Author: BSEE Hugo E Reyes
A three-phase power system is subjected to PHASE FAULTS as well as GROUND FAULTS. Faults involving just one of the phase conductor and ground are called GROUND FAULTS. Faults involving two or more phase conductors, with or without ground, are called PHASE FAULTS.
On the other hand, it may be pointed here that fault current for a single line-to-ground fault (SLG), depends on the system grounding and the distance to the fault, but also on the level of generation in the power system. Other factor is the arc resistance that is nothing but the arc that is formed due to flashover of insulator string.
It may occurs that the fault current for a single line-to-ground fault, may be less than the load current. The lowest current fault magnitude corresponds to a SLG located at the end of the backed-up line section under minimum generation conditions. Under these scenarios, the ground overcurrent element must have maximum sensitivity in order to detect high-resistance faults.
2. COMMON METHODS OF GROUND FAULT DETECTION
2.1. RESIDUAL CONNECTION
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 Figure 1 depicts the basic residual connection scheme.
Figure 1 Residually Connected Ground Relay
In this scheme, the phase overcurrent elements (50P/51P) are connected to the phases of the Wye-connected current transformers (CTs) and the ground overcurrent element (50N/51N) is connected to the Wye Neutral. Thus, the residual connection allows the ground fault relay to measure the sum of the phase currents, which is equivalent to the residual current (3.I0). With electromechanical relays this scheme needs four relays; nowadays, with digital relays this scheme needs just one relay.
The pickup current of the phase overcurrent relay (51P) needs to be set above the maximum load current. On the other hand, the ground fault element (51N) needs to be set above the maximum zero-sequence unbalance current, which is typically no more than 10 to 15 % of the phase current.
Unequal performance of current transformers during heavy phase faults or initial asymmetrical motor starting currents may produce false residual currents causing ground fault relay (GFR) operation, an instantaneous GFR with a higher pickup should be substituted, or the time overcurrent relay should have a larger time dial or pickup setting.
2.2. ONE CURRENT TRANSFORMER (CT) IN THE GROUNDED NEUTRAL
A ground fault relay (GFR or 50N) that is connected to a current transformer located in the grounded neutral of a Power Transformer or AC Generator, provides a convenient, low-cost method to detecting ground fault currents. This scheme is widely applied on 5 kV up to 15 kV systems where low-resistor grounding is frequently used, and the ground fault current is as low as 200 Amperes. The GFR can be set to minimum values of current pickup and time delay to be selective with load-side feeder GFRs. This method is also applied on solidly grounded systems, 480 Volts, 3 phase/ 3 Wire or 3 phase /4 wire systems. One advantage that this scheme provides is the fact that false residual currents do not occurs and do not cause relay operations.
The Figure 2 is a typical scheme of a neutral connected GFR.
2.3. ZERO-SEQUENCE GROUND FAULT RELAY
The Figure 3 depicts a typical zero-sequence ground fault relay scheme.
Figure 3 Zero-sequence Ground Fault Relay
One advantage of this scheme is that the CT Ratio (CTR) is not dictated by the load current, it avoid the possible difficulties of unequal individual CTs saturation. The disadvantage is the limitation of the size of conductors that can be passed through the window of the toroidal CT. The standard ratio for toroidal CTs are: 50/5 and 100/5.
The zero-sequence CT is commonly used with a 0.25 Amperes instantaneous ground overcurrent element (50G). The combination provides a primary pickup of 5 Amperes, rather than 2.5 Amperes.3. GROUND OVERCURRENT PROTECTION
The lowest fault current (IF-1Ø), returning through the Earth to the Substation Neutral, corresponds to a single-line-to-ground fault located downstream of the backed-up power line section under minimum generation conditions.
ü Ground overcurrent relays have more sensitivity than phase overcurrent relays, because they are usually set at 10% to 20% of the sensitivity of phase relays.
ü Ground overcurrent relays (50G or 50N) usually can be set and coordinated independently of the phase overcurrent relays (50P/51P).
ü The application and coordination of ground overcurrent relays are the same as for phase relays.
ü As with instantaneous phase overcurrent relays (50P), instantaneous ground trip (50G or 50N) can be used to improve relaying, particularly for close-in ground faults.
ü Ground relays are not affected by out-of-step conditions.
ü The higher zero-sequence line impedance Z0L, as compared with the positive-sequence line impedance Z1L, may allow to use a high-set ground overcurrent element and make coordination easier than for phase faults.
ü The zero-sequence isolated system may make coordination easier.
ü The NEC Art. 230.95 requires the implementation of a ground overcurrent relay at least at the supply end of a low-voltage systems if the neutral is solidly grounded, and voltages in range of more than 150 volts to ground but not exceeding 600 volts phase-to-phase for each service disconnect rated 1,000 Amperes or more.
ü THE MAXIMUM SETTING OF THE GROUND-FAULT PROTECTION SHALL BE 1,200 AMPERES, and the maximum time delay shall be one (1) second for ground fault currents equal or greater than 3,000 Amperes.
ü There is no minimum setting for ground overcurrent relays. Even though, for neutral connection schemes a range of 200 A to 400 A is widely used.. On the other hand, for zero-sequence ground fault schemes a range of 30 mA to 50 A is frequently used.