This article describes a nuisance trip of sensitive earth fault (SEF) protection due to an unexpected source of earth fault current within a 22kV installation. The nuisance tripping was solved by making the SEF protection directional. The direction element considered the angle between the residual current and the residual voltage.
Sensitive Earth Fault Protection – SEF
In Victoria, the Victorian Service and Installation Rules (VSIR) describe the state-specific requirements for connecting to the networks of local utilities. Some requirements are onerous, including the requirement for Sensitive Earth Fault (SEF) protection on 100% underground cable networks.
SEF is, (as the name suggests), more sensitive than regular earth fault protection. Typically, pickup will be in the range of 5-10A, with longer delays than standard earth fault protection, in the order of 1 second.
Its primary purpose is to detect fallen overhead conductors. If a conductor falls on to extremely dry ground, the fault impedance can be very high and only a small amount of zero sequence current will flow. Hence the need to a more sensitive earth fault protection.
In a medium voltage underground cable system using earth-screened cable and solidly-earthed metal-enclosed switchgear, high impedance faults are rare. An installation can develop partial discharge (PD), but PD is characterised by tiny discharges and does not yet count as a fault. When it does develop into a fault, typically the fault develops very quickly and is of relatively high earth fault magnitude due to earth resistance requirements for such installations. Complaints aside, the VSIR are the rules and SEF protection must be provided.
The use of the zero sequence (core-balance) CT is recommended for SEF. At such low pickup values, SEF protection is prone to CT summation error. You can use a very low ratio (e.g. 10/1A) zero sequence CT as CT saturation is not a concern if regular earth fault protection is implemented using the phase CTs. If there is a high zero-sequence current, earth fault will pick up in the phase CTs and clear the fault.
Mystery SEF Trips
A new 22kV installation suffered a SEF trip when there was an earth fault on the utility network. The fault was on a different feeder, but the faulted feeder was on the same zone substation bus as this installation.
When an earth fault occurred on the utility network, the faulted phase voltage was depressed and the two un-faulted phases rose:
Source of Zero Sequence Current
In medium voltage systems containing a significant length of underground cable (e.g. utility networks), directional SEF protection is standard. This prevents false trips due to capacitive current during unbalanced voltage. Medium voltage cable develops significant capacitance across the insulation between the phase conductor and the earth screen. Long runs of underground cable have enough capacitance to allow zero-sequence current to flow during an unbalanced fault. This zero sequence capacitive current can exceed the SEF pickup:
Consider the phasor. If the same cable capacitance is connected between each phase and earth, the resulting capacitive currents will not cancel out. This is a potential source of zero sequence current (and the nuisance SEF trip).
Consultation of the ever-handy Olex Catalogue reveals that typical 22kV XLPE cable (take 185mm2) has conductor to screen capacitance of 0.269 uF/km.
Calculating the cable capacitance from the 2km of cable within the site (downstream of our SEF CTs) did not yield enough capacitance to result in 5A of zero sequence current sufficient to exceed the SEF pickup during unbalanced faults. There was another source of zero sequence current somewhere. All the transformers within the site had delta primaries and therefore could not contribute zero sequence current. We finally identified the source of the mystery zero sequence current in a dry-type transformer that had been fitted with surge arrestors and surge capacitors.
The surge capacitors fitted to the dry type transformer were connected in star, and thus needed to be considered as a potential source of zero sequence current during unbalanced voltage conditions.
Directional Torque Control on SEF Element
To solve the nuisance tripping, the SEF protection was made directional. The protection relay examines the angle between the residual voltage, V0 and the zero sequence (residual) current, IN. From the angle, it determines whether the fault is upstream or downstream.
For a capacitive (upstream) fault, the residual current leads the residual voltage by approximately 90 degrees:
Whereas for a resistive fault (internal = trip on SEF), the residuals are around 180 degrees apart:
Always consider all potential sources of zero sequence current. Having all delta transformer primary windings does not prevent zero sequence current flow due to unbalanced voltages.