Restoring the string inverter itself "technology"

    The maintenance of the series inverter is usually replaced by the power station operation and maintenance personnel directly, which requires low skills and high maintenance efficiency. But the centralized maintenance is must by the factory technical personnel to the scene, to the skill request is very high, the maintenance efficiency is low. The advantages of this type of maintenance are obvious, especially in remote areas or overseas. An obvious fact can be proved: the vast majority of inverter manufacturers, rarely sell centralized inverter to foreign countries, but almost all manufacturers, all the group of inverter sold to foreign countries. The reason is very clear, the maintenance cost abroad is too high, maintenance personnel go out costly. Spare parts of the machine are also a problem. It is not easy to keep them locally and it is not feasible to carry them with you. When we arrive at the site, we find that the spare parts are not suitable and have to be sent back from China.

    See, from the effects of the fault of power station group also has obvious advantages: string type inverter hypothesis set of string type inverter and centralized inverter failure rate is 1%, 1 mw power station with two centralized inverter, 40 units set of string type inverter, according to the set of string type inverter mean time to repair for 2 hours, the centralized for 12 hour (considering each manufacturer is not the same as the response time, centralized the actual repair time could be much longer), set of string type inverter capacity loss caused trouble only 1/6 of the centralized.

    Myth 2: power station harmonics will increase with the number of series inverter machines

    Harmonic refers to the voltage or current component of the current whose frequency is an integer multiple of the fundamental wave. It generally refers to the current component of the periodic non-sinusoidal current which is Fourier decomposed and deducted from the fundamental wave. Harmonic current will generate harmonic voltage drop on the short circuit impedance of the power grid and affect the voltage output waveform (user terminal voltage = stable voltage of the power grid - harmonic voltage drop).

    The main sources of power network harmonic are three aspects: first, the quality of power generation is not high harmonic generation; Second, the transmission and distribution system produces harmonics; Third, the harmonics produced by the electric equipment, among which the harmonics produced by the electric equipment are the most.

Belongs to the power equipment, inverter itself on the output voltage is not controlled, on grid voltage, electric current is pumped into the grid, this way of working less effect on the power grid voltage harmonic (except if triggered resonant grid inverter), so the measure of photovoltaic power station and network, power quality in power grid voltage harmonic under the condition of meeting the required 5%, focus on the current harmonics of the inverter output. The current harmonic of inverter is mainly related to the following factors:

    (1) output voltage waveform quality: the output voltage in the control algorithm of the inverter is sine wave, and when there is distortion in the output PWM wave modulated by the inverter, the output harmonic and control effect of the inverter will be affected. Increasing the switching frequency and the output PWM level number is helpful to reduce the distortion rate of PWM waveform. The series inverter with high switching frequency and three levels has more advantages than the centralized inverter with low switching frequency and two levels.

    (2) software control bandwidth: the higher the switching frequency of the inverter, the wider the control bandwidth, and the more adequate suppression of harmonic current in a wide range. To ensure stability, the control bandwidth of the inverter is usually 1/10 of the switching frequency. The switching frequency (about 16kHz) of the series inverter is much higher than that of the centralized inverter (the two-level inverter is 3kHz, and the three-level inverter can achieve about 8k), so the control bandwidth is wider and the control ability for low-order harmonics is stronger. With high control frequency, the harmonic of the power grid can be detected in the control loop, and the suppression program of low-frequency harmonic is added to make the output current harmonic of the inverter do better than the voltage harmonic of the power grid.

    (3) grid-connected filter performance: the high-frequency components of the output current other than the control bandwidth need to be filtered by relying on the filter. The series inverter generally adopts LCL filter, which has the advantages of strong high-frequency harmonic attenuation ability and little influence of grid-connected impedance.

    (4) parallel machine harmonic cancellation ability: the distance between a square array and multiple serial inverters and the booster transformer is different, so the line impedance will be different. The line impedance will be equivalent to the inductance of L2 in the grid-connected LCL filter, and different filter parameters will change the phase of harmonics. When multiple serial inverters work in parallel, the harmonic components will partly cancel each other due to the phase difference, thus reducing the overall harmonic value of the system.

    It can be seen from the above four points that the output current harmonic of the series inverter is no worse than that of the centralized inverter in principle. Because of its higher working frequency, it is completely possible to add a harmonic suppression algorithm into the algorithm to ensure that the output current harmonic is not disturbed by the harmonic of the power grid, which is more advantageous than the centralized inverter.

    Myth 3: the number of series inverters connected in parallel is large, which causes resonance and makes the system unstable

The multi-machine parallel system of the inverter is composed of photovoltaic cell array, multiple inverters, transmission and distribution equipment and power grid. The inverter and transmission and distribution equipment are highly nonlinear, and the power input photovoltaic cell array and the output power grid may also have a large disturbance, so the whole system is very complex. The unreasonable design may lead to the oscillation between multiple inverters and between inverters and power grid, which may lead to the disconnection of inverter protection and even cause personal and property losses. The causes of resonance are various and not directly related to the number of devices

    Common parallel resonances in grid-connected inverters are divided into two situations:

In the first case, when the inverter operates in parallel, the common impedance at the grid-connected end of its output triggers the multi-machine resonance between the parallel inverters. In the parallel system, when the output current of one of the inverters contains harmonics, the harmonic component will generate harmonic voltage drop in the loop and affect the grid-connected terminal voltage of the other inverters in parallel. When the voltage harmonic is close to the control frequency of the inverter, it may lead to multi-machine parallel resonance. This kind of resonance is more common in parallel inverter systems with lower operating frequencies. The working frequency of centralized inverters is 3~8kHz, while the working frequency of series inverters is higher than 16kHz. Therefore, such resonance is more likely to occur in parallel centralized inverters.

  In the second case, the port of the inverter has a filter capacitor, which, together with the leakage inductance of the transformer, forms the LC network. If the high harmonic contained in the output current of the inverter happens to coincide with the resonance frequency of the LC network, resonance will be generated. At this time, if the power grid also contains high-order harmonics of the same frequency, the oscillation will be intensified, resulting in the voltage oscillation of the power grid. Such resonance is difficult to be encountered in large ground power stations with clean power grid, while it may occur in distributed low-voltage grid-connection occasions when the power grid contains high order harmonics due to the complex local load.

    These two kinds of resonances are essentially caused by the high harmonic in the output of the inverter itself. The fundamental method to suppress resonance is to improve the control of the inverter and the design of LC filter to ensure that the output side of the inverter does not contain high-frequency harmonics. For a large power station with a series inverter, generally 1 ~ 2MW is designed to form a grid-connected unit, which is connected through isolation transformer. The isolation transformer will play a good decoupling role between the MW units to ensure that the MW units will not affect each other. In MW unit interior, multimachine parallel, because the set of string type inverter switching frequency is higher, generally amounting to more than 16 KHZ control bandwidth is also relatively wide, the general has reached around 2 KHZ, and the harmonic wave of power grid is generally not more than 2 KHZ, within a set of string type inverter control bandwidth, set of string type inverter can be in the control loop to join these harmonic suppression algorithm, makes the inverter on the frequency of harmonic response, can effectively prevent the occurrence of resonance, thus ensuring the stability of the system.

    Myth 4: low voltage transversal performance of a series inverter is worse than that of a centralized inverter

    The so-called low wear/zero wear is that the inverter detects the voltage drop of the power grid, maintains the power grid in a short period of time, and supports the restoration of the power grid as soon as possible through reactive power output. When the power grid voltage is not completely dropped to zero, it is considered in the standard that the power grid voltage falls below 5%, which is called zero penetration, because the inverter still needs to detect the phase of the power grid in order to send reactive power to support the power grid. The key point of the inverter's response to low penetration is that the inverter can detect the voltage sag in time and then make corresponding response according to the internal algorithm. In a grid-connected unit, the impedance of the ac cable is small, and the inverter can detect the drop of the power grid and react in time. Therefore, low penetration is completely an independent behavior of the inverter, and there is no need for any linkage between the inverters. The low penetration characteristics of the power station are not necessarily related to the number of inverters.

    Myth 5: interference between multiple serial inverters will lead to the island can not be protected

   Island is when power networks for when because of the failure, accident, natural factors or tripping outage maintenance reasons the interruption of power supply, photovoltaic (pv) grid inverter failed to real-time detect the state of power outages and will cut off the mains network itself, still continue to a certain proportion to grid electricity, by solar energy grid generation system and the surrounding the load on the formation of a electric power company is unable to grasp the self power island. As can be seen from the definition, there are two conditions for grid-connected photovoltaic inverter to form an island: the inverter system is separated from the power grid; The inverter output power is matched with the local load, which causes the output voltage to maintain the output continuously, thus forming isolated power supply operation.

    The anti - islanding protection scheme of inverter is divided into active anti - islanding protection scheme and passive anti - islanding protection scheme. The passive scheme detects the island effect by detecting the abnormal voltage or frequency of the ac output terminal of the inverter. In this scheme, each inverter detects the power grid, and there will be no mutual interference between multiple inverters. The active scheme monitors the corresponding changes in voltage, frequency and impedance of the system by intentionally introducing disturbance signals to determine the existence of the power grid. The active protection scheme mainly includes frequency offset, impedance change caused by current pulse injection, power line carrier communication, etc. In the active island scheme, if there are inverters of different manufacturers in a grid-connected unit, there may be a phenomenon that the direction of disturbance signal is inconsistent and the active island scheme is affected.

    Standard authentication process, the island's test is very strict, testing institutions are building the resonance frequency of 50 hz LC resonance network testing island, to ensure that the inverter in these extreme cases on island protection, inverter in order to meet the requirements of the standard, passive anti island alone is not enough, must increase active scheme of the island. However, in the reality of grid-connected, LC resonant network with resonant frequency exactly 50Hz is almost impossible to be encountered, and the inverter can achieve the purpose of protection by passive anti-islanding means such as voltage and frequency detection. A Dutch study organization issued a report shows that although there are a lot of European power station group is string type inverter, and active island between different manufacturers of inverter scheme may not be consistent, but only by passive island scheme can realize the protection, and passive island scheme will not interfere with each other, so the actual power station in the case of no problems because of the island.