ADVANCED LIGHTNING CURRENT GENERATORS

Lightning current impulse circuit researches have u sed various schematics for diverse impulses, which makes several problems for lightnin g current impulse generator fabrication with a suit able cost. In addition, errors of several lightning current impul se math models have not met the standards. This wor k presents solutions to determination of parameters for a spec ific lightning current impulse circuit and a lightn ing current impulse math model which is in Matlab environment w ith high accuracy.


Researching on effects of lightning current
impulses is important to selection of lightning-strokeprotective devices and overvoltage calculation on grid. Lightning current circuit researches have applied various schematics for diverse impulses, which makes several issues for lightning current impulse generator fabrication with a reasonable price. Furthermore, some of proposed lightning current impulse generator physical models have the front and half-value errors greater than the standard ones [2]. Therefore, it is necessary to research and propose a lightning current impulse generator model generating various wave shapes with high accuracy and suitable price.
This work presents the approximate method of quickly calculating basic parameters of lightning current impulse generator and the error-evaluating method of correcting the front error and the halfvalue error as the standards.
In addition, lightning current impulse math models for wave shapes 8/20µs and 4/10µs are proposed in Matlab environment. [3].   can be rewriten as equation (2): To achieve a standard lightning current, parameters p and t 2 must be selected correctly. Then based on equations (3) and (4), the resistance, inductance and capacitance of the circuit can be estimated.
( The equation (2) shows that functions and generate front and tail wave shapes, respectively.
When applied with the approximate method, the wave shapes can be presented in Figure 3.
In the period of tail time, it is assumed that . Equation (2) can be rewritten as below: Therefore, i(t) = 0,5.I m if t = t s -t ñs . So: Hence: Similarly, in the period of front time, it is assumed that current can be present as below: The amplitude of current reaches 0.1I m at t 10% and 0.9I m at t 90% .Therefore: Solving the system equations above, the parameter p can be estimated as equation (6): Based on equation 5 and 6, the result of estimating the parameters is shown in Where: t a is the estimated front time, t a =1.25*(t 90% -t 10% ); t b is the estimated half value time, t b =t 50% -t 10% +0.1t a .
It is assigned e 1 , e 2 and e are front error, halfvalue error and accumulated error, respectively.
Parameters p and t 2 must reach the conditions (7) and (8): Among values (p,t 2 ) passing conditions (7) and (8), the values reaching the condition "d=0" mean the errors pass the standard. If "d=0"-reaching values are available, the value with the minimum accumulated error is the best option. In the case that no value (p,t 2 ) supports condition "d=0", the value having the minimum accumulated deflection will be chose as the best option.
Based on the error-evaluating method, round values R, L and C are presented in Table 3. Through this result, wave shapes 8/20µs and 4/10µs are only two conditions not achieving the requirements, and the errors are lower than proposed models [2].

Heidler equation
Heidler equation is one of equations that used to express lighting current impulses [4]: Where: I m is peak current (kA); is increasing current time coefficient (µs); is decreasing current time coefficient (µs) ; µ is peak-current-adjusting coefficient.
Applied with the approximate method, in the period of front time, it is assumed that .

TAÏ P CHÍ PHAÙ T TRIEÅ N KH&CN, TAÄ P 14, SOÁ K4 -2011
Trang 89 Solving the systems of equation (10), parameter can be estimated base on equation (11): Similarly, in the period of tail time, assume that , the Heilder equation can be rewritten as below: At t= t s -t ds the current value reaches a half of the peak value. Hence: Errors of the lightning currents based on equation (10) and (11) are shows in Table 4. Through this   Correcting flowchart is presented in Figure 4, and