System grounding is basically a process of connecting the neutral of the power system (generator, transformer or network) to the earth. Either solid connection or resistive or reactive or left ungrounded, it is this choice that determines the behavior of the system during faults. The magnitude of the earth fault current, transient over voltages and thus the system insulation levels also depends upon the system earthing greatly.
The protection system, is also designed based on the system grounding be it the relay setting or the grounding resistor or neutral grounding reactor’s coordination. In substations, industries and domestic installations, system grounding leads to stable voltage, limiting the equipment stress.
Table of Contents
Objectives of system grounding
Limiting over voltages: Grounding the system neutral, prevents the shift of neutral point during the fault and hence the phase voltages are also prevented from rising to line voltage ( √3 x phase voltage ). Thus the transient over voltages are supressed and voltage of the equipment remains within the basic insulation level ( BIL ).
Stable system voltage reference: When a system is ungrounded, each phase conductor has a capacitance with the ground which creates a reactive path with the ground. The system neutral floats to satisfy the capacitive network. Whenever a fault occurs, the neutral shifts as the other phases capacitance must satisfy KCL and KVL making the voltage in the healthy phases go unstable. Clamping the neutral to earth locks the neutral voltage to 0 Volts and the capacitance effect can no longer shift the neutral. This makes the system voltage stable.
Check the magnitude of the Ground fault current: With the help of solid, resistive or reactor grounding, the earth fault current can be set to desirable values. This helps in keeping the touch and step voltage within safe limits while also ensuring the fault current to be high enough for the protective devices to detect and trip.
Ensure proper operation of protective relay: Grounding defines the zero sequence network, which is important for the earth fault relays. Correct system grounding enables the reliable measurement of the zero sequence current, enabling faster and selective protection.
I0 = zero sequence current component = Ia+Ib+Ic = Sum of three phase current.
For a balanced system, I0= 0 but during a ground fault I0 is not equal to 0, this provides a measurable signal for the relay.
Reduces insulation stress: An installation with the system grounding prevents severe phase to ground voltage rise when encountered with faults and switching surges. Controlled voltage stress extends the transformer cable’s and the switchgear insulation life.
Types of system grounding
Ungrounded system: There is no connection between the neutral and the ground. Therefore, during the single line to ground fault, the fault current is limited to capacitive charging current of the healthy phases.
If = 3ωCp-gVphase
Where C is the capacitance between phase and earth, V is the phase to neutral voltage, ω = 2πf, f is the frequency.
This results in very low value of fault current. Floating neutral allows line to earth voltages on the healthy phase to rise above the nominal approaching 173 % approx. of the original, which can seriously stress the insulation. Also, high voltage arc can damage the equipment if the fault finds a path through the dust and vapours. This system is typically practiced in process industries where continuous operation is prioritised more than insulation stress.
Solidly grounded system: In this system grounding system, the neutral is directly grounded with negligible impedance. This results in higher fault currents and thus requires fast protective relays. Solid grounding keeps the healthy phase near the normal voltage with respect to the earth as the neutral has no fluctuation. This stabilizes over voltage and ensures predictable voltage distribution. The protection coordination is also simple because high fault current simplifies the relay selection with reliable overcurrent relay tripping and zero sequence measurements. This system grounding is common in high voltage substations, transmission, and distribution networks.
Resistance grounding: There are two types of resistance grounding discussed in brief
Low resistance grounding (LRG): In this system grounding, the neutral is connected to a low value resistor for limiting the fault current to a value ranging from 100 A to few KA. In this process the resistor (Neutral grounding resistor) inserted between the neutral and ground brings down the ground fault current, high enough for reliable relay detection but low enough to prevent the equipment damage. It offers less damage than solid grounding, better protection than ungrounded system, avoids transient overvoltage while maintaining the service reliability. It is widely used in the medium voltage industries and commercial sector.
High resistance grounding: In this type of system grounding, the neutral is connected to earth via a high value resistor for limiting the system ground fault current to 5 to 10 A. This fault current is very low just enough for detection. During the ground fault, the system is allowed to continuously operate with a ground fault alarm activated. The system is used in industries where continuous operation is emphasized over immediate shutdown like process and chemical plants. This system behaves like ungrounded system but without any overvoltage transients.
Reactance grounding: In this system grounding, the neutral is grounded via a reactor which limits the ground fault current and stabilizes the system voltage via controlled neutral displacement during a single line to ground fault. The reactor limits the AC current via inductive reactance (XL = 2πfL) and does not convert the fault current into heat. It provides stronger voltage suppression during the fault and supports arc suppression. Typical fault current range in this case is 100 A to few KA depending upon the reactor design. It is used in medium voltage networks where controlled fault current is required.
Resonant Grounding (Petersen Coil): In this system grounding, the neutral is connected through a tuned inductance (Petersen coil) whose inductive current cancels the system’s capacitive earth fault current during a single line to ground fault. In 3-phase system, each phase has a capacitance to earth. During single line to ground fault, the capacitive ground fault current IC = 3 ω Cp-g Vphase
The Petersen coil provides the inductive current IL = VPhase/ ωL.
The coil is so tuned that IL = IC , which gives L = 1/ (3ω2C)
The coil therefore nearly cancels out the IC leaving small residual current of 2-10A, which minimizes the step and touch voltage with arc suppression. It Is widely used in overhead distribution networks (11 to 33 KV) because network capacitance is large enough to cause arcing ground.
How to chose the correct system grounding
System voltage level
LV: Solid system grounding is preferred for fast fault clearing and safety.
MV: System grounding varies depending upon network type.
HV: The neutral is always solidly grounded through transformer neutral as the insulations and protection requirement demands high fault current for instantaneous clearing.
Desired magnitude of earth fault current
If the requirement is fast and definitive protection, chose solid grounding.
If the aim is to limit the arc damage and protect sensitive equipment, chose low resistance grounding.
If the aim is continuous operation avoiding immediate shutdown, chose high resistance system grounding.
If the aim is to minimize the arc flash in the overhead distribution line, use a Petersen coil.
Neutral availability
Star connected LV side (Yyn, Dyn) : Any system grounding is possible.
Delta secondary: No neutral, need zigzag transformer to apply grounding.
Ungrounded system: May intentionally avoid grounding for continuity but face overvoltage risk.
Protection system requirements
Directional Earth fault: Solid, low resistance grounding or Petersen coil system grounding.
Restricted earth fault: Solid or low resistance system grounding.
Sensitive earth fault: High resistance grounding or Petersen coil grounding.
Arc flash sensitive load: High resistance grounding or Petersen coil.
Type of load
Industrial plant: Prefers high resistance grounding to avoid trips and transient over voltage.
Substation: Prefers solid grounding for fast tripping.
Petrochemical or mines: Prefer low resistance grounding to avoid high arc flash.
Rural overhead lines: Prefer Petersen coil to avoid feeder tripping due to tree contacts and lightning.
This article is a part of the Safety and Earthing page, where other articles related to topic are discussed in details.
