ELECTRICAL EASY

The oil can only be filled in the transformer which satisfies the standard specifications mentioned by the manufacturer of transformer. Normally Transformer is supplied with gas filled i.e. the Main Tank of Transformer is filled with Nitrogen gas under some specified pressure to prevent any degradation of the insulation of core and winding. But we cannot keep Transformer Main Tank filled with nitrogen for a period more than three months and hence oil filling becomes important. Oil filling in Transformer also becomes important if we want to store the Transformer for more than six months. The oil filling is done in the main tank under vacuum. The large Transformers are generally designed to withstand the full vacuum for long periods. Oil filling and filtration of oil is carried out simultaneously. Oil sampling is done during the process of oil filtration and if sample result for Dielectric Strength and moisture comes under limit then filtration of oil is stopped. Figure below show...

Variable Frequency Transformer or simply VFT is quite a new technology to connect two asynchronous Grids. Asynchronous Grids mean two power systems operating at two different frequencies. The world’s first VFT was installed and commissioned in the year of 2004 at Hydro- Quebec’s Langlois substation, where it is used to exchange up to 100 MW of power between the asynchronous power grids of Quebec (Canada) and New York (USA).  First thing to note here that though the name suggests it to be Transformer but construction wise it is like a Slip Ring Induction Motor. The basic concept behind the VFT is a rotary transformer with three phase windings on both rotor and stator. A motor and drive system are used to adjust the rotational position of the rotor relative to the stator, thereby controlling the magnitude and direction of the power flowing through the VFT. VFT is a controlled bi-directional device allowing flow of power in both directions i.e. from one Grid to another and vice ...

The transformer which handles voltages and currents in the form of pulse are called Pulse Transformer. Pulse Transformer is mostly used in Power Electronic circuits as an Isolating Transformer to isolate source and load. It is also used in television, radar, digital computers etc. The main functions of the Pulse Transformer are as below: 1)       For changing the amplitude of voltage pulse 2)       For inverting the polarity of the pulse 3)       For coupling different stages of pulse amplifier 4)       As an Isolation Transformer It shall be noted here that the turn ratio of an Isolation Transformer is 1:1. The Pulse Transformer core is made of Ferrite. The input voltage to a Pulse Transformer is of discontinued type as shown in figure below. The most and basic requirement of such transformer is that it should reproduce input voltage at its primary to secondary as acc...

Load Test of a Transformer is basically carried out for determining the maximum temperature rise of the Transformer. However is not viable to conduct this test by connecting the secondary of the Transformer to rated load. In case of small Transformer, rated load can be connected to the secondary of Transformer but for large Transformer, rated load capable of consuming rated power is not easily available and also this will lead to wastage of energy. Therefore the best and smart way to load a Transformer is to conduct Back to Back Test or Sumpner’s Test. It shall be noted here that, Back to Back Test or Sumpner’s test can also be conducted for calculating the efficiency of a Transformer but it is better to calculate the efficiency of Transformer using Open Circuit Test and Short Circuit Test of Transformer as this will give more accurate result. In this post we will focus on Back to Back Test or Sumpner’s Test of single phase Transformer. The Back to back Test on single phase tr...

Magnetic Oil Level gauge or MOG is provided in a Power Transformer to indicate the level of insulating oil in the Conservator . A decrease in level of Transformer Oil in Conservator is an indication of leak and therefore MOG level is an important parameter to be monitored. Normally all Transformers are provided with an expansion vessel called Conservator , to take care of expansion in the oil volume due to rise in temperature, when the load on the Transformer increases or due to increase in ambient temperature. The oil level in the Conservator  consequently goes up and conversely it fall when temperature or load reduces. It is essential that oil level in the conservator tank to be maintained at a pre-determined minimum level. Therefore all large Transformers are fitted with a Magnetic Oil Level Gauge, which are incorporated with mercury switches which operates when oil level in the Conservator goes below minimum level and generate an alarm or Trip contact. You may also like to re...

As we know that Transformer   Open Circuit Test is conducted on LV side and that Short Circuit Test on HV side. To understand the reason behind this, we will consider a Single Phase Transformer of rating 3300  / 220 Volt, 33 kVA. Thus the voltage at the LV side of this Transformer is 220 Volt. Therefore, for Open Circuit Test on LV side the range of Voltmeter will be 220 V. Now, Full Load Current  = (33x1000) / 220 = 150 A So, excitation current = 4% of Full Load Current                                    = 0.06x150 = 6 A Here note that Excitation Current is taken as 4% of the Full Load Current as the range of excitation current is 2-6%. Also, the range of Wattmeter will be 220 V and 6A. We see that the rating of instruments required for the testing are standard and easily available. Furthermore, usin...

Open Circuit Test and Short Circuit Tests are two important tests which are carried out on a Transformer to determine its equivalent circuit parameters, Voltage Regulation and Efficiency. Short Circuit Test   Short Circuit Test of Transformer is performed on HV side and the supply voltage is so adjusted that rated current flows through the shorted secondary. As rated current is flowing through the shorted secondary that means that rated current will also flow in the primary because of Transformer action. The supply voltage required for the flow of rated current in the shorted secondary is around 2-12% of rated voltage. Thus the supply voltage i.e. primary voltage is very less which in turn means that core losses during short circuit test will be negligible (as core loss is directly proportional to the square of primary voltage.). Thus short circuit test gives us information of Ohmic loss of Transformer and the power measured by the Wattmeter is Ohmic loss. An equivalent circuit of...

Open Circuit Test and Short Circuit Tests are two important tests which are carried out on a Transformer to determine its equivalent circuit parameters, Voltage Regulation and Efficiency. Though theses parameters can also be found using the physical dimension of Core and Winding detail but using Open Circuit and Short Circuit Tests are quite easy and simple. The circuit diagram for Open Circuit Test is shown below. As clear from the figure below, Voltmeter, Ammeter and Wattmeter are connected in LV side of the Transformer and HV side is left open. Rated LV voltage is applied to the LV side of the Transformer and the reading of Voltmeter, Ammeter & Wattmeter is noted for further analysis. As the Transformer Secondary i.e. HV side is kept open therefore Transformer will only take excitation current to set up magnetic flux in the core. Therefore Ammeter A will read Excitation Current Ie which is around 2-6% of the full load current. As Ie is very less therefore, Primary leakage imped...