Protection of LV/LV transformers

LV/LV transformers are generally in the range of several hundreds of VA to some hundreds of kVA and are frequently used for:

Changing the low voltage level for:
- Auxiliary supplies to control and indication circuits
- Lighting circuits (230 V created when the primary system is 400 V 3-phase 3-wires)
Changing the earthing system for certain application such as safety services, medical IT, or certain loads having a relatively high capacitive current to earth (computer equipment) or resistive leakage current (electric ovens, industrial-heating processes, mass-cooking installations, etc.)

LV/LV transformer shall be protected against short-circuit and overloads.

When energized, a LV/LV transformer draws a very high inrush current. This very high inrush current (and its time/current characteristics) should be taken into account when selecting the overcurrent protection, to avoid nuisance tripping.

Notes:

LV/LV transformers may be supplied with embedded protective systems, such as over-temperature sensors. Refer to transformer manufacturer documentation.
In the particular cases of LV/LV safety isolating transformers at extra-low voltage, an earthed metal screen between the primary and secondary windings is frequently required, according to circumstances, as recommended in European Standard EN 60742.

Transformer-energizing inrush current

When energized, LV/LV transformers draw very high inrush currents which must be taken into account when choosing overcurrent protection devices. (see Fig. N33)

The peak value of the first current wave often reaches 10 to 15 times the rated rms current of the transformer, and may even reach values of 20 to 25 times the rated current for transformers < 50 kVA. This transient current decreases rapidly, with a time constant 𝜏 of the order of several ms to severals tens of ms.

This can be compared to the maximum short-circuit current, which is the nominal current divided by the short circuit impedance voltage of the transformer in %. For example, when the short circuit impedance voltage is 5%, the short circuit current is In / 5% = 20 In, e.g. the same order of magnitude as the transformer energizing inrush current. The main difference is that the inrush current when energizing decreases very fast.

Fig. N33 – Transformer-energizing inrush current

Note: the actual magnitude of the current peak can vary a lot, and depends on:

The value of voltage at the instant of energization
The magnitude and polarity of the residual flux existing in the core of the transformer
The characteristics of the load connected to the transformer

Protection for the supply circuit of a LV/LV transformer

The selection of the protective device for the supply circuit of a LV/LV transformer must avoid the possibility of incorrect operation due to the inrush current when enegizing the transformer, described above. It is therefore necessary to use:

Selective (i.e. slighly time-delayed) circuit-breakers, such as Compact NSX with electronic trip-unit

(see Fig. N34)

Fig. N34 – Tripping characteristic of a Compact NSX with electronic trip-unit

Where:

I_n = nominal current of the transformer,
I_r = Long Time Protection (overload) setting of the circuit breaker
I_sd, t_sd = Short-Time Protection (short-circuit) setting of the circuit breaker, which should be selected according to the time/current characteristics of the transformer energizing inrush current,
I_i = Instantaneous Protection (short-circuit) setting of the circuit-breaker

Circuit-breakers having a very high magnetic-trip setting, such as Compact NSX with TMD thermal-magnetic trip unit or Acti 9 curve D

(see Fig. N35)

Fig. N35 – Tripping characteristic of a Acti 9 curve D

Example

(see Fig. N36)

A 400 V 3-phase circuit is supplying a 125 kVA 400/230 V transformer (In = 180 A) for which the first inrush current peak can reach 12 In (value provided by the transformer manufacturer), i.e. 12 x 180 = 2160 A.

This current peak value corresponds to a thermal equivalent rms value of 2160 / √2 = 1530 A.

A Compact NSX250N circuit-breaker with I_r setting of 200 A and I_sd setting at 8 x I_r (= 1600 A) would therefore be a suitable protective device.

Fig. N36 – Example

Typical electrical characteristics of LV/LV 50 Hz transformers

These values are given as an example, always refer to the manufacturer's technical data.

3-phase (≤ 80A)
kVA rating	5	6.3	8	10	12.5	16	20	25	31.5	40	50	63	80
No-load losses (W)	100	110	130	150	160	170	270	310	350	350	410	460	520
Load loss at rated power (W)	250	320	390	500	600	840	800	1180	1240	1530	1650	2150	2540
Short-circuit voltage (%)	4.5	4.5	4.5	5.5	5.5	5.5	5.5	5.5	5	5	4.5	5	5

3-phase (≥ 100A)
kVA rating	100	125	160	200	250	315	400	500	630	800
No-load losses (W)	570	680	680	790	950	1160	1240	1485	1855	2160
Load loss at rated power (W)	3700	3700	5900	5900	6500	7400	9300	9400	11400	13400
Short-circuit voltage (%)	5.5	4.5	5.5	5	5	4.5	6	6	5.5	5.5

1-phase
kVA rating	8	10	12.5	16	20	25	31.5	40	50	63	80	100	125	160
No-load losses (W)	105	115	120	140	150	175	200	215	265	305	450	450	525	635
Load loss at rated power (W)	400	530	635	730	865	1065	1200	1400	1900	2000	2450	3950	3950	4335
Short-circuit voltage (%)	5	5	5	4.5	4.5	4.5	4	4	5	5	4.5	5.5	5	5