Ground-fault & protection relaying
While ground-fault protective schemes may be elaborately developed, depending on the ingenuity of the relaying engineer, nearly all schemes in common practice are based on one or more of the methods of ground-fault detection discussed in this article.
Distribution circuits that are solidly grounded or grounded through low impedance require fast clearing of ground faults. This need for speed is especially true in low-voltage grounded wye circuits that are connected to busways or long runs of metallic conduit.
The problem involves sensitivity in detecting low ground-fault currents as well as coordination between main and feeder circuit protective devices.
The discussions relate to various forms of ground-fault protection to prevent excessive damage to electrical equipment with current sensitivity in the order of amperes to hundreds of amperes.
- Residually connected overcurrent relays
- Core balance sensing of feeder conductors
- Detection of ground-return current in the equipment grounding circuit
- Differential relaying
A residually connected ground relay is widely used to protect medium-voltage systems. The actual ground current is measured by CTs that are interconnected in such a way that the ground relay responds to a current proportional to the ground-fault current.
The term residual in common usage is normally reserved for three-phase system connections and seldom applied to single-phase or multiple-signal mixing.
The basic residual scheme is shown in Figure 1. Each phase relay is connected in the output circuit of its respective CT, while a ground relay connected in the common or residual circuit measures the ground-fault current.
In three-phase three-wire systems, no current flows in the residual leg under normal conditions because the resultant current of the three CTs is zero.
This situation is true for phase-to-phase short circuits also. When a ground-fault occurs, the short-circuit current, returning through earth, bypasses the phase conductors and their CTs, and the resultant current flows in the residual leg and operates its relay.
Without the neutral-conductor CT, the current in that conductor would appear to the ground relay as ground-fault current, and the ground relay would have to be desensitized sufficiently to prevent opening under unbalanced load conditions.
The selectivity of residually connected relays is determined by the CT ratio and the relay pickup setting. The CT ratio must be high enough for normal load circuits.
Also to be considered are the unbalanced primary currents in each phase, the sum of which may contain sufficient asymmetrical current to cause a trip during a motor start.
For this reason, sensitive ground-fault protection schemes do not use instantaneous trips when a residually connected circuit is involved. If ground-fault protection with greater sensitivity is needed, the core-balance method should be considered (see next paragraphs).
The core-balance method is based on primary current phasor addition or flux summation. The remaining zero-sequence component, if any, is then transformed to the secondary.
The core-balance CT or sensor is the basis of several low-voltage ground-fault protective systems.
The principle of the core-balance CT circuit is shown in Figure 2.
All load-carrying conductors pass through the same opening in the CT and are surrounded by the same magnetic core.
Core-balance CTs are available in several convenient shapes and sizes, including rectangular designs for use over busbars.
This method can be more sensitive than the residual method because the sensor rating is large enough for the possible imbalance, not for the individual conductor load current.
Under normal conditions (i.e., balanced, unbalanced, or single-phase load currents or short circuits not involving ground – if all conductors are properly enclosed), all current flows out and returns through the CT. The net flux produced in the CT core is zero, and no current flows in the ground relay.
When a ground fault occurs, the ground-fault current returns through the equipment grounding circuit conductor (and possibly other ground paths) and bypasses the CT. The flux produced in the CT core is proportional to the ground-fault current, and a proportional current flows from the CT secondary to the relay circuit.
If only phase conductors are enclosed and neutral current is not zero, the transformed current is proportional to the load zero-sequence or neutral current.
Systems with grounded conductors, such as cable shielding, should have the CT surround only the phase and neutral conductors, if applicable, and not the grounded conductor.
By properly matching the CT and relay, ground-fault detection can be made as sensitive as the application requires.
The speed of the relay limits damage and may be adjustable (for current or time, or both) in order to obtain selectivity. Many ground protective systems now have solid-state relays specially designed to operate with core-balance CTs. The relays in turn open the circuit protective device. Power circuit breakers, MCCBs with shunt trips, or electrically operated fused switches can be used.
The last category includes service protectors, which use circuit breaker contacts and mechanisms, but depend on current-limiting fuses to interrupt the high available short-circuit currents.
Fused contactors and combination motor starters may be used where the device-interrupting capability equals or exceeds the available ground-fault current.
Figure 3 shows a typical termination of a medium-voltage shielded cable. After the cable is pulled up through the core-balance CT, the cable jacket is removed to expose the shielding tape or braid. Jumpering the shields together, the connection to the ground is made after this shield lead is brought back through the CT.
Between multiple shield ground connections on a single conductor cable, a potential exists that drives a circulating current, often of such a magnitude as to require derating of the cable ampacity.
When applying the core-balance CT, the effects of this circulating current should be subtracted from the measuring circuit.
Ground-return relaying is illustrated in Figure 4. The ground-fault current returns through the CT in the neutral-bus to ground-bus connection.
For feeder circuits, an insulating segment may be introduced in busway or conduit, as shown in Figure 4, and a bonding jumper connected across the insulator to carry the ground-fault current. A CT enclosing this jumper then detects a ground fault.
This method is not recommended for feeder circuits due to the likelihood of multiple ground-current return paths and the difficulty of maintaining an insulated joint.
A generic term, ground differential is used for a variety of schemes that utilize phasor or algebraic subtraction or addition of signals. The currents may be produced by any of the methods discussed in sections 1 through 3.
Figure 5, for example, shows a ground differential protection for single-phase center-tapped loads and is similar to the residual method.
One core-balance sensor could detect ground faults in a plurality of loads (see Figure 6). Ground-differential relaying is effective for main bus protection because it has inherent selectivity.
With the differential scheme (see Figure 7), core-balance CTs are installed on each of the outgoing feeders and another lower ratio CT is placed in the transformer neutral connection to ground.
All CTs should be carefully matched to prevent improper opening for high-magnitude faults occurring outside the differential zone.
Bus-differential protection protects only the zone between CTs and does not provide backup protection for feeder faults.
Factors that affect the sensitivity of ground-fault sensing include the following:
- Circuit-charging current drawn by surge arrestors, shielded cables, and motor windings
- The number of coordination steps between the branch circuit and the supply
- Primary rating and accuracy of the largest CT used to supply residually connected relays in the coordination
- How well the CTs used for residually connected relays are matched
- Burden on the CTs, in particular the burden of the residually connected relay (Solid-state and some induction disk relays have burdens of 40 VA for a 0.1 A tap.)
- Maximum through-fault current and its effect upon the CTs, with selected relays for phase and residual connections
- Fault contact resistance
- Location of conductors within core-balance transformers
Reference // IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Std 242)