Electronics > Power/Renewable Energy/EV's

Adding a GTI to my PV water heating system blog

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Having finally gotten the water heater to work reliably (see other blog) my thoughts have turned to a grid tie inverter to use the surplus PV energy once the water tank has reached it's set temperature (62-64degC).
I have already had a go at my own design but got bogged down in the control system trying to resolve the complex feedback compensation issues caused by the output LCL filter poles (this was/is a high frequency ferrite transformer design), I cannot afford a copy of Mathcad :(. So I decided to cut my losses and side step the issue and use a commercial unit instead.

Wary of the instant blow up versions available from certain parts of the world and needing more than a Kw to use the PV power I watched a well known auction site for well known names and came up with a Fronius IG15 (I deliberately avoided transformerless designs). Fortunately using my existing Semi-sepic converter I can adapt the PV voltage to practically any converter input range.

The next issue I looked at was how to avoid exporting excessive power and wearing my equipment out for no gain (I cannot claim payment for anything exported). To this end I have designed a system that measures power and direction at the meter and sends this information to the existing Sepic converter, that in turn will adjust the amount of power it makes available to the GTI to minimize the export dynamically as household loads change.

Cool huh, I will let you know if it works in due course, meanwhile I have to wait a few days for the GTI to arrive that I shall teardown and bench test for starters :)

One thing I did note about commercial GTI's is there is NO information or firmware available for the hobbyist, all of that is jealously guarded by the companies and there service agents.

IG15 Teardown
Made a bit difficult due to no component designators and the necessary conformal coating obscuring some component part numbers, I should also mention the boards are at least 4 layer and the conformal coating makes probing difficult, however :)
Starting at the power connectors they are screw type with no pressure plates so a bit wire unfriendly (EDIT sorry they have pressure plates), the multiple DC inputs are all connected in parallel.  Immediately behind the connector bulkhead lies the FILTER board containing common mode chokes and various caps for both AC & DC sides, I notice the Y caps are Wima yellow ones that I have encountered exploding before.
Following the AC path from the FILTER to the AC board it is connected to a small 12Vrms mains trafo via an opto-triac, this could be for any number of purposes. The main power path passes through a relay to a large 4.7uF X capacitor and through two huge pot core potted chokes via a current sensor (LEM HXN 20-P) to an Hbridge using IXFX48N50Q 500V mosfets. Each half of the bridge is driven by an HCPL-A314J twin opto-igbt driver, power is derived from an isolated winding on the APU to be described later.
The DC-buss has three 470uF/450V reservoirs in turn fed via a large choke from a bridge rectifier formed of four unidentified TO220 diodes. High frequency AC comes from the chassis mounted main transformer.
The main transformer is Litz wound secondary first then two primaries, windings are right to the cheeks so the only way the 4mm creepage can be met is if the Litz is itself insulated to re-enforced standard (commonly known as TIW).
The DC board (underneath the AC) drives the transformer primaries with what appears to be another H-bridge with unidentifiable IR mosfets driven in exactly the same manner as already described. The feed to this bridge comes via a current transformer (Pulse F1S115NL) from a set of three 390uF/500V reservoirs that are connected directly to the DC feed from the FILTER board. The transformer primaries are linked to the Hbridge through two power relays and I speculate this is to allow series/parallel operation to better cover the wide input range.
The APU consists of a small ferrite transformer driven by an ST B7NK80Z 800V mosfet. Power comes from the DC feed from the FILTER board. There are many rails, at least a floating +15V for the AC board, +15 & +5V for the DC & AC boards.
The main processor is a TI TMS320LF240 DSP, there is also an Altera EPM30302 PLD almost certainly doing PWM & sine modulation. There is another ~28 lead wide soic but I think its just some kind of power regulator going by the number of power tracks to it.
The INTERFACE board is located behind the LCD display at the bottom of the chassis and carries 4 option board connectors (all empty). It is connected to the DC board (processor) by a ribbon cable labelled LTG-BUSS and also to the LCD via another ribbon. The option connectors are potentially dangerous as they seem to have a grid feed from the FILTER board via a connector on this board labelled AC. For some reason this board has at least two chips with associated crystals. One is an NXP P89V664 (flash based 80C51 MCU). The other is a TI TL16C550 UART (why, surely MCU has one ?). There are also about 7 SSI cmos logic chips.
Finaly the LCD board has several buttons, a pretty LCD and a NXP PCF8576CT I2C LCD controller.
Remaining pictures in next post.

IG15 Teardown, remaining pics

capt bullshot:

IG15 Bench Test
First the equipment, a bunch of transformers fed by a Variac and ballasted by an electric fire, rectified with reservoirs form the PV source (see pics). The grid is simply connected through a power meter.
In a nutshell I find this thing quirky but then it is the first pro grid-tie I have encountered, also there is a nasty warning about never disconnecting the DC while on load, hmm so not very fault tolerant then  :palm:
The APU starts up at about 140V but in unhappy mode displaying "wait ps" and a flashing panel symbol. Pushing it up to >170V gets you past that and into the extremely long (3 minutes) "startup" phase, at the end of this it zaps the PV's with a ~1Amp current pulse and if they drop below 170V you get "power low" that lasts for 30 seconds then sends you back to the slow "startup" again.
If your source impedance/starting voltage is low enough to get past the current zap it will actually start regulating nicely and delivering output power (you also get a nice green led), the lowest the MPP will drive the input voltage is 160V and if the panel falls to ~155V you go back to "power low". Anytime you are naughty the led glows orange (assuming the APU is alive).

Operationally that's it but what I want to know is if someone hits the big DC breaker I have on the wall like all sensible people does this stupid thing blow up  >:D ??


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