BACKGROUND Aground fault"is a condition where. due to component failure. an inadvertent path to earth ground or other system components is formed such that uncontrollable current or hazardous high voltage can exist within the product or system. Such conditions can lead to destruction of the product and life safety risks (e. g.. electrocution) to personnel who come in contact with the system components. Due to these life safety risks, safety standards such as the National Electrical Code (NEC) and Underwriters Laboratories (UL). among others, require that such ground faults be detected in high voltage and high power equipment and appropriate remedial action taken when ground faults are detected.

In one common scenario of a ground fault, current flows within insulated conductors enclosed in an underground (buried) metallic conduit. These conductors may be feeding a transformer at a nominal current flow of 1. 000 RMS AC amps. The conductors and associated equipment are protected from overcurrent damage by a circuit breaker or fuse. The circuit breaker or fuse is typically set to open at a current above the maximum nominal current in the system. In such a system, should the cable insulation within the conduit fail, electrical current will also flow to the conduit and subsequently toward the earth ground. However, due to the relatively poor electrical conductivity of the steel conduit and the surrounding soil, the resulting (fault) current in the conductor may not be sufficient to trip the overcurrent protective device.

Consequently, the fault current will heat the conductor. conduit, adjacent insulation or other components. As the insulation is typically composed of a polymer with an ignition temperature around 300C, a fire will start in the conduit. The fire will spread to other equipment and potentially destroy the entire installation.

Besides the obvious hazard of fire. there is another safety hazard associated with ground faults. The current flowing in the conductor gives rise to a voltage proportional to the product of the current and the conductor's resistance to ground.

Thus. the conduit in the above example. and other equipment electrically attached to i.. will be energized with a voltage high enough to be hazardous to personnel who come in contact with it. Both the hazard of fire and the hazard of contact with energized surfaces are the basis of the cited life safety standards and requirements.

SUMMARY The invention features a safe and reliable approach for detecting ground fault current in an electrical system, such as utility recovery systems connected to utility power networks.

In a general aspect of the invention, a ground fault system for use with an electrical svstem connected to a utility power network includes a circuit interruption device. positioned to receive a ground fault current flowing either through a DC portion and an AC portion of the electrical system ; and a controller, responsive to the circuit interruption device, which in response to excess fault current flowing through the circuit interruption device, disconnects the electrical system from the utility power network. The electrical system includes a power converter connected between the DC portion and the AC portion with the ground fault system electrically connected to the power converter.

Another aspect of the invention relates to an electrical system for connection to a utility power network, including a power converter connected between a DC portion and an AC portion of the electrical system and the ground fault system described above.

Embodiments of these aspects of the invention may include one or more of the following features.

The converter is an inverter, which converts the DC signals carried in the DC portion to the AC signals carried in the AC portion. The circuit interruption device is a fuse. The fuse includes an auxiliary switch. which disconnects the electrical system from the utility power network.

The controller is electrically isolated from the interruption device. for example. using an optical coupler. A clamping network is used to limit the voltage of signals received by the optical coupler.

The electrical system may be a utility recovery device having a DC power source (superconducting magnet, capacitor bank, battery) and an inverter.

In another aspect of the invention a method of protecting a utility recovery system connected to a utility power network includes the following steps. The flow of excess ground fault current through either of the DC portion and the AC portion is detected. Flow of current through the utility recovery system is then prevented in response to determining the flow of excess ground fault current. The utility recovery system is generally of the type including a DC portion for carrying DC signals and an AC portion for carrying AC signals.

Among other advantages, the ground fault system provides a simple. relatively low cost approach for detecting fault currents in electrical systems which carry both DC and AC currents. For example, utility recovery systems used in conjunction with utility power systems to supply a power (and in some cases absorb power) from distribution network of the utility power network. Such voltage recovery systems are used to stabilize voltage on the utility power network (e. g., D-SMES applications) or to maintain sufficient power to a factory load connected to the distribution network due to momentary sags on the utility power network (e. g., PQ IVR applications). A D-SMES (Distributed Superconducting Magnetic Energy Storage) device is used to protect transmission systems from instability and collapse while a PQ IVR device protects large industrial facilities from momentary voltage sags on the utility power network.

Both are products of American Superconductor Corporation, Westboro. MA.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects. and advantages of the invention will be apparent from the description and drawings and from the claims.

DESCRIPTION OF DRAWINGS Fig. ! is block diagram of a utility recovery system connected to a utility power system.

Fig. 2 is block diagram of a ground fault system of the utility recovery system of Fig. 1.

Fig. 3 is block diagram of the utility recovery system of Fig. 1 showing a ground fault located in the AC portion of the system and the ground fault current path.

Fig. 4 is block diagram of the utility recovery system of Fig. 1 showing a ground fault located in the DC portion of the system and the ground fault current path.

Fig. 5 is a perspective view of a fuse used by the ground fault system of Fig. 2.

DETAILED DESCRIPTION Referring to Fig. 1. a utility recovery system 10 is shown connected in shunt with a utility distribution line 12 of a utility power network. In this embodiment, the utility recovery system depicted is a distributed SMES or D-SMES module. As will be discussed in greater detail below, utility recovery system 10 includes a ground fault detection system 100 connected between the mid-point of an inverter array 14 and earth ground to detect the presence of ground fault current in both the AC and DC portions of the system. If the magnitude of the ground fault current is above a predetermined threshold, ground fault detection system 100 quickly ensures that the current is interrupted. thereby reducing the risk of damage (e. g.. due to fire) to the utility recovery system as well as reducing risk of injury (e. g., electrical shock) to personnel in the vicinity of the system.

Inverter array 14 includes twelve 250 Kwatt inverter modules 16 connected in a 3 X 4 matrix arrangement. Each inverter module in each of the four rows of array 14 converts DC voltage delivered from a superconducting magnetic energy storage unit 18 to AC voltage, passes the AC voltage through a filter 20 and provides the filtered AC voltage to distribution line network via a summing transformer 22.

Each power transformer 22 is a 24.9 KV/480 V three-phase oil filled pad mount transformer having a nominal impedance of 5.75% on its own base rating. The power transformers are generally mounted outdoors adjacent to the system enclosure with power cabling protected within an enclosed conduit (not shown). Depending on the distribution

voltages carried on distribution line 12. transformer voltages other than 24. 9 kV are available. Positioned between each of the four rows of the inverter modules and transformers 22 is a shunt-trip circuit breaker 24. Each circuit breaker 24 (or switchgear unit) provides over-current protection between power transformers 22 and inverter array 14 and. in this embodiment, is rated at 480 V. 900 A RMS continuous per phase with 65 kA interruption capacity. The circuit breakers are generally mounted adjacent to the inverter unit enclosures. As will be described below circuit breakers 24 serve as the primary disconnect means for safety and maintenance purposes.

Superconducting magnetic energy storage unit 18 includes an energy storage magnetic coil 26 positioned within a containment vessel 28 of a cryogenic refrigerant unit. Containment vessel 28 maintains magnetic coil 26 in liquid helium. and is fabricated of two austenitic stainless steel vessels separated by a vacuum insulated space. In the embodiment shown, the cryogenic refrigerant unit includes two Gifford-McMahon type compressors (not shown), operating in concert. to maintain operating temperatures within vessel 28 and to re-liquify any helium vapor building up within the vessel. No helium (liquid or gaseous) circulates outside vessel 36 under normal operating conditions. The external, room temperature, refrigeration system gasses are not interchanged with the internal helium supply. The system design permits continued system operation, with one or both compressors inoperable. for a minimum of 48 hours.

Superconducting magnetic coil 26 is fabricated from superconducting cable formed from niobium-titanium copper matrix wire, cabled into a mechanically stable form. and insulated prior to winding. A superconducting magnetic coil well-suited for use with utility recovery system 10 is available from American Superconductor Corporation. Westborough, MA. A magnet power supply 38, for charging magnetic coil 26 is available from Dynapower Corporation of South Burlington. VT. In this embodiment, power supply 38 delivers DC power to magnetic energy storage unit 18.

Superconducting magnetic energy storage unit 18 interfaces with inverter array 14 through steering diodes 40 which ensure that power flows only from the energy storage unit to the inverter arrav and not in the opposite direction. In certain applications. a magnet interface including a voltage regulator is connected between the energy storage unit and inverter array.

Referring to Fig. 2. ground fault detection system 100 includes a fuse 102 connected between the mid-point of inverter array 14 and earth ground. In this embodiment. fuse 102 has a 100A rating which establishes the fault threshold of the system. That is, when a ground fault current exceeding 100 A flows through fuse 102. the fuse opens ("blows") and ground fault system operates to interrupt power to voltage recovery system 10 in a safe and reliable manner. In parallel with fuse 102 is a opto- coupler 104 which receives the voltage across fuse 102 through a voltage detection circuit 106. Voltage detection circuit 106 includes a pair of back-to-back zener diodes 108. which are driven by current through a 5. 6 KS2 current limiting resistor 110. Zener diodes 108 establish a safe voltage capable of driving opto-coupler 104 through a 470 ohm resistor 111. Opto-coupler 104 is optically coupled and electrically isolated from a microcontroller 112. Microcontroller 112. is connected to a relay 1 4 which in turn. is connected to the shunt-trip mechanism circuit breakers 24.

More specifically, in operation, when a ground fault current in excess of the fault threshold flows through fuse 102, fuse 102 opens. A relatively high voltage appears across the blown fuse and is reduced to a safe operating voltage received by network 106 and opto-coupler 104 which generates an optical signal. Microcontroller 112. in response to the optical signal, generates a trigger signal for energizing relay 114 to activate the shunt-trip operation of circuit breakers 24.

It is important to note that ground fault system 100 is appropriately positioned to detect ground fault current originating from the AC portion of voltage recovery system 10 (i. e., from the multiple AC power feeds) as well as from the DC portion (i. e., superconducting magnetic energy storage unit) of the voltage recovery system.

An important and advantageous feature of ground fault system 100 relates to the stacked nature of inverter array 14. Specifically, the 480 VAC output of the uppermost row of inverter modules 16 has a DC offset voltage of 1, 125 Volts from the center of the Wye connected 480V windings of transformer 22 to ground. This is marked as point"X"in Fig. 1. Likewise, the row of inverter modules 16 has a 375 VDC offset to ground on its 480 VAC output. The problem is that the main circuit breakers 24 are rated to break the 480 VAC component of the output voltage but are not rated to the high voltage DC component. Were the circuit breaker to attempt to interrupt. 1. 125 VDC. for example, the internal contact separation would be insufficient to extinguish (break) the current arcing across the open contact of the circuit breaker. likely leading

to failure of the circuit breaker. Ground fault system 100 solves this problem by utilizing fuse 102 to provide both DC and AC interruption capability of a voltage greater than is available with the system. Since, by design, fuse 102 clears before the main circuit breakers open. the interruption of the DC voltage and ground fault current takes place within the fuse. The circuit breakers now only need to disconnect the 480 VAC current for which they are rated.

For example, referring to Fig. 3, a ground fault is shown occurring due to an electrical short within the conduit leading to (or within) one of transformers 22.

Ground fault current (represented by arrow 30) flows through one or more of inverter modules 16 of inverter array, along mid-point 15. and then to ground fault detection system 100. As discussed above, if the ground fault current exceeds the predetermined fault threshold (in this case, 100A), fuse 102 of ground fault detection system will open. thereby triggering disconnection of circuit breakers 24.

Referring to Fig. 4, a ground fault is shown occurring due to insulation failure within superconducting magnet 26 or associated cabling connected to the magnet.

Even in this case, ground fault current (represented by arrow 32) flows through one or more of inverter modules 16 of inverter array, along midpoint 15. and then to ground fault detection system 100. In the event of a ground fault occurring within the DC portion of the circuit, it is generally necessary to protect the DC power source. For example, in this embodiment, superconducting magnet 26 is discharged in response to a detected ground fault. In this case, microcontroler 112 can be used to control magnet 26 to discharge its stored energy. If the DC power source is a batterv or fuel cell. microcontroller 112 can be used to control a DC rated relay or circuit breaker with shunt-trip capability to disconnect the source from the system.

Referring again to Fig. 2, in this embodiment and in accordance with safety agency standards, ground fault system 100 provides redundancy in the event that the control electronics described above should fail. In particular. an electromechanical switch 120 is used to open circuit breakers 24 thereby protecting the utility recovery system from damage. In particular, electromechanical switch is activated when fuse 102 opens and redundantly activates the shunt trip mechanism associated with circuit breakers 24. Switch 120 can also be used to trigger discharging or disconnection of the DC power source.

Referring to Fig. 5. fuse 102 and electromechanical switch 120 are shown combined in a single unit 130. Fuse 102 includes a pair of terminals 132a. 132b connected to the inverter array and earth ground, respectively. Switch 120 includes an actuation mechanism 134 for opening the contacts 136 leading to the shunt trip mechanism along a redundant path.

Other embodiments are within the scope of the claims. For example. in the above embodiment, ground fault system is shown in use in conjunction with a D-SMES system. The ground fault system. however. can be used in other utility recovery systems associated with utility power systems. For example, the ground fault system can also be used in a PQ-IVR system in which the utility recovery system is used to support a factory load connected to a distribution network in the event of momentary sags of power on the utility powder network.

In addition, although ground fault system 100 described above used a voltage detection and opto-coupler arrangement, in certain applications, the microcontroller can be connected in a more direct fashion to the fuse without the need for electrical isolation.

Still other embodiments are within the scope of the claims.