Earthing transformer for power system

It is mandatory, within industrial power systems, to make use of a protection system designed with the scope of dealing with the faults that could happen on the power line during the service life on any of the installed appliances.

In the moment a short circuit appears on the power supply, the immediate consequence is the generation of a really high current, which may be hard to control and drive towards the ground without causing too much damage around the system. These currents have to be interrupted as soon as possible, and need to be immediately deviated to a safe sink while the deputed protections (switches, breakers, fuses and so on) are not active yet: the scope of this is in first instance to protect the personnel, and, as second goal, to preserve the appliances that have not been interested by the fault in the first moments of its occurrence.

FDUEG can tackle all these needs with its offer of neutral earthing (or grounding) transformers (NET). These devices are explicitly designed to be installed in industrial plants where, through a medium voltage power line, any power level is carried around the facility. They respond with a very small impedance in case fault currents appear, and direct them towards the ground so that they can be almost completely removed from the appliances. In the meantime, the protection devices along the line may have enough time to operate.

FDUEG designs MV earthing transformers (grounding transformers) for power systems, constructive features

A neutral earthing transformer is characterized by two windings, of which the primary can be either connected as zig-zag or star with neutral; this usually can be loaded by a medium voltage level, with typical values of 11 kV, 15 kV or 20 kV. The star point that results can be connected to ground directly or through an impedance; this last one can be either a simple resistor, rated for the current and voltage levels that appear in case of fault, or coil whose inductive value is tuned to resonate with the distributed capacitance at the system’s frequency (Petersen coil).

The secondary winding, on the other hand, can sport the following connections:

• Star, if the primary is zig-zag connected with an impedance between star point and ground. The secondary side can be given the possibility to carry some amount of continuous power to auxiliary equipment or local appliances;
• Open delta, closed on a resistor rated for low voltage, to act as homopolar impedance. In case of fault, this parameter is reflected on the primary side through magnetic coupling, and it is magnified by the transformer’s turn ratio.

Exploiting any of these allows to attain a control upon fault currents, reducing them as requested by technical specifications and design. Both connection configurations can be set up with any vector group, provided it is available for the kind of connection that has been used.

Based on the setup adopted for the electrical configuration of the windings, neutral earthing transformers are available with three- or five-columns magnetic core (for the latter, only the three inner columns are actually wound). From a point of view related to mechanical containment and cooling, FDUEG proposes dry type vacuum pressure impregnated (VPI) transformers; as alternative, they can be immersed in mineral or vegetable oil, inserted in a customized cabinet, compliant to IP65 degree, that can be treated according to the needs of the installation environment.

All FDUEG products satisfy the relative IEC standards for transformers.

Critical aspects during the operations

Apart from the behavior of the device in case of fault on the power line, this kind of transformer can be used to make available a grounding connection (if the voltage level allows it) to insert single-phase loads. Given the vector group of the primary windings, the start point will also be relatively stable in case of asymmetric power supply in the feeding lines.

A neutral earthing transformer can be given a continuous rating, to keep into account third harmonics (and multiple of three) and the homopolar current it is constantly interested by. They will find a path to be used to reach the ground, without overloading the power supply or other loads present in the system. By limiting these disturbance currents, the operational temperatures decrease, reducing in turns the progressive degradation of the insulating material present on the windings.

It is only for really short periods that a neutral earthing transformer has an important role on the line where it is installed, especially if compared with the duration of its service life. Therefore, it has to sport the least possible footprint, also considering the usually limited available area in a typical industrial facility.

On the other hand, a transformer of this kind has to sustain extremely quick load variations as soon as a line fault appears: this implies that there is a need to exploit a product whose mechanical stability is remarkable, together with a long thermal time constant; this last parameter makes it resilient, from a heat management point of view, with respect to fault currents through their windings.

Another important feature for neutral earthing transformers involves their resistance to chemically aggressive environments, since they are usually exploited in industrial plants. It has to be considered how this kind of device needs to maintain its ideal working condition for the whole service life of the plant where it is inserted.

Efficiency-wise, a neutral earthing transformer is basically always fed with its rated voltage, thus it generates no load losses as any other transformer. The design proposed by FDUEG aims at reducing this continuous power consumption (which in turns diminishes the overall costs), while also ensuring that the magnetic core always works below its saturation threshold.

Rating parameters

FDUEG designs, manufactures and delivers neutral earthing transformers suitable for power lines with 36 kV voltage rating. Higher voltage levels can be analyzed if needed, assisting the customer throughout the whole design phase.

The homopolar impedance is tailored so that the transformer limits the current to the requested value, also considering the impedances installed (either at the star point or in the open delta). The tolerance imposed on the final value of this parameter is such that the maximum tolerated current is never exceeded.

All transformers produced for this purpose are studied to resist an indefinite number of fault instances, without interrupting their service or reducing their performance while connected to the line. Especially for oil immersed models, special cabinets are available with enhanced resistance to chemical action of an environment or particular mechanical stresses.