Offshore Substation in India : A case study on planning of associated transformation capacity and Reactive compensation schemes for evacuation of power generated through off shore wind system-Gujrat

GW non-fossil fuel energy capacity by 2030. Among the various sources of non-fossil fuel based energy generation, offshore wind energy is also a potential source of energy generation in India.

Offshore wind energy involves producing electricity using wind turbines located in oceans or seas. Since wind speeds are typically stronger over water than on land, these offshore wind farms can generate more electricity for the same amount of installed capacity. India has a long mainland coastline. The coastline is present all across the western, southern, and eastern parts of mainland India.

The Government of India introduced the National Offshore Wind Energy Policy on October 6, 2015, to promote offshore wind power in the country. This policy outlines guidelines for developing offshore wind projects within 200 nautical miles from the coastline, covering the area within India's Exclusive Economic Zone (EEZ). Preliminary studies using satellite data have identified eight potential offshore wind zones each in Gujarat and Tamil Nadu. Ministry of New And Renewable Energy (MNRE) has identified about 30 GW Offshore wind potential each off the coast of Gujarat and Tamil Nadu. Initially 5 GW Offshore wind potential each at

Gujarat (CUF – 38%) and Tamil Nadu (CUF – 48%) has been prioritized for implementation wherein 2 GW transmission capacity (1 GW each off the coast of Gujarat and Tamil Nadu) will be developed in the 1st Phase. The offshore projects planned are divided into different phases.

Under Phase-1, Transmission system for evacuation of 500MW off the coast of Gujarat will be implemented by Power Grid Corporation of India Limited. The development of transmission and evacuation infrastructure for 500 MW capacity of offshore wind energy project in Gujarat includes the construction of Offshore pooling station and Onshore substation, 220kV AC

Submarines cables etc. Submarine cables are used to carry power from Wind zone to offshore substation and for carrying power from Offshore substation to Onshore substation.

This paper discusses in detail the various ratings of transformers required in offshore and onshore substation for the evacuation of power from offshore wind system. Generation of power from the wind energy system is at 0.9-1.5 kV which will be stepped up at a level of 66kV. The onshore substation is at 220 kV voltage level. Also, for reducing the losses, the network voltage is required to be stepped up and Step- up transformer is required to be provided at the offshore substation. Offshore submarine array Cable of 66kV is used for evacuation of wind energy system from different arrays to offshore substation which is approximately 15kms. Also, the 220 kV submarine cable is used for connection from offshore substation to onshore substation which is approximately 35kms. Submarine Cable by its capacitance generates reactive power, thus there will be overvoltage at offshore and onshore substation. To compensate for the overvoltage at offshore and onshore substations, sufficient reactive power compensation is required to be provided at both the buses of onshore and offshore substation. The reactive power compensation can be provided through Fixed Shunt Reactors, Variable Shunt Reactors (VSR),

STATCOM etc. The paper discusses in detail the effect of reactive power compensation on bus voltage using combination of different reactive power techniques. Simulation outputs using

Phasor Domain Transient (PDT) stability software on All India model are provided in the paper to understand the effect of reactive power compensation on bus voltage profile. The paper also describes in detail about the design aspects of transformers and reactors used in the onshore and offshore substation.

Following variables have been considered while evaluating the adequacy of reactive power requirements:

1. Variation in generation dispatch.

2. Variation in Grid strength.

3. Minimum-generation scenario.

Based on the study results, discussion will be done on the voltage stability being achieved with the combination of static and dynamic reactive power compensation devices.