Substation Ground Grid Interconnection to Distribution System Neutral

Key Takeaways

• Why interconnecting a substation ground grid to the distribution neutral can dramatically reduce Ground Potential Rise (GPR).

• How neutral interconnection lowers induced voltages on communication cables by nearly 50%.

• What impedance threshold ensures compliance with the 5 kV GPR safety limit.

• How touch and step voltages remain within safe limits under both configurations.

• Practical recommendations for bonding, surge protection, and ongoing impedance monitoring.

⏱ Estimated reading time: 6–7 minutes.

1. Introduction

This study investigates whether a substation ground grid should be interconnected to the distribution system neutral. The site under consideration includes a joint-use communications cable installed on the same pole line as a 4.16/2.4 kV distribution line for a distance of 300 m.

Two alternatives are analyzed:

1. Alternative 1 – Interconnection: Substation ground grid bonded to the utility system neutral. (Measured neutral impedance: Zn = 1.4 Ω).
2. Alternative 2 – No Interconnection: Entire fault return current flows through earth.

The study evaluates Ground Potential Rise (GPR), touch and step voltages, and induced voltage on communication cables for both alternatives, in accordance with applicable electrical safety standards and recommended practices.

2. System and Ground Grid Data

The system and supply line data, substation main grounding grid and substation ground grid expansion given below.

2.1 Electrical System Data

System and supply line data

2.2 Substation Grounding Data

The ground grid data are provided in the following tables.

Substation main grounding grid

Substation ground grid expansion

3. Grounding Performance Without Neutral Interconnection

*(I_g) = V_n/(Z_s + R_g)

Observation: Even with an extended ground grid, GPR remains above the 5 kV code limit. Therefore, the local distribution utility was approached to grant permission for interconnecting the substation ground grid to the available neutral associated with the 4.16/2.4 kV distribution system.

4. Neutral Interconnection Analysis

The fall-of-potential method was used to measure the utility system neutral impedance. The results are shown in following figure. Results were plotted and the utility system neutral impedance value was found to be:

• Zn = 1.4 Ω at 62% probe distance.

By connecting the grounding grid to the utility system neutral, the resulting circuit model can be represented as follows:

The GPR was calculated using various utility neutral impedance values Z_n to establish the threshold at which the allowable limit is attained:

With the neutral resistance at 1.4 W, the calculated GPR value was found to be 3947V, which is below the required code limit. The threshold for the GPR to be below 5000V is 1.916 W.

• Threshold: To maintain GPR ≤ 5,000 V, Zn ≤ 1.916 Ω.
  Measured Zn = 4 Ω → acceptable.

5. Induced Voltage on Communication Cables

Since there was a joint-use installation on the pole line carrying the system neutral for a distance of 300m, the utility granted permission to interconnect the substation ground grid to the system neutral and requested that the induced voltage on the communication cables be calculated. The following calculations were conducted to find out the values of the induced voltage with and without the ground grid interconnection to the system neutral.

5.1 Alternative 1 – Interconnected Neutral (Zn = 1.4 Ω)

The calculated currents in the phase, neutral and earth return are based on the ground grid analysis. The induction calculations will be based on the currents corresponding to a system neutral impedance of 1.4 W.

Current components representation

The total fault current is the vectorial sum of the return current through the system neutral and the return current through the earth. They can be represented as follows:

Vectorial representation of ground fault current components

The numerical values of the current components are:

Induced voltage due to the portion of current returning through the neutral conductor can be calculated as:

	Da’x	Distance between communication cable and neutral conductor	4′
	Dax	Distance between communication cable and faulted phase conductor	16′
	In	Neutral current during fault	2816Ð0° (A)

Induced voltage due to the portion of current returning through the earth can be calculated as:

	Dex	Distance between communication cable and equivalent earth return	De – Dax
	Dax	Distance between communication cable and faulted phase conductor	16′
	De	Distance between faulted phase conductor and equivalent earthreturn
	Δ	Earth resistivity	Ω.m
	f	System frequency	60 (Hz)

Since Δ varies, D_e will vary and hence D_ex. Giving Δ different typical values following results are obtained:

The magnitude of Induced voltage can be calculated as below which is approximately between 130 to 160V.

5.2 Alternative 2 – No Interconnection

This alternative calculates the induced voltage in the communication cable if the system neutral and the substation ground grid are NOT interconnected. The entire fault current will return through the earth.

The system impedance at the substation	7.21Ð82.1° (W)
The resistance of main substation ground grid	5.66 (W)
The magnitude of the total impedance (grid + system)	9.75 (W)
The magnitude of the phase to ground fault current (If)	2605 (A)

Since Δ varies, D_e will vary and hence D_ex. Giving Δ different typical values following results are obtained:

The induced voltage is approximately 270 to 420V (higher than alternative 1).

Observation: Interconnection reduces induced voltage by ~50% for all resistivity values.

6. Conclusion and Recommendations

Conclusion:

• Interconnecting the substation ground grid to the distribution neutral (Zn = 1.4 Ω) reduces GPR from above code limits to 3,947 V, meeting safety requirements.
• It also reduces induced voltages on communication cables from ~270–420 V to ~130–160 V.
• Touch and step voltages remain within permissible limits.
• For Zn ≤ 1.916 Ω, GPR remains ≤ 5 kV.

Recommendations:

1. Bond substation ground grid to distribution neutral under continuous monitoring of neutral impedance.
2. Bond communications cable messenger strand to neutral at 300 m intervals. It should be also noted that a phase to ground fault on the 44kV line will be similar to alternative 2 in this case; resulting in high induced voltages on communication circuits.
3. Install surge protection at communication entry points.
4. Periodically verify Zn and Rg to ensure compliance.
5. Update maintenance procedures to reflect interconnection configuration