What is Grounding Transformer?
The earthing transformer, also known as the grounding transformer, is used to provide a neutral point in electrical power systems where one is not naturally available. This can be necessary in situations where the neutral point is not available due to the use of a delta connection or when the neutral is desired on the busbars.
The earthing transformer serves two main purposes: it provides a low-impedance path to the neutral, and it helps to eliminate overvoltage's caused by arcing grounds, static induced charges, and lightning surges by discharging them to ground.
There are two main ways to use an earthing transformer to create a neutral point:
1. Using a zig-zag earthing transformer
2. Using a star-delta earthing transformer
1. Zig-Zag Earthing Transformer:
The zig-zag transformer is a type of grounding transformer that is used to provide a low-impedance path to ground in electrical power systems. This helps to protect the system components from fault currents and reduce the stress on the voltage during a fault condition.
The transformer has three limbs, each with two identical windings. One set of windings is connected in a star configuration to provide a neutral point, while the other ends of these windings are connected to the second set of windings. The direction of current in the two windings on each limb is opposite to each other.
Under normal operating conditions, the total flux in each limb is very small, so the transformer draws a small magnetizing current. However, during a fault condition, the impedance of the grounding transformer is very low, allowing it to provide a low-impedance path to ground and limiting the fault current.
To further limit the fault current, a resistor is often connected in series with the neutral grounding. This resistor is designed to handle the rated current for a short period of time (usually 10 seconds) and is sized for a short-time kVA rating.
Overall, the zig-zag transformer serves an important role in protecting the power system by providing a low-impedance path to ground and limiting fault currents.
Application of zig-zag transformer:
- Grounding transformer: The zig-zag transformer can be used as an earthing transformer in a delta-connected (no neutral terminal) system or an ungrounded star-connected (three terminal star) system where the neutral is not available for grounding. In a delta-connected transformer, there is no path for zero-sequence components, and no protection can be performed for these components, which can increase stress and heating in the windings. The zig-zag transformer provides a neutral point and a path for zero-sequence components during a line-to-ground fault, allowing protection to be activated in the event of this fault. In the absence of a grounded neutral, the voltages of healthy phases would increase to the line-to-line voltage level, stressing the insulation on equipment. The zig-zag transformer not only helps with protection but also reduces voltage stress under symmetrical fault conditions.
- Power electronic converters: In power electronic converters, the zig-zag transformer is used to eliminate the DC magnetizing component introduced due to improper firing angles of power electronic components (such as SCRs). The improper firing angles may introduce a DC magnetizing component, which is canceled in each limb of the zig-zag transformer due to the opposite direction of the DC magnetizing component of the currents flowing in the windings on the same limb.
- Earthing reference or earthing transformer: The zig-zag transformer offers a low-impedance path to zero-sequence components during fault conditions, so it can be used as an earthing transformer with an earthing reference. If the earthing current needs to be limited during fault conditions, a suitable resistor can be placed in the zig-zag transformer's neutral terminal.
- Harmonic voltage cancellation: The zig-zag transformer can also be used to cancel out some of the harmonic voltages present in a system due to the opposite connection of the winding coils.
Advantages of Zig-Zag Transformer:
There are several advantages to using a zig-zag transformer compared to other types of grounding transformers, such as the Scott transformer:
- Cost: Zig-zag transformers are generally less expensive to manufacture than Scott transformers.
- Low impedance to zero-sequence currents: The zig-zag transformer provides a low-impedance path to zero-sequence currents, which can be beneficial in protecting the power system from fault currents.
- Harmonic voltage suppression: The zig-zag transformer can be used to cancel out some of the harmonic voltages present in a system, which can help to improve the overall power quality.
- Isolation between ground and component: The zig-zag transformer provides a perfect isolation between the ground and the component, which helps to protect the system from fault currents and reduce the stress on the voltage during a fault condition.
2. Star-Delta Earthing Transformer:
The star-delta earthing transformer is a type of grounding transformer that is used as an alternative to the zig-zag transformer when it is not available. Like the zig-zag transformer, it is used to provide a low-impedance path to ground in electrical power systems, helping to protect the system components from fault currents and reduce the stress on the voltage during a fault condition.
In a star-delta earthing transformer, the primary winding is connected in a star configuration, with the neutral point grounded. The terminals of the primary winding are connected to the three-phase ungrounded system. The secondary winding is connected in a delta configuration, but it is not connected to any external circuit (it operates unloaded).
Under normal operating conditions, the star-delta earthing transformer offers a high impedance to current. However, during a ground fault, it provides a low-impedance path for the fault currents. It is important to ensure that only one earthing transformer is operated at a time on the same supply system, and that short-circuit protection is provided in each line of the transformer.
The selection of the type of grounding for an electrical power system depends on various factors, such as the type of system, the voltage levels, and the potential for transient overvoltage and ground-fault currents. Some general considerations for the selection of grounding include:
- Transient overvoltages: The type of grounding used should be able to handle the transient overvoltages that may be developed in the system, such as those caused by lightning strikes or other sources of high-energy transients.
- Ground-fault current magnitude: The grounding should be able to limit the magnitude of the ground-fault current as a percentage of the three-phase fault current. This helps to protect the system components and reduce the stress on the voltage during a fault condition.
- Dip-in-line voltage: The grounding should be able to minimize any dip in the line voltage that occurs during a fault condition. This helps to maintain power quality and reduce the risk of equipment damage or interruption of service.
Generally, solid grounding (also known as "high-resistance grounding" or "impedance grounding") is used for low-voltage systems up to 600V. This type of grounding uses a high-resistance path to ground to limit the fault current and protect the system components. For voltage levels up to 11KV, resistance grounding is often used. This type of grounding uses a resistor in series with the grounding path to limit the fault current and reduce the stress on the voltage during a fault condition.