The Globecore Transformer Drying Plant US-6 is capable of drying and improving cellulose paper insulation properties by heating uniformly by combined thermal Heater and LFH Converter and removing moisture from the insulated Winding-Core Assembly of the repaired distribution transformer of 11/0.4 kV with capacity up to 500 KVA) and 33/11 kV power transformer of capacity up to 26 MVA. Oil tank assemble and oil filling work of transformer will be done at outside of the Drying Chamber.

The plant will have provision to input the startup data/ parameter of transformer initial condition in control screen of SCADA system that are used in determining time requirement of different drying stages. These data are mainly design data of the transformer like i) Weight of cellulose paper insulation, ii) Voltage rating of primary winding, iii) Current density of each winding, iv) Type of material of each winding, v) Cold resistance of the LV & HV windings, vi) Ambient temperature etc. Drying process cycle will be controlled considering these parameters to accomplish with minimum amount of time.

Proper drying cycle of justified period will be ensured with uniform drying (Heating & vacuum pumping) of the entire insulated Winding-Core Assembly of transformer for full load of Drying Plant i.e. to reach final insulation properties of minimum moisture content (i.e. 0.5% moisture content) from maximum moisture content in the cellulose paper insulating material in a batch.
 
The Drying Plant is combination of application of Chamber heating by electric thermal plate heating system, Transformer winding heating system by variable voltage Low Frequency Heating (LFH) Converter, Hot air circulation and ventilation system, variable pressure vacuum & cooling system which are controlled by a core controller in coordination with all other controller to ensure at least Seven processing phases consists of followings:
 
     1) Measurement / leak test phase:
The startup data/ parameter of transformer initial condition shall be input in control screen of SCADA system and shall be used in determining time requirement of different drying stages. A vacuum leak test of Drying Chamber shall be performed.
     2) Heating up & Stabilization Phase-1: With the Drying Chamber at atmospheric pressure with the aeration valve open, the transformer windings are heated by the combined thermal plate heater and LFH Converter current circulating in the transformer windings. Simultaneously, dry hot air (125 0C) is circulated through the tank to preclude a condensation build up in the winding-core assembly and to heat up the non-coil insulation parts. After reaching the average winding temperature (100 0C) measured by resistance, the temperature stabilization time is to allow winding temperature to equalize throughout all winding and insulation parts. The speed of heat up must be limited to 1 0C/min because of high moisture content in the insulation so as to limit depolarization of the cellulose insulation and to ensure that the no- coil insulation parts are properly heated.
     3) Intermediate Pressure Reduction (IPR) Phase -1: After the stabilization time has expired and the transformer windings have been reached to 1000C, hard vacuum is pulled on the drying chamber. During this process a high percentage (75%) of total moisture is removed from the insulation. The voltage class and weight of the cellulose insulation in the transformer set the IPR vacuum phase length of time. The purpose of this vacuum phase is to remove the evaporating moisture driven from the cellulose insulation by the heat.
     4) Heating up & Stabilization phase-2:
The vacuum is released and Chamber is raised to atmospheric pressure with aeration valve open. The transformer primary windings are heated the combined thermal plate heater and to temperature 110 0C by the low frequency current circulating in the transformer windings. After reaching the average winding temperature (110 0C) measured by resistance, the temperature stabilization time is to allow winding temperature to equalize throughout all the winding and insulation parts. After this phase vacuum is not released so that there is no chance to enter moisture in the Chamber.
     5) IPR Phase -2:
After the stabilization time has expired and the transformer windings have been reached to 1100C, a second hard vacuum is pulled on the drying chamber. During this process majority moisture is removed from the insulation.
     6) EV Phase:
The vacuum is pulled to 30 millibar and then the winding temperature is heated to 120 0C average determined by winding resistance.
     7) Fine Vacuum Phase:
The average winding temperature is maintained at 120 0C with low frequency current, while the vacuum on the Chamber is continued to be pulled until the fine vacuum process time is expired and final vacuum shall be maintained at 0.2 millibar for the desire time. At this stage heating is removed and water extraction rate meter determines the desired level of drying of insulation is obtained.
During drying, autoclave inside pressure is auto-adjusted according to temperature of drying object. It creates best water-evaporation condition for drying object. High temperature in the winding (1200C depends upon the class of insulation) and low vacuum in the Chamber (0.2 millibar) will be maintained for effective removal of moisture from the winding insulation at the final stage. Heat will be transferred through radiation and air flowing caused by pressure-changing in autoclave and there shall not be core- rusting problem. Drying rate, heating and vacuum shall be uniform and properly configured to avoid the flash over during high temperature heating and low/ fine vacuum pulling of moisture at the same time.

Default settings of Temperature, Vacuum Pressure, Hot Air Circulation will be configured for all Drying Stages and also shall have provision of customizing within certain range of settings according to user needs.
All auxiliaries/ accessories/ Instruments/ equipment / sub-assembly / sub-system capacity (Vacuum pump-motor, Hot Air Circulating fan-motor flow and power capacity, Chiller Cooling Capacity, LFH Converter, Thermal Heater heating capacity etc.) shall properly design with optimized margin to ensure the maximum requirement of Drying plant for achieving full capacity of the Plant.

The Floor mounted Electrical Cabinet will be integrated with PLC with 14 inch Display Panel, Relay protection and SCADA monitored with HMI (Computer, Mouse, Keyboard, color laser printer etc.) system, whole system is automatically operated as well as manual provision ensuring protection and safety in each case. LFH Converter voltage, current; Winding temperatures, Moisture extraction rate, vacuum levels, hot air circulation speed, improvement in insulation parameters will be displayed in HMI of SCADA and Display Panel and time based process Trend Chart for the same in the HMI. Instantaneous, nominal-protection-alarm settings value of various field data / parameters, historical processing data / parameter and trend shall be displayed in the Display Panel of the Electrical Cabinet and HMI of SCADA system, shall have provision to copy in the laptop and print out. Panel thickness will not be less than 1.62mm.