AU2015202730A1 - Solar Power Export Limiter - Google Patents

Abstract

Abstract A solar power export limiter is disclosed. The system compares the current required to power a locally connected load and the current generated by a locally generated Photovoltaic (PV) Grid Connected (GC) power system. The solar power limiter utilises Pulse Width Modulation (PWM) techniques to modify the output power capacity of a PV panel or array prior to the solar inverter of the local PV power generation system. The PWM may be externally set to enable full power generation with power in excess of local load requirements sent or exported into the reticulated electrical grid or a percentage of available power sent or exported to the grid or it may be set such that no excess power generated locally is sent or exported to the grid. When set to restrict power generation to load requirements only, the solar power limiter will track the local load requirements in real time and alter the local PV power generation to match the requirements of the local load. ca 0* z0 caa ca c.3 0" -'o 0~ ~ Ji0 aL

Description

1 Title of Invention: Solar Power Export Limiter Technical Field: Solar Power Generation Background Art: [0001] The generation of electricity using photovoltaic (PV) cells is well known. The generation system for grid connect (GC) or grid tie (GT) systems is accomplished by the use of PV cells connected to an electronic inverter which converts the direct current (DC) generated by the PV cells to an alternating current (AC) suitable for interconnection to the reticulated power grid. These inverters associated with the PV system employ proprietary algorithms to enable the inverter to extract maximum power from the PV cells. [0002] These algorithms allow the inverter to track the available PV voltage and current in real time and develop maximum power from those variables. This process is known as Maximum Power Point Tracking (MPPT). [0003] The developed AC output is connected to the local load as well as the reticulated grid such that any excess power generated above that required to satisfy the local load is returned to the grid. The inverters accomplish this by increasing the output voltage of the inverter to promote the flow of excess current to the connected grid. [0004] These inverters also monitor the grid voltage and frequency to ensure that the measured values are constrained within defined limits. If these limits are exceeded then the inverter automatically shuts down the AC output until such time as the parameters are again within those defined limits.

2 [0005] The inverters also shut down the AC output in the event that the grid fails. This is to prevent the PV system from generating power and applying that power to a possible grid fault situation and also to allow repairs to be carried out on the grid safely without the possibility of power being applied to the grid without the supply authority being aware of such an event. This process is known as "anti-islanding" and is a mandatory condition of GT/GC inverter. Summary of Invention: [0006] These GT/GC inverters can lead to voltage instability and a higher than normal grid voltage because the inverters must increase the output voltage so that the excess power from that particular site is sent or exported to the grid. If there are a number of PV systems connected to the grid in close proximity then a positive voltage feedback process is begun. [0007] The purpose of this invention is to prevent the GT/GC PV system from exporting excess power generated at a site to the grid without affecting the ability to satisfy the local connected load. This restriction could be controlled by the supply authority such that if extra grid power were required then the supply authority could allow these GT/GC PV systems fitted with a Solar Power Limiter (the invention) to export all excess or in fact a percentage of the excess power generated by the PV system to the grid. [0008] The invention will not interfere with the anti-islanding functions or the MPPT algorithms inherent in the inverter. [0009] There are some companies who provide their PV inverters the capability of load tracking and preventing excess power being sent to the grid. These systems interact with the MPPT proprietary algorithms of their inverters to affect the inverter output power. One such method is described in patent number 2011213747 and patent number 2009353975. Another is the Power Router with Dynamic Feed-in Limiter manufactured by Nedap . However these technologies are proprietary inverter specific.

3 [00010] The invention herein disclosed will control all PV inverter systems and will not require access to proprietary algorithms or software. The unit is designed to be add-on to existing PV solar systems as well as incorporated in new installations without compromising the integrity or required safety features of any connected hardware. Brief description of drawings: A particular preferred embodiment of the invention will now be described with reference to the drawings in which: Figure 1 is a block diagram of the interconnection of modules that comprise the complete PV generation system. Fig 2 is a block diagram of the preferred embodiment of the series control module. Fig 3 is a block diagram of the preferred embodiment of the sense module. Description of Embodiments: [00011] With reference to Fig 1the preferred embodiment consists of the series control module (2) connected between the PV panel or a string of panels (1) and the inverter (3) and a sense module (4) connected inductively to the load bus (5) and to the solar inverter output bus (6). The sense module is also connected via a cable to the series control module. [00012] In one embodiment of the invention and with reference to Fig 2 the series control element of the series control module consists of a P junction 4 Insulated Gate Bipolar Transistor (IGBT) whose collector is connected to the positive output of a PV panel or string of panels. The emitter of the IGBT is connected through a series diode to the positive DC input of the DC- AC inverter. The gate of the IGBT is controlled by a Pulse Width Modulator (PWM) circuit in the series control module that causes the IGBT to turn on and off to alternatively pass and block current flowing from the PV panel or string to the inverter. This process reduces the available average power from the PV panel or string to the inverter and the inverter will then use MPPT to reduce the power output of the inverter. [00013] The PWM can vary the ratio of the on time to the off time of the IGBT. The ratio is determined by the information sent from the sense module. The PWM changes the ratio such that the current measured at the solar inverter output is equal to the current measured going to the load. [00014] If a plurality of separate PV panels or strings is used in the installation then there exists a plurality of IGBT's, one for each separate panel or string. There also may exist a plurality of PWM circuits. [00015] The preferred embodiment is that the series control element be capable of controlling up to two strings of panels and therefore has two IGBT's with separate drivers and one PWM to control both IGBT drivers at the same time. [00016] The PWM control circuit utilizes information from the sense module to alter the on to off times of the IGBT's in real time and therefore track the changes in the power required by the connected load in real time. The PWM circuitry may totally turn one IGBT off and only control the remaining string in order that the generated current from the inverter matches the connected load requirements. [00017] The negative output of the PV panel or string of panels is connected to the negative input terminal or terminals of the DC-AC inverter.

5 [00018] In another embodiment the series control element consists of a Power Metal Oxide Substrate Field Effect Transistor (Power MOSFET). [00019] The preferred embodiment of the sense module will now be described. [00020] The sense module measures the instantaneous current in the output cable of the solar inverter using a split core current transformer of a capacity capable of correctly measuring the maximum expected current from the solar inverter and expresses this value as a proportionate voltage at its output. [00021] The sense module of the preferred embodiment measures the instantaneous current in the cable to the connected load using a split core current transformer of a capacity capable of correctly measuring the maximum expected current to the local load and expresses this value as a proportionate voltage at its output. [00022] These two AC measurements are converted to a DC voltage by a RMS to DC converter and these two DC voltages are then compared using two oppositely connected comparator operational amplifiers (Fig 3). [00023] This produces an error voltage from each of the comparators representing either higher load current than solar generated current or higher solar generated current that required load current. There may also be no output from the comparators indicating balanced load and solar generating power. This information is fed back to the series control module to change the PWM characteristics to achieve a balance of solar generated capacity to load requirements. [00024] Another embodiment may use a split core current transformer to measure the current in the cable to the local grid and convert it to a DC voltage and send that DC voltage to the series control module for inclusion in the PWM control process.

6 [00025] Yet another embodiment of the sense module may use Hall Effect device to sense the currents in the solar inverter, connected load and grid current cables. [00026] Another embodiment may use an analogue to digital converter to process the sensed values and utilize a Peripheral Interface Controller (PIC) or other programmable device to interpret and derive the controls for the PWM. [00027] Another embodiment may allow local or remote control of the series control module or the sense module to enable an authority to allow full power, partial power or no power to flow to the reticulated grid. [00028] Another embodiment may incorporate the sense module inside the same housing as the control module.

Claims (19)

1. A solar power limiter system comprising a series control module coupled to receive power from Photovoltaic (PV) panels or panels connected in a string and the said control module is also coupled to send power to an inverter; a sense module magnetically coupled to both the output from the solar generator system and to the input of the locally connected load and the output from the said sense module is coupled to the series control module.

2. The series control module of claim 1 comprising a series control element consisting of an Insulated Gate Bipolar Transistor (IGBT) that has the collector connected to the PV panels or in the alternative, a PV string and said IGBT that has the emitter connected to the anode of a diode and that the cathode of said diode is connected to the inverter.

3. The gate of the IGBT of claim 2 is connected to the output of a Pulse Width Modulator (PWM) circuit that has a plurality of inputs and a plurality of outputs.

4. The PWM of claim 3 may be connected to an external manual control to set or vary the operation of the PWM.

5. The series control module of claim 2 may contain a plurality of series control elements.

6. The series control module of claim 2 may contain a plurality of PWM circuits.

7. The sense module of claim 1 comprising a first split core current transformer connected to the input of a first RMS to DC converter that has the output connected to a first comparator operational amplifier non-inverting input and connected to a second comparator operational amplifier inverting input and a second split core current transformer connected to the input of a second RMS to DC converter that has the output connected to the second comparator operational amplifier non inverting input and connected to the inverting input of the first comparator operational amplifier and the output of the first comparator 8 operational amplifier and the output of the second comparator operational amplifier is connected to inputs of the PWM circuit.

8. The sense module is connected to the PWM module using a dedicated cable to carry power to the sense module and control signals back to the PWM module.

9. The sense module may be connected to the PWM module by a wireless system and deriving power locally.

10. The sense module may be connected to the PWM module using Power Line Communications and deriving power locally.

11. The sense module may be connected to the PWM module using Ethernet over Power and deriving power locally.

12. The series control element of claim 2 implemented using a Metal Oxide Substrate Field Effect Transistor (MOSFET).

13. The series control element of claim 2 and the PWM of claim 3 implemented using a purpose designed integrated circuit.

14. The PWM circuit of claim 3 implemented using discrete integrated circuits and discrete components.

15. The PWM circuit of claim 3 implemented using a Programmable Interface Controller (PIC)

16. The PWM of claim 3 implemented using a programmable logic device.

17. The sense module of claim 7 implemented using Hall Effect current sensors.

18. The sense module of claim 7 implemented using Analogue to Digital (AD) converters and a PIC.

19. The sense module of claim 7 implemented using a programmable logic device (PLD).