TECHNICAL FIELD

This invention relates to a system for reducing risk of fire by a municipal power grid. In particular, the invention relates to a circuit for electrically isolating a fault in the grid infrastructure and diverting current away from the source of a fault.

BACKGROUND

Modern civilization is reliant on the wide-scale distribution of affordable power. This power is distributed in the form of electricity to our homes and businesses by an expansive power distribution network including power plants, substations, and power lines. This power, typically in the form of three phase electricity, is commonly mounted atop power poles that branch out from power plants and into our communities and down our suburban streets. Although incredibly safe, the power lines are sometimes susceptible to fault events such as trees, mylar balloons, car accidents, natural disasters, etc. For example, trees sometimes can grow in directions and make direct connect with power lines, automobiles sometimes collide with power poles, and tornadoes sometimes topple power lines to the ground. In each of these circumstances, high voltage lines may come in contact with potentially flammable materials and cause ignition.

A typical situation is shown inFIG. 1which illustrates a transformer100with a primary102and a secondary120in the form of a grounded wye. The wye secondary includes three coils or windings130a-130c, each of which is connected to one line of a three phase power line110a-110con the outputs of the secondary120. The coils130a-130cconnected at a common point to form a neutral, N. When one line1110cof the three phase power line comes in contact with an object150, for example, a fault current runs from ground, through the neutral, through the third power line130c, and back to earth through the object150. The fault current, FC1, is enabled, in part, by the transformer ground connection.

Concerning fault currents can arise in ungrounded transformers, as well, as illustrated in the ungrounded wye secondary120shown inFIG. 2. When an object150comes in contact with one110cof the three phase wires, the fault current that runs to ground through the object may be supplied by the other two lines (phases)110a,110b. Where these wires110a-110care part of an overhead power line, the overhead lines are capacitively coupled to the ground. The longer the power lines, the greater the capacitance. This capacitance, represented by capacitors140a,140b, may then complete the path for current through the object150via the transformer's windings. As such, the ungrounded transformer120actually facilitates the flow of the fault current FC2through the object.

In the scenarios shown inFIGS. 1 and 2, the fault current may result in a risk of fire ignition or other consequences. There is therefore a need for a power distribution network that reduces this fault current and fault energy to minimize the fire ignition risk for a category of power circuits in some circumstances.

SUMMARY

The invention in some embodiments features a novel system for reducing ground fault currents in a municipal power grid, thereby reducing the risk of igniting a fire where ground faults occur in powerline networks. This includes faults related to powerlines contacting or in arcing proximity to grounded equipment/objects, foreign objects bridging connections between a power line to grounded equipment/objects, and a powerline in contact with earth. The circuit comprises an isolation transformer, a neutral connection, a current compensating device, and a protection system. The isolation transformer includes a primary side connection to the three phase input from the power grid, and a secondary side connected to a section of distribution circuitry. The isolation transformer effectively has a one-to-one ratio between the input and output line-to-line voltages. The phase angle is constant which results from a delta-delta or delta-zigzag transformer. The current compensating device is connected to the neutral and configured to redirect a substantial portion of a fault current to a ground electrode instead of the ground fault connection location. The current compensating device may comprise of inductor(s), an arc suppression coil, and/or an inverter tuned to match the capacitance of the three phase outputs. When delta-delta isolation transformer connection is used, a grounding bank connected wye-grounded delta can be employed to provide a neutral connection for an additional current compensating device. The protection system includes a circuit breaker. e.g., an automatic recloser with a relay, is configured to clear the fault and transmit alerts if necessary. The relay is configured to measure a voltage of the isolation transformer from the transformer neutral or the neutral of a grounding transformer.

In some embodiments, the current compensating device comprises of inductor(s) or an arc suppression coil connected to the transformer neutral and a ground electrode. In other embodiments, the current compensating device comprises an inverter configured to inject current to redirect some or all of the fault current to the grounding electrode. In some embodiments, the isolation transformer comprises the neutral and the current compensating device is connected to the isolation transformer neutral. In other embodiments, the circuit includes a grounding bank with a neutral, and the current compensating device is connected to the neutral of the grounding bank.

The protection system comprises of a fault interrupting device(s), measurement devices, and a relay(s) Examples of fault interrupting devices include circuit breaker(s), automatic reclosers, and fault interrupters. Examples for measurement devices include current transformers, potential transformers, and resistive or capacitive sensors. Relays are typically connected to accept the measurement devices and provide control signals to the fault interrupting devices. The protection system may further include at least one fuse.

The present invention may be installed in proximity to fire hazard areas to reduce the probability of the power grid unintentionally igniting a wildfire or other fire from single phase ground faults to the high voltage power line. It is believed, using the present invention, that the risk of starting a fire from single phase to ground faults is reduced by approximately 90 percent relative to conventional bare wire electric distribution systems in operation currently.

