Power Transformer Neutral Grounding: Purpose and Design

Whether or not the neutral point of a power transformer should be grounded, and where to ground it, seems to be a step that is completed smoothly during wiring. In fact, it is a design matter that must be determined on the drawing before a transformer is incorporated into the system. The English word “power transformer neutral grounding” is also often written as “power transformer neutral earthing” in British English, referring to the same thing. It determines where the fault current goes, how the protection is acted upon, and whether the secondary system can stably establish a reference to ground.

The neutral point comes out of the star connection

The two most commonly used connections for three-phase transformers are star and triangle. Different connections will have different relationships between line voltage, phase voltage, line current, and phase current. The star connection connects the ends of the three windings to the same common point. This common point 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. Without such a common point in the triangle connection, there is naturally no neutral point to draw out.

The triangle — star combination commonly found in power distribution systems takes advantage of this: the high-voltage side is connected to form a triangle, and the low-voltage side is connected to form a star. The low-voltage side then has an additional neutral point, which also affects the channel of the zero-sequence current and the direction of the harmonic. It should be noted that this does not mean that all distribution systems necessarily adopt this connection method, and establishing a neutral point is only one of the many functions that transformers may perform in the system.

Grounding provides a ground reference for the system

The primary effect of grounding the neutral point is to provide a clear reference to the ground for the system. With this reference, the current can form a low-resistance loop in the event of a ground fault, allowing the protection device to operate accurately and timely. In turn, if the secondary side is not grounded for a long time, it becomes 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 mark is an inseparable parameter here. It expresses the connection and phase displacement of the high and low voltage windings at the same time, and is an important basis for judging the parallel connection, grounding, protection and harmonics of the system. Visible grounding is never an isolated action; it must be viewed alongside the connection group and protection settings.

Two kinds of grounding, two different design lines

When talking about grounding, it is easy to confuse two concepts: neutral point grounding, which deals with how the working circuit establishes a reference to ground; and protective grounding, which deals with how the transformer housing and the exposed conductive part of the load are connected to the protective conductor. They belong to two design lines and cannot replace each other or be mixed.

When dropped on equipment, the grounding terminal, neutral point lead-out, shielding terminal and housing protection grounding points should be clearly marked in the drawings and nameplates 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 fixed once during the design phase

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.

The case of isolated transformers deserves a separate mention. Its secondary side can remain floating as designed, or a new grounding reference can be established; whether it is grounded or not 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. That is, the ground adjusts the electrical behavior of the system, not the isolation relationship between the windings.

Go through the grounding items one by one before putting into operation

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.