1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to such electrical switching apparatus including a plurality of Rogowski coils. The disclosed concept also pertains to methods of calibrating electrical switching apparatus including a plurality of Rogowski coils.
2. Background Information
A Rogowski coil is an electrical device for measuring alternating current (AC) or high speed current pulses. The Rogowski coil consists of a helical coil of wire, wound around a nonmetallic core.
Another type of Rogowski coil includes first and second helical coils of wire (loops) electrically connected in series with each other. The first loop is wound with a substantially constant winding density in a first direction around a core that has a substantially constant cross section. The second loop is wound with a substantially constant winding density in a second direction around a core that has a substantially constant cross section. A conductor (e.g., a power line) whose current is to be measured traverses through the loops. A voltage may be induced in the coil based on the rate of change of the current running through the power line. Rogowski coils may have other configurations as well. See U.S. Pat. Appl. Pub. No. 2009/0115427.
Pat. Appl. Pub. No. 2009/0115427 also discloses that a Rogowski coil may include an air core (or other dielectric core) rather than an iron core, which gives the coil a low inductance and an ability to respond to fast-changing currents. Further, the Rogowski coil typically is highly linear, even when subjected to large currents, such as those of low voltage and medium voltage power lines. By forming the Rogowski coil with equally spaced windings, effects of electromagnetic interference may be substantially avoided.
The voltage that is induced in the Rogowski coil is proportional to the rate of change of current in the conductor. The output of the Rogowski coil is usually connected to an integrator in order to provide an output signal that is proportional to current.
For sensitive ground fault detection applications, two sets of Rogowski coils are electrically connected in series. The first set of Rogowski coils is used for motor protection and the second set of Rogowski coils is used for ground fault protection.
Power systems are grounded to prevent transient overvoltage caused by arcing ground faults. However, if a power system is solidly grounded, then the ground faults can be relatively very high (e.g., without limitation, thousands of amperes) and cause extensive damage to conductors, motors and other loads. Most power systems have adopted a middle ground of using 200 ARMS or 400 ARMS grounding resistors, which are 10 second rated at a 4160 VACRMS level. This limits the damage and allows ground fault detection systems to be relatively simple and provide positive detection without nuisance tripping.
High resistance ground systems limit the ground current to 10 ARMS to 25 ARMS. The purpose of high resistance grounding systems is to limit damage due to ground faults to conductors and motors of the power system while controlling transient overvoltages due to arcing ground faults. Often, users of the power system will only alarm on ground faults and plan an outage of the equipment with the ground fault rather than shutting down the process in the middle of a batch, thereby destroying lots of incomplete product.
Typical medium voltage power systems are grounded to limit collateral damage to equipment due to overvoltage during a ground fault. Most of these systems limit the current flowing in the ground fault for one or more of the following reasons: (1) to reduce the burning and melting effects in faulted electric equipment, such as switchgear, transformers, conductors and rotating machines; (2) to control transient overvoltages while at the same time avoiding the shutdown of a faulty circuit on the first occurrence of a ground fault (e.g., high-resistance grounding); and (3) to limit the damage and allowing less expensive repair often avoiding having to replace the equipment by setting a ground fault relay 2 (FIG. 1A) to trip at ground fault current levels of as low as 5 ARMS.
FIG. 1A shows a ground return method, which does not discriminate as to which starter load (not shown) has a fault. This is used as a backup in case a ground fault is not cleared by an individual starter (not shown) or if a ground fault occurs in the power bus or conductor (not shown) upstream of a starter. The ground return method looks at the ground current as it enters the transformer neutral 4 and is used for a power system (not shown) including many different starters and circuit breakers (not shown).
As shown in FIG. 1B, a zero sequence method uses a separate current transformer 6, through which pass all three phases 8,10,12 (e.g., without limitation, motor leads; transformer primaries) and the neutral 4. The corresponding magnetic fields cancel and only the imbalance of current (i.e., the ground current) becomes the secondary current. Such a separate current transformer 6 is relatively large. Since the size of medium voltage starters decreases with each generation as newer technologies remove heat and improve insulation systems, finding space in the starter (not shown) for such a separate current transformer is problematic.
FIG. 1C shows another method of ground fault detection called the differential current or residual method. The secondaries of all of the phase and neutral current transformers 14,16,18,20 are paralleled and the imbalanced current flows in a ground fault relay section 22 of the loop. When using current transformers with magnetic cores, there are errors due to differences in the current transformers such as saturation and winding errors. This can cause a false indication of a ground fault of up to 5% of the corresponding phase currents. When a three-phase motor starts, the current is six to eight times the full load current. Typical starters are designed up to 800 ARMS. This means that there can be currents flowing up to 6400 ARMS. A 5% error means that this method can give a false indication that a 320 ARMS ground fault is present when none is there. However, it is desirable to set the ground fault relay 22 to trip at ground fault current levels as low as 5 ARMS.
There is room for improvement in Rogowski coils.
There is further room for improvement in electrical switching apparatus including Rogowski coils.