Medium Voltage technical guide - Schneider Electric

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Switchgear defi nition Medium voltage circuit breaker Example 1: b For a circuit breaker with a minimum opening time of 45 ms (Top) to which we add 10 ms (Tr) due to relaying, the graph gives a percentage of the aperiodic component of around 30% for a time constant τ1 = 45 ms: %DC = e – (45 + 10) 45 = 29.5% Example 2: b Supposing that %DC of a MV circuit breaker is equal to 65% and that the symmetric short-circuit current that is calculated (Isym) is equal to 27 kA. What does Iasym equal? Iasym = Isym 1 + 2 %DC 100 b Using the equation [A], this is equivalent to a symmetric short-circuit current at a rating of: 36.7 kA 1.086 = 27 kA 1 + 2 (0.65) 2 = 36.7 kA = 33.8 kA for a %DC of 30%. b The circuit breaker rating is greater than 33.8 kA. According to the IEC, the nearest standard rating is 40 kA. 2 [A] DE59050EN DE59051EN b Automatic reclosing cycle Assumption: C order as soon as the circuit breaker is open, (with time delay to achieve 0.3 s or 15 s or 3 min). Closed position Contact movement Current flow Current flow Energizing of opening release Open position Dead time Open-close time Remake time Reclosing time Final arc extinction in all poles Separation arcing contacts in all poles Energizing of closing circuit Rated short-circuit breaking current (cf. § 4.101 IEC 62271-100) Contact touch in all poles Contact touch in the first pole Start of current in the first pole 52 AMTED300014EN.indd Time The rated short-circuit breaking current is the highest value of current that the circuit breaker must be capable of breaking at its rated voltage. b It is characterised by two values: v the r ms value of its periodic component, given by the term: “rated short-circuit breaking current” v the percentage of the aperiodic component corresponding to the circuit breaker’s opening time, to which we add a half-period of the rated frequency. The half-period corresponds to the minimum activation time of an overcurrent protection device, this being 10 ms at 50 Hz. b According to IEC, the circuit breaker must break the rms value of the periodic component of the short-circuit (= its rated breaking current) with the percentage of asymmetry defined by the graphs below. Percentage of the aperiodic component (%DC) as a function of the time interval (t) %DC 100 90 80 70 4 = 120 ms 60 (special case time constant) 50 40 30 20 1 = 45 ms 10 (standardised time constant) t (ms) 0 10 20 30 40 50 60 70 80 90 t: circuit breaker opening duration (Top), increased by half a period at the power frequency (Tr). b As standard the IEC defines MV equipment for a time constant of 45 ms, for a peak value of maximum current equal to 2.5 • Isc at 50 Hz or 2.6 • Isc at 60 Hz. In this case use the τ1 graph.
DE59052 Switchgear defi nition Medium voltage circuit breaker I (A) IMC AMTED300014EN.indd IDC IAC t (s) b For low resistive circuits such as generator incomers, τ can be higher, with a peak value of maximum current equal to 2.7 • Isc. In this case use the τ4 graph. For all time constants τ between τ1 and τ 4, use the equation: – (Top + Tr) %DC = 100 • e τ1, …, 4 b Values of rated short-circuit breaking current: 6.3 - 8 - 10 - 12.5 - 16 - 20 - 25 - 31.5 - 40 - 50 - 63 kA. b Short-circuit breaking tests must meet the five following test sequences: Sequence % Isym % aperiodic component %DC 1 10 ≤ 20 2 20 ≤ 20 3 60 ≤ 20 4 100 ≤ 20 5* 100 According to equation * For circuit breakers opening in less than 80 ms. IMC Making current IAC Periodic component peak value (Isc peak) IDC Aperiodic component value %DC % asymmetry or aperiodic component IDC IAC – (Top + Tr) • 100 = 100 • e τ1, …, 4 b Symmetric short-circuit current (in kA): Isym = IAC 2 b Asymmetric short-circuit current (in kA): Iasym 2 = Isym 2 • I 2 DC Iasym = Isym 1 + 2 %DC 100 Rated Transient Recovery Voltage (TRV) (cf. § 4.102 IEC 62271-100) This is the voltage that appears across the terminals of a circuit breaker pole after the current has been interrupted. The recovery voltage wave form varies according to the real circuit configuration. A circuit breaker must be able to break a given current for all transient recovery voltages whose value remains below the rated TRV. b First pole-to-clear factor For three-phase circuits, the TRV refers to the pole that breaks the circuit initially, in other words the voltage across the terminals of the first open pole. The ratio of this voltage to a single phase circuit voltage is called the first pole-to-clear factor, it is equal to 1.5 for voltages up to 72.5 kV (isolated neutral of the supply circuit). 2 53
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Page 46 and 47: Design rules Protection index DE590
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Page 50 and 51: U 1.0 B 0.9 Rated insulation level
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Page 66 and 67: DE58034 Switchgear defi nition LPCT
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Page 78 and 79: DE59063EN Standards IEC - ANSI/IEEE
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Medium Voltage technical guide - Schneider Electric

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