Solar Inverter

solar inverter The main function of solar inverter is to convert battery's Direct Current (DC) into pure sine wave Alternative Current (AC) to feed home compliances.

Solar AC power system is consisted of solar panels, charger controllers, inverters and rechargeable batteries, while solar DC power system is not included inverters. The inverter is a power conversion devices, which can be divided into self-excited oscillation inverter and external excited oscillation inverter.

Solar Inverter Advantages

The diversity of the building will lead to the diversity of solar panels installation, in order to make the highest conversion efficiency of solar energy and consider the building's appearance at the same time, it requires the diversify of the inverter to achieve best solar energy conversion.

Central inverters

Central inverter system is generally used for large photovoltaic (PV) power plants (> 10kW), lots of parallel PVs are connected to the same DC input of the central inverter. Generally, large power inverter adopts three-phase IGBT power modules, smaller power inverter adopts field effect transistors, and use DSP switching controller to improve the quality of the output electricity, to make it close to sine wave power. The featured advantage of central inverter is high efficient power and low cost. However, the influence of the parallel PVs and partial shading will result in low efficiency and output capacity of the entire PV system, and the whole photovoltaic power generation system's reliability is affected by one poor working photovoltaic cell group. The latest research is using space vector modulation control, as well as the development of new inverter topology connections to obtain high efficiency under partial load conditions. On SolarMax central inverter, there is an additional PV array interface box to monitor each PV string, suppose there is a PV string working improperly, the system will transmit the information to remote controller, and stop it by remote control to avoid the affection of this PV string to the whole PV system.

String inverters

String inverter becomes the most popular inverter on the international market now. String inverter is based on modularization, each PV string (1kW-5kW) goes through an inverter, with maximum power tracking peak at the DC terminal, and grid-tied at the AC side. Many large-scale photovoltaic power plants use string inverters. The advantages are without impact of the difference between the modules and shadow, and reducing the mismatch between the PV module's optimum operating point and inverter, thereby increasing the power generation. These technical advantages not only reduce the system cost, but also increase the whole system reliability. Meanwhile, the introduction of "master-slave" concept, so that if a single string power cannot make a single inverter work, connect several PV strings together, to make one or several PV strings work to produce more power. The latest concept is making several inverters to form a "team" to replace "master - slave" concept, which makes the whole system motor reliable. (related article String inverter VS Micro inverter.)

Multi string inverters

Multi string inverter is putting the advantages of central inverter and string inverter together, avoiding its weaknesses, which can be applied to several kilowatts PV power stations. Multi string inverters include various individual peak power tracking and dc-to-dc converter, these dc currents go through a common DC to AC inverter, the convert to ac to the grid. The PV modules different rating values (eg: different power ratings, different string number of each string, different manufacturers etc. ), different sizes or different technologies, different directions (eg: East, South and West ), different inclination or shading, all can be connected to the same inverter, while each string is working in their maximum peak power. Meanwhile, the DC cable length is reduced, and minimizes the shading effects of strings and the losses of the strings difference.

Module inverter

Module inverter is connecting each PV module with one inverter, and each module has an individual maximum peak power tracking, so that the module fit better with the inverter. Module inverter typically used for 50W to 400W photovoltaic power stations, total efficiency less than string inverter. As module inverter is in parallel at the AC side, which increases the wire complexity of the AC side and difficult in maintenance. Another important thing needs to be addressed is how to connect to grid more effectively, the simplest way is connecting to a common AC outlet directly, thereby reducing the equipment and installation cost, but different areas have different safety standards around the grid and may not allow to do so, the power companies are likely to oppose the power plant connect to an ordinary socket directly.

Solar inverter efficiency

Solar inverter efficiency refers to: as the demand for renewable energy, solar inverter (photovoltaic inverter ) market is growing, and these inverters require high efficiency and reliability. Solar PV inverter general structure is shown as following Figure, with three kinds of inverters for option. Sunlight shine on the solar modules connection in series, each module comprises a string solar cells in series. The direct current (DC) voltage generated by solar module is on several hundred volts, the value is determined by illumination condition, the battery temperature and the modules number.
Solar PV inverter structure
The primary function of solar inverter is converting DC voltage to a stable value. This function is achieved by the boost converter and need a boost switch and boost diode. In the first structure, it's an isolated full-bridge converter after boosting. The role of full-bridge transformer is to provide isolation. The second full bridge converter on output side is used for converting the first full bridge converter's DC into AC voltage. Its output is filtered by an additional double contact relay switch before connecting to AC power grid, the purpose is to provide safe isolation in case of a failure and at night isolate with the power grid. The second structure is non-isolated solution, the AC voltage is generated by DC voltage boosting directly. The third structure is using power switch and power diode to integrate the boost and AC generating functional in a dedicated topology, although the conversion efficiency of solar panels is very low, it's important to make it close to 100%. In German, a 3kW south-facing roof mounted module can generate approx 2550 kWh electricity annually. If the inverter efficiency increase from 95% to 96 %, then it's able to generate additional 25kWh per year. The use of additional solar modules to generate 25kWh electricity will cost as high as an inverter price. Since the cost of the inverter won't increase if the efficiency increase from 95% to 96%, so it's an inevitable choice to invest high efficient inverters (related article: Solar PV and Solar Thermal system).

Solar Inverter Functions

The inverter not only has DC-AC switching function, it also has a maximum function to carry out the usefulness and functionality of the solar cells. Sum it up, the inverter has initiative operation and stop, maximum power point tracking (MPPT) control function, anti-independent operation function (for grid tied system), active voltage regulation function (for grid tied system), DC detection function (for grid tied system), DC ground fault detection function (for grid tied system). Here introduce initiative operation and stop functions, maximum power point tracking (MPPT) control function.

1. Initiative operation and stop: in the morning after sunrise, solar radiation intensity strengthen; gradually, the output of solar cells also increase, when the power reach the inverter required output power, then the inverter starts operation initiatively. During operation, the inverter monitors solar cell module's output in every moment, as long as the output power of the solar cell module is greater than the inverter's required output power, the inverter will operate continuously until sunset shutdown, the inverter can operate even in rainy days. When the output of solar modules becomes smaller, the inverter output is close to 0, the inverter will in a standby status.

2. Maximum power point tracking (MPPT): the output of solar cell modules is changeable according to solar radiation intensity and the solar cell modules' temperature. As the voltage of solar cell modules decreases when current increases, thus there is a best point of maximum power. The solar radiation intensity is changeable, obviously the maximum power point is also changeable. Related to these changes, make the solar cell modules in the maximum power point, the system obtains the maximum power output from the solar cell modules, which means MPPT control. The most important feature of the inverter in solar power system is integrated maximum power point tracking (MPPT) function.

Solar Inverter Types

Base on application types

1) Common Inverter
DC 12V or 24V input, AC 220V, 50Hz output, power capacity from 75W to 5000W, and some inverters have AC -DC switching function like UPS.

2) Inverter / charger integration
In this type of inverter, user can use various forms of power to feed the AC loads: if AC power available, inverter using AC power to feed the loads, or changing battery; if AC power no available, using battery to feed AC loads. This inverter can be used with a variety of powers, like batteries, generators, solar panels and wind generators etc.

3) Post and Telecommunications Dedicated Inverter
High quality 48V inverter for post and telecommunications, good quality, high reliability, modular (1kW module) inverter, and has N +1 redundancy, scalable(from 2KW to 20KW) function.

4) Aviation, military dedicated inverter
Such inverters are 28V DC input, with AC output: 26V AC, 115V AC, 230V AC, the output frequency: 50Hz, 60Hz and 400Hz, output power ratings from 30VA to 3500VA. And there are aviation dedicated DC-DC converters and inverters.

Base on output waveform types

1) Square wave inverter
The inverter outputs square waveform is called square wave inverter. The inverter circuit of these inverters are not exactly the same, the common feature is simple circuit, less power switches. The design power generally between one hundred watts to one kilowatts. The advantages of square wave inverters are: simple circuit, cheap and easy maintenance. The disadvantage is, due to the square-wave voltage contains a lot of high harmonics, which will result in additional losses in the loads with core inductor or transformer, and have interferences to some communications devices. Besides, square wave inverters voltage range is not wide enough, protection function is not perfect, high noise and other shortcomings.

2) Step wave inverter
This inverter output step wave AC voltage waveform. Different step wave inverters output different waves, the step number may have great differences. The benefits of step wave inverter are, the output waveform is significant improved compare with square wave, high harmonic content decreased, it's close to sine wave form when the wave steps reach 17, which called modified sine wave inverter. It has high efficiency when there is no transformer output. The disadvantage is that the step wave superimposed line uses more power switches, some of them require multiple DC power input lines. This brings troubles to solar cell phalanx grouping and wiring, and the balance charging of the battery. In addition, step wave voltage still has some high-frequency interference to radio communications devices.

3) Sine wave inverter
Sine wave inverter output voltage waveform is sine wave. The benefits of sine wave inverter are, output waveform is good, low distortion, less interference to radio communication devices and low noise. In addition, comprehensive protection functions, high overall efficiency. The disadvantages are: complicate lines, require high technical repair cost.

Above three types of inverters classification is helping photovoltaic and wind power generation systems designers and users to make a right inverter selection.

Solar Inverter Performance

There are lots of technical parameters to describe an inverter performance, here we only list the technical parameters for the evaluation of an inverter briefly.
  • Inverter working environmental: altitude less than 1000m, air temperature less than 0 ~ +40 °C.
  • DC power condition. DC voltage input range: battery rated voltage values ± 15%.
  • Rated output voltage. In specified input power conditions, under rated current, the inverter outputs rated voltage fluctuation: Single phase 220V ± 5%, three phase 380 ± 5%.
  • Rated output current. In specified output frequency and load power factor, the inverter output rated current value.
  • Rated output frequency. In specified conditions, the fixed frequency inverter output frequency 50Hz ± 2%.
  • Inverter maximum harmonic content. In resistive loads, the maximum harmonic content of the sine wave inverter output voltage should be ≤ 10%.
  • Inverter overload capability. In specified conditions, in a short time, the capability of the inverter output current exceeds the rated current. Inverter overload capability should meet certain requirements under specified load power factor.
  • Inverter efficiency. In rated output voltage, output current, and specified load power factor, the ratio of the inverter's output active power and input active power (or DC power).
  • Load power factor. The allowable variation range of the inverter load power factor, recommended value is 0.7 to 1.0.
  • Asymmetric load. In 10% asymmetrical loads, fixed frequency three-phase inverter output voltage asymmetry should be ≤ 10%.
  • The asymmetry of output voltage. In normal operating conditions, each phase load symmetrical, the asymmetry of the output voltage should be ≤ 5%.
  • Solar inverter starting characteristic. In normal operating conditions, under full load and no load running conditions, the inverter should have 5 times consecutive starts.
  • Protection function. The inverter should be set: short circuit protection, over current protection, over voltage protection, under voltage protection and phase loss protection.
  • Interference and anti-interference. In specified normal operating conditions, the inverter can withstand a certain electromagnetic interference. The inverter anti-interference and electromagnetic compatibility should comply with the relevant standards.
  • The noise of infrequent operation, monitoring and maintenance inverters should be ≤ 95db; regular operation, monitoring and maintenance inverters should be ≤ 80db.
  • Display. The inverter should provide display of AC output voltage, output current, output frequency and other parameters.

Solar cell characteristics

It's very important to understand solar cell's (PV cell) different characteristics before developing solar inverter system. Solar PV cell diagram is shown as right Figure. Rp and Rs are parasitic resistances, in ideal condition, they are infinity and zero. Solar PV cell

Sun light intensity and temperature can affect solar PV cells operating characteristics significantly. Current is proportional to the sun light intensity, but the impact of sun light changes to operating voltage is very small. However, the operating voltage is affected by temperature. Battery temperature increase will reduce the operating voltage, but has less impact to current. Following figure illustrates the effect of temperature and sun light intensity on PV modules.

The impact of sun light intensity on battery output power is greater than the impact of temperature changes. It's applicable to all common PV materials. The important result of these two effects combination is, solar PV cell's power will decrease with lower light intensity and(or) temperature rise. solar PV module electrical characteristics

Maximum power point (MPP)

Solar cell can work in a wide range of voltages and currents. By the resistive load on the battery from zero (short circuit) continues increase to high value (open circuit) to determine MPP. MPP is operation point of the maximum value of V x I (voltage multiply current), and reach maximum power in the sun light intensity. The output power is zero in short-circuit (PV voltage equal to zero) or open circuit (PV current is equal to zero).

High quality single crystal silicon solar cells can generate 0.6 volts open circuit voltage in the temperature 25 °C. In full light and air temperature is 25 °C, the temperature of solar cell may be close to 45 °C, which makes the open circuit voltage drops to about 0.55V. With the increases of temperature, the open circuit voltage continuous declines until the PV module short circuit.

The maximum power of the solar cell in temperature of 45 °C usually is usually produced under the conditions of 80% open circuit voltage and 90% short circuit current. The solar cell short-circuit current is almost proportional to the intensity of illumination, while the open circuit voltage may only drop 10% when the illuminate intensity decreases 80%. Lower quality solar cells' voltage will drop faster in the case of the current increase, thereby decrease the output power from 70% to 50 % or even 25%.

Following figure shows the relationship of solar PV panel output current and output power in a certain sun light intensity to operating voltage. solar PV module MPPT characteristics

Solar inverter must ensure PV module working on MPP at any given time, to obtain the maximum energy from the PV modules. Using maximum power point control loop to achieve this purpose, the control loop is also known as maximum power point tracker (MPPT). To achieve high percentage MPP tracking, it needs PV modules output voltage ripple as small as enough, so that the solar PV modules current changes small when it works near the maximum power point.