Solar Projects With ₹7.01/kWh Tariff Face Higher Curtailment in Tamil Nadu: POSOCO

Power System Operation Corporation Limited (POSOCO) has submitted a report on renewable energy curtailment in Tamil Nadu following the Appellate Tribunal For Electricity’s (APTEL) orders. The tribunal had ordered POSOCO to verify in detail the data presented by the stakeholders and submit the report.

Based on the report, APTEL had directed the Tamil Nadu State Load Despatch Center (TNSLDC) and Tamil Nadu Generation and Distribution Corporation (TANGEDCO) to compensate the solar developers for curtailment of power at 75% of the tariff listed in the power purchase agreement (PPA) along with 9% interest.

The POSOCO had been directed to report whether the Tamil Nadu State Load Despatch Center had intentionally curtailed power or whether the curtailment was due to grid safety precautions.

POSOCO analyzed the data from March 1, 2017, to June 30, 2017, and observed that 16 out of 56 power generators, comprising approximately 68% of the total installed solar capacity in the state (totaling 1,052 MW), had submitted the data. Of the 16, only ten generators submitted complete data.


POSOCO deducted the actual generation from the irradiance-based estimated generation to arrive at the proportion of curtailment. Since solar power was only available throughout the day, solar and non-solar periods were compared.

A curtailment comparison between solar and wind power projects was made for blocks with solar generation exceeding 10 MW and wind power generation exceeding 500 MW.

POSOCO confirmed that it completed the investigation, validated the data, and reached conclusions based on the facts available. Based on the extensive study, the following findings were reported by POSOCO:

  • Of the 1,140 blocks (total curtailed blocks), 55 blocks (4.82%) had a frequency greater than 50.05 Hz. Grid frequency within 49.90 Hz – 50.05 Hz is considered safe and does not call for security measures.
  • In terms of grid frequency, only 5.26% (60 out of 1,140 blocks) curtailment was justified from a grid security standpoint.
  • Most solar power generators with a tariff of ₹7.01 (~$0.095)/kWh faced more curtailment in terms of instances and percentage generation compared to other solar power generators.
  • Frequency was over 50.00 Hz in 427 blocks (37.45%) of 1,140 blocks. The state was under-drawing in 350 of the 427 blocks. In 60 of the 350 blocks, there was no margin for backing down thermal and hydro generation to absorb renewable energy.
  • Wind and solar energy generation were not curtailed simultaneously (during the same day or month); however, wind and solar power were equitably curtailed to a large extent over the four months. Equitable curtailment in this context means curtailment across the board without discriminating generators based on the tariff, source of generation, or any other factor.
  • In terms of generation (MW), from the solar generators’ standpoint, 52% of time blocks where wind energy was curtailed were equitable. Meanwhile, from the wind generators’ perspective, 48% of time blocks where solar energy was curtailed were equitable.

Comparing the curtailment of non-renewable facilities to renewable energy was considered inappropriate by POSOCO. It stated that there were difficulties in furnishing some data related to state-owned thermal stations and renewable energy solar forecasts due to the non-implementation of intra-state forecasting and scheduling regulations.

Hydropower potential in the state was found to be limited when compared to other states in India. Hydropower is used to bridging the variations of renewable energy in real-time grid operation daily and seasonally and found to be utilized prudently.

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