences (From 1993 to 1998)
  To review existing technical guidelines, local regulations and operational results of grid interconnection with building- integrated and other dispersed PV systems to aid Subtask 20 in defining existing guidelines and producing concepts for new requirements and devices.

• Subtask 20: Definition of Guidelines to be Demonstrated (From 1993 to 1998)
  Utilizing the results of Subtask 10 and a questionnaire, existing technical guidelines and requirements to be demonstrated will be defined, and concepts for new requirements and devices will be developed; with safety, reliability, and cost reduction taken into consideration.

• Subtask 30: Demonstration Test Using Rokko Island and/or Other Test Facilities (From 1993 to 1998)
  To evaluate, by demonstration tests, the performance of existing and new technical requirements and devices defined in Subtask 20.

• Subtask 40: Summarizing Results (From 1993 to 2001)
  To summarize the results of Task 5 and to produce a general report for all participating countries of Task 5, as well as for the ExCo members.

• Subtask 50: Study on Highly Concentrated Penetration of Grid Interconnected PV Systems (From 1999 to 2001)
  To assess the net impact of highly concentrated PV systems on electricity distribution systems and to establish recommendations for both distribution and PV inverter systems in order to enable widespread deployment of solar energy.

Overview of past activities

• Subtask 10
  Surveys of the existing guidelines and regulations for grid connection of PV systems and the difference of electrical distribution systems in Task 5 participating countries were completed as a Task 5 internal report open to IEA member countries. The report on inverter and related protection equipment was distributed to Task 5 participants as an internal Task working document. The summary of PV operating experiences of participating countries was included in the Task 5 summary report.

• Subtask 20
  Research results on subjects important for the interconnection of PV systems were summarized as an official IEA report. The following topics are covered in the report:
  • Harmonics
  • AC-Module
  • Multiple Inverters and AC Grid
  • Grounding of Equipment in PV Systems
  • Ground-fault Detection and Array Disable for PV Systems
  • Overvoltage Protection
  • Electro Magnetic Compatibility
  • External Disconnect
  • Reclosing
  • DC Injection and Isolation Transformer

• Subtask 30
  Demonstration tests for harmonics, islanding, PV output variation etc. were conducted using the Rokko Test Facility in Japan. The Subtask 30 Report, summarizing demonstration test results by using the Rokko Test Facility, was completed and published as an official IEA-PVPS report. Contents of the report include, harmonics caused by PV inverters, measurement of islanding, distribution line short circuit fault, AC/DC mixing fault, the effect of a PV system’s output fluctuation and other topics.

• Subtask 40
  The Summary Report of Task 5 Activities from 1993 to 1998 was produced as official IEA-PVPS report. Status reports and other management reports for the Executive Committee were also prepared in this subtask.

• Subtask 50
  Surveys for the latest PV system grid interconnection technologies were conducted and reported. These surveys included PV system grid-interconnection and design guidelines or national standards, updated information of inverters and interconnection devices and PV system testing certification and utility inspection and maintenance methods.
  Research on islanding related problems has been conducted in highly penetrated PV systems conditions. The probability of load and PV system output matching was determined using real measurement and the risk of islanding was defined. Islanding detection methods and test circuits for islanding detection performance were summarized and reported.
  Important factors to decide on the limit of PV system penetration in distribution networks were listed and analysis based on distribution line voltage limit was conducted. Measures to stretch the limit of PV penetration and some financial aspects of PV penetration were also discussed. The power value and capacity value of PV system penetration were evaluated using different customer load profiles.

Summary of Task 5 - Subtask 50: Conclusions

• Activity 51
  
  1. Revised Survey for the Grid-Interconnection Guidelines or Standards
     The main conclusion of this survey is that many countries recognized that it is required to have the PV specific or inverter interfacing specific standard for grid-interconnection of PV systems apart from the standard for ordinary rotating generators grid-interconnection standards. Requirements for the safe and reliable grid-interconnection of PV systems are the common issues for the standards, although there are differences in the approach because of the difference of technical boundary conditions (layout of grid, grounding philosophy etc.) in different countries. Therefore, even if it will be difficult to achieve full harmonization of international standards, it seems possible to reach a consensus in at least 90 % of all topics.

  2. Revised Survey for the Inverter Technologies and Related Control and Protection Interfacing Technologies
     The technology for inverter circuits fell into almost the same configuration and controlling concept within the different manufacturers.
     Improvement of performance was observed with higher conversion efficiency, lower harmonic current injection, higher power factor operation etc. Moreover, reduction of cost, volume and weight of inverters have been achieved, when compared with the survey of three years ago. Sufficient control of power and protection from grid fault has been achieved by using software-based operations that allow low cost control and the protection of the inverter.
     Finally, the difference between countries and manufacturers became smaller.

  3. Recommendations for the Certification Tests for Safe and Reliable Installation of Grid-Interconnected PV Systems
     Recommended test items for manufacture product certifications, laboratory type tests, utility onsite initial starting tests and periodic inspections were listed with the objective of the tests, testing procedures, notes and criteria for each test. Examples of testing circuits were also listed. These results will serve as a good reference for the certification of PV systems and will be useful for manufacturers, utilities, testing laboratories and standard making bodies.

• Activity 52
  1. Probability of Islanding by PV System Penetration in Practical Distribution Systems
     By actual measurement of the distribution line load variation and the PV output variation, the probability of load matching or occurrence of islanding was estimated. It was found that load matching never occurs when the total capacity of PV systems are under the minimum load of distribution system during daytime, corresponding to around 400 Wp PV system installation per every house. Large scale penetration of a PV system causes the possibility of load matching and the chance is highest when PV systems are installed around 900 Wp per every house. However, the actual probability is very small, even when the large margin of load mismatch was considered. The probability of load matching for more than 1 second is less than 1,0 E-5/sec and load matching for more than 5 seconds is less than 1,0 E-6/sec; this corresponds to 130 and 13 times per year. Load matching greater than 10 seconds will occur very seldom. Considering that the probability of loss of main (LOM) is about 6,3 E-8/sec (twice per year), the probability of islanding is far more reduced to 8,3 E-6/year. As a conclusion, islanding is no issue for the probability base.

  2. Risks Associated with Islanding
     The final risk associated with islanding was calculated by considering the protection failure caused by operator failure, inverter protection failure, insufficient installation etc. As a result, it was found that the risk of electric shock associated with islanding is far less than the risk that already exists for network operators and customers. The additional risk presented by islanding does not materially increase the existing risk, as long as the risk is managed properly.

  3. Islanding Detection Methods
     Various types of islanding detection methods were summarized; including voltage and frequency detection, passive methods (frequency change rate, voltage phase jump and voltage harmonic change etc.) and active methods (impedance measurement, frequency shift etc.). The theory of operation, strengths and weakness and more importantly, the non-detection zone (NDZ) of each method were described. The non-detection zone means the condition in which the detection system cannot detect islanding because of the sensitivity and the theory of operation.
     The non-detection zone decides the allowable load mismatch for islanding operation and the effect on the probability of islanding.
     Testing circuit configurations and the procedures of testing were also recommended.

• Activity 53, 54
  1. Maximum Penetration Capacity of PV Systems in Distribution Networks
     The maximum penetration capacity of PV systems in distribution networks were evaluated by considering the distribution line voltage criteria. Allowable penetration capacity depends on the load profile of the distribution system, especially by the minimum load during daytime. At the minimum load condition, no PV penetration is expected in the most severe case. However, only a slight increase of load allows for a significant increase of PV penetration.

  2. The Value of PV Systems
     The value of PV systems was evaluated by the flattening effect of the load curve and the reduction of joule loss of distribution lines. It was found that the value of PV systems is greatly dependent on the load profile of distribution line in which PV systems are interconnected.
     The load profile for an urban area, which has peak consumption in the daytime, for example, an office building, has the advantage of PV penetration; while the load profile for a residential area has little advantage of PV penetration.

In conclusion, IEA PVPS Task 5 has performed important studies on topics related to grid interconnected PV systems. The reports published by Task 5 will serve as valuable information dissemination tools for utility people, PV industries, standard making bodies and customers. These reports will aid in them in understanding the problems and finding solutions for grid interconnection of PV systems. One example is that the grid interconnection guideline or standard of participating countries has been improved to distinguish the PV systems or inverter interface generation systems from rotating generators since the difference of protection requirements has been learned. Although there are differences of grid conditions by countries, requirements for the protection of grid connection for PV systems fall into almost the same concept. One of the most important points that Task 5 showed was that the probability of islanding is very small and costly protection systems for preventing islanding are not needed any more.

At the end of Task 5, the need for wider research in grid connection issues including all kinds of distributed generation systems in one distribution network was recognized. Similar to the way PV systems can be influenced by the presence of other types of generators, future distribution systems are expected to encounter analogue situations. While the international collaboration within Task 5 has contributed strongly to the understanding of such situations for PV power systems, in future, such issues are best dealt with as a cross-cutting issue for international cooperation between different energy technologies
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Industry involvement

Reports produced in 2003

• Report IEA PVPS T5-05: 2002 ; “Grid Connected Photovoltaic Power Systems: Survey of Inverter and Related Protection Equipments.”

Contact information

• Mr. Tadao ISHIKAWA
  CRIEPI
  2-11-1 Iwato-kita Komea-shi
  JPN - 2018511, Tokyo
  Email: ishikawa@criepi.denken.or.jp