“Ground faults can be very dangerous. Short-circuit grounding in a directly grounded system can generate large currents, damage the equipment and cause it to stop running. Ground faults can also produce arc flashes, which can cause serious injury to nearby personnel and damage to equipment. The danger of arc flash is the reason that warning labels must be affixed to the electrical cabinet and that anyone working on the energized panel must wear appropriate personal protective equipment (PPE). In addition, the arc flash may not have enough current to quickly trip the overcurrent protection device.
How to use sensitive relays to find ground faults that are difficult to find in VFD resistance grounding systems?
Ground faults can be very dangerous. Short-circuit grounding in a directly grounded system can generate large currents, damage the equipment and cause it to stop running. Ground faults can also produce arc flashes, which can cause serious injury to nearby personnel and damage to equipment. The danger of arc flash is the reason that warning labels must be affixed to the electrical cabinet and that anyone working on the energized panel must wear appropriate personal protective equipment (PPE). In addition, the arc flash may not have enough current to quickly trip the overcurrent protection device.
One way to reduce or eliminate many ground fault problems (including most arc flash events) is to use a high resistance ground (HRG) system where the neutral point of the transformer (star-connected transformer or generator X0 point or zigzag transformer The artificial neutral wire) is connected to the earth through the neutral ground resistance (NGR), as shown in Figure 1. NGR limits the ground fault current to a low value, and in many cases, will allow the system to continue to operate. By eliminating phase-to-ground faults, the HRG system eliminates 95% of the arc flash hazard. The mining industry has long required the use of HRG systems.
Figure 1: In a high resistance grounding system, the neutral point of the transformer (or the artificial neutral line of the zigzag transformer) is grounded through the neutral point grounding resistance.
Even though high-resistance grounding brings great benefits in improving safety, there are still some important things to consider, including many factors that may make detecting low-level ground faults difficult. Some of these factors will be exacerbated by the use of frequency converters (VFD).
This article will introduce some aspects of how the HRG system works, discuss the various challenges faced by ground fault monitoring in the HRG system, and show how to solve these technical challenges.
Detection of ground faults
Since the HRG system can prevent ground faults from causing overcurrent trips, there must be a way to detect when a ground fault occurs. The best way to detect a ground fault in a grounded system is to use a current-sensing ground fault relay (GFR), which uses a magnetic potential balance zero sequence current transformer (CT or ZSCT) to detect currents that should not exist. In this way, selective coordination can be carried out and ground faults can be found.
Any through-type current transformer will become a magnetic potential balance zero sequence CT when all current-carrying conductors pass through the CT window. If there is no ground fault, the output of CT is zero. If there is a ground fault, the current will not increase to zero, and the difference can be detected and used to signal an alarm or trip the faulty part.
Many factors may limit the ability to measure low-level ground fault currents. These factors include system capacitance, unbalanced single-phase load, current sensor limitation, low frequency operation, harmonic components and VFD carrier frequency.
All electrical systems have capacitance to ground. Although the capacitance is actually distributed on the system, it is usually modeled as a “lumped” value, as shown in Figure 2. On a grounded system, if all three-phase capacitances are equal, the magnetic potential balance CT reading together will be zero, but if the capacitance or the phase-to-ground voltage are not equal, the CT will have a non-zero output. Unless the trip threshold is increased, it will cause a fault trip and reduce the ability to detect low-level ground faults. This explains to a large extent the reason why it is impossible to achieve personnel protection levels in industrial systems.
Figure 2: If the distributed capacitances of all three phases are equal, the magnetic potential balance CT reading of the three phases together is zero, but if the capacitance or the phase-to-ground voltage are not equal, the CT will have a non-zero output value.
When the phase of the ungrounded system is short-circuited to ground, current will flow through the capacitor to the other two phases to ground, as shown in Figure 3. Note that the magnetic potential balance CT at point “1” will measure the charging current, and the magnetic potential balance CT at point “2” will measure zero. The charging current may be large enough for the ground fault relay to act on the unfaulted feeder. In order to avoid this so-called sympathetic trip, the protection of these feeders must be set above the level of the charging current; if sympathetic operation is acceptable, the protection level can be set lower.
If the line-to-line voltages of the three phases are unequal, the current through the phase capacitors will also be unequal, resulting in a steady-state zero-sequence current. This voltage imbalance may be the result of an unbalanced single-phase utility load. Its impact is usually small, but it may affect the detection of low-level ground fault currents. It is worth noting that in the absence of current leakage to ground, the unbalanced load current will not cause a ground fault trip, because the phase current in the magnetic potential balance CT will increase to zero.
Current mutual inductance limit
Current transformers have their own limitations for low current and high current. At the low current end, there is a small primary current that will produce an output, and under certain conditions, this makes it necessary to use a dedicated ZSCT. At the high current end, there may be a problem of core saturation, and the output is no longer proportional to the primary current. Even if the primary current is not excessive, if the main conductor is poorly placed or there is a large inrush current, the transformer core part may enter a saturated state, thereby preventing the balance current from adding correctly and generating an output when there is no zero sequence current. One way to reduce this effect is to use a flux regulator, which is a magnetic sleeve suitable for CT windows, which can reduce local saturation. It is also helpful to center the phase conductors in the CT window and tie them together with ABC, ABC, etc. (instead of AA, BB, CC, etc.).
Figure 3: When one phase of an ungrounded system is short-circuited to ground, current will flow through the capacitor to the ground of the other two phases. This charging current may be large enough to cause the mutual inductance of the unfaulted feeder to trip.