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 DC-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.
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 see SMPS topologies). 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.
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.