With the close proximity of the photovoltaic DC1500V system, the selection issue has also been put on the agenda. In August 2015, the Three Gorges New Energy 30MW Photovoltaic Power Plant project took the lead in adopting the DC1500V DC photovoltaic circuit breaker. This is the first time that DC1500V DC circuit breakers have been used in batches in large-scale terrestrial photovoltaic power plants. Today we will share with you our selection ideas for the DC1500V PV system.
The rated operating current, rated operating voltage, and breaking capacity of the three major indicators of the circuit breaker should focus on the rated working voltage and rated operating current in the photovoltaic system, and the breaking capacity can be used as a reference indicator.
The selection of rated operating voltage and rated operating current shall ensure that the circuit breaker is protected and does not malfunction. The main selection of circuit breakers in PV systems is based on several major aspects such as component parameters, number of clusters, altitude, peak irradiance, extreme low temperature, and margin. The module parameters and the number of cluster blocks are the main calculation basis. The altitude, irradiance peak, and extreme low temperature should be considered together with the design margin calculation. The rated operating voltage is mainly related to the component parameters and the number of strings, and the altitude and the low temperature are taken into consideration in the design margin. The rated operating current is considered together with the peak irradiance and the empirical margin. Our selection concept is based on the rated operating voltage and rated operating current. First we talk about the system voltage, and then we talk about the current.
We selected the components of a well-known domestic component factory that has passed UL1500V certification as the reference sample for calculation. The component power was 260W to 275W, and the module efficiency was 16.8%. It is important to point out that the sample parameters of the component plant are atmospheric AM1.5, irradiance 1000 W/m2, parameters under the temperature of 25°C, and the field peak data is quite different from the above conditions. This is the focus of the margin design calculation. Consider aspects. The component parameter selection focuses on the three main parameters of the component, 1. Maximum operating voltage; 2. Maximum operating current; 3. Maximum open circuit voltage.
Let's first discuss voltage calculations:
The main influence of system voltage is the arrangement of components and the number of single string components. The core value of the DC1500V system should be to improve the system efficiency and effectively reduce the DC transmission and reverse the cost. At present, our mainstream single-string assembly uses more 2*11, this program is the optimal cost plan at present. The DC1500V system does not change the system on the power generation side and the AC side. Therefore, the DC1500V solution should retain the current components to mainstream the scheme and increase the number of single-blocks to achieve a higher system voltage. Based on the above reasons, we recommend that the best solution for the DC1500V system string arrangement and number of blocks is 3*11, which can achieve greater results in terms of cables, combiner boxes, and inverters without changing the scheme of the component array. Cost reduction. If we determine the number of single-block components, then the system voltages are very good.
Is the data obtained from Chart 2 the actual peak? This is certainly not the case. There are two major influencing factors for system voltage. At altitude and temperature, the arc extinguishing performance of the circuit breaker is first discussed from the altitude. The biggest challenge to the circuit breaker in the voltage problem is arc extinguishing, and the higher the voltage, the greater the difficulty. The circuit breaker parameter experiment environment is based on the atmospheric AM reference of 2000 meters above sea level. The air above 2000 meters is relatively thin, and the arc extinguishing ability of the circuit breaker decreases linearly with the elevation. We can calculate the derating coefficient of the rated working voltage for calculation purposes. . According to the analysis of the data gathered by the St. Electra team for many years, the large-scale ground power stations in China occupy an altitude of 1,500 to 3,000 meters. Therefore, it is recommended to consider 10% of the designed capacity for altitude derating so that it can cover most of the project elevations.
In addition, the ambient temperature has a great influence on the output voltage of the module. From 25° to -10°, the output voltage of the module is a steep rising curve, and the voltage rise changes less than -10°. The voltage temperature coefficient of the module is -0.35%/k (different from some manufacturers). We recommend 30*0.35%=10.5% for the temperature coefficient margin. We consider the factors of the two margins of integrated altitude and temperature. The voltage design margin is 20%. The following is the recommended margin system voltage after correction:
From the above figure, we find that by using the peak value calculation system operating voltage maximum below 1250V, the use of DC1500V rated voltage photovoltaic circuit breaker can meet the system requirements. However, it is worth noting that the system corrects the maximum open circuit voltage by more than 1.5% of the maximum rated effective operating voltage of the circuit breaker. Although this is only a corrected result and does not represent the actual peak value, the open circuit voltage exceeds the maximum circuit breaker after an altitude of more than 3000 meters. The effective operating voltage, system open circuit voltage should not exceed the maximum effective operating voltage of the circuit breaker is the basic code of our selection.
Secondly, let's look at the choice of current. The current selection method is the mainstream of the DC1000V system's calculation of the preferred value of the circuit breaker after each string of 12A. This program is also the result of my research at the end of 2011. There is nothing wrong with the DC1500V system, but this result can no longer be used. The increase in the efficiency of components is the main reason for the declining price of components in recent years. That is, the output of higher power in the same unit area does not increase but the power of the components increases. This will inevitably increase the voltage and current output of the module at 260 W. In the above photovoltaic system, the rated operating current of the circuit breaker shall be gradually considered, and the increase of the current has nothing to do with the DC1500V or DC1000V system, which is a problem caused by the increase of the output parameter of the module.
We recommend a simple speed algorithm with a nominal maximum operating current of *150% for the current selection of photovoltaic circuit breakers. According to the Sanon Electric team's follow-up survey database from 2010, 130% of the experience margin design is a critical value. , it is very easy to accidentally jump.
There are three reasons why the circuit breaker recommended margin is 50%. 1. Effect of irradiance: The current of the module is the reference of the irradiance of 1000. The peak irradiance of the area with good irradiation is around 1200, so it is consumed at least. With a 20% design margin, the northwestern region is the easiest to surpass in the second quarter. 2. The installation environment of the equipment is rather poor, the heat dissipation is poor, and the internal temperature of the equipment is very high. This has an effect on the derating of the circuit breaker. The actual measured temperature on the site has found that the maximum temperature exceeds 70°. 3. The temperature rise control of circuit breakers varies greatly among different manufacturers. The temperature rise of our PV circuit breakers after series connection does not exceed 60K, and generally exceeds 70K. Unqualified products exceeding 80K are also popular, and the main cause of temperature rise exceeding 80K is There is no welding method in the series section, and the copper row screw installation is too hot. In 2012, the series temperature rise of a certain Korean brand circuit breaker product in the northwest region could not satisfy the use of a large area of ​​erroneous tripping. Our recommended current margin precision design choice is 30% experience margin + (peak irradiance/1000-1) * 100% = project actual current design margin, simple calculation in accordance with 50% calculation
Finally, we summarize our point of view: Photovoltaic DC1500V system we recommend a single component 3 * 11 = 33 block program, the convergence box outlet and inverter input circuit breaker operating voltage selection DC1500V, current minimum selection 225A, if the series is used Using screws to install copper strips and other non-welded connections, the current recommended to re-election to a large file to 250A. Selection of photovoltaic circuit breakers recommend that you use the peak parameters as a calculation basis.
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