FIELD OF THE INVENTION
The present invention generally relates to protection modules for achieving optimum service of station transformers with multiple secondary windings. More particularly, the invention relates to a system for detection and prevention of station transformer failures due to overheating of core clamp plates under unbalanced load conditions.
BACKGROUND OF THE INVENTION
A station transformer is the first major terminal equipment to be installed during commissioning of a power generation plant. The station transformer supplies power to the auxiliary equipment during start-up operation of the generator unit or when the plant is not generating power. The station transformers are generally designed with multiple secondary LV windings to have economical circuit breaker ratings and maintain supply to diverse loads. A plurality of independent secondary coils are arranged linearly one above the other on the same core limb to meet the impedance requirements. The clamp plates are important structural components of the transformers and hold the laminated core intact. They are located between the windings and the core and attract large leakage flux when secondary windings are loaded. The normal component of leakage flux intersects the clamp plate surfaces near the winding ends and induces eddy currents in the clamp plate resulting in localized overheating. This phenomena is more pronounced in case of station transformers with linearly arranged multiple secondary windings, especially under unbalanced load conditions, as the clamp plate surface gets exposed to a larger leakage flux emanating from the expanse between the secondary windings.

The prior art with reference to clamp plate overheating, emphasize on design optimization of the clamp plates for reduction of losses, by choosing non-magnetic material or providing slits to break the eddy current loops. Systems for online measurement of top oil and winding temperatures are being widely used, while the clamp plate hot spots are generally ignored in the power art solutions, owing to relatively lower losses. But in case of station transformers, even for optimized slotted non-magnetic clamp plate designs, the temperatures can still reach at dangerous values when the imbalance in loading of the secondary windings is very high. The prior art do not address these issues, although failure of the station transformer due to overheating of clamp plates are reported.
The station transformers supply power to the auxiliary equipment, during starting up of power generator units or when unit is not in generation mode, and cater supply to diverse loads. They transfer the power from the grid connected to the primary HV winding to multiple secondary LV windings arranged linearly one above the other on same limb to supply independent diverse loads. The operating load factor of station transformers is low and highly non-uniform. The unequal loading of transformer secondary windings can result in very high clamp plate hot spot temperatures, due to concentration of high eddy currents in the clamp plate regions exposed to the stray flux between the linearly arranged LV coils and at the winding ends. Instances of transformer failures due to overheating of clamp plates were reported from sites. The core clamp plate design is very critical in case of station transformers and it is very important to accurately estimate the current densities and hot spot temperatures under critical loading conditions to ensure safety of the equipment.

Magnetic shielding structure for iron core clamping member of transformer CN 202034186 U - proposes a way for reduction of leakage field and resulting hot spots for power transformers, but this is not applicable to station transformers with split winding configurations.
Transformer hot-spot temperature monitor
US 4623265 A - This invention relates generally to a method and apparatus for determining the temperature of the hot spot in a transformer, and more particularly, to such a method and apparatus employing the transformer current and top-oil temperature to determine the hot-spot temperature. This method estimates average hot-spot temperature by indirectly measuring winding currents and is not applicable to localized overheating to clamp plates due to induced eddy currents.
Network for online monitoring of power transformer at intelligent substation A2903912A1- This patent presents online monitoring of a power transformer at an intelligent substation monitoring online, the gas and micro-water dissolved in oil, sleeve pipes, winding optical fibre temperature, local discharge, iron core grounding current, infrared temperature, vibration, load capacity, etc. But, it does not monitor the clamp plate temperature.
Loading guide for oil-immersed power transformers
IEC 60076-7- The standard presents the guidelines for loading of transformers under conditions and the effect of temperature rise on relative ageing rate and insulation life of the equipment.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to a protection module for optimum service of station transformer with multiple secondary windings.
Another object of the invention is to propose a system for detection and prevention of station transformer failures due to overheating of core clamp plates under unbalanced load conditions, which detects the limiting operational conditions for a station transformer and trip the equipment before the clamp plate temperature approaches the threshold value.
A still another object of the invention is to propose a system for detection and prevention of station transformer failures due to overheating of core clamp plates under unbalanced load conditions, which is enabled to self-compute and display the rate of reduction in the transformers operational life due to over-heating of the clamp plates particularly under unbalanced loading conditions of the secondary windings.
Yet another object of the invention is to propose a system for detection and prevention of station transformer failures due to overheating of core clamp plates under unbalanced load conditions, which is capable to issue guidelines to the operator to re-distribute the loads on secondary windings to minimize the load unbalance and limit the clamp plate hot spot temperatures, and thereby improve the service life of the transformer.

A further object of the invention to propose a system for detection and prevention of station transformer failures due to overheating of core clamp plates under unbalanced load conditions, which allows economical operation of the station transformers for short duration over loads, or rescheduling of loads to ensure continuous operation of the equipment within the safe limits, by integrating the clamp plate hot spot temperatures corresponding to elapsed time including computation of overload allowance based on operating conditions.
A still further object of the invention is to propose a system for detection and prevention of station transformer failures due to overheating of core clamp plates under unbalanced load conditions, which enables accurate measurement of hot spot temperatures occurring on the clamp plate through optimal location of the fibre optic sensors.
SUMMARY OF THE INVENTION
The present inventors carried-out extensive experimentation on station transformer clamp plate models above 100MVA rating, to co-relate the effect of material properties, clamp plate dimensions, slit configurations and loading conditions on hot spot temperatures. It was observed that loading conditions have a greater impact on the clamp plate hot spot temperatures. The estimated hot spot temperatures are found to be in good agreement with the values obtained by measurements. It was found that the clamp plate losses and hot spot temperatures were higher by about 40% when single LV winding is loaded in comparison with the values obtained when both LV windings are loaded. The present invention proposes a system for detection and prevention of station transformer failures due to overheating of core clamp plates under unbalanced load conditions. The system continuously monitors the clamp plate hot spot

temperatures and trips the equipment in case of dangerously high temperatures. The system also indicates the rate at which the transformer life is being consumed with reference to the unbalanced loading of LV windings, which can be used as a tool to improve the service life of the equipment. Investigations using numerical techniques on coupled electromagnetic-thermal station transformer models were carried out, and it was observed that the loading conditions have a direct bearing on the clamp plate hot spot temperatures. High temperatures were estimated even for non-magnetic clamp plate designs with optimal slits, under non-uniform loading conditions where one of the secondary LV winding is fully loaded while the other is left open. The maximum temperatures estimated under the stringent loading condition were near to the limiting values given by IEC standard, at which the equipment is likely to fail. Though equal loading of station transformer secondary windings do not result in higher clamp plate hot spot temperatures, but unequal loading can lead to failure of equipment. Hence, a system with a protection module, for detection of core clamp plate overheating in station transformers with multiple secondary windings, and trip the equipment at limiting conditions has been proposed for optimizing the service life of the equipment. The proposed invention also indicates the rate at which the transformer life is being consumed with reference to the unbalanced loading of LV windings and it can be used as a tool to improve the service life of the equipment.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Schematic of station transformer and its protection module Figure 2 - Electromagnetic model of station transformer Figure 3 - Connection diagram of station transformer windings

Figure 4 - Temperature distribution across core clamp plate when both LV windings are loaded
Figure 5 - Temperature distribution across core clamp plate when only LV1 winding is loaded
DETAILED DESCRIPTION OF THE INVENTION
The clamp plates of the station transformer clamp the main legs of the transformer core together and also hold the windings in place through the end frames fastened to the clamp plates. The clamps plates are located very close to the windings and attract maximum leakage flux, the normal flux field induces eddy currents in the clamp plates resulting in higher load losses and overheating. The leakage in the magnetic field increases with loading of transformer secondary, and the normal component of the leakage flux, which is significantly higher at the winding ends, induces large eddy currents in the corresponding clamp plate surfaces resulting in localized overheating. This phenomena is more critical in case of station transformers which have multiple secondary windings arranged linearly one above the other on a single core limb.
The station transformers are required to supply power to the auxiliary equipment during start-up operation in power stations and also other lighting loads, cranes, drives, workshops etc. The station transformers are designed with multiple secondary winding arrangement to have economical circuit breaker ratings. The independent secondary coils are arranged one above the other on the same core limb to meet the impedance requirements, as shown in the sectional view of typical 120 MVA station transformer in Fig. 2 and circuit connections with reference to loading of independent secondary windings are shown in Fig. 3.

Because of operational requirements, at times when only one of the secondary windings is fully loaded, overheating of the clamp plates and rise of hot spot temperatures was observed on the surface exposed between the two linearly arranged LV windings, due to enormous rise in the normal component of the leakage flux owing to the lower reluctance path through air than the yoke. Instances of failure of station transformers due to prolonged overheating of clamp plates were reported in literatures.
Though the contribution of induced eddy current losses in the transformer clamp plates is only a small portion of the total load losses, but higher concentration of eddy current densities in specific regions of the clamp plate can lead to transformer insulation failure due to overheating of the clamp plates. Hence, it is very important to estimate the current densities and hot spot temperatures in clamp plates with reference to the loading conditions. Coupled electromagnetic and thermal simulations were carried out on the station transformer models using numerical techniques. The hot spot temperatures computed under full load balanced and unbalanced conditions of loading LV windings is shown in Figures 4 & 5 respectively. It is observed that the operation of the transformer under single LV winding full load condition lead to hot spot temperatures exceeding 180°C and failure of equipment, as per IEC guidelines on current and temperature limits.
The operating load factor of the station transformers is low and highly non-uniform. Though unequal loading of the secondary windings do not significantly affect the health of the station transformers when operated for short durations, but prolonged operation and overheating can lead to transformer failure. Therefore, a continuous monitoring of the clamp plate temperatures will enable the transformer protection system, when the hot spot temperatures rise above the

critical value especially under prolonged non-uniform loads, and trip the equipment before reaching the limiting condition.
According to the invention, a protection module for station transformers with multiple secondary windings is implemented using a plurality of fibre optic sensors (TPT-62), which are installed at the critical regions on the clamp plate surfaces for continuous monitoring of hot spot temperatures. The proposed protection scheme is shown in Fig. 1. The analog output current from the RTDs is converted to voltage signal using an input card and an ADC converts the analog signal to digital signal before feeding them to the processor. The station transformer service life optimization program processes the input data and sends appropriate instructions to the control room through an output card. The operating load factor of the station transformers is governed by the power requirements of auxiliaries in power station, which is highly non-uniform and can lead to overheating of the clamp plates. The protection module monitors the clamp plate hot spot temperatures and enables the operator to re-distribute the loads on secondary windings to minimize the clamp plate overheating or trip the transformer in case of critical conditions. The proposed system offers economical operation of the transformer by allowing short duration over loads and also enables the operator to reschedule the loads based on the operating hot spot temperatures and allowable values.

WE CLAIM :
1. A system for detection and prevention of station transformer failures due to overheating of core clamp plates under unbalanced load conditions, comprising : a protection module having :
• at least three fibre optic sensors disposed on each of the core limbs of the transformer;
• a DC power supply system;
• a relay system consisting of an input card, a processor card, and an output card with contacts; wherein the protection module integrated on a station transformer;
• a trip system to isolate the station transformer; and
• a control module for optimization of loading the transformer secondary windings, wherein analog output current from the RTDS is converted to voltage signals using said input card, wherein an analog to digital converter (ADC) converts the analog voltage signals to digital voltage signals, wherein said processor loaded with service-life optimization programme of the transformer, the processor receiving the digital voltage signals, processing the signals and transmitting instructions via said output card to the transformer control panel, and wherein the control panel operator based on the transmitted data initials redistribution of the loads on the secondary windings to minimize over-heating of the clamp plates or trips the transformer in case the over-heating data of the clamp-plates exceeding a threshold value.

2. The system as claimed in claim 1, wherein the system indicates the actual rate at which the station transformer life is being reduced, due to overheating of the clamp plates under unbalanced loading conditions of the secondary windings.
3. The system as claimed in claim 1, wherein said plurality of fibre optic sensors disposed on the critical regions on the clamp plates surfaces which allows an accurate measurement of hot spot temperatures occurring on the clamp plate.