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NF50-HP Datasheet(PDF) 85 Page - Mitsubishi Electric Semiconductor |
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NF50-HP Datasheet(HTML) 85 Page - Mitsubishi Electric Semiconductor |
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85 / 105 page  84 instantaneous supply voltage (V), according to the phase angle at the instant of circuit closure. This charge results in a large surge current. If the circuit is closed at the peak (Em) of the supply voltage (V), the surge current (i), according to transient phenomena theory, is: 2 Em i = t 2L R – ε C 4L – R2 2L sin t C 4L – R2 From Fig. 7.12, the maximum value (im) is: Em im = R – ε C L R arctan C C 4L – R2 4L – R2 and appears at time t = t0 where: 2L t0 = R arctan C C 4L – R2 4L – R2 Although V is not constant, τ0 is extremely small, so that V = Em can be assumed for the transient dura- tion; similarly, the conduction time can be assumed as 2 τ0. Thus, an MCCB for use in a capacitive circuit must have an instantaneous-trip current of greater than im x 2 τ0. Example: MCCB selection for a 3-phase 230V 50Hz 150 kVA capacitor circuit. From Table 7.4, C = 0.9026 x 10–2 (F) and I = 377(A). The values of R and L in the circuit must be esti- mated, and for this purpose it is assumed that the short-circuit current is approximately 100 times the circuit capacity – i.e., 50,000A. Z = R2 + (2 πfL)2 ∴ 50,000 = 3 Z V thus: Z = = 2.66 x 10–3 3 x 50,000 230 since: Em = obtained from their respective formulas as, V = 188, im and τ0 can be 3 2 and assuming: then: 2 πfL = 2.60 x 10–3 Ω thus: R = 5.21 x 10–4 Ω L = 8.29 x 10–6 (H) = 5 R 2 πfL im =6200A τ0 = 4.27 x 10–4 (sec). Since current-flow duration is approximately 2 τ0, an MCCB is selected with a latching time of 0.001 seconds at 6200A. The Type NF630-SP is suitable, having a latching time of 0.0029 seconds at 10,000A. Even with a shorter latching time, tripping is unlikely under the application of the above current, but selec- tion of an MCCB with an instantaneous-trip current of greater than M2 6200 = 4400A is recommended for an adequate safety margin. Such an MCCB will be rated at 600A. Accordingly, in this example the Type NF630- SP, rated at 600A, is selected. Table 7.4 is a basis for selection, but since, in cases where the short-circuit capacity of the circuit is considerably higher than that of the MCCB, spurious tripping due to the switching surge may occur, it is also necessary to make calcu- lations along the lines of the above example. Em Vc iL R C Fig. 7.11 PF Correction Capacitor Vc i im τo FIg. 7.12 Currents and Voltages 7.7 MCCBs for Thyristor Circuits Both overcurrent and overvoltage protection must be provided for these elements. MCCBs can be used effectively for overcurrent, although application de- mands vary widely, and selection must be made care- fully in each case. Overvoltage protection must be provided separately; devices currently in use include lightning arresters, dischargers, RC filters and oth- ers. 1. MCCB Rated Currents A primary factor determining the rated current of the MCCB to be used is the question of AC-side or DC- side installation. AC-side installation permits a lower rating, which is a considerable advantage. Fig. 7.13 shows both AC and DC installation (MCCBs 1 and 2); Table 7.5 gives a selection of circuit formats and cur- rent configurations; using this table it is possible to determine the MCCB rating for either MCCB 1 or 2, as required. The current curve of the thyristor (aver- age current is usually given) and the tripping curve of the MCCB should be rechecked to ensure that there is no possibility of overlap. When an overcurrent is due to a fault in the load, causing a danger of thermal destruction of the circuit elements, either AC or DC protection is adequate, provided the parameters are properly chosen. When |
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