Grid Connected PV System

Go analog. You are overcomplicating a simple process.

Here is a link to most of my GTI design work I have put online for public use.

electro-tech-online. Alternative Energy Section, GTI Threads.

Wow TCM did not know you were so into alternate energy knew about alt fuels not power I will read your other posts for ideas for my current solar work. Can't sell back to utility but can charge bulk bank to down load energy to EV. Seems to be best use of extra energy instead of credits we wont use unless our system has a catastrophic failure, like tornado,thunder/electrical storm, or baseball sized hail which we have here in Indiana from time to time. I will check out your other posts. Duke. P.S.tried to post as off topic but will not let me for some reason SORRY.

I have been playing with AE all of my life and have been into power electronics all along the way as well.

Some years ago I went back to college a second time for EE and while at NDSU in Fargo ND I saw that they had one professor trying to design his own grid tie system which according to the students that worked with it he had a pile of money tied up in the design and it never really worked all that well.

I on the other hand at that point had already designed, built and thoroughly tested a GTI system that could do everything his could and more with high reliability that could literally be built out any old battery charger transformer and $25 - $30 in parts from Radio Shack!

I met the guy once and told him what he was doing wrong and how it could be done simply and reliably but was given the blind eye deaf ear treatment because according to him I had not yet taken classes by so and so or him and thusly could not know the intricacies of how grid tie synchronisation, control, and blah blah blah worked.

Some time later when I got into online forums I figured the best revenge was to publish a lot of my basic work out into open source for the whole world to have for free where anyone can use it or improve /modify my designs to fit whatever they want based on my design theory!

It has always bewildered me how I can design such devices just for sh!ts and giggles just to keep myself from being bored while people with full doctorates in EE can't grasp the concepts. They're too educated to know better I guess?

Oh well his loss and the worlds gain.

Grid tie was not in my line due to the fact that I am dyslexic and need to be very careful and purposeful/deliberate when I type or start to read type or, build something. So complicated electronics was not in the cards for me just not worth the energy expended to complete a project simpler to buy. Also here in Indiana you have to use electric vender's equipment. Just found out since I last posted this that our phone company just purchased a power company, after talking with them they may be willing to purchase extra power when available. I might be able trade power for telephone service since I have five phone lines for my business beats not getting anything for it.. Anyway I am using amphoras panels just so I don't have the issue of debri or bird droppings stopping the panels from making power and was thinking of covering the panels once in place with Polycarbonate plastic and running an inert liquid through the panels to cool the panels to extend their lifetime and possably a little domestic hot water.and possably a little amplification of the light energy going through the liquid. Any thoughts Duke

The digital way uses a PLL to lock to the line phase and frequency. You can always add / remove a small offset to the reference angle to adapt to internal delays.

The amplitude and the phase of the voltage reference will have to be changed to keep the current in phase. This is called vector control and it is well published. (do not confuse with PWM vector modulation).

You will be taxing the PIC quickly with these algo if you are not careful. Start with simple controls.

Good luck.

abba ji.here is the schematic of PLL

http://en.wikipedia.org/wiki/Phase-locked_loop follow the link to read more about it.

regards

dear umer i am an automation engineer ,after the googling i got the information which i will like to share with you.

Operating a renewable energy system in parallel with an electric grid requires special grid-interactive orgrid tie inverters (GTI). The power processing circuits of a GTI are similar to that of a conventional portableDC-AC converter that operates as a stand-alone device. The main differences are in their control algorithm and safety features. A GTI basically takes a variable voltage from a DC source, such as solar panels array or a wind system, and inverts it to AC synchronized with the mains. It can provide power to your loads and feed an excess of the electricity into the grid. Depending on power and voltage levels, GTIs circuits normally have from one to three stages. A conceptual power train schematic diagram below illustrates the principles of operation of a three-stage grid tie inverter. Such a topology can be useful for low-voltage inputs (such as 12V) in grounded systems. The control circuits and miscellaneous details are not shown here. As I mentioned above, there are also two-stage and single-stage configurations for the examples.

The input voltage is first raised by the boost converter formed with inductor L1, MOSFET Q1, diode D1 and capacitor C2. If a PV array is rated for more than 50V, generally one of the input direct current busses has to be grounded per National Electric Code®. The NEC® however allows some exceptions which we will discuss below. Although in theory either of two busses can be connected to earth, usually it is a negative one. It is important to remember that if DC input has a conduction pass to ground, the output AC conductors in utility-interactive configurations should be isolated from DC. In our example, a galvanic isolation is provided by a high frequency transformer in the second conversion stage. This stage is a basically a pulse-width modulated DC-DC converter. The schematic above shows a full bridge (also known as H-bridge) isolating converter comprised of Q2-Q5, T1, D2-D5, L2, and C3. For power levels under 1000 watt it could also be a half-bridge or a forward converter (for more details seeSMPS types). Some commercial models use low-frequency (LF) transformer in the output stage instead of a high frequency one in the DC-DC section. With such a method, input is converted to 60 Hz AC, and then a LF transformer changes it to a required

level and provides isolation at the same time.

The equipment with an LF transformer has a significantly larger weight and size, but it will not inject a DC component into the load. Here is a lesser known detail: UL 1741 does allowtransformeless inverters and exempts them from dielectric voltage withstand test between input and output. Therefore the isolating stage can be eliminated. It is important to note that the conductors from PV array in non-isolated designs can't be bonded to earth. NEC® 690.41 allows ungrounded configurations is they comply with Article 690.35. The transformerless inverters of course feature lower weight and cost. They are especially popular in Europe where ungrounded electrical systems are common. However, because of the lack of the galvanic isolation, these models present potential electrical hazards. In such a setup if a person touches a terminal of the PV panel or the battery, he will appear under AC line potential. The transformeless systems require additional protection devices per NEC® Article 690.35 and special warning labels placed wherever energized circuits may be exposed during service.

T1 can be a so-called step-up type to amplify the input voltage. With a step-up T1, the first stage (boost converter) may be omitted. However, high turns ratio leads to large leakage inductance. This in turn causes voltage spikes on the FETs and rectifiers as well as other undesirable effects.
The regulated converter provides a DC-link to the output AC inverter. Its value must be higher than the peak of the utility AC voltage. For example, for 120VAC service, the Vdc should be >120*√2=168V. Typical numbers are 180-200V. For 240VAC you would need 350-400 V.
The third conversion stage turns DC into AC by using another full bridge converter. It consists of IGBT Q6-Q9 and LC-filter L3, C4.

The IGBTs Q6-Q9 work as electronic switches that operate in PWM mode. This topology requires anti-parallel freewheeling diodes to provide an alternate path for the current when the switches are off. These diodes are either included within IGBTs or added externally. By controlling different switches in the H-bridge, a positive, negative, or zero potential can be applied across inductor L3. The output LC filter then reduces high frequency harmonics to produce a sinewave.

Any grid tie power source has to synchronize its frequency, phase and amplitude with the utility and feed a sinewave current into the load. Note that if inverter output (Vout) is higher than utility voltage, the GTI will be overloaded. If it is lower, the GTI may sink current rather than source it. In order to allow a limited current flow into the loads as well as back into the line, "Vout" has to be just slightly higher than the utility voltage. Usually there is an additional coupling inductor (Lgrid) between GTE and the mains that "absorbs" the extra AC voltage. It also reduces the current harmonics generated by the PWM. A drawback of "Lgrid" is it introduces extra poles in the control loop, which potentially may lead to the system instability. Because the grid acts as a source with a very low impedance, normally, a GTE is designed to work as a current controlled source, rather than a voltage source.

In solar applications, to maximize the system efficiency, a GTI also has to meet certain requirements defined by the photovoltaic panels. Solar panels provide different power in different points of their volt-ampere (V-I) characteristic. The point in the V-I curve where output power is maximum is called maximum power point (MPP). The solar inverter must assure that the PV modules are operated near their MPP. This is accomplished with a special control circuit in the first conversion stage called MPP tracker(MPPT).

A GTI also has to provide so-called anti-islanding protection. When mains fails or when its voltage level or frequency goes outside of acceptable limits, the automatic switch should SW quickly disconnect the system output from the line. The clearing time depends on the mains conditions and is specified by UL 1741. In the worse cases, when utility voltage drops below 0.5 of nominal, or its frequency deviates by +0.5 or -0.7 Hz from the rated value, GTI should cease to export power back to the grid in less than 100 milliseconds. An anti-islanding can be accomplished for example via AC undervoltage or output overcurrent detection functions. Our example depicts a system with power backup option: when contactor SW opens, the GTI will supply critical loads connected to the sub-panel.

The implementation of control algorithm of grid tie inverters is quite complex and normally is done with microcontrollers. The hobbyists are often searching the web for a complete schematic of a grid tie inverter. Unfortunately, it is almost a fruitless task- a GTI is not a hobbyist project. Also note, it may be illegal to connect any non-UL approved power generator into the grid-connected wiring. In any case, the manufactures of GTIs obviously will not reveal the details of their designs. Even if you could find a complete schematic, it would be useless without the controller source code. The only design information for hobbyists with a source code that I found online was a 100W home brewed GTI. I made no attempt to review this design though. For engineers, there is an application note AN3095 by ST Micro. It provides a complete solar inverter circuit diagram and a design guide for a 3000 watt photovoltaic inverter.

kindly reff to the link ,mentioned below

http://solar.smps.us/grid-tie-inverter-schematic.html

with love