Claims:WE CLAIM:
1. An apparatus for preventing failure of MF (Medium Frequency) transformer due to loose connection at its Primary side terminals in incremental pipe bending machines, comprising of:
A PTC Thermistor (29) connected to a copper bus bar link (17) in primary terminal tapping (12 and 13) to sense the temperature at Copper bus bar link (17);
A pair of shielded cables (C & D) for connecting the Thermistor output to a Temperature controller (30);
Said temperature controller (30) for setting the safe temperature value and to compare the actual temperature;
control cables for interconnecting the changeover contact output in said temperature controller (30) with the Emergency circuit (A-B) in an Auxiliary control panel (23);
2. The apparatus as claimed in claim 1, wherein said PTC thermistor (29) is connected to said copper bus bar (17).
3. A method for preventing failure of MF (Medium Frequency) transformer due to loose connection at its Primary side terminals in incremental pipe bending machines, wherein:
A PTC Thermistor (29) connected to a copper bus bar link (17) in primary terminal tapping (12 and 13) to sense the temperature at Copper bus bar link (17);
A pair of shielded cables (C & D) for connecting the Thermistor output to a Temperature controller (30);
Said temperature controller (30) for setting the safe temperature value and to compare the actual temperature;
control cables for interconnecting the changeover contact output in said temperature controller (30) with the Emergency circuit ( A-B) in an Auxiliary control panel (23);
said emergency circuit (A-B) breaks when the temperature at said copper bus bar link (17) increases beyond safe value.
, Description:FIELD OF THE INVENTION
The present invention generally relates to an apparatus to prevent failure of Medium Frequency (MF) transformer. The present invention in particular relates to an apparatus to prevent failure of MF (Medium Frequency) transformer in incremental pipe bending machine due to loose connection at its Primary side terminals.
BACKGROUND OF THE INVENTION
Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, through heat generated in the object by eddy currents. Induction bending enables precision control of the conditions for heating and cooling narrow sections of steel pipe to ensure more stable quality and shape.
An Induction heating process has the following advantages: (1) Free shape bending (No bending die is needed), (2) Highly accurate processing, (3) Small-radius bending of heavy-walled steel pipes, (4) Bending for even mechanical properties e.g. strength and toughness.
The incremental Pipe bending machine is used for making pipe bends and they operate on the principle of Induction heating. These bent pipes are used in Thermal power plants and they carry steam / water at high pressure & high temperature.
The incremental pipe bending machine is used for bending of Pipes with Various Materials, diameter & thickness. This machine can produce pipe bends with bending radius from 400mm to 4600mm of Pipe OD 150mm to 720mm having thickness of 5 to 100mm.
For bending the pipes, Induction heating principle is employed in the machine. The 3 phase, 415V, 50 Hz, AC Voltage input supply is rectified into DC Voltage by using rectifiers. By using IGBT (Insulated Gate Bipolar Transistor) Inverter this DC voltage is converted back to AC voltage. The output frequency of AC Voltage is varied by suitable adjustment of the Gate triggering pulses to these IGBTs and the required frequency is based on the job specifications like thickness, material composition, etc.
The load being an Inductive coil, consumes lots of lagging reactive power from the power supply lines, which is undesirable. In order to compensate this, the capacitors which supply leading reactive power are used in parallel with the load. These capacitor banks are available in stages and required stage can be selected by the operator.
These capacitors & inductive coil form a parallel tank circuit. The resonant frequency of this tank circuit (the frequency at which inductive reactance cancels capacitive reactance and the circuit is purely resistive) is determined from the values of inductance ‘L’ & capacitance ‘C’. The machine controls are designed in such a way that, the frequency of the inverter AC output is automatically adjusted to match the resonant frequency formed by the tank circuit. Only at this condition, effective power is transferred to the load.
The inverter is designed to operate between its lower & higher frequency of operation. Adjustment of inverter frequency to match resonant frequency is possible only within these limits. Whenever, the resonant frequency value goes out of these two limits, the inverter trips on alarm. The remedy is to adjust the values of inductance and capacitance to bring the resonant frequency within these two limits.
However adjusting the value of ‘L’ is not possible as it is determined by the application and adjusting the value of ‘C’ is limited with respect to available capacitor stages. In such cases, one more option is provided in the machine in the form of MF transformer to get the effective power.
The Primary role of MF transformer is to match the load impedance (in this case, ‘tank circuit’) in accordance with the circuit impedance such that always maximum possible power from the source is transferred to the load. The load impedance is matched by adjusting the MF transformer Turns ratio (N2 / N1).
To avoid handling of high current at tap changing, these tap changing provisions are provided in the primary side of MF transformer. The tap changing operations are performed as and when required by the operator according to the power requirement.
In the existing method of Tap changing provisions at Primary, the required tapping is connected by means of copper links by using stainless steel bolts by the operator. The existing arrangements by the machine manufacturer showing the terminal arrangements at the primary side of the MF transformer is shown in Fig.1 & the schematic electrical circuit for the Emergency switch circuit is shown in Fig.2.
In this manual connection, there is always a chance for loose connections which may lead to enormous heat generation at these terminals as it carries high current. Whenever these loose connections are left unnoticed, then the heat generated leads to flash over at transformer terminals. This leads to MF transformer failure resulting into more maintenance cost, man hour and machine hour loss due to machine breakdown and productivity loss.
Hence, a suitable and cost effective mechanism is required at these tap changing points to monitor the temperature and to give necessary warning / alarm when it exceeds the allowable value.
US7089145 A - TAP CHANGER CONDITION DIAGNOSING
The Invention was related to continuous monitoring of the condition of the oil immersed tap changers in power transformers and gives the diagnosis results to the operator and it is not related to the proposed filed of invention.

US6052060 A - TEMPERATURE MONITOR FORELECTRICAL SWITCHGEAR
The Invention was related to continuous monitoring of dielectric oil temperature used in electrical switchgears & transformers which are circulated through a pump and it is not related to the proposed filed of invention.
US4745571 A - MODULAR ELECTRONIC TEMPERATURE CONTROLLER
The Invention was related to controlling the temperature of liquid cooled power transformers by electronically computing the winding temperature from the actual top oil temperature and the incremental additional temperature resulting from transformer load current and it is not related to the proposed filed of invention.

US4192174 A - TRANSFORMER PROTECTIVE DEVICE
The Invention was related to measuring the conditions such as temperature, pressure in a sealed liquid dielectric filled transformer and it is not related to the proposed filed of invention.

US4654806 A - METHOD AND APPARATUS FOR MONITORING TRANSFORMERS
The Invention was related to microprocessor-based transformer monitoring system to provide continuous on-line monitoring and analysis of transformer operation related to transformer load and condition and it is not related to the proposed filed of invention.
OBJECTS OF THE INVENTION
The object of the present invention is a method for preventing failure of MF (Medium Frequency) transformer due to loose connection at its Primary side terminals.
Another object of the present invention is a method for preventing failure of MF (Medium Frequency) transformer due to loose connection at its Primary side terminals in incremental pipe bending machines.
Yet another object of the present invention is a method for preventing failure of MF (Medium Frequency) transformer due to loose connection at its Primary side terminals in incremental pipe bending machines and the machine is controlled through microprocessors such as PLC (Programmable Logic Controller).
A still further object of the invention is a method for preventing failure of MF (Medium Frequency) transformer due to loose connection at its Primary side terminals in incremental pipe bending machines and the machine is operated in Auto operation.
SUMMARY OF THE INVENTION
The MF transformer handles very high current; hence, the core, primary & secondary windings are water cooled. Based on the power requirement, the required tapping position is selected at primary side by the operator. There are 6 terminal groups. 2 groups form one polarity and the remaining 4 groups form the other polarity of the primary winding. The required terminals within a group are connected through a shorting link made of copper. The shorting link is connected through Stainless steel bolts.
The power from Inverter output section is conducted through water cooled conductors and they are terminated in copper bus bars at MF transformer primary terminal. Through the shorting link this copper bus bar is get connected to require tapping points.
The machine is controlled by a Programmable Logic Controller. For ensuring safety of the people working around the machine, Emergency switches are provided at various locations in the machine. These Emergency switches are electrically connected in series and acting any one of these switches will halt the machine operation and prevent any accident.
When a change in output power is required, the operator will remove the copper shorting link at the MF transformer Primary terminals by loosening the stainless steel bolts and place the copper shorting link at the new tapping point & tighten the stainless steel bolts.
Due to congested space at primary terminal side with lots of bus bars & water cooled conductor circuit, difficulty is faced in ensuring proper tightness at the connection terminals. Also, the machine vibration during operation leads to further loosening of these stainless steel bolts over a period of time.
Due to high handling current, heat gets developed at these loose connection points and when it left unnoticed leads to major breakdown and safety hazards.
To prevent damages in MF transformer at its primary terminals due to loose connection, a new apparatus and method has been developed by introducing a PTC (Positive Temperature Co-efficient) Thermistor at these connection terminals & by introducing a temperature controller. The PTC thermistor senses the temperature continuously and the feedback in terms of resistance is given to temperature controller. When the measured temperature exceeds the safe value, the temperature controller changes its output contact. This output contact is suitably sandwiched in the machine Emergency circuit & the contact change due to temperature raise will break the Emergency circuit, which is otherwise closed. Thus, the machine is halted for correction& prevented from a major damage and safety hazard.
BRIEF DESCRIPTIONS OF THE ACCOMPANYING DRAWINGS
The proposed invention will be better understood by the following description with reference to the accompanying drawings:
Figure 1 – shows the schematic view of existing arrangement at the Primary side terminal of MF transformer.
Figure 2 – shows the schematic circuit of Emergency circuit connected to auxiliary control panel.
Figure 3 – shows the schematic view of modified arrangement at the Primary side terminal of MF transformer.
Figure 4 – shows the modified schematic circuit of Emergency circuit connected to auxiliary control panel.
Figure 5 – shows the newly introduced temperature controller circuit.
DETAIL DESCRIPTION OF THE INVENTION
Ref. to Fig. 3, the MF transformer according to the present invention consists of the following functional elements:
The transformer is housed in a solid steel casing (1). At the Primary terminals side, salt free treated water is circulated through the primary windings (2,3,14,15) andthe secondary winding is cooled by circulating the water through its conductors (10,11). The core which houses the primary & secondary winding is also cooled by circulating water through it (21,22).
For making connection to the required windings, tapping has been taken out at different points in the primary winding and provisions are available (5,8,12,13,16,19). Out of the available provisions, (5) & (8) forms one polarity of the primary winding and based on the power requirement either one of them will be connected through copper bus bar link (6) using stainless steel bolts (31).
Tapping provisions (12,13,16,19) forms other polarity of the primary winding and based on the power requirement one of them will be connected through copper bus bar link (17) using stainless steel bolts (31).
A di-electric sheet (4) is provided in front of the tapping provisions at the Primary side to safely make the power connections. For the two different polarity of the primary windings two separate copper bus bars (7,18) are used and they are placed on the top side of the di-electric sheet (4). These copper bus bars are separated with an insulating material.
The water cooled power cables coming from the Inverter section are connected to said two copper bus bars (7 and 18) at (9) & (20) respectively, through suitable nipples & clamping rings. Due to loose connections at (6) & (17) heat is generated and when it left unnoticed, leads to a major fault and safety hazard like flash over & short circuit.
A PTC thermistor (29) is connected in the copper bus bar link (17). The output of the PTC Thermistor is connected to the Temperature controller (30) through two shielded cables (C) & (D) (Fig. 5).
The sub components in the machine are monitored and controlled through an auxiliary control panel (23) as shown in Fig.2. One of the monitoring functions performed by the Auxiliary control panel is ensuring the healthiness of the Emergency circuit (24, 28). The emergency switches at various locations of the machines (25, 26, 27) are connected in such a way to form a closed loop under normal operating conditions.
The Temperature controller (30) in Fig.(5) has a change over contact through its output terminals (A) & (B) and the contact status changes based on the set temperature value & actual temperature value measured through the PTC Thermistor. This output contact status is suitably sandwiched in the Auxiliary control panel Emergency circuit (A) & (B).
Thus, if due to some loose connections, the temperature at copper bus bar link (17) increases beyond the safe value, then the Emergency circuit loops break at (A) & (B), thus halting the machine function and preventing further heat development and damage at the MF transformer terminals. The drawbacks said in the background of the invention are eliminated.
The proposed invention as narrated herein above should not be read and construed in a restrictive manner, as some modifications, adaptations and alterations are possible within the scope and limit of the invention, as defined in the encompassed appended claims.