Protection of LV/LV transformers

These 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 method of earthing for 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 transformers are generally supplied with protective systems incorporated, and the manufacturers must be consulted for details. Overcurrent protection must, in any case, be provided on the primary side. The exploitation of these transformers requires a knowledge of their particular function, together with a number of points described below.

Note: 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

At the moment of energizing a transformer, high values of transient current (which includes a significant DC component) occur, and must be taken into account when considering protection schemes (see Fig. N33).

Fig. N33 – Transformer-energizing inrush current

The magnitude of the current peak 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
Characteristics of the load connected to the transformer

The first current peak can reach a value equal to 10 to 15 times the full-load r.m.s. current, but for small transformers (< 50 kVA) may reach values of 20 to 25 times the nominal full-load current. This transient current decreases rapidly, with a time constant θ of the order of several ms to severals tens of ms.

Protection for the supply circuit of a LV/LV transformer

The protective device on the supply circuit for a LV/LV transformer must avoid the possibility of incorrect operation due to the magnetizing inrush current surge, noted above.It is necessary to use therefore:

Selective (i.e. slighly time-delayed) circuit-breakers of the type Compact NSX with electronic trip-unit (see Fig. N34) or

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

Circuit-breakers having a very high magnetic-trip setting, of the types Compact NSX or Acti 9 curve D (see Fig. N35)

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

Example

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, i.e. 12 x 180 = 2,160 A.

This current peak corresponds to a rms value of 1,530 A.

A compact NSX250N circuit-breaker with Ir setting of 200 A and Im setting at 8 x Ir would therefore be a suitable protective device.

A particular case: Overload protection installed at the secondary side of the transformer

(see Fig. N36)

Fig. N36 – Example

An advantage of overload protection located on the secondary side is that the short-circuit protection on the primary side can be set at a high value, or alternatively a circuit-breaker type MA (magnetic only) can be used. The primary side short-circuit protection setting must, however, be sufficiently sensitive to ensure its operation in the event of a short-circuit occuring on the secondary side of the transformer.

Note: The primary protection is sometimes provided by fuses, type aM. This practice has two disadvantages:

The fuses must be largely oversized (at least 4 times the nominal full-load rated current of the transformer)
In order to provide isolating facilities on the primary side, either a load-break switch or a contactor must be associated with the fuses.

Typical electrical characteristics of LV/LV 50 Hz transformers

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
Full-load losses (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
Full-load losses (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
Full-load losses (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