It comprises theme-based units in which the English language is presented in an active context. The book has been written based on the needs and topics relevant to students at C1 and C2 levels. Reactivate your Grammar & Vocabulary for C1/C2 examspresents grammar and vocabulary in a clear and concise way. a section dedicated to English idioms after every three units.a variety of vocabulary exercises covering word formation, phrasal verbs, prepositions and contextualised lexical items.a variety of grammar exercises that provide students with ample practice.clear and simple grammar explanations with an emphasis on examples.Reactivate your Grammar & Vocabulary for C1/C2 exams features: Students are able to consolidate the grammatical structures and vocabulary required for the C1/C2 level examinations by means of a range of exam-based tasks as well as generic exercise types. Reactivate your Grammar & Vocabulary for C1/C2 exams presents grammar and vocabulary in a clear and concise way. The frequency range for tan delta test is generally from 0.1 to 0.01 Hz depending upon size and nature of insulation.Reactivate your Grammar & Vocabulary for C1/C2 So to keep the power requirement for this dissipation factor test, very low-frequency test voltage is required. Therefore, required apparent power for tan delta test would become high enough which is not practical. At high frequency as capacitive current, is large, the amplitude of the vector sum of capacitive and resistive components of electric current becomes large too. The resistive component is nearly fixed it depends upon applied voltage and conductivity of the insulator. If the frequency of the applied voltage is high, then capacitive reactance of the insulator becomes low, hence the capacitive component of electric current is high. A loss angle analyzer is connected with tan delta measuring unit to compare the tan delta values at normal voltage and higher voltages and analyze the results.ĭuring the test, it is essential to apply test voltage at a very low frequency. The tan delta controller unit takes measurement of tan delta values. If the value of tan delta appears good enough, the applied voltage is raised to 1.5 to 2 times of normal voltage, of the equipment.
A very low-frequency test voltage is applied across the equipment whose insulation to be tested.įirst, the normal voltage is applied. The cable, winding, current transformer, potential transformer, transformer bushing, on which tan delta test or dissipation factor test to be conducted, is first isolated from the system. Now, from the diagram above, it is cleared, the ratio, I R to I C is nothing but tanδ or tan delta. Now, total leakage electric current I L(I c + I R) makes an angle δ (say) with y-axis. resistive component of leakage current, I R will also be along x-axis.Īs the capacitive component of leakage electric current I C leads system voltage by 90 o, it will be drawn along y-axis. In the vector diagram above, the system voltage is drawn along the x-axis. Sometimes it is also referred to as dissipation factor. This ratio is commonly known as tanδ or tan delta. For good insulator, this ratio would be quite low. In another way, the healthiness of an electrical insulator can be determined by the ratio of the resistive component to the capacitive component. Hence, it is needless to say that, for good insulator, this resistive component of electric leakage current is quite low.
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