PHOTOVOLTAIC ARRAY GROUND FAULT DETECTION METHOD FOR UTILITY-SCALE GROUNDED SOLAR ELECTRIC POWER GENERATING SYSTEMS

• US 20110199707A1
• Filed: 02/15/2011
• Published: 08/18/2011
• Est. Priority Date: 02/16/2010
• Status: Active Grant

• Current Assignee: Parker Intangibles LLC (Parker-Hannifin Corporation)
• Original Assignee: Greenvolts, Inc.
• Inventors: SELCHAU-HANSEN, VIGGO, HINMAN, BRIAN, KAZEMI, HOSSEIN
• Granted Patent: US 8,564,916 B2
• Time in Patent Office: Days
• US Class Current: 361/47
• CPC Class Codes: H02H 3/33 using summation current transformers H02H3/347 takes precedence
• CPC Class Codes: H02H 7/1222 responsive to abnormalities in the input circuit, e.g. transients in the DC input
• CPC Class Codes: H02H 7/20 for electronic equipment for converters H02H7/10; for electric measuring instruments G01R1/36; for dc voltage or current semiconductor regulators G05F1/569; for amplifiers H03F1/52; for electronic switching circuits H03K17/08
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers

Abstract

Various methods and apparatus are described for a photovoltaic system. In an embodiment, a hybrid grounding circuit as well as a ground fault monitoring circuit are in the inverter circuitry with its switching devices that generate three-phase Alternating Current (AC) voltage. The three-phase AC voltage is supplied to a utility power grid interface transformer, where a primary-side common node of the Utility Power grid interface transformer is connected to Earth ground. Each inverter has 1) its own set of isolation contacts to connect as well as isolate this particular inverter from the utility grid interface transformer, and 2) control components in the ground fault monitoring circuit for controlling operation of the isolation contacts based off a presence of the ground fault detected by the ground fault monitor circuit for that inverter. The inverter circuit receives a DC voltage supplied from its own set of ungrounded Concentrated PhotoVoltaic modules.

Claims

1. An apparatus for a photovoltaic system, comprising:

a hybrid grounding circuit as well as a ground fault monitoring circuit in inverter circuitry with switching devices that generate three-phase Alternating Current (AC) voltage supplied to a utility power grid interface transformer, where a primary-side common node of the Utility Power grid interface transformer is connected to Earth ground, where each inverter has

     1) its own set of isolation contacts to connect as well as isolate this particular inverter from the utility grid interface transformer, and

     2) control components in the ground fault monitoring circuit for controlling operation of the isolation contacts based off a presence of the ground fault detected by the ground fault monitor circuit for that inverter, where the inverter circuit receives a DC voltage supplied from its own set of ungrounded Concentrated PhotoVoltaic (CPV) modules.

2. The apparatus for a photovoltaic system of claim 1, further comprising:

where the ground fault monitoring circuit is configured to detect the presence of the ground fault in the ungrounded CPV modules that supply Direct Current (DC) power to grounded inverter circuit, where the inverter circuitry uses a Space Vector Modulated bridge switches, typically generating 480 V AC three phase power, that directly couple to the utility power grid transformer, without connection through an isolation transformer and then to the utility grid transformer, in where a neutral wire of the primary side of the utility power grid interface transformer is referenced to Earth ground, and where multiple solar arrays, each with their one or more inverter circuits, directly couple their three phase AC output to the same utility grid transformer.

3. The apparatus for a photovoltaic system of claim 1, where multiple solar arrays, each with their one or more inverter circuits, directly couple their three phase AC output to the same utility power grid interface transformer, and

where each inverter circuit in the grounded system has its own ground fault monitoring circuit, where the ground fault monitoring circuit detects the presence of the ground fault via a change in 1) voltage, 2) current or 3) both and the detection method occurs via a differential sense method.

4. The apparatus for a photovoltaic system of claim 1, where in the hybrid grounding circuit when in operation producing AC, the inverter circuit gets referenced to 1) the primary-side common node of the Utility Power grid interface transformer which is connected to Earth ground during the operation of the inverter in this hybrid grounding circuit and 2) referenced to Earth ground by a grounding switching device in an input circuit of the inverter circuitry prior to the switching devices of the inverter producing three phase AC power.

5. The apparatus for a photovoltaic system of claim 1, where the isolation contacts and control components of the ground fault monitoring circuit are configured to prevent the 1) disconnection of the entire solar power generating system or 2) disconnection of an inverter group from the utility power grid interface transformer due to a ground fault occurring in an individual inverter circuit or its associated CPV modules, by a localization of the ground fault to 1) a specific inverter circuit from an inverter group coupling to the utility grid power transformer or 2) even more specifically, a specific set of CPV modules feeding a specific inverter circuit, which also reduce corrective maintenance costs.

6. The apparatus for a photovoltaic system of claim 1,

where the inverter circuit receives a bipolar DC voltage supplied from its own set of CPV modules,

where a switching device in the input of the inverter circuit is used to create the common reference point for the positive VDC and the negative VDC inputs from the PV array, and

where the ground fault monitoring circuit localizes of the ground fault to a specific set of CPV modules feeding a specific inverter circuit by when the detected ground fault voltage has a negative voltage component, then the ground fault is coming from the set of CPV modules supplying the negative DC voltage;

and likewise when the detected ground fault voltage has a positive voltage component, then the ground fault is coming from the set of CPV modules supplying the positive DC voltage.

7. The apparatus for a photovoltaic system of claim 1,

where the inverter circuit receives a bipolar DC voltage supplied from its own set of CPV modules, and

where the hybrid grounding circuit includes an input DC grounding circuit located in each inverter circuit that electrically couples to a string of CPV cells from the solar array and the primary-side common node of the Utility Power grid interface transformer connected to Earth ground, where the electrical components in the input DC grounding circuit include 1) one or more switching devices, one or more load resistors, and one or more over current devices to cause the DC power from that string of CPV cells to be connected to Earth ground when the inverter circuit is not producing three phase AC power out, and the electrical components in the input DC grounding circuit also includes 1) a contact 2) switch or 3) both to create a dynamic common zero Volts DC reference point for the bipolar DC voltage supplied to that inverter from its set of CPV modules.

8. The apparatus for a photovoltaic system of claim 1, where the ground fault monitoring circuit contains a DC imbalance sensing circuit to assist in fault protection in determining which set of CPV modules supplying power to the common utility grid transformer has a ground fault both 1) prior to inverter start up and 2) during the inverter circuit operation while producing the three phase AC voltage.

9. The apparatus for a photovoltaic system of claim 1, where the ground fault monitoring circuit is configured to detect PV array ground leakage before the isolation contacts of the inverter circuit connect to the utility power grid interface transformer, which prevents disconnecting all of the inverter circuits in the entire system due to a ground fault on a single set of CPV modules.

10. The apparatus for a photovoltaic system of claim 1, where the hybrid grounding circuit in the inverter circuit uses relays to maintain the set of CPV modules from the solar array at a safe voltage while the inverter circuit is disconnected from the grid by connecting a DC output of the set of CPV modules feeding that inverter through a switching device to an Earth ground.

11. The apparatus for a photovoltaic system of claim 10, where the hybrid grounding circuit in the inverter circuit couples the set of CPV modules to an Earth Ground in the input of the inverter circuit through the switching device, a load resistor, and an over current device, and where the where switching device may actuate, A) by electrically opening a contact or switch, B) electrically by closing a contact or switch, C) by the switching device starting to conduct, and D) by any combination of the three, to create an electrical path.

12. The apparatus for a photovoltaic system of claim 10, where the hybrid grounding circuit in the inverter circuit has DC common reference point control relay actuates one or more contacts to create a common DC reference point for a bipolar DC voltage input from the CPV modules of a solar array while the inverter circuit is turned off, and also while the inverter logic is powered but the inverter circuit is disconnected from the grid and not operating, and when in these conditions, the inverter bridge switches are open, resulting in an electrical open circuit at each CPV array pole, and consequently, no current flows in the CPV solar array even if well-illuminated by sunlight because a complete ground path cannot be established, and

where an input of the ground fault monitoring circuit of each inverter circuit is equipped with a residual current monitor (RCM).

13. The apparatus for a photovoltaic system of claim 12, where the ground fault monitoring circuit detects when a ground leak occurs in the CPV modules, a ground current will flow through the CPV modules, ground fault resistance, a load resistor, a fuse, and the switching device completing the ground path between the normally ungrounded CPV modules and Earth ground, and with the ground fault current flowing, the ground fault monitoring circuit can detect the presence of the ground fault on the ungrounded CPV array, and in addition, when the fault current exceeds the fuse rating, then the fuse will open and interrupt the fault current;

however the voltage from the fault will now be sensed on an input legs of a voltage differential sense circuit.

14. The apparatus for a photovoltaic system of claim 13, where the ground fault monitoring circuit detects when the inverter is scheduled to put be on-line, the inverter controller interrogates the open fuse sense lines and when the inverter controller senses any voltage between the two open fuse sense lines, a fuse has opened, indicating a ground fault in the CPV solar arrays, and the inverter controller keeps the inverter circuit off-line by keeping the isolation contacts open.

15. The apparatus for a photovoltaic system of claim 14, where the ground fault monitoring circuit detects no fuse failure, the inverter controller then puts the inverter on-line with the grid by closing the isolation contacts, and also energizes the PV array DC common reference point control relay CRG to interconnect and float the CPV array DC voltage midpoints, where the system ground is now established by the system transformer common being connected to Earth ground, and the residual current monitor is now active and responsive to future CPV array ground leaks, and when a ground occurs in a CPV array, the ground fault is sensed by the associated residual current monitor, and the solar array with the ground fault and its inverter circuit are isolated from the utility grid transformer but the transformer continues to receiver AC voltage from the other inverter circuits coupling to that transformer and continues to produce power.