This is based on my own experience. I visited Chandrapur-Padghe HVDC Transmission System at Chandrapur HVDC Super Power Station on 25/7/2016.
So here I am mentioning what I noticed in that plant.
- It is located in Maharastra, India owned by Maharashtra State Electricity Board. It is constructed by ABB and BHEL.
- It was commissioned in the year 1999. The total length of the line between two stations is 752 Kms using 1980 towers.
- The capacity of the line is 1500 MW(each line is 750MW), ±500 KV and it is Bipolar Link type HVDC system.
- AC voltage is 400 KV.
- At Chandrapur Rectifier Link is there.
It consists of the following major equipment.
These are used to nullify the effect of harmonics and to inject reactive power around 800 MVAR capacitors, Inductors and Resistors are housed.
- Converting house:
Convert AC to DC by using Converting Transformer and Rectifier.
- Converting Transformer:
It is a two winding transformer which is 400KV/500KV
Input is star type and output is star and delta. The input is taken from 400KV HT Line and the two input transformers are connected in parallel with neutral grounded for each. The output at 500KV is given to thyristor valves. Here the windings are connected in star and delta with neutral point ungrounded.
Each three phase converter gives the 6 pulse output.
- Thyristor Valves:
These are two numbers of structures which are hanging from top.These are two three phase converter bridges connected in series to form a twelve pulse converter unit. That implies these are 6 pulse to 12 pulse converters. Thyristors are cooled by using water.Liquid cooling using deionized water is more efficient and results in the reduction of station losses.
Optical Fibre Communication is the new method employed here. PLCC is stand as backup.
This need only 4 cables one for transmitter and one for receiver, remaining two are standby.
- Neutral Grounding:
Neutral is grounded far away from the substation taken away through 33KV line. It is due to during fault conditions the neutral currents are high so there is corrosion problem which disturbs the substation earthing so it is isolated in another village.
HVDC system is operating in three modes:
It has two conductors, one positive and the other negative. Each terminal has two sets of converters of identical ratings, in series on the DC side. The junction between the two sets of converters is grounded at one or both ends. Normally, both poles operate at equal currents and hence there is zero ground current flowing under these conditions.
When two lines are healthy bipolar mode is operated. Half of the power is transmitted through each conductor.
2. Monopolar with Ground Return:
Monopolar has one condutor usually of negative polarity and uses ground return.If line itself need to be maintained then the line is isolated and the return path is taken through ground.
3. Monopolar with Metallic Return:
If we want to do maintenance of converting transformer then it is isolated and the current can be passed from the ground return path to a metallic return path provided by the HVDC conductor of the faulty pole.