- Inrush current
At the moment of starting the motor, the required current is much larger than the rated value. This larger inrush current can be provided by the battery, however the MPP tracker can also provide this larger inrush current by other means. A PV array may operate at high voltage and low current, where if the voltage is lowered, the current will rise, generating additional starting current with the same power. When the motor reaches the rated speed, the voltage is increased and the current is decreased. The rated parameters of the inverter must meet this additional starting current requirement. The allowable surge current value and duration are indicated in the inverter specification. The grid-connected interactive inverter does not need to provide inrush current to the grid, because this is not the function of the grid-connected interactive inverter, and the grid can provide inrush current for the starting of the motor.
- Charge Controller
The charge controller is an electronic voltage regulator, which is used to regulate the voltage and current during charging, which can fully charge the battery and avoid overcharging or undercharging. Overcharging occurs when a battery is fully charged but is still being powered by a photovoltaic array or other power source. The charge controller can operate in conjunction with the MPPT function, which is usually embedded in the controller. A microprocessor chip can be used to execute different control algorithms for battery connection and disconnection and to control the setpoints. The charge controller manages the current between the charging source, the battery and the load by measuring the charge level and temperature of the battery, and then adjusts the set point to achieve the optimal charge and discharge of the battery. The charger can limit the charging current to a “trickle charge” level, preventing overcharging and maintaining the battery’s charge level. The charge controller can also short the PV array, open the charging loop, or shunt current to an “unloaded load” such as an electric water heater or blower. MPPT charge controllers can input higher voltages from the PV array (or other power source) than the battery pack requires, reducing current flow and shrinking wire gauges. The charge controller then adjusts the charge voltage. Want to learn more about charging and discharging batteries? click here to open.
2.1 Parallel controller
There are two basic types of charge controllers: parallel and series. The parallel controller shunts the charging current from the battery by short-circuiting the output of the charging power supply. When the output terminal of the power supply is shorted, the current flows through the corresponding shunt switch, shorting the battery. And when the shunt switch is open, current flows through the battery. The PV array can be safely short-circuited because its short-circuit current is limited by the PV modules. This means that the short-circuit current of PV modules is also limited by the current capacity of the modules. Short-circuiting other types of power sources such as batteries or grid generators is prohibited because when these devices are short-circuited, dangerous amounts of current can be generated. The adjustment switch located in the parallel controller can operate automatically, and through the work of the regulator module, the charging current can be increased or decreased.
Figure 1 shows the schematic diagram of a parallel switching regulator. The regulation control module monitors and detects the power of the battery, and opens or closes the shunt switch under the control of the control program. Current will flow to the battery or be shunted by the switch. The anti-reverse diode prevents the current from being backfilled by the battery when the charging power supply does not generate charging current, and also prevents the battery from short-circuiting when the switch is closed.
On rainy days or at night, the photovoltaic array becomes a load for the battery, and when the wind turbine stops, the generator also becomes a load. At this time, if there is no anti-reverse diode, the battery current may flow back to the generator. Wind turbines cannot be short-circuited during operation, so short-circuit switches are not suitable. The device uses a power transistor to replace the mechanical switch, which has the advantages of low on-resistance and high off-resistance, so the power loss of the device itself is very small.
The parallel controller uses a short-circuit switch to separate the charging current into pulses, and the interval time of the current pulses determines the size of the charging current. An alternative to parallel regulation is to replace switches with resistive elements such as resistors, as shown in Figure 2. This resistive element decreases or increases the current according to Ohm’s law I=V/R. If R is small, the current flowing into the cell from the PV array is small, and the (shunt) current (Is) of the parallel branch is approximately equal to the current (Ia) of the array. If R is large, the current flowing into the battery of the photovoltaic array is large, and the current (Is) of the parallel branch is approximately equal to zero. By increasing or decreasing the resistor value in small increments, the charge current (Ic) can be controlled in a smooth and linear manner. This type of regulator is called a shunt linear regulator. Linear regulators are less efficient than switching regulators because resistive elements dissipate a lot of power, and switching regulators generate pulses that can cause noise in audio equipment, while linear regulators do not have these side effects. In some applications, fast charging of the battery can be achieved because the charging current of the linear regulator is smoother and more stable. When the resistance of the parallel device increases, the current in the parallel branch decreases while the charging current increases
Figure 3 shows the linear relationship between parallel branch resistance and charging current.
2.2. Serial controller
Series controllers have a regulating switch in series between the charging source and the battery (see Figure 4). The switching algorithm controls the switching on and off according to a certain order, which is similar to the parallel regulator. When the switch disconnects the power supply, the charging current is interrupted. When the battery is nearly full, the adjustment control module detects the battery level and adjusts the on-off cycle of the switch. Similar to shunt-type regulators, series-type controllers can also use resistive elements instead of switches to reduce or increase the charging current by changing the resistance value according to Ohm’s law. when the power supply
When the voltage is low, the regulating switch is turned off, preventing current from flowing back from the battery to the charging source. When charging power is restored, the switch turns the circuit back on. Anti-reverse diodes are not required since the series switch prevents backflow of current.
Read more: How much do you know about inverter system applications?