A system is effectively grounded when its ground connection has low enough impedance that a ground fault cannot raise voltages above the limits set for the connected equipment. The IEEE definition uses sequence impedances: the ratio X0/X1 must be positive and below 3, and R0/X1 must be positive and below 1. Under these conditions the coefficient of grounding stays at or below 80%. If a phase faults to ground and the system is not effectively grounded, equipment on the healthy phases can see up to 173% of nominal voltage. Effective grounding keeps this to roughly 125% to 138%, depending on the utility system. Utilities expect distributed generation and storage sites to match their level of grounding, so the site does not weaken the utility's grounding. Some utilities therefore require AC-side grounding of PV inverters. Effective grounding can be provided in two ways. One is a grounding reactor in the transformer neutral. The other is a grounding bank, built from a zig-zag or wye-delta transformer, which passes zero-sequence current easily but blocks positive-sequence current. A single grounding bank at the point of common coupling can effectively ground a whole multi-inverter plant, so the inverters can be connected without a neutral. Grounding banks are usually rated only for short-term duty and need overcurrent protection. The inverters should be interlocked so they shut off when the grounding bank breaker opens. Effective grounding also gives fault current a low-impedance return path, so utility ground-fault protection can operate faster. Related topics are symmetrical components, transformer winding configuration and protection coordination.
related_toc1 drafted · lead IEEE 142-2007 (Green Book) proposed
The IEEE Green Book defines effective grounding through zero-sequence to positive-sequence impedance ratios and suggests two grounding schemes.
sec=h6 h=df3ebcbd6c0bsec=h5 h=338071d5dee9fig=9 h=5b6e3941354ec2 drafted · lead IEEE 1547 proposed
PV plants must meet IEEE 1547 and local utility interface rules. Some utilities also require AC-side grounding of the inverters to match their own grounding scheme.
sec=h2 h=0777d4c89a85c3 drafted · lead Symmetrical components
Effective grounding is stated as ratios of zero-sequence to positive-sequence impedance. Grounding banks give a low-impedance path to zero-sequence voltage.
sec=h6 h=f7484046292esec=h6 h=df3ebcbd6c0bsec=h2 h=0777d4c89a85c4 drafted · lead Transformer winding configuration
Whether a transformer configuration provides effective grounding is judged case by case by each utility.
sec=h14 h=0d98a148bbbbsec=h18 h=dce03f8db182sec=h5 h=9abaa87be95asec=h5 h=338071d5dee9c5 drafted · lead Protection coordination
Solid grounding can let large fault currents flow, which can desensitize overcurrent coordination. Effective grounding speeds up utility ground-fault protection.
sec=h5 h=338071d5dee9sec=h3 h=e32722ef1cc0c6 drafted · lead Transient grid overvoltage event
Systems that are not effectively grounded can see temporary overvoltage. One mitigation is a local 3V0 relay scheme.
sec=h6 h=df3ebcbd6c0bsec=h3 h=e32722ef1cc0sec=h25 h=9c2e61fc1e4ec7 drafted · lead Medium voltage transformer
A grounding reactor in the transformer neutral, or a grounding bank on the primary side of the MV transformer, can provide effective grounding.
sec=5 h=98c779e01729sec=h5 h=338071d5dee9review note: support check: partial - Grounding reactor in neutral and grounding bank on the MV transformer primary side are supported, but the reactor is described for a transformer neutral generally, not specifically the MV transformer.
c8 verified · fact Solectria Effective Grounding Design Tool proposed
Used to calculate the grounding bank impedance and current ratings.
sec=5 h=98c779e01729fig=12 h=d43b060d84a6verified by: review-workflowverified on: 2026-10-24
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