UNITIII
UNITIII
UNITIII
INTRODUCTION
NEED FOR A.C. SYSTEMS AND A.C. GENERATION FOR TESTING: Generation of Electrical Power in large Hydro/ Turbo Generators (60 MW, 100MW, 200 MW, 500 MW) is with a.c. only. Since, d.c. generators have limitation of rating due to commutator problems and it is not economical to use d.c. generators for large power stations. Transmission of Electrical Power over Long Distance is by EHVAC Transmission System (220kV, 400kV etc) Transmission of large blocks of power (500 MW, 1000 MW) requires high voltages. The electric power (P) transmitted on an overhead a.c.line increases approximately with the surge impedance loading (or) the square of the system voltage. (P= V2 / ZL). Figure shows an overview of the voltage levels and the duration of these different types of over voltages.
BY V.BALAJI, AP/EEE, DCE 3
INTRODUCTION
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Lightning Overvoltages
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O v ervo lta g e (p .u .)
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Switching Overvoltages
Operating
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Operating Overvoltage s
1E-01 1E+00 1E+01 1E+02 1E+03 1E+04 1E+05 1E+06
1E-07
1E-06
1E-05
1E-04
1E-03
1E-02
Duration (s)
INTRODUCTION
Every type of over voltages corresponds to special tests, which are defined in different International and National Standards. In order to evaluate the performance and the ability of power apparatus to withstand continuous system voltage (50 Hz) and short time over voltages (LI and SI), they have to be tested either at the factory or at HV Test Laboratories as per the relevant Standards. Since, alternating voltages can be easily stepped up and stepped down by means of Power Transformers bulk power transfer and distribution is generally by means of EHVAC system. For testing of Power Apparatus of such HVAC and EHVAC system voltages, AC & DC Test Generation Equipment are BY V.BALAJI, AP/EEE, DCE required for ascertaining their satisfactory operation. 5
INTRODUCTION
NEED FOR D.C. SYSTEMS AND D.C. GENERATION FOR TESTING: HVDC systems are used for long distance (above 800km) high power (above 1000 MW) transmission systems. For asynchronous Ties As Back to Back Stations Underground or Submarine Cabling The other applications of HVDC systems are for electrostatic precipitation (ESP in Process Industries as HVR High Voltage Rectifiers), Electrostatic Painting (HCR- High Current Rectifiers), Medical Equipment (X Rays). For testing of Power Apparatus of such HVDC systems, DC Testing Equipments are required for ascertaining their satisfactory operation/ functioning.
BY V.BALAJI, AP/EEE, DCE 6
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RECTIFIER CIRCUITS USED FOR HV DC GENERATION At F and beyond, since the supply voltage is greater
capacitor voltage , diode D starts Vmax. Thus, while each pulse of diode current lasts less than a half cycle, the load receives current more continuously from C. During one period T = 1/f of the a.c. voltage, a charge Q is transferred to the load RL. Q = T iL (t)dt = T VRL(t) / RL dt = IT = I/f where I is the mean value of the d.c. output iL(t) and VRL(t) the d.c. voltage. It is also evident that this charge is supplied by the capacitor over the period T when the voltage changes from Vmax to Vmin over approximately T. (neglecting the conduction period of the diode) If the voltage of the capacitor is V and it decreases by an amount dV in time dt, then charge delivered by the capacitor is dQ = CdV dQ = CdV = -C (Vmax- Vmin) Q= C(Vmax- Vmin) Q= 2 VC V= IT/2C= I/2fC
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Allilones Circuit
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Cockroft-Walton Circuit
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Contd.,
Thus in a 3 stage generator, Vn = 3I/fC V2 = {2*3+(3-1)}I/fC V3 = {2*3+2*2+1}I/fC Hence in an n stage generator V1 = nI/fC Vn-1 = I/fC{2n+(n-1)} Vn-2 = I/fC{2n+(n-1)+(n-2)} Similarly, V1 = I/fC{2n+2(n-1)+2(n-2)+ +2*3+2*2+1} V = Vn + Vn-1 + + V1
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The last terms of all the terms are added first. Likewise the last term of the remaining term is added. [n+(n-1)+(n-2)++2+1]+ [2n+2(n-1)+2(n-2)++2*2]+ [2n+2(n-1)++2*4+2*3]+ [2n+2(n-1)++2*4]+ [2n+2(n-1)+2(n2)++2*5]+[2n] Hence rearranging the terms and simplifying we have T=n + 2(n-1) n (n2 n) = 2n n n2 (Sum of squares of the first n natural numbers) T = 2n{n(n+1)/2}{n(n+1)(2n+1)/6}
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ELECTROSTATIC GENERATOR
An isolated sphere is the most favorable electrode shape and will maintain a uniform field E with a voltage of Er where r is the radius of the sphere. As the h.t. electrode collects charges its potential rises. The potential at any instant is given as V = q/C where q is the charge collected at that instant. The moving belt also distorts the electric field and therefore, it is placed within properly shaped field grading rings.
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CASCADED TRANSFORMER
High voltage transformers for testing purposes are designed purposely to have a poor regulation. This is to ensure that when the secondary of the transformer is short circuited (as will commonly happen in flash-over tests of insulation), the current would not increase to too high a value and to reduce the cost. In practice, an additional series resistance (commonly a water resistance) is also used in such cases to limit the current and prevent possible damage to the transformer.
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Resonant Transformers
The resonance principle of a series tuned L-C circuit can be made use of to obtain a higher voltage with a given transformer
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Trigatron gap
The third sphere arrangement described for the trigger arrangement is not very sensitive. A better arrangement is to have an asymmetrical gap arrangement. The trigatron gap is such an arrangement which is in general use.
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OR
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