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earth protection system - neutral grounding

earth protection system - neutral grounding

Since the power transmission network is three-wire, and with regard to the transformer secondary winding connection by triangle, in case of phase-to-ground connection, there is no path for current to flow through the earth back to the source so no fault will be found in the network. This necessitates creating an artificial neutral point. This can be done by connecting three similar coils, connected in star, and earthing their zero point, but the drawback with the design is that if a voltage imbalance exists in three phases, the star neutral point will produce voltage. Of course, it is possible to create balance in the star windings by adding three coils attached in a triangular fashion. The design is employed in some cases, but it is better to use a zigzag transformer. The zigzag transformer form stable neutral with a voltage close to zero. Additionally, the zigzag transformer leg impedance can be calculated in a manner that when the phase is connected to the ground, the current does not exceed a certain value. Therefore, in addition to providing an artificial zero point, the zigzag transformer also restricts the short-circuit current to the ground. By installing a relay on the neutral path, connections to the ground can be detected and controlled.

Neutral point ground resistors (NGRs) are used to connect the transformers and generators neutral point to the ground. These resistors are usually placed between the equipment star neutral point and the ground. The resistors’ main purpose is to limit transient over-voltages that flow through the neutral point of a transformer or generator to a safe degree during a fault event to prevent damage. The fault current is detected via protection relays and are then eliminated.

NGR capacity is determined by the specifications of the network protection system. These specifications can include maximum current, ground potential, and minimizing fault damage. It is designed according to IEEE-32 standard based on resistance network information. The resistor bank consists of a series of detachable parts consistent with the details of the design and the total voltage is distributed across these parts. Finally, all resistors are fully tested in accordance with routine testing procedures. The resistors tank has sufficient creep length regarding the operating voltage, and proper construction allows the resistor to withstand temperatures up to 760 ° C during a ground fault.

Neutral transformer is able to generate a neutral point for another transformer, which is typically the case in three-phase transformers where the neutral point is not available or transformers or generators with a triangular connection are used. Thistype of transformer is called a neutral transformer or a ground transformer. A neutral transformer can include two windings. The primary winding is connected in zigzag and the secondary winding is star-connected. It can also have only one winding and as a three-phase autotransformer have interlinked connections in star-shaped or zigzag windings. These transformers have three-column cores transformer with two balanced windings placed on each core. One side of the primary coil is connected to ground to create a neutral point and the other side is connected to the secondary coil as shown below. Therefore, further study of what a neutral transformer is can help to better understand how the device operates.

MECO manufactures both systems.

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