Photovoltaic off-grid power generation system is mainly used to solve the basic electricity consumption problem of residents in areas without electricity or less electricity. The photovoltaic off-grid power generation system is mainly composed of photovoltaic modules, brackets, controllers, inverters, batteries and power distribution systems. Compared with the photovoltaic grid-connected system, the off-grid system has more controllers and batteries, and the inverter directly drives the load, so the electrical system is more complicated. Since the off-grid system may be the user's only source of electricity, and the user is highly dependent on the system, the design and operation of the off-grid system should be more reliable.
Common design issues for off-grid systems
There is no unified specification for photovoltaic off-grid systems. It should be designed according to the needs of users, mainly considering the selection and calculation of components, inverters, controllers, batteries, cables, switches and other equipment. Before designing, the preliminary work must be done well. It is necessary to first understand the user's load type and power, the climatic conditions of the installation site, the user's electricity consumption, and the demand before making a plan.
1. The voltage of the module and the voltage of the battery should be matched. The PWM controller solar module and the battery are connected through an electronic switch. There is no inductance and other devices in the middle. The voltage of the module is between 1.2 and 2.0 times the voltage of the battery. If It is a 24V battery, the input voltage of the component is between 30-50V, the MPPT controller has a power switch tube and an inductor and other circuits in the middle, the voltage of the component is between 1.2-3.5 times the voltage of the battery, if it is a 24V battery, The component input voltage is between 30-90V.
2. The output power of the module should be similar to the power of the controller. For example, a 48V30A controller has an output power of 1440VA, and the power of the module should be around 1500W. When choosing a controller, first look at the voltage of the battery, and then divide the component power by the voltage of the battery, which is the output current of the controller.
3. If the power of one inverter is not enough, multiple inverters need to be connected in parallel. The output of the photovoltaic off-grid system is connected to the load. The output voltage and current phase and amplitude of each inverter are different. If the terminals are connected in parallel, an inverter with parallel function should be added.
Common problems when debugging off-grid systems
1 The inverter LCD does not display 01
Failure analysis
There is no battery DC input, the inverter LCD power supply is powered by the battery.
02 Possible reasons
(1) The battery voltage is not enough. When the battery first leaves the factory, it is generally fully charged, but if the battery is not used for a long time, it will be slowly discharged (self-discharge). Off-grid system voltages are 12V, 24V, 48V, 96V, etc. In some applications, multiple batteries must be connected in series to meet the system voltage. If the connecting cables are not properly connected, the battery voltage will be insufficient.
(2) The battery terminals are reversed. The battery terminals have positive and negative poles, generally red is connected to the positive pole, and black is connected to the negative pole.
(3) The DC switch is not closed or the switch is faulty.
03
Solution
(1) If the battery voltage is not enough, the system cannot work, and the solar energy cannot charge the battery, you need to find another place to charge the battery to more than 30%.
(2) If it is a problem with the line, use a multimeter to measure the voltage of each battery. When the voltage is normal, the total voltage is the sum of the battery voltages. If there is no voltage, check whether the DC switch, wiring terminal, cable connector, etc. are normal in turn.
(3) If the battery voltage is normal, the wiring is normal, the switch is turned on, and the inverter still does not display, it may be that the inverter is faulty, and the manufacturer should be notified for maintenance.
2 The battery cannot be charged
01 Failure Analysis
The battery is charged by the photovoltaic module and the controller, or the mains and the controller.
02 Possible reasons
(1) Component reasons: the component voltage is not enough, the sunlight is low, and the component and DC cable connection is not good.
(2) The battery circuit wiring is not good.
(3) The battery is fully charged and reaches the highest voltage.
03 Solutions
(1) Check whether the DC switches, terminals, cable connectors, components, batteries, etc. are normal in turn. If there are multiple components, they should be connected and tested separately.
(2) When the battery is fully charged, it cannot be recharged, but different batteries have different voltages when fully charged. For example, a battery with a rated voltage of 12V has a voltage between 12.8 and 13.5V when fully charged. The specific gravity of the electrolyte when the battery is fully charged is related. Adjust the maximum voltage limit according to the battery type.
(3) Input overcurrent: The charging current of the battery is generally 0.1C-0.2C, and the maximum is not more than 0.3C. For example, a lead-acid battery 12V200AH, the charging current is generally between 20A and 40A, and the maximum cannot exceed 60A. The component power should match the controller power.
(4) Input overvoltage: The input voltage of the module is too high, check the voltage of the battery board, if it is really high, the possible reason is that the number of strings of the battery board is too many, reduce the number of strings of the battery board
3 The inverter shows overload or cannot start 01
Failure analysis
The load power is greater than the inverter or battery power.
02 Possible reasons
(1) Inverter overload: If the inverter overload exceeds the time range, and the load power exceeds the maximum value, adjust the load size.
(2) Battery overload: The discharge current is generally 0.2C-0.3C, the maximum does not exceed 0.5C, 1 12V200AH lead-acid battery, the maximum output power does not exceed 2400W, different manufacturers, different models, the specific values are also different .
(3) Loads such as elevators cannot be directly connected to the output terminal of the inverter, because when the elevator is descending, the motor reverses, which will generate a back electromotive force, which will damage the inverter when it enters the inverter. If an off-grid system must be used, it is recommended to add a frequency converter between the inverter and the elevator motor.
(4) The starting power of the inductive load is too large.
03 Solutions
The rated power of the load should be lower than that of the inverter, and the peak power of the load should not be greater than 1.5 times the rated power of the inverter.
Battery FAQ
1 Short circuit phenomenon and reasons
The short circuit of the lead-acid battery refers to the connection of the positive and negative groups inside the lead-acid battery. The short-circuit phenomenon of lead-acid batteries is mainly manifested in the following aspects:
The open circuit voltage is low, and the closed circuit voltage (discharge) quickly reaches the termination voltage. When a large current is discharged, the terminal voltage drops rapidly to zero. When the circuit is open, the density of the electrolyte is very low, and the electrolyte will freeze in a low temperature environment. When charging, the voltage rises very slowly, always staying low (sometimes dropping to zero). During charging, the temperature of the electrolyte rises very quickly. During charging, the electrolyte density rises very slowly or hardly changes. No bubbles or gas appears late when charging.
The main reasons for the internal short circuit of lead-acid batteries are as follows:
The quality of the separator is not good or defective, so that the active material of the plate passes through, resulting in virtual or direct contact between the positive and negative plates. The displacement of the separator causes the positive and negative plates to be connected. The active material on the electrode plate expands and falls off. Due to the excessive deposition of the fallen active material, the lower edge or side edge of the positive and negative plates is in contact with the sediment, resulting in the connection of the positive and negative plates. A conductive object falls into the battery, causing the positive and negative plates to connect.
Phenomenon and causes of 2-pole sulfation
The plate sulfation system is the lead sulfate that forms white and hard lead sulfate crystals on the plate, and is very difficult to convert into active substances during charging. The main phenomena after sulfation of lead-acid battery plates are as follows:
(1) The voltage of the lead-acid battery rises rapidly during the charging process, and its initial and final voltages are too high, and the final charging voltage can reach about 2.90V/single cell.
(2) During the discharge process, the voltage decreases rapidly, that is, it drops to the termination voltage prematurely, so its capacity is significantly lower than that of other batteries.
(3) During charging, the temperature of the electrolyte rises rapidly and easily exceeds 45°C.
(4) During charging, the density of the electrolyte is lower than the normal value, and bubbles occur prematurely during charging.
The main reasons for the sulfation of the plate are as follows:
(1) The initial charging of lead-acid batteries is insufficient or the initial charging is interrupted for a long time.
(2) The lead-acid battery is not sufficiently charged for a long time.
(3) Failure to charge in time after discharge.
(4) Often overdischarge or small current deep discharge.
(5) If the electrolyte density is too high or the temperature is too high, lead sulfate will be deeply formed and difficult to recover.
(6) The lead-acid battery has been put on hold for a long time, and it is not used for a long time without regular charging.