Power Transformer Neutral Earthing Explained

Whether and how to ground the neutral point of a power transformer is a problem that many people encounter when they first come into contact with a substation or distribution system. In Chinese, it says that the neutral point is grounded. The corresponding English expression is power transformer neutral earthing, which is also often written as power transformer neutral grounding. The two expressions refer to the same thing. Let’s break this down and explain it clearly below.

Where does the neutral point come from

Three-phase transformers are often connected in two ways: star and triangle. The connection methods are different, and the relationship between line voltage, phase voltage, line current and phase current is also different. When connecting in a star shape, the ends of the three windings are connected to the same common point, which is the neutral point; at this time, the line voltage is √3 times the phase voltage, and the line current is equal to the phase current. Triangular connections have no such common points, and therefore no neutral points to draw out.

In power distribution systems, triangle — star is a very common combination. The high-voltage side is connected in a triangle, and the low-voltage side is connected in a star. This provides a neutral point on the low-voltage side and also affects the channel of zero-sequence current and the direction of harmonics. However, it should be noted that not all distribution systems necessarily adopt this connection method.

Why Ground the Neutral Point

The role of a transformer in a power supply system is not only to convert voltage. It may also undertake tasks such as electrical isolation, establishing a neutral point, and impedance conversion. The specific tasks it undertakes depend on its position and structure in the circuit. When the neutral point is introduced, whether to connect to the ground and where to connect to the ground are things that must be determined during the system design stage.

Establishing a reference to ground for the neutral point mainly affects how the fault current flows and how the protection is coordinated. After the neutral point is grounded, the system has a clear grounding reference. In the event of a ground fault, the current can form a low-resistance loop, allowing the protection device to operate accurately. In turn, if the secondary side is not grounded for a long time, it is a floating ground system. The voltage of any phase conductor to ground will be affected by the distributed capacitance, measuring instrument impedance and load connection, and the value is not fixed.

The coupling group designation is crucial here. It expresses the connection and phase displacement of both high and low voltage windings, and is an important basis for determining the parallel connection, grounding, protection and harmonics of the system. So grounding is never as simple as just connecting a wire.

Neutral point grounding and protective grounding are two different things

Two concepts should be distinguished here: neutral point grounding refers to how the working loop ground reference is established, and protective grounding refers to how the transformer housing and the exposed conductive part of the load are connected to the protective conductor. The two are different design dimensions and cannot be confused.

Isolation, floating ground, independent neutral point, and protective grounding, each of these words has its own meaning. For example, for an isolation transformer, its secondary side can remain floating as designed, and a new grounding reference can also be established; whether it is grounded does not change the fact that the primary and secondary windings are physically separated from each other, but it will actually change the path and protection method of the fault current.

On actual equipment, the grounding terminal, neutral point lead-out, shielding terminal and housing protection grounding point should be clearly marked in the drawings and nameplates respectively to avoid mixing functional grounding, shielding grounding and protection grounding. The protective grounding conductor must be connected to the exposed conductive part of the shell and load according to the design. The prescribed protective grounding cannot be removed in pursuit of so-called complete isolation; the grounding disconnection made for noise cancellation will cause the exposed conductive part to lose its low-resistance protection path if a fault occurs.

The grounding point must be determined at one time

Whether the secondary neutral point is grounded and where to place the grounding point must be determined at the system design stage. It cannot be repeatedly connected in multiple locations, nor can it be suspended for a long time without evaluation.

Multi-point grounding, accidental grounding, or overlapping of external signal lines may damage the originally designed floating or single-point grounding structure, causing circulation, misoperation, and even the risk of electric shock. Therefore, grounding is based on design and cannot be taken care of based on experience.

How to check when putting into operation and maintenance

Whether the grounding is in place must be checked item by item before operation. Before putting into operation, it should be confirmed that the phase sequence, connection group, protection value, cooling, temperature control and grounding are correct. Before the initial power supply, it should also be confirmed that the terminal fastening, insulation, grounding continuity and protection settings are correct. The range to be covered by the ground is not only the neutral point, but also includes the approved ranges of the body, housing, designated ground point of the iron core, accessories and cable shielding. The connection between the neutral point and the protective ground must be implemented according to the system ground design.

Wiring, grounding, protection testing and maintenance should be carried out by personnel with appropriate qualifications based on nameplates, wiring diagrams and local specifications. No specific wiring steps are developed here, because the neutral point grounding must be determined in combination with the on-site single-wire diagram, system short-circuit level and protection cooperation. It is not safe to give so-called standard practices outside of specific projects.