Transformers, as crucial equipment in the power system, are filled with scientific charm in their working principles and application scenarios. Transformers, like a skilled translator, convert one voltage language into another, enabling smooth power transmission between different grids. So, what are the advantages and disadvantages of neutral grounding and non-grounding systems for transformers? Many electrical professionals may feel confused. Today, I will share some knowledge in this regard.

▌01 Advantages and disadvantages of transformer neutral grounding and non-grounding systems

1. Advantages and disadvantages of transformer neutral grounding system:

(1) Advantages: For a power system with a neutral grounding system, if a single-phase grounding occurs, the voltage of the other two phases does not rise, which can reduce the insulation level of the entire system. Additionally, single-phase grounding generates a large short-circuit current Is, enabling protection devices (relays, fuses, etc.) to act quickly and accurately, thus improving the reliability of protection.

(2) Disadvantages: For systems with a grounded neutral point, due to the high single-phase short-circuit current Is, switches and electrical equipment need to be chosen with a larger capacity, and it can also cause system instability and interference with communication lines;

2. Advantages and disadvantages of transformer neutral ungrounded system:

(1) Advantages: For systems where the transformer neutral point is not grounded, the interference to communication is minimized due to the limitation of single-phase grounding current; furthermore, single-phase grounding allows operation for a period of time, enhancing the reliability of power supply.

(2) Disadvantages: For systems where the neutral point of the transformer is not grounded, when one phase is grounded, the voltage of the other two phases to ground increases by a certain multiple, which can easily cause breakdown at weak insulation points, resulting in a two-phase grounding short circuit.

▌02 Grounding methods for various voltage levels

In high-voltage or ultra-high-voltage systems of 110kv and above, a neutral grounding system is generally adopted. The purpose is to reduce the insulation level of electrical equipment and eliminate the asymmetry caused by continuing to operate after a single-phase grounding.

2. The factory’s power supply system operates with voltages ranging from 1kv to 35kv and typically employs a neutral non-grounded system. Due to the short power supply distance of the factory, the capacitance to ground is small (Xc is large), and the single-phase grounding current is low. This allows for operation over a period of time, enhancing the system’s stability and power supply reliability, with minimal interference to communication.

In coal mine shafts, countries such as China and West Germany prohibit neutral grounding, primarily for safety reasons, as it reduces the single-phase grounding current. However, even small single-phase grounding currents are not allowed in coal mine shafts. Therefore, leakage detection relays are installed in coal mine shafts, which can quickly cut off the power supply when the insulation impedance of the power grid to ground drops to a dangerous value, or when a person touches a phase conductor or a phase of the power grid is grounded, thus preventing electric shock and leakage accidents and cutting off the faulty equipment in advance.

3. For power supply systems with voltages below 1kv (380/220 volts), except for certain special cases (such as underground mines and swimming pools), the vast majority are neutral-grounded systems, primarily to prevent the risk of electric shock due to insulation damage.

▌03 Protective grounding and protective neutral connection of electrical equipment

1. Protective grounding of electrical equipment

(1) Protective grounding

The principle behind connecting the metal casing of electrical equipment to the grounding body via a grounding wire is the shunt principle (as shown in Figure 1). Since the human body resistance Rr is much greater than the grounding resistance Rd, Ir is less than Id. Protective grounding is suitable for power supply systems where the neutral point of the transformer is not grounded. However, in dry locations, electrical equipment with an AC voltage of 50V or less, or a DC voltage of 110V or less, may not have its metal casing grounded; in dry locations with poor conductive surfaces such as wood or asphalt, electrical equipment with an AC rated voltage of 380V or less, or a DC rated voltage of 440V or less, may not have its metal casing grounded unless otherwise specified (it should still be grounded in explosion-hazardous locations).

When electrical equipment is at a high place, protective grounding measures should not be taken, otherwise the earth potential will be directed to the high place, which will increase the risk of electric shock.

(2) Points to note when conducting protective grounding

In a power supply system supplied by the same transformer (with neutral point not grounded), all electrical equipment should not be individually grounded, but should form a protective grounding system. This not only reduces the grounding resistance but also prevents the hazards caused by different phases of different electrical equipment simultaneously touching the shell (grounding). After forming a protective grounding system, the two-phase short-circuit current mainly flows through the grounding grid, thus increasing the value of the two-phase short-circuit current and ensuring reliable operation of the overcurrent protection device.

2. Protective neutral connection of electrical equipment

(1) Protective neutral connection

Due to the fact that neutral points of low-voltage networks (380V/220V) are not grounded in only a few occasions, such as mines and swimming pools, most low-voltage power grids adopt a three-phase four-wire power supply system with grounded neutral points. Equipment operating in such power grids requires its metal casing to be closely connected to the neutral wire, known as protective bonding, as shown in Figure 2. The purpose of protective bonding is also to ensure safety. When a phase of the equipment touches the casing, it causes a single-phase short circuit, prompting the protective device to act quickly and cut off the faulty equipment.

Based on the combination of neutral wire and protective wire, protective neutral connection can be divided into the following three situations:

1) In the entire system, the neutral line N and protective line PE are combined, as shown in Figure 2. This is typically applicable to places where the three-phase load is relatively balanced and the single-phase load capacity is relatively small.

2) The neutral line N and the protective line PE are separated in the entire system, as shown in Figure 3. That is, the equipment enclosure is connected to the protective line PE. Under normal circumstances, no current flows through the protective line, so the equipment enclosure is not charged.

3) A part of the system adopts a combination of neutral wire and protective wire, with dedicated protective wires used locally.

(2) Issues to be noted in protective neutral connection:

1) For lines powered by the same generator or transformer, it is not allowed to have some equipment protected by grounding and other equipment protected by connecting to neutral.

2) Ground one or multiple points along the neutral line, which is known as double grounding, to ensure the reliability of the protective grounding device. However, double grounding only serves to balance electrical potentials. Therefore, it is essential to avoid breaking the neutral line as much as possible. Careful construction and maintenance of the neutral line are required.