The LV installation which forms the subject of this study is shown in figure 1 above. This study includes discrimination and coordination of protection between the LV equipment and the protection located upstream of each MV/LV power supply transformer.
The selections refer to Schneider Electric circuit breakers (Masterpact NW and Compact NSX type).
The installation includes 2 medium voltage 20 kV incoming lines protected by a fuse, each equipped with an MV/LV transformer with characteristics 20 kV/410 V, 1600 kVA, and an incoming LV circuit breaker (A) or (B). A section switch (C) can be used to operate both parts of the installation together or separately, in order to optimize availability of power in the event of failure of one of the two incoming lines.
- Dimensioning the protective equipment
- Selecting devices to ensure discrimination
- Discrimination between circuit breakers (F) and (D)
- Discrimination between circuit breakers (G) and (E)
- Discrimination between circuit breakers (E) and (C)
- Discrimination between circuit breakers (D) and (C)
- Discrimination between circuit breakers (C) and (B) or circuit breakers (C) and (A)
- Discrimination between circuit breakers (A) or (B) and MV fuses
1. Dimensioning the protective equipment
Determining the nominal current for the LV incoming lines:
1600 kVA at 410 V corresponds to a nominal current of 1,600,000 / (410 × √3) = 2253 A.
Incoming devices with a rating of 2500 A are therefore chosen.
The nominal current for the MV incoming lines is:
In = 1,600,00 / (20 000 × √3) = 46 A
1.3 Breaking capacity for the various devices
Each transformer has a short-circuit current Isc equal to 36 kA (current linked to the transformer power and short-circuit voltage). When the section switch is closed, downstream of devices (A) and (B) and ignoring the busbar impedances, the short-circuit current is 2 × 36 = 72 kArms
Given the cable impedances, the short-circuit current crossing the circuit breakers located at (F) and (G) is no more than approximately 50 kA.
The breaking capacity required for each device must be determined according to the short-circuit current values at different points in the installation.
- CBs (D) and (E) must have a breaking capacity higher than 72 kA,
- CBs (A), (B) and (C), a breaking capacity higher than 36 kA is adequate.
- CBs (F) and (G) must have a breaking capacity of 50 kA minimum.
2. Selecting devices to ensure discrimination
Discrimination is determined by comparing the characteristics of each circuit breaker with those of the protective device (circuit breaker or fuse) located immediately upstream.
Once the characteristics of these circuit breakers have been established, one can work back up the installation, ensuring discrimination between circuit breakers 2 by 2 (downstream circuit breaker/upstream circuit breaker).
- At F: In = 185 A; Isc = 50 kA
A circuit breaker with a rating of 250 A is suitable, for example a Compact NSX 250 H (breaking capacity 70 kA at 415 V).
- At D: In = 700 A; Isc = 72 kA
A circuit breaker with a rating of 800 A is suitable, for example a Compact NSX 800 L or a Masterpact NT 08 L1 (breaking capacity 150 kA at 415 V).
- Discrimination mechanism
Device (F) is very limiting (the maximum current which can cross it is 22 kApeak for a prospective short-circuit of 50 kArms), and this circuit breaker therefore allows “pseudo-time” discrimination with circuit breaker (D).
This discrimination is improved by applying the “SELLIM” principle (discrimination principle which allows both discrimination and current limitation.) to circuit breaker (D). This device, which is also limiting (with low EDW to ensure very good current limiting), enables total discrimination between (F) and (D) since, according to the SELLIM principle of discrimination, the device on (D) does not trip on the 1st current wave.
Note: – The “SELLIM” function is systematically included in Micrologic – Schneider Electric trip units, and automatically activated on the devices concerned.
- At G: In = 330 A; Isc = 50 kA
A circuit breaker with a rating of 400 A is suitable, for example a Compact NSX 400 H (breaking capacity 70 kA at 415 V).
- At E: In = 750 A; Isc = 72 kA
The same circuit breaker (current limiting) can be used as for (D), but since the NSX 400 H current limiting is weaker than on the NS 250 H, this combination will not be totally discriminating.
To achieve this discrimination, a selective circuit breaker must be selected, for example Masterpact NW 10 H2 (In 1000 A, breaking capacity 100 kA at 415 V, Icw = 85 kArms /1 s). In addition, the current limiting power of device (G) enables, if necessary, pseudo-time discrimination.
- Discrimination mechanism
Since the Icw (85 kA) is less than the breaking capacity (100 kA), this device has an instantaneous self-protection release (DIN) with a threshold of 170 kApeak.
With an Isc = 72 kArms, the maximum current at (E) is 72 × 2.3 = 165 kApeak. Since the DIN threshold is therefore never reached, no trip will be generated which would hinder discrimination. Moreover, in the event of a short-circuit at (G), the maximum current, which corresponds to an Isc of 50 kA, will be limited for (G) to 30 kApeak!
Discrimination will therefore be total, as long as device (E) is fitted with a trip unit with an instantaneous threshold higher than 30 kApeak, say 30/r = 21 kArms = 21 In, and that the short-time release delay is set on the 0.1 s band.
This discrimination is not essential if both incoming lines are operational (since opening of the section switch does not interrupt the power supply via (A) and (B)). Conversely, it is essential if incoming line (B) is non-operational.
- Value of the nominal current In at (C):
To offer the maximum flexibility, the section switch devices have identical dimensions to the incoming devices, ie. In = 2500 A.
As Isc = 36 kA, a selective circuit breaker placed at (C) allows time discrimination with (E) and even more with (D) which is current limiting, for example a Masterpact NW 25 H1 (In 2500 A, breaking capacity 65 kA at 415 V, Icw 65 kA/1 s).
- Reason for this selection
Since the Icw for the device equals the breaking capacity, it does not incorporate an instantaneous self-protection release. Time discrimination can therefore be applied without restriction up to the breaking capacity.
The circuit breaker (C) must therefore be fitted with a selective trip unit, with its instantaneous release set to the “Off” position, and the short-time delay on the 0.2 s band (since the short-time release delay of circuit breaker (E) is set on the 0.1 s band).
The solution chosen for discrimination between (E) and (C) is also suitable between (D) and (C) since (C) is totally discriminating up to its breaking capacity.
(A) and (B) are selective devices, without a self-protection instantaneous release. Here too, time discrimination applies up to the breaking capacity, with for (A) and (B): Their instantaneous release set to the “Off” position and their short-time delay set on the 0.3 s band (since the short-time release delay of circuit breaker (C) is set on the 0.2 s band).
To analyze this discrimination, we need to compare the trip curves for LV circuit breakers and MV fuses. To do this, transpose the MV fuse curve to LV, by multiplying the current scale by the transformer ratio, or here 20,000/410 = 48.8 (see figure 2).
Discrimination is considered with 2 types of trip unit:
- Standard selective trip unit, and
- Trip unit with IDMTL curves.
- Long-time threshold
No problem, the non-tripping limit current for the fuse is well above the limit current for circuit breaker tripping. The long-time threshold can therefore be set to maximum (ie. Ir = In).
Long-time delay and short-time threshold
The blowing characteristic for MV fuses has a much steeper slope than that of the long-time delay release (LT) tripping, with a slope of I2t (see fig. 2). To avoid the curves intersecting, the long-time delay (tr), or short-time threshold (Isd), must be set to sufficiently low values.
A good compromise in this example consists of setting tr = 12 s (at 6 Ir, in a range generally going from 1 to 24 s), and Isd = 4 Ir (in a range from 1.5 to 10 Ir).
These values allow the passage of peak currents at switch-on or starting currents for loads located downstream without false tripping. A detailed study needs to be undertaken on these loads. With a higher short-time delay threshold, 5 Ir for example, tr should be reduced to 4 s.
- Short-time delay
As the short-time delay is set on the 0.3 s band, to ensure discrimination with the devices downstream as indicated above, the fuse and circuit breaker curves cross at around 10 In (see fig. 2). Discrimination between the circuit breaker and the MV fuse is therefore limited to approx. 25 kArms, for a maximum short-circuit current Isc of 36 kArms.
With these trip units, it is possible to select the slope of the long-time curve. In this case, we can opt for the “HVF” (High Voltage Fuse) slope, which is the closest to that for the fuse (slope of I4t).
With a delay at 6 Ir of 2s, better immunity to high transient currents (peak currents at switch-on or starting) is possible, in the zone for currents between 5 and 10 Ir, since the short-time threshold can be set at any desired value up to 10 Ir (see fig. 2).
Reference // Discrimination with LV power circuit breakers by JP. Nereau (Schneider Electric)