TNERC Clarifies Single/Three-Phase Inverter Usage in Rooftop Solar Systems up to 4 kW

Plants up to 4 kW capacity can be connected to 240V single-phase service connection or 415V three-phase service connection

December 31, 2021


Tamil Nadu Electricity Regulatory Commission (TNERC) heard a petition filed by the Solar Energy Developers Association, Salem, seeking clarification on ‘Low Tension (LT) Connectivity’ covered in the ‘Order on Rooftop Solar Generation’ issued on March 25, 2019.

The Association requested directions for inverters in rooftop solar systems (up to 4 kW) irrespective of whether it is connected to a single-phase service connection or a three-phase service connection at the consumer’s end.

The ‘Order on Rooftop Solar Generation’ issued on March 25, 2019, covered consumers’ eligibility under net metering, commercial arrangements, metering standards, location, LT connectivity, standards, operation and maintenance of solar power generators, etc.

The respondents of the petition included – Tamil Nadu Generation and Distribution Corporation (TANGEDCO), the Director Distribution, and Tamil Nadu Energy Development Agency (TEDA).

The Association had filed the present petition to redress its members’ grievances – all solar energy system developers in Tamil Nadu. It requested the Commission to direct the respondents to implement the original order issued on March 25, 2019, claiming that association members were seriously discriminated against by the director distribution’s misinterpretation of the Commission’s Order concerning LT connectivity.

According to the 2019 order, the maximum capacity for interconnection with the grid at a specific voltage level would be governed by the supply/distribution code (amended from time to time).

For Solar PV plants with inverters up to 4 kW capacity, the interconnecting voltage level can be 240V – single-phase or 415V three-phase as per the consumer’s option. For solar PV plants with inverters above 4 kW capacity, the interconnecting voltage level must be 415V – three-phase only.

Tamil Nadu - Interconnecting Voltage levels of Distributed Generating Sources Relevant to Capacity Range

The petition further states that wrong interpretations would make the entire rooftop solar program unviable, resulting in a substantial financial loss to the installers.

However, TANGEDCO had not issued any operating instructions to the field level officers. Due to a lack of clarity among the field level officers, the Solar Energy Developers Association members faced various difficulties.

The Association had sent a detailed representation to the director distribution seeking clarification on various issues. As per the director distribution’s instruction dated November 15, 2019 – if a domestic consumer connected to a 415V three-phase service connection wants to install a 1 kW rooftop solar system, they will have to install a three-phase inverter instead of a single-phase inverter, which is contrary to the original order.

In a letter dated July 21, 2020, the director distribution had stated that the original order (Order on Rooftop Solar Generation’ issued on March 25, 2019) only highlighted the voltage levels for solar consumers and not the usage of single-phase or three-phase inverters.

However, three-phase inverters with less than 4 kW capacity are not available in the market; even if it is procured, the cost of such inverters would be high, resulting in the entire rooftop solar program being commercially unviable. The benchmark cost of rooftop solar systems notified by the Ministry of New and Renewable Energy (MNRE) could not be met if 3 phase inverters are mandated for a system less than 4 kW.

Single-phase inverters work efficiently for a solar system of capacity less than 4 kW. But if a single-phase inverter is used in any one of the three-phases, it can cause an imbalance in the load circuit.

Usage of single-phase inverters for single-phase service connections and three-phase inverters for three-phase service connections are being insisted to balance the consumption and the generation across the phases while maintaining secure, quality, and stable power supply to the consumers.

However, TANGEDCO is insisting usage of single-phase inverters in single-phase service connections and three-phase inverters in the three-phase service connections to reduce the loss due to imbalance and to maintain quality power supply.

Commission’s decision

The Commission struck down the conditions imposed by the TANGEDCO to employ three-phase inverters for single-phase rooftop solar systems up to 4 kW. TNERC stated that a ‘licensee’ has no jurisdiction in contemplating additional connectivity conditions, not to mention the unreasonable commercial implications on the consumer.

The Commission further clarified that the solar power generation unit up to 4 kW could be connected to 240V single-phase service connection or 415V three-phase service connection at the consumer’s option as clearly mentioned in Commission’s Order No.3 dated March 25, 2019.

This month, the TNERC had issued draft amendments to the Electricity Supply Code 2004 for bi-directional meter billing for rooftop solar systems. Stakeholders’ comments and suggestions are sought on the amendments. The Commission has added a new clause for assessing billing in cases where there is no bi-directional meter, or the bi-directional meter is defective in low tension (LT) rooftop services.

TNERC has also ruled that consumers applying for additional solar capacity should adhere to the gross metering methodology as long as the installed solar capacity does not exceed the sanctioned load of the consumer’s premises.