Precautions for installation and use of photovoltaic DC miniature circuit breakers

Precautions for installation and use of photovoltaic DC miniature circuit breakers

Photovoltaic DC miniature circuit breakers are used as photovoltaic power distribution, and the role of DC miniature circuit breakers is particularly prominent. So how can we use DC circuit breakers safely?
1. Check whether the wiring is correct after the DC miniature circuit breaker is connected. It can be checked with the test button. If the circuit breaker can be disconnected correctly, it means that the leakage protector is installed correctly; otherwise, the circuit should be checked to eliminate the fault;
2. After the circuit breaker is disconnected due to the short circuit of the line, it is necessary to check the contacts. If the primary contact is severely burned or has pits, it needs to be repaired; the four-pole leakage circuit breaker (DZ47LE, TX47LE) must be connected to the neutral line. To make the electronic circuit work properly;
3. After the leakage circuit breaker is put into operation, the user should check whether the circuit breaker usually operates through the test button every time after some time; the leakage, overload, and short circuit protection characteristics of the circuit breaker are set by the manufacturer and cannot be adjusted at will so as not to affect performance;
4. The function of the test button is to check the running state of the circuit breaker in the state of closing and energizing after it is newly installed or operated for a certain period. Press the test button; the circuit breaker can be disconnected, indicating that the operation is regular and can continue to be used; if the circuit breaker cannot be disconnected, it indicates that the circuit breaker or the circuit is faulty and needs to be repaired;
5. If the circuit breaker is disconnected due to the failure of the protected circuit, the operating handle is in the tripping position. After finding out the cause and eliminating the fault, the operating handle should be pulled down first to make the operating mechanism "re-buckle" before the closing operation can be performed;
6. The load wiring of the leakage circuit breaker must pass through the load end of the circuit breaker. It is not allowed that any phase line or zero line of the load does not pass through the leakage circuit breaker. Otherwise, it will cause artificial "leakage" and cause the circuit breaker to fail to close, resulting in a "Mistake."
Due to the continuous improvement of photovoltaic DC circuit breaker technology,
How does a PV DC circuit breaker work in a PV system?
To understand the workflow of the photovoltaic DC circuit breaker, it is necessary first to understand the workflow of the entire photovoltaic system:
When the photovoltaic DC system is working, it relies on the function of the solar module square array to convert the solar energy into adequate electrical power. Under the action of the photovoltaic controller, the output voltage is stabilized, and the connection with the DC system is realized. Suppose the voltage output by the solar module meets the voltage requirements of the DC system. In that case, the AC contactor at the input end of the charger will be automatically disconnected under the control of the photovoltaic controller, and the photovoltaic power supply will complete the power supply to the DC system of the substation. Correspondingly, suppose the output voltage cannot meet the voltage requirements of the DC system. In that case, the output work will automatically stop under the control of the photovoltaic controller, and at the same time, the AC contactor at the input end of the charger will also be closed. At this time, the charger completes the DC system power supply work of the substation. The photovoltaic controller and the charger work alternately under this working principle to realize automatic switching. 
Photovoltaic DC circuit breakers generally comprise a contact system, an arc extinguishing system, an operating mechanism, a release, and a casing.
The working principle of the photovoltaic circuit breaker is as follows:
  • When a short circuit occurs, the magnetic field generated by the large current (generally 10 to 12 times) overcomes the reaction force spring.
  • The release pulls the operating mechanism to act.
  • The switch trips instantaneously.
When overloaded, the current becomes more extensive, the heat generation increases and the bimetallic sheet deforms to a certain extent to promote the action of the mechanism (the more significant the current, the shorter the action time). Now there is an electronic type, which uses the transformer to collect the current of each phase and compares it with the set value. When the wind is abnormal, the microprocessor sends a signal to make the electronic release drive the operating mechanism to act.
The function of the DC circuit breaker is to cut off and connect the load circuit, cut off the fault circuit, prevent the accident from expanding, and ensure safe operation. The high-voltage circuit breaker needs to break 1500V arcs with a current of 1500-2000A. These arcs can be stretched to 2m and continue to burn without being extinguished. Therefore, arc extinguishing is a problem that high-voltage circuit breakers must solve. The principle of arc blowing and arc extinguishing is mainly to cool the arc to reduce thermal dissociation.
On the other hand, lengthen the arc by blowing the angle to strengthen the recombination and diffusion of charged particles. At the same time, the charged particles in the arc gap are blown away, and the dielectric strength of the medium is quickly restored. Low-voltage circuit breakers, also known as automatic air switches, can be used to switch on and offload circuits and can also be used to control motors that start infrequently. Its function is equivalent to the sum of some parts of electrical appliances such as knife switch, overcurrent relay, voltage loss relay, thermal relay, and leakage protector. Therefore, it is an essential protective electrical appliance in the low-voltage distribution network. 
1. The rated working current, rated working voltage, and breaking capacity of the circuit breaker should focus on the rated working voltage and rated working currently in the photovoltaic system. The breaking capacity should be used as a reference index. The selection of rated working voltage and rated current should ensure that the circuit breaker protection is reliable and has no malfunction. The choice of circuit breakers in photovoltaic systems is mainly based on the parameters of the modules, the number of strings, altitude, peak irradiance, shallow temperature, margin, etc. The parameters of the modules and the number of lines are the primary calculation basis; length, irradiance peak, the external temperature should be considered together with the design margin measurement. The rated working voltage is mainly directly related to the component parameters and the number of strings, and the altitude and low temperature are considered in the design margin. The rated working current is regarded with the irradiance peak value and the empirical margin. Our selection ideas are based on the rated working voltage and rated working currently. First, let's talk about the system voltage, and then talk about the current.
2. We choose a module from a well-known domestic module factory that has passed UL1500V certification as a reference sample for calculation; the module power is 550W to 530W, and the module efficiency is greater than 20%. It should be noted that the sample parameters of the component factory are atmospheric AM1.5, irradiance 1000W/m², and temperature 25°C. Therefore, the field peak data is quite different from the above conditions, which is critical in calculating the margin design aspect. Component parameter selection focuses on three main parameters of the component: 1. Maximum operating voltage; 2. Maximum working current; 3. Maximum open-circuit voltage.
First, let's discuss the calculation of voltage: 
STC peak power (Pmax) 550W 545W 540W 535W 530W
Best working voltage (Vmp) 42.05V 41.87V 41.75V 41.57V 41.39V
Best working current (lmp) 13.08A 13.02A 12.94A 12.87A 12.81A
Open circuit voltage (Voc) 49.88V 49.69V 49.54V 49.39V 49.24V
Short circuit current (Isc) 14.01A 13.96A 13.89A 13.83A 13.76A
Component conversion efficiency 21.3% 21.1% 20.9% 20.7% 20.5%
Component operating temperature -40 °C to +85 °C
Maximum system voltage 1500V DC (IEC)
Maximum series fuse current rating 25A
Power tolerance 0/+5W

Table 1: PV Module Parameter Table
Test data Environmental indicators: (atmosphere AM1.5, irradiance 1000W/m², temperature 25°C)
The primary influence of the system voltage is the arrangement of components and the number of modules in a single string. The core value of the DC1500V system should be to improve the system efficiency and effectively reduce the cost of DC transmission and inverter. At present, our mainstream single-string component arrangement uses 2*11 more, and this solution is the optimal cost solution at present. The DC1500V system does not change the design on the power generation side and the AC side, so the DC1500V solution should retain the current mainstream solution of component arrangement and increase the number of single-string blocks to achieve higher system voltage. Based on the above reasons, we recommend that the best solution for the string arrangement and number of blocks of the DC1500V system is 2*13 so that based on the key without changing the module array, it is possible to achieve greater efficiency in the three aspects of cables, combiner boxes, and inverters—cost reduction. If we determine the number of component blocks in a single string, the system voltage behind it is perfect.
Component power 550Wp 545Wp 540Wp 535Wp 530Wp
Maximum working voltage 1093.3 1088.62 1085.5 1080.82 1076.14
Maximum open circuit voltage 1296.88 1291.94 1288.04 1284.14 1280.24

Table 2: 26-module string reference voltage
Test data Environmental indicators: (atmosphere AM1.5, irradiance 1000W/m², temperature 25°C)
Are the figures in Table 2 the actual peaks? Unfortunately, this is not the case. Two main factors affect the system voltage. Altitude and temperature, the arc extinguishing performance of the circuit breaker is first discussed from the size. The biggest challenge of the voltage problem to the circuit breaker is arc extinguishing. The higher the voltage, the more difficult it is. The experimental environment of circuit breaker parameters is based on the atmospheric AM benchmark at an altitude of 2000 meters. Above 2000 meters, the air is relatively thin, and the arc extinguishing capacity of the circuit breaker decreases linearly with the increase of altitude. For the convenience of calculation, it is converted into the derating factor of the rated working voltage. According to the data analysis collected for many years, the altitude of large-scale ground power stations in China is 1500 to 3000 meters, so it is recommended to consider 10% in the design margin of altitude derating, which can cover the altitude of most projects.
In addition, the ambient temperature dramatically influences the component's output voltage. The component's output voltage between 25°C and -10°C has a steep rise curve, and the voltage rise changes less after -10°C. The voltage temperature coefficient of the component is -0.36%/k (different manufacturers are slightly different). In terms of the temperature coefficient margin, we recommend considering 42*0.36%=15.12%. We recommend the system regarding the two margin considerations of altitude and temperature. The voltage design margin is 20%. The following is the recommended system voltage after the margin correction:
Component power 550Wp 545Wp 540Wp 535Wp 530Wp
Maximum working voltage 1311.96 1306.344 1302.6 1296.984 1291.368
Maximum open circuit voltage 1556.256 1550.328 1545.648 1540.968 1536.288

Table 3: System correction voltage of different power components of photovoltaic DC1500V system
From the above table, we found that using the peak data to calculate that the maximum operating voltage of the system is below 1320V, a photovoltaic circuit breaker with a rated operating voltage of DC1500V can meet the system requirements. However, it is worth noting that the maximum open-circuit voltage of the system correction exceeds the maximum rated effective working voltage of the circuit breaker by 1.5%. Although this is only the corrected result and does not represent the actual peak value, the open-circuit voltage will exceed the maximum open-circuit voltage of the circuit breaker after the altitude exceeds 3000 meters. Therefore, effective working voltage the system open circuit voltage should not exceed the maximum effective working voltage of the circuit breaker is the basic rule of our selection.
Secondly: let's look at the selection of current. The quick calculation method of taking the optimal value of the circuit breaker after calculating each string of 12A in the DC1000V system is the mainstream method. There is nothing wrong with the calculation method in the DC1500V system, but this result can no longer be used. The improvement of module efficiency is the main reason for the decline in module prices in recent years; that is, higher power output in the same unit area, the module area does not increase—still, the power increases, which will inevitably increase the module voltage and current output at 400W. In the above photovoltaic systems, it is necessary to gradually consider increasing the rated working current of the circuit breaker. The recent increase has nothing to do with the DC1500V or DC1000V system. This is a problem caused by the improvement of the output parameters of the components.
Component power 550Wp 545Wp 540Wp 535Wp 530Wp
Maximum operating current 13.08 13.02 12.94 12.87 12.81
Maximum operating current after correction 19.62 19.53 19.41 19.305 19.215
24 sinks 1 maximum working current 470.88 468.72 465.84 463.32 461.16

Table 4: Maximum operating current calculation table
For the current selection calculation of photovoltaic circuit breakers, we recommend a fast and straightforward algorithm of the nominal maximum working current of the module * 150%. In 2016, the follow-up survey results showed that the 130% empirical margin design is a critical value, prone to false trips. Accident.
There are three reasons for the recommended margin of 50% for circuit breakers:
.Irradiance impact: The current parameter of the module is the benchmark for irradiance of 1000W/m². The peak irradiance in areas with good irradiation conditions is about 1200W/m², consuming at least 20% of the design margin. Accessible to super send.
.The equipment installation environment is relatively harsh, the heat dissipation is poor, and the internal temperature of the equipment is very high, which has an impact on the derating of the circuit breaker. The field measurement found that the highest temperature exceeded 70℃.
.There is a big difference in the temperature rise control of circuit breakers of different manufacturers. The temperature rise of our photovoltaic circuit breakers after being connected in series should not exceed 60K, generally above 70K. Unqualified products exceeding 80K are also popular. The main reason for the temperature rise exceeding 80K is the series connection. Part of the welding method is not used, and the heating of the copper bar screws is too high.
In 2012, a Korean brand circuit breaker product in the northwest region was still vividly remembered because the series temperature rise could not meet the use of large-scale false trips. Therefore, the recommended accurate design selection of the current margin is 30% empirical margin + (peak irradiance/1000-1) * 100% = actual current design margin of the project, and the simple, quick calculation is calculated according to 50%.
Finally, a summary: The photovoltaic DC1500V system recommends a single-string module of 2*13=26 pieces. The working voltage of the combiner box and the inverter inlet circuit breaker is DC1500V, and the minimum current is 500A. For non-welded connection methods such as a row, it is recommended to select a higher current to 630A. It is recommended that you use the peak parameters as the calculation basis for selecting photovoltaic circuit breakers.

Leave A Reply