Lakvijaya Power Station

Lakvijaya Power Station(Norochcholai)

Us...

In this article, I mainly focus what I had experiences during the 2 weeks (08/08/2016 to 19/08/2016). On the first day we were assign to 3 main sections of the plant that are I&C(Instrumentation and controlling) Unit,EM(Electrical Maintenance) Unit, and Operation Unit. Our group was 10 members from G5 & G6 groups.But HRM was 3 groups. Suditha Shamika (Me), Kavinda Geeth, Janith Chamikara and Kanchana Lakmal are together.

Background

Lakvijaya Power station is the one and only coal power plant in Sri Lanka. It feed 900MW power to the national grid by 3 of 300MW generators. It is about more than 40% of electricity demand in the country.There are many various negative misconceptions in the society. In my opinion most of the politicians spread them for their personal cartels. We had a talk with Eng.Shami Mudunkotuwa about how should be electricity generation in future. In there I realized as a 3rd world country power is the most significant sector for developments. But the major problem is the against of the internationals and internal politicians and ignorant eco-friendly people. You can make your own view by analyzing whats really happen in Sri Lanka.

Technology

You can get an idea about how is this plant working --->

 Technology of Lakvijaya Power Station

Basically I try to give some technical tips what I could get during the internship period in there.

Main transformer tap changer issue 
In the main transformer HV side(220kv) is connected to the 220kv GIS. Because of that when grid is loaded 220kv voltage is vary with time frequent. So the main transformer should be maintain 220kv on Hv side constant. to do this there can be used 2 methods.

The idea for the Lakvijaya coal power project was successfully completed with the help from China National Machinery and Equipment Import & Export Corporation (CMEC) as main contractor and project funded by EXIM Bank of China. This is the largest proposed power plant in Sri Lanka (3 x 300 MW) located in Norochchole in Puttalam district. Option 1- When 20kv is changed in UAT LV side voltage(6kv) will be changed because UAT has off-load tap changer.If 20kv is changed it will be harmful to motors and auxiliary components.So the option 1 can't be implemented.Change the LV side voltage (20kv) by using AVR

Option 2- So this is the best fit way to overcome this issue. Standard tap changing is 1000 times per year.But in here there was happen around 11000 times in past 3 years. Due to that reason tap changer should keep up properly.  (Note- Transformer issues will be in here)

  Coal Handling

The process of coal handling is very much important to power generation by the plant. The coal is transported from the respective country by ships (65 tons per ship) and they are kept anchored about 4-5 km away from the shore. Then, the coal is transferred in to barges (long flat-bottomed boats) by which the coal is transported to the jetty.

After coal is transported to the jetty by the barges, there is a conveyor belt system to transfer coal to the bunker bay where it is stored before feeding the furnace. The coal is first unloaded to conveyor belt No. 0 directly from the barge with the help of coal unloaders. The plant is provided with two coal unloaders each having unloading capacity of 500 tons per hour. Conveyor belt No. 0 runs directly to the junction tower at which the coal travelling path divides into two. These two paths are called the emergency path and the normal path. The normal path is numbered as conveyor belt No.6. The emergency path is used under an emergency situation where the normal path cannot be used due to malfunctioning. This arrangement is needed because delays in unloading process would cause heavy demurrage by the shipping companies to CEB.

Quality of coal is checked on samples taken at every hour at the junction tower itself. Then the normal path runs into a stockpile where a bucket wheel stacker reclaimer is installed to reclaim coal in to the conveyor or stack coal in to a stock pile. The Normal path or the conveyor belt 6 ends at transfer tower 3 where an iron separator is installed to separate iron from coal. After this, the coal is fed using conveyor belt No.3 in to the crusher house where the coal is crushed so as to reduce the particle size.

Thereafter, the coal is taken to the transfer tower 1 with the help of conveyor belt No. 2. The coal brought in should be maintained around 10% moisture content immediately before use. There exist five mills which are used to pulverize (grind) coal into further small particles. Actually coal is fed only in to four ball mills for pulverizing process and the 5^th mill acts as a standby. In here, the coal is pulverized. Then the pulverized coal particles are conveyed to the furnace in the boiler by means of primary air.

Boiler

The Lakvijaya Power Plant has the largest boiler in the country. It was manufactured in China by Harbin Boiler Company. Its efficiency is 88.58% and a capacity of 1025 t/hr. The sub critical pressure of the boiler is 173 bar and the temperature of steam at super heater outlet is 541 ⁰C. The boiler arrangement consists of burners, super heaters, re-heaters, an economizer and an air pre heater. Two numbers of induced draft fans are installed in the boiler in order to create negative pressure inside the boiler. This is necessary to expel flue gas in the boiler easily. There is a booster fan as well, to support the process of the expulsion of flue gas. Circular firing system is employed in the furnace and the temperature in the furnace is maintained below 1200 ⁰C.

Turbine    

There are three turbines in deferent levels for each units. They are High pressure (HP) turbine, Intermediate Pressure (IP) turbine & Low pressure (LP) turbine.

Transformer

There are three phase main transformer which is rated as 20 kV/220 kV, 360 MVA & an auxiliary transformer which is rated as 20 kV/6 kV for each unit. And the primary winding is delta connected & the secondary winding is star connected & solidly earthed.

There is a problem. Standard number of tap changing per year is 1000 times. But in Norochcholai there was happen around 11000 times in past 3 years. Due to that tap changer get failed. In main transformer HV side (220 kV) is connected to the 220 kV GIS. Because of that when grid is loaded 220 kV voltage is vary with time frequent. So the main transformer should be maintained 220 kV in HV side constant. For this there are 2 methods.

       I.                        Change LV side voltage (20 kV) by using AVR

    II.                       Change tap position of main transformer

According to first option, when 20kV is changed in Unit Axillary Transformer (UAT) LV side voltage (6 kV) will be changed because UAT has off load tap changer. So 6 kV can’t be kept constant. If 20 kV is changed. It will be harmful to motors and other auxiliary components. Then option 1 can’t be implemented. So option 2 is the best fit way to overcome this problem. To keep tap changer in better state, they have to do maintenance frequent and well.

Bottom Ash Handling

The residual after coal combustion is called bottom ash and is about 20% of the total ash produced during combustion. A submerged scraper conveyor belt system conveys the bottom ash from the furnace. This bottom ash is used as land fill material.

Fly Ash Handling

This is the fine particles generated during coal combustion and they escalates with flue gas. If the fly ash is mixed with the atmosphere, it will cause a huge environmental impact. Therefore a special mechanism has been installed to grab fly ash from the flue gas which is called the electrostatic precipitator.