Neutral Grounding Transformer (NGT) Calculator

Excel Calculator

What is Neutral Grounding Transformer (NGT)?

When a transformer is used to provide a neutral point for grounding in a system where the neutral point of the three phase system is unavailable or the transformers or generators are delta connected, the transformer is referred to as neutral grounding transformer (NGT).

What is the difference between NGR and NGT?

The NG cubicle is used to install a Neutral Grounding Transformer (NGT) and a Neutral Grounding Resister (NGR) for grounding/earthing neutral current during an unbalanced three phase load of a generator, which is primarily necessary for protecting the generator winding.

How to Calculate NGT Rating?

The following overview describes the generator’s power, capacitance, resistance, & Report Word current ratings to provide proper rating and protection for operational conditions.

Generator & Capacitor Ratings

The generator has high power and voltage ratings. It has an intrinsic capacitance per phase, and extra linked capacitances adds to the overall capacitance per phase. The total capacitance is computed by adding an additional capacitance component to each phase.

Impedance & Current Calculations

The total capacitance throughout all phases generates a given capacitive reactance, which leads to a specific capacitor current. A computed tolerance is determined after considering field forcing.

Voltage & Insulation Ratings

The Next Generation Transformer (NGT) has a specific voltage rating and insulation level. The power rating of the NGT is derived by multiplying the field forcing allowed by the voltage rating, and the result is rounded to the nearest standard rating.

Resistance & Current Calculations

The corresponding primary resistance has been established. The secondary resistance is calculated based on the secondary voltage selected. After calculating for the transformer’s resistance, the external resistance is calculated.

The current rating is computed using the power rating and external resistance to yield a specific current value.

Final Ratings

• Next Generation Transformer (NGT) Rating: Rated for a specific power level for a limited time.
• Neutral Grounding Resistors (NGR) Rating: Rated with a specified resistance value and tolerance.

Formula

To calculate the ratings of a Neutral Grounding Transformer (NGT), apply the formulas listed below:

NGT kVA=Field Forcing Allowance X NGT Voltage (kV)

Calculation Steps 

Step-1: Set the primary resistance to the predicted capacitive reactance.

Step-2: To calculate the secondary resistance, multiply the primary resistance by the square of the secondary voltage-to-primary voltage ratio.

Step-3: Subtract the transformer resistance from the secondary resistance to calculate the external resistance.

Step-4: To get the current rating, divide the NGT kVA rating by the external resistance & take the square root.

Step-5: To get the NGT kVA rating, multiply the field forcing allowed by the NGT voltage.

These formulas and calculation steps provide a complete technique for computing the parameters required for a Neutral Grounding Transformer.

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