Failure Analysis Of Central Inverters

The solar power industry has grown enormously in the last decade. At present, the installed capacity of India is 15.6 GWp which is expected to reach 20 GW by 2017-18. Amidst the huge number of projects being planned, it is possible to predict the number of central inverters in solar sector to be installed.

If we delve in a solar power plant, the most important equipment to come across are Inverters. These Inverters convert DC power from solar array to AC power which is fed to transformer for stepping up followed by the grid. Generally, solar inverters come with 5 years warranty with full load efficiency of around 98%. However, buyers must be cautious while selecting inverter for their solar plant as reliability of inverters for solar generation is utterly important. Let’s go through a case study which throws light on keeping a tab on reliability of inverters.

Case:

The study focuses on reliability of Inverters installed in a Solar Power generation plant. A set of inverters have been studied and errors/issues have been analysed for enhancement and future references.

Description:

Study of Identical Inverters across the solar plants were carried out. Considering the performance of 150 identical central inverters with 1MW capacity across 5 regions, of which, at least 80% of the inverters are working for over a year on the field.

This reliability study is based on the statistical measures of Mean Time to Failure (MTTF), Mean Time Between Failure (MTBF), Mean Time To repair (MTTR) and Failure rate.

Terms & Definitions:

  • MTBF: Average time difference between two consecutive failures.
  • MTTR: Average time taken for repair of a specific failure.
  • MTTF: The anticipated timespan for a device/product to last in an operation.

It is like MTBF, only difference being MTTF is used for non-repairable product and MTBF is used for repairable ones.

  • FAILURE RATE: Number of failures over a time-period. Failure rate is reciprocal of MTBF.

RESULT  

MTBF (hrs)59.14
MTTR (hrs)1.18
MTTF (hrs)60.32
FAILURE RATE/hr0.02

FAILURE RATE/day

0.41
FAILURE RATE/month12.17

Study shows a pretty reliable number for this inverter as the failure rate is moderate.

Choosing right inverters and maintaining them by O&M team is extremely important for overall plant performance. There is also an external factor called ground fault which impacts the inverters performance.

— By Madhushree Sinha- Electrical Engineer

2 thoughts on “Failure Analysis Of Central Inverters

  • Being a Retired Solar Scientist and an electrical engineer, I request you to pl try to answer the following questions Questions regarding PV power
    * Me , RSSharma,former Extra& Ultra High Voltage Engineer, founder member of 400KV sustem of Rajasthan ( 1982-89)
    * Who demonstrated CSP first time in India October 1985.
    * Then, joined Ministry of Nonconventional Energy Sources/ now MNRE in July 1989. Dedicated 27 Yrs for CSP R&D. After generating 3PATENTS in CSP retired in July 2006, as Solar Thermal Power Scientist F, MNRE.
    * As you are an expert in PV Electrification , therefore , wish to send you few questions regarding PV Electrification.
    * As I found you an experienced person, you maybe the person to answer
    * My questions are ;
    * 1- whether, the fixed PV generation WITOUT BATTERY STORAGE is Grid friendly?
    * 2- Can it feed additional power to the underfrequency grid?Reactive power
    * 3- Can it absorb the excess power from grid without loosing its generation , while grid is in over frequency? Synchronous power.
    * 4- in case of fixed PV ,How can we increase the average annual efficiency<7% and Plant Load Factor 20-21% to , parallel with 12.5-21% and + 60% in case of CSP ?
    * 5- Can PV plant deliver power on demand during evening peak and night?
    * 6-Whether the ROOF TOP PV programme will be justified in the unstable grid(S) of India, as there will be no way to evacuate power without availability of distribution Grid.
    * 7- Is India, not loosing Forex Researves and filling the Forex coffer of China by importing manipulated CHEAP PV COMPONENTS from CHINA, instead of installing its own OEM Febs?
    * I am raising these questions since last few months to various Electrical professors of IITs, PV system manufacturers and eminent personalities in PV field, but NO REPLY from any of them
    * You are highly qualified instead of me , having + 34 Yrs combined practical experience in both EHT & UHV transmission and Solar Power including biomass gasifire integration with CSP in 2004.
    * You may be aware that world’s one of the strongest GERMAN GRID WAS FAILED ( tripped) on 12th June 2011 due to over frequency and caused lot of revenue loss to them and by restricting PV power generation below 50.5Hzs,
    * some of the feed in terrif plants have to loose revenue as they are cutoff from the grid .
    * I will be thankful for your expert reply
    * With Regards
    * Radhey S. Sharma
    * rssharma108@rediffmail.com
    * Pl be kind enough for replying my questions.
    *

    • hello sir, these are the answers of your question..
      1. PV without battery is of course grid friendly. it will be a on grid system.
      2. if the grid voltage increases +/- 2% of standard voltage level solar inverters will try to match up with that voltage level to some extent by delivering reactive power but beyond the max limit inverter will trip. these condition in solar inverters called HVRT
      3. same as prev. mentioned condition , if grid voltage goes down inverter will be in sync to some extent by absorbing reactive power, this condition is called LVRT(low voltage ride through).
      4. efficiency of a solar plant measured as its performance ratio. this PR value will decrease with time. in a well maintained plant you can get 20-21% PLF.
      5. If you have storage system then only you can use solar power during night , else in grid tie system it is not possible to use solar power directly.
      6. Yes in India it is challenge, if there is no grid there is no way to evacuate your solar generated power in on grid solar system. but again, if it is a hybrid system or off grid system then you have a option for it.
      7. Polysilicon manufacturing is an expensive and complex process. Due to lack of raw material, technology, unskilled labors, and overall infrastructure India is still having cell manufacturing capacity of 1.4GW. mainly cost effectiveness is not there in pv cell manufacturing industry.

      hope this will serve your purpose …

      thankyou
      regards
      Madhushree

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