What is a Grounding Transformer or Earthing Transformer?

What is a Grounding Transformer or Earthing Transformer?

A grounding transformer is also known as an earthing transformer. 

This is a type of auxiliary transformer that generates an earthing fault current (when one occurs) to neutral for relay protection in 3-phase electric power systems. 

It provides a grounding / earthing pathway to either an ungrounded wye (star) (or) a delta-connected system.

Grounding Transformers are designed to provide an artificial neutral point for the ungrounded three-phase electrical systems.

An earthing transformer’s kVA rating is determined by the normal 

• Line-to-neutral voltage & 
• Size of the fault current 

over a set period of time, which is often seconds or minutes. 

In the case of a line-to-ground fault, these transformers provide a straight path to ground, limiting fault currents & transient overvoltages.

Grounding Transformers commonly handle short-circuit ground current until the circuit breaker resets the fault. 

As a result, they have short time ratings.

In three-wire systems, utility distribution networks & some power electronic converters use grounding transformers as neutral points. 

The diagram below shows a 3-phase 2-winding transformer with windings 1 and 2 coupled in zig-zag.

A three-wire system with a single-phase load between phase C & ground is shown. 

The ground and grounding transformer neutral return the load’s current I to the source. 

The grounding transformer has a low zero-sequence impedance and a high positive-sequence impedance due to the zig-zag connection and opposite upper and lower winding polarities. 

Thus, only zero-sequence current can travel via each of the 3 windings. A zero-sequence current is a set of 3-phase currents with the same amplitude and phase. 

The neutral current I splits into 3 equal currents I/3. The grounding transformer’s three currents are equivalent, keeping the neutral point stable & the line-to-neutral voltages balanced.

Three 1:1 voltage-ratio two-winding transformers model the grounding transformer. 

Consider six identical windings:

R – Winding Resistance

X – Winding Leakage Reactance

R_mag, X_mag – Magnetizing Branch Parallel Resistance and Reactance

Grounding transformer positive-sequence impedance Z₁ & zero-sequence impedance Z₀ are:

Z₁ = R₁ + jX₁ = 3[jR_magX_mag / (R_mag + X_mag)]

Z₀ = R₀ + jX₀ = 2(R + jX)

Zero-sequence reactance X₀ is the grounding transformer’s most essential parameter. Keep reactance X₀ low to reduce voltage imbalance.

IEEE Standard 142 (Green Book): Recommended Practice for Grounding of Industrial & Commercial Power Systems.

What is the Purpose of Grounding Transformer?

Grounding transformers can provide a grounding path for an ungrounded system or be utilized when the system’s neutral line fails and becomes unavailable. 

The reason for employing a grounding transformer (or) its functionalities are as follows:

• When the transformer is grounded, reduce the magnitude of transient overvoltage.
• Provide a reasonably low impedance grounding path for transformer such that the system’s neutral point remains at or near the ground potential.
• Provide a current source during a line grounding failure.
• In special cases, connect a phase-neutral load.

Why are Grounding Transformers necessary?

It is necessary to properly ground various power system components because it increases their accessibility, basic insulation level, overvoltage tolerance & short circuit withstand capabilities. 

However, some portions of the system may lack the necessary neutral connection for grounding requiring the installation of a second component such as a grounding transformer.

Grounding transformers make grounded neutral connections for the ungrounded 3- phase systems, such as delta-connected & ungrounded wye systems. 

If a single line-to-ground fault occurs in such an isolated (or) ungrounded three-phase system, the zero-sequence fault current will have no return path due to the lack of a grounded connection. 

In these conditions, the system will continue to function, but the fault will cause the voltages on the unfaulted lines to rise by a factor of √3 (1.73). 

This would cause a 173% overstress on the transformer’s insulation and other system components. 

Furthermore, voltage spikes are typically controlled using solid-state devices such as metal-oxide varistors (MOVs). 

However, even in the absence of substantial flashover voltage, such devices could be harmed by the heating effect of leakage currents under fault conditions.

It is clear how the system could be harmed in this case if proper fault tolerance is not provided. 

As a result, the grounding transformer can prevent such a condition from occurring by connecting the fault current to a grounded line. 

In short, grounding transformers are employed for:

• Connecting phase-to-neutral loads to the system.
• Create a path for the ground-fault zero-sequence currents to flow via in the case of a line-to-ground fault.
• Provide a pathway for the triple harmonics produced by the excitation current that flows if an ungrounded transformer is powered.
• Maintain the system’s neutral to near-ground potential by effectively shorting the 2 via a low-impedance channel between neutral and ground.
• Protect your equipment against overvoltage transients generated by restriking ground faults. 

Characteristics of Grounding Transformer

• Grounding transformer characteristics include being in a no-load state for extended periods of time during regular power system operation. 
• When a grounding short circuit happens, it is temporarily overloaded.
• Since the load current flowing via the transformer winding is zero-sequence, the transformer’s zero-sequence impedance is more essential.

Types of Grounding Transformers

Grounding transformers are often classified into two types based on connections and windings:

Based on Connection

1. Solidly Grounded System
2. Resistance Grounded System

Based on Winding

1. Wye (Ynd)-Connected Winding
2. Zig-Zag (Zn)-Connected Winding

Based on Connection

1). Solidly Grounded System

A solid grounding system requires connecting a power system component to a grounding transformer. 

This is a simple installation that adds several safety features to an already ungrounded system, but it lacks current-limiting capacity as the grounding transformer itself has minimal resistance.

2). Resistance Grounded System

In order to assist in restricting the amplitude of ground fault currents, a neutral ground resistor is typically added to grounding transformer to create a more secure system. 

We refer to this configuration as a resistive grounding system. When installing resistive grounding, selecting the resistor’s ohm value is a sensitive step.

Since it needs to be both high enough to stop huge fault currents from passing through the system and low enough to minimize thermal damage.

Based on Winding

1). Wye (Ynd)-Connected Winding

A Wye-connected grounding transformer with two windings is more extensively used and popular among consumers for various reasons:

It is widely assumed that the parts of a double-winding transformer are simpler to replace (or) upgrade after damage.

Designers are more proficient with the construction of double-winding transformers than the zig-zag arrangement. As a result, double-winding transformers will increase in demand and widespread use.

A Wye-connect double-winding transformer may be utilized for secondary loading and metering, but a zigzag design cannot.

2). Zig-Zag (Zn)-Connected Winding

The shape of the zig-zag connection enables it to restrict the flow of third harmonics. 

It also does not need a secondary delta-connected winding to function, which reduces the transformer’s cost and size.

Zig-zag transformers are difficult to design.

Not every manufacturer will supply zig-zag grounding transformers.

IS 3043: Code of Practice for Earthing.

Advantages of Grounding Transformer

• The power supply’s high level of dependability is one of the advantages of using a grounding transformer.
• The amount of interference that occurs with the communication & signal system is negligible.

Disadvantages of Grounding Transformer

• Insulation requirements and significant costs.
• Arc grounding can lead to overvoltage.
• The choice of grounding relay protection is complex.
• Resonance overvoltage of an electromagnetic voltage transformer.
• The disconnection may result in resonant overvoltage.

How to choose a Grounding Transformer?

System demands must be considered while choosing a grounding transformer. 

Throughout the selection process, consider these important parameters:

1). Primary System Voltage

System voltage is essential while choosing a grounding transformer. Lightning surge resistance is another voltage factor for grounding transformer selection. Basic impulse level of the transformer.

2). kVA Rating

Grounding transformers only operate briefly during ground failures. They are cheaper and smaller than continuous duty transformers of the same kVA.

3). Duration & Fault Current

This is the total fault current expected in a ground fault. This current must be grounded by the transformer. Thus, the transformer must sustain 100–1000 Amps of current for a certain time without being destroyed by the current flow (or) heating impact.

4). Constant Neutral Current

Grounding transformers are sized by current, not kVA. The continuous neutral current must travel via the neutral circuit without tripping any safety gear.

5). Impedance

The impedance can be ohms per phase (or) a percentage. In case of a single line-to-ground fault, the unfaulted phases’ voltages should not exceed the transformer’s and system equipment’s temporary overvoltage withstand capabilities.

6). Primary Coil Connection

The transformer’s primary connection type: Wye (or) Zig-zag must be specified.

7). Auxiliary Load

Wye & Zig-zag primary winding transformers have auxiliary loading size considerations. This is important for choosing and operating a transformer.

8). Secondary Connection

The transformer’s secondary winding’s connection type (Delta or Wye) & voltage ratings should also be stated.

Grounding Transformer Design Considerations

The transformer design must address several more concerns which includes:

• Transformer bodies need special paint.
• Consider wherever the transformer will be put and used such as indoors (or) outside. Outdoor transformers need extra protection.
• Before selecting a transformer fluid, consider the application and distinctive properties of silicone, mineral oil & natural ester fluids. Design adjustments should support the chosen choice.
• Think on whether the site needs a live or dead front design for connectivity. Terminals can be concealed or exposed on the body such as on the sidewall (or) under a cover.
• Temperature rise is usually 65°C, however location, ventilation & application can affect it. It should be modified inside the design if needed.
• Consider environmental factors like site elevation and build protective devices to assist with the transformer work in all conditions.
• Resistance grounding requires that the neutral ground resistor’s rated voltage match the grounding transformer’s line-to-ground voltage.
• Additionally, check its current and duration ratings against the grounding transformer.

External Earth Fault (E/F) Stability on Delta Side of Star-Delta Power Transformer

An Earth Fault (E/F) in delta system outside Current Transformer (CT) sites would cause current distributions in the differential CT secondaries avoiding the operational coils, if the Delta earthing transformer is outside the protective zone.

Zig-Zag or interconnected star grounding transformers have high normal magnetizing impedance, but E/F currents flow in same-core windings to cancel ampere turns and lower impedance.

A zigzag-connected earthing transformer can produce an artificial neutral point in a delta-connected electrical power transformer.

This core transformer has three limbs. Every phase winding in the zigzag connection is split in half. The transformer core has half coiled on one limb & half on the other.

• First half of Red phase winding is wound on core’s first limb and the second half on the third.
• The first half of Yellow phase winding is on the 2nd core limb, the 2nd half on the 1st limb.
• The first half of Blue phase winding is on the core’s 3rd limb, and the second half is on the 2nd limb.

All three windings link at a neutral point. If any phase in a delta-connected system fails, the zero sequence fault current circulates through earth.

Normally, the earthing transformer winding voltage is 1/√3 times the system’s rated per phase voltage.

When a single line to ground fault happens on any phase of the system, the earth fault current’s zero sequence component flows in the earth & returns to the electrical power system via the earthing transformer’s earth star point It’s split evenly over three phases. 

As seen in the image, the currents in two winding halves in the same core limb run in opposite directions. 

The magnetic flux created by these 2 currents will resist and negate each other. Since fault current does not increase flux, there is no change in dφ/dt, indicating no choking impact on fault current flow. 

Thus, the zigzag earthing (or) grounding transformer sustains the rated supply voltage at normal current and when a solid single line to ground fault current runs through it.

Earthing (or) grounding transformers are designed for line-to-line voltage.

This transformer’s current rating is its maximum neutral current in Amps (Amperes) in fault state for a given time. 

Transformers constructed to carry maximum fault current safely usually last 30 seconds.