A GRID TIE INVERTER FOR SOLAR SYSTEMS

Operating a renewable energy system in parallel with an electric grid requires special grid-interactive or grid tie inverters (GTI). The power processing circuits of a GTI are similar to that of a conventional portable power inverter. The main differences are in their control algorithm and safety features. A GTI basically takes a variable DC voltage from the 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 one to three stages. The simplified schematic diagram below illustrates the principles of operation of a three-stage grid tie inverter. Such power train can be useful for low-voltage inputs (such as 12V) in grounded systems. The control circuits and various details are not shown here.

The input voltage is first raised by the boost converter formed with inductor L1, MOSFET Q1, diode D1 and capacitor C2. If PV array is rated for more than 50V, one of the input DC busses has to be grounded per National Electric Code®. Usually a negative bus is connected to earth. Note that output AC conductors in utility-interactive configurations should not be grounded. In our example, a galvanic isolation between the input and output 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) converter in the second stage. For power levels under 1000 watt it could also use a half-bridge or a forward converter (for more details see SMPS topologies). Some commercial models use low-frequency output transformer instead of a high frequency one in a DC-DC stage. With such method input voltage 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 output LF transformer have larger weight and size, but it will not inject a direct current into the load. Here is a little known detail: UL 1741 allows transformeless 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 between the DC and AC, these models present potential electrical hazards if a person touches a terminal of a PV panel or the battery. They require a special warning label placed wherever energized circuits may be exposed during service.

The transformer T1 can be a so-called step-up type to amplify the input voltage. With a step-up type, 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 voltage 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.
PWM produces sinewave

The IGBTs Q6-Q9 work as electronic switches that operate in PWM mode. They usually contain internal ultrafast anti-parallel diodes. By controlling different switches in the H-bridge, a positive, negative, or zero voltage can be applied across inductor L3. The output LC filter reduces high frequency harmonics to produce a sinewave voltage.

A 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 voltage (Vout) is higher than utility voltage, the GTI will be overloaded. If it is lower, GTI would sink current rather than source it. In order to allow a limited current flow into the loads as well as back into the grid, "Vout" has to be just slightly higher than the utility voltage. Usually there is an additional inductor (Lgrid) between GTE output the grid that "absorbs" 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 may lead to the system instability. Because the grid acts as a source with a very low impedance, the GTE normally works as a current 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 grid fails or when utility voltage level or frequency goes outside of acceptable limits, the automatic switch SW quickly disconnects 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.

The implementation of control algorithm of grid tie inverters is quite complex and is normally done with microcontrollers. The hobbyists are often searching the web for the 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 complete 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 design guide for a 3000 watt photovoltaic inverter.

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