Some time ago, the AQSIQ exposed a list of unqualified inverters . In this unsettled photovoltaic circle, it has stirred up waves. The author looked at the reasons for the failure, and also looked at the list of manufacturers (there are many listed companies in the list, and there are also many well-known manufacturers whose products are below 10KW). The reason is basically judged: the reason is not the inverter manufacturer, in the power grid.
Most inverters have no problem in urban applications, but in rural areas, especially in rural areas that have not undergone rural network transformation, problems will occur frequently.
1.1 The following is a typical case because the low voltage cannot be connected to the grid.
“After a certain inverter is installed in a farmer's house, one of them is installed separately in one farmer's house. The other two 5KW inverters are installed in another farmer's home. After three weeks of stable operation, the three inverters are stable. At the beginning, three inverters started to start repeatedly, and in the case of high weather (12 noon - 15 pm), the basic stop work does not generate electricity."
Reason: Because the village is a big village. And the village's voltage is low. The temperature is still relatively suitable during installation and is smoothly connected to the grid. After a week or so, the local temperature increased, and most of the households opened air conditioners at home. When the electricity consumption was large, the local grid voltage was low. Exceeded the lower limit of the inverter's AC voltage output range (187-270VAC). Another week later, heavy rain fell on the project site, the electricity consumption in the village became less, and the voltage of the grid met the inverter voltage requirements, and the system worked normally. â€
Many companies encounter this situation will increase the voltage of the inverter, from the original (178-270) VAC to (140-270) VAC, companies with overseas markets will adjust the "safety" to "Australia" Safety". As a result, the sampling is unqualified.
1.2 Another situation is that the voltage exceeds the upper limit, causing the inverter to not work.
“When several inverters in a poverty alleviation project are connected in parallel, the inverter will have reconnection due to excessive output voltage, and there will be problems such as abnormal noise, which will affect the user's power generation.â€
Why is this happening? We do a calculation.
First, the resistance of the line, that is, the impedance from the inverter end to the transformer end (here, according to the pure resistance calculation, the AC power is more complicated)
R=Ï*L/S copper resistivity 0.0175, aluminum resistivity 0.0283 (using aluminum wire for project site)
The local grid connection point is 1400 meters away from the transformer L=1400M
Transmission line diameter 95mmS=95
R=Ï*L/S=0.0283*1400/95=0.4Ω
There are 13 machines on each fire line in the project. When the sun is strong, the average output current of one machine is 10A, then the current of 13 machines is 130A. At this time, the voltage can be raised up to 130*0.4=52V. ! That is, the voltage from the transformer to the inverter terminal will be raised by 52V. According to the test results in the field, at noon, the voltage on the low voltage side of the transformer reaches 240V, and the voltage rises to 240+52=292V at the output of the inverter, and the actual voltage rise will be higher. The inverter voltage range is exceeded, so the error will be reconnected. When the light is weak, the output current of the inverter is small, the voltage rise is small, and the voltage at the output of the inverter is within the normal range, and normal power generation can also be performed. This is also the reason why the scene is sometimes reconnected more frequently.
Inverter manufacturers will also raise the upper limit of the voltage (178-270) VAC, sometimes to 320VAC. As a result, the sampling will be unqualified.
1.3 Problems encountered in PV poverty alleviation
Due to the wide distribution and development of rural power grids in the country, some problems and characteristics of power quality of rural power grids have brought certain challenges to photovoltaic power generation , especially the operation of inverters. At the same time, the current PV inverter standards are more based on the ground. Power plants, developed regional power grids, so there is a certain degree of incompatibility, which requires photovoltaic power design end, EPC end, etc. to make reasonable design and consideration, to obtain maximum power generation stability and efficiency.
For photovoltaic power generation, rural power grids have the following power quality characteristics that easily affect the normal operation of the inverter:
1) Large line loss
The effective cross-section of the line conductor is much smaller. Many distributed photovoltaic power generation units rely on the existing transmission and distribution facilities for power station construction. In some areas, due to the early construction, the existing facilities have low capacity, and the load (photovoltaic power generation capacity, for the power grid) Is a reverse load) Inaccurate expected, resulting in too small wire cross-section selection, high aluminum wire resistivity, less power supply point construction, low power distribution line power supply radius, when the power supply radius exceeds a certain length (reasonable It is recommended that the maximum is not more than 0.5km, the distribution should be located close to the load center.) The line loss is too large, causing the inverter side voltage to be much higher than the PCC (common access point) voltage, resulting in the inverter voltage. The range is higher than the legal requirement range (CQC voltage range 187V-252V), the inverter frequently trips, the power generation is small or can not work normally;
2) Three-phase voltage imbalance The electrical level of the rural power grid is low. When the design or capacity is increased, the three-phase load is unbalanced, causing the midpoint to shift, resulting in phase voltage imbalance. The load is light, the phase voltage is high, and the load is heavy. The voltage is low, and some areas have exceeded the requirements of the “Three-phase voltage allowable imbalance†required by the national standard, resulting in the inverter not working properly;
3) The distribution capacity is small, the system impedance is high, and the load is overloaded, resulting in low voltage. When the load is low, the voltage is high. The voltage in the single-day period has obvious large-scale changes, exceeding the allowable deviation of the supply voltage. Scope, the main transformer of the substation adopts the load-carrying automatic voltage-regulating transformer to alleviate this problem; the distribution capacity and the length of the distribution line have a serious problem, which makes the system impedance relatively large, and the significant problem caused by the large system impedance is The stability of the system in which the inverters are connected in parallel is reduced and the harmonics are amplified. The more the number of single points is more obvious, the inverter manufacturer usually limits the transmission distance from the AC end of the inverter to the PCC point, and the total number of parallel connections.
1.4 Personal advice on rural PV poverty alleviation:
The current characteristics of rural power grids are poor grid, large line losses, and the need for safe power consumption. The line loss refers to the power loss in the line and transformer during the transmission of electrical energy by the power supply system.
Line loss has the following performances:
(1) Leakage of equipment: Most township enterprises and farmers purchase non-standard electrical equipment that has been eliminated from the city at a cheap price. They have been in disrepair for many years and have serious leakage.
(2) Line aging: concentrated in the demolition line, the small-diameter wire has a heavy load for many years, accelerates aging, is difficult to weather, has more leakage, and has a large voltage drop.
(3) Equipment overload: Most farmers lack the knowledge of electrical appliances on the one hand, and save money on the other hand, which makes the distribution transformer, motor and line overload operation and heat consumption increase.
(4) The capacity of the distribution transformer is not suitable, and the loss is high: due to the long supply radius of the line, the voltage quality is poor, the line loss is large, and the power consumption is serious.
(5) Private pull-up, poor indoor wiring: rural busy seasons, gathering occasions, private pull-up, illegal use of electricity; indoor wiring installation private installation, aging is not repaired, serious electricity catching.
(6) Management confusion: The performance is out of control in the areas of common electricity theft, inaccurate metering, power consumption, and confusion of accounts. ã€1】
Due to the above factors, from a technical point of view, for rural areas that are remote and have not been transformed by rural power grids, it is not suitable for one household and one inverter. Instead, it should be planned as a grid connection point to reduce the amount of inverter used. From other perspectives, the Chinese have a characteristic of "not suffering from ignorance and suffering from inequality." If users who are far away from the transformer have problems every day, the benefits are relatively reduced, and it is difficult to guarantee that they have no opinions.
2. Some misunderstandings about inverter understanding
I don't know when it started, photovoltaic design began to popular inverter over-match. There is nothing wrong with this, but everyone is over-prone.
Let's take the example of the best Hefei Sunshine inverter in the industry. The parameters of the SG500MX are as follows:
First, why is the maximum input voltage not 1200V, not 1100V? Because the mainstream market support bus capacitance maximum pressure rating is 1100V, our IBGT generally 1200V / 1400A, in accordance with the bucket theory, we have to follow the input voltage inverter design pressure rating of the capacitor, the inverter the upper limit is the DC voltage 1000V (100V left margin can extend the life of the capacitor). We did a series-parallel component design, do not deliberately removed to open-circuit voltage components with 1000V, 900V or 950V with it.
Secondly, why the upper limit of the MPPT voltage of 850V is the lower limit is 460V? SG500MX the output AC voltage is 315V, 315 is an average value of the AC voltage, the average value of the relationship with the maximum value 1.41 * 315 = 445V.
Why limit is 850V, the problem there is a little complicated. This involves the loss of the inverter IGBT.
The loss of IGBT mainly includes on-state loss and switching loss.
The on-state loss mainly depends on the on-state saturation voltage drop Vce(sat) of the IGBT. The switching loss mainly depends on the on-time Ton and the off-time Toff. Different frequencies and different circuit modes, the total loss of these two kinds of losses The ratio is different. When the operating frequency is low, the on-state loss is the main one. It is recommended to select the IGBT with low saturation voltage drop. When the operating frequency is high, the switching loss is the main one. It is recommended to select the fast IGBT with short tail current.
Modules with larger currents increase switching speed and reduce switching losses. The switching loss includes turn-on loss and turn-off loss. In the series resonant circuit, it is easier to achieve zero voltage turn-on when the IGBT is turned on. At this time, the turn-on loss is extremely small; when the IGBT is turned off, it is close to zero current turn-off. It can be done, and the turn-off loss is also small at this time. The reduction of the gate resistance RG shortens the turn-on time Ton and the turn-off time Toff, and reduces turn-on and turn-off losses, but increases surge voltage and electromagnetic interference.
Therefore, the higher the voltage, the larger the switching stress of the IGBT, and the switching loss becomes high. This is why the inverter is the least efficient at 850V as shown in the figure below.
The same is true for string inverters. It is not the number of components connected in series, the better, but the best. I hope that PV designers will learn to look at the efficiency graph of the inverter. Don't study 24 pieces, or 23 pieces. Instead, the configuration is more efficient when looking at the highest and lowest ambient temperatures, otherwise the meaning of the string inverter is lost.
[1] Yan Mingjun. Discussion on technical measures for the transformation of Jinrong rural power grid [期刊论文]-China Rural Water and Hydropower 2005(11)
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