1.Reliablility

1.1Better tolerance in temperature rise - longer life cycle of the product
The main insulation material is the patent-Nomex that produced by Du Pont Company. It is Class C insulation material (220¡æ), meaning the transformer can operate normally where the ambient temperature is 40¡æ, average temperature rise is 150K and the highest temperature tolerance is 30¡æ.

Our excitation transformer is the Class H (maximum running temperature is 180¡æ), the design average temperature rise is lower than 100K, but the average temperature rise of general Class H product is 125K. Therefore, it has better tolerance in temperature rise so that it will guarantee the service life of product.

1.2  Over-excitation ,anti-high frequency radiation
   When the excitation transformer is in use, the system will tent to overexcite. Large-scale excitation transformer requires to assuring two times of excitation voltage when the generator's end voltage drops to 0.8 times of rated voltage. To maximize the safety and the reliability, we can calculate the excitation transformer secondary rated voltage according to strong excitation condition, the valid value of the excitation transformer secondary rated current can be worked out by the maximum continuous excitation system output, and then we can determine the maximum continuous excitation power.

1)   Because the excitation transformer winding current is not sinusoidal waveform (contains high-order harmonics pulses waveform), it will cause the imbalance between rectifier transformer capacity and the DC transmission power. The copper loss and iron loss produced by the harmonics current are greater than those produced by winding rated current, and secondary winding capacity is greater than primary's during poly-phase rectification. Therefore, compare with ordinary distribution transformers, excitation transformers must meet the higher requirements in terms of electric density, magnetic field density, temperature rise, capacity and overload ability.

In our design, not only we installed the static electric isolation shield between primary and secondary windings, also adopted higher winding insulation level than ordinary distribution transformer.

2) Our excitation transformer adopts Nomex?insulation material. In that Nomex?have the remarked anti-high frequency radiation ability, it will intact by 800Mgy plasma radiation, and its electric and mechanical ability will never be changed even after 8 times radiated. Such as the high frequency transformer insulation material of microwave oven is Nomex?so high frequency radiation will hardly shorten the service life of product.

3) When loaded with high-order harmonics, transformer will produce high heat point temperature. Usually we will adopt the solution of temperature drop to compensate and thus, increase the sizes of the equipments and waste resources. The dry type transformer, which uses Nomex?insulation system has better heat tolerance, can endure high heat and therefore prolong the life of the transformer.

1.3 High anti-short circuit and thunder impulse

   The low voltage winding is foil type and the high voltage winding is multi-layer cylinder type. Just consider from the structure, the anti-sudden short circuit ability is so far stronger than the spiral pancake winding together rolled by several flat wires or cylinder winding. Furthermore, our products also have been processed by VPI (Vacuum Pressure Varnishing Technology), so that guarantee the mechanical characteristics.

1.4 Low partial discharge
    Partial discharge means that there is just discharge partly in the effect of electric field, but not be struck, and this phenomenon is partial discharge.

    Normally partial discharge is caused by compact electric field in the inner or on the surface of the insulator. Generally, this discharge phenomenon is constant impulse less than 1s. However, it also possibly appears as a continual phenomenon, for instance the non-impulse discharge in the gas medium. Normally, this type of discharge cannot be tested.   

    During 2001-2003, in Thailand, there were so many epoxy resin dry type transformers crazing, partial discharge higher suddenly, which disturbed the daily life seriously. Therefore, Thailand government revised the dry transformer standard. The new standard definitely prescribes that dry transformer must pass the partial discharge test before selling. Meanwhile the same problems also emerged in Europe. However, the most dry type transformers were insulated by Nomex?material, which could solve such problems because of the perfect electrical and mechanical characteristics itself. Therefore, American never made any regulations about the partial discharge on dry type transformer.  

The different insulation materials tend to the different attitudes for the same problem.

Compared with other insulation materials, the inductivity of Nomex?is very low.

The relationship between the electrical field strength and the insulation inductivity is as follows:

Epoxy resin  ¦Å=4.5      Nomex? ¦Å=1.5        Air  ¦Å=1

U=10000V;  :B 0=B1

Voltage U=10000V; Distance:B 0(Air)=B1(Solid)

(1)  In the system of Epoxy resin and air
1818.18V; Epoxy resin load: 1818.18V
8181.82V;  Air load: 8181.82V
As the place where there has air can become the part where lead the whole insulation system to be broken because of the huge voltage.

(2) In the system of Nomex@ and air

Nomex@ load: 400V;
Air load: 600V

If there is air in system, the voltage of Nomex?and air system will reduce 26.7% rather than epoxy resin and air system's. Because of the inductivity of Nomex?is close to the air's, so the distribution of the electric field is evenly, thus it cannot produce the partial discharge even when there are air bubbles in the insulation system.

The air bubbles in the insulator interior or on its surface are the most possible things to result in partial discharge. For the inductivity of air is always lower than solid and liquid's. Under electrical field, distribution of electric field strength is in reverse proportion to inductivity.

That means if there are air bubbles exist in liquid or solid medium, the electric strength in air bubble will higher than the electric strength in surrounding medium. The strike electric strength of air bubbles is always lower so much than liquid and solid's under the atmospheric pressure, so air bubble discharge firstly, while other medium still keep insulation. Then the partial discharge will happen.

The dry type transformer insulated by Nomex?can avoid the partial discharge happen because the inductivity of Nomex?is so close to air inductivity that improves the electric field distribution

2. High adaptability
    Because the installation place where the excitation transformer employed have the high humidity (hydroelectric station) and the serious defilement (power plant), the humidity-proof and defilement-proof measures should be given appropriate consideration in the process of transformers' manufacture so as to keep it running normally.

2.1  Anti-sudden cold and anti-sudden heat ability
    Du PonTTM ReliatraN?dry type excitation transformer passed C2 "Climatic Class" in European independent laboratory (CECI) in Milan, Italy. The procedures of test is that putting the transformer into a laboratory (690m3), dropping the temperature of laboratory from + 25¡æ to - 25 ¡æ within 8 hours, and keeping - 25 ¡æ for 12 hours; at that time, the surface of iron has frozen by frost about 2 mm. Then, putting into the 2 times rated current from LV side, and raising temperature from  - 25 ¡æ to 165 ~ 170 ¡æ quickly. Then all the frost turns to water pearl and steam completely. Afterwards, drop the temperature to 25?/FONT>C, start the insulation test with 75% rating voltage and partial discharge test.