Solar-battery integrated DC system
A DC bus is connected to a PV array, a battery or both, to power a grid connected inverter load on the bus. A system and method is described for controlling current flow on a DC bus regardless of the type of inverter. A converter receives a DC battery voltage. A switching module controls current flow through the converter. A current generator generates a reference current in response to a reference voltage. A comparator connected at the output of the current generator compares the reference current with a battery current and outputs a signal to a controller. The controller generates a reference value. A PWM generator modulates the switch module to control current flow and energy flow between the DC bus and the battery. The current from the DC bus to the inverter is controlled to allow proper grid-connected operation regardless of inverter type.
1. A solar-battery integrated DC system for controlling current flow on a direct current (DC) bus, comprising:
a controller comprising:
a pair of input terminals for receiving a DC battery voltage from a battery;
a switching module for controlling a bi-directional current flow between the battery and the DC bus, the switching module having a pair of output terminals and photovoltaic (PV) and Battery voltage and current sensors;
a control path for controlling the switching module, the control path comprising:
a current generator to generate a reference current in response to a reference voltage detected by the voltage sensor; and
a comparator connected at the output of the current generator, the comparator configured to compare the reference current with a battery current and output a signal to a controller module; the controller module configured to generate a reference value; and
a pulse width modulating generator configured to receive the reference value and modulate the switch module to control current flow and energy flow between the DC bus and the battery to adjust the controller output voltage to match the DC bus;
wherein the DC bus is further connected to a PV array; and
wherein the DC bus is connected to an inverter, the inverter controlling alternating current (AC) to inverter output terminals.
2. The system of claim 1 , wherein the controller module is a proportional-integral controller.
3. The system of claim 1 , wherein the controller comprises a micro-controller, the micro-controller programmed to:
a. Set type of conventional off-the-shelf (COTS) Inverter
b. Set Operating Mode
c. Set Desired Battery Current Value
d. Adaptively control bidirectional DC/DC Converter voltage and current to present appropriate Current-Voltage characteristic at inverter input for desired Battery current and generate reference value for DC/DC converter
e. Modulate the switch module and maintain desired battery current direction.
4. The system of claim 3 , wherein the micro-controller is further programmed to:
1. Monitor Battery and determine if Battery or Cells are Compromised
2. Remove Compromised Battery from Operation.
5. The system of claim 3 , wherein the type of COTS inverter is selected from the group consisting of maximum power point tracking (MPPT) and DROOP.
6. The system of claim 3 , wherein the Set Operating Mode is selected from the group consisting of PV only, Battery only, PV+Battery—Charging and PV+Battery—Discharging.
7. The system of claim 1 , wherein the controller is configured to control the switching module to switch a power flow to charge the battery from the DC bus in response to sensing that the inverter is isolated from the alternating current (AC) grid.
8. The system of claim 1 , wherein the converter is controlled to provide a low frequency ripple to the DC bus.
9. The system of claim 1 , wherein the battery being connected in parallel with a first capacitor, and in series with an inductor; and the switching module having a first directional switch connected between the inductor and a first converter output terminal, and a second directional switch connected between the inductor and a second converter output terminal.
10. The system converter of claim 7 , further comprising a second capacitor connected in parallel across the first directional switch and the second directional switch, and between the first converter output terminal and the second converter output terminal.
11. The system of claim 1 , further comprising the switching module is controlled by the control path for distributing a bi-directional flow of power to charge battery in response to the PV array generating energy without an inverter connected load; and support the DC bus voltage when DC bus voltage drops below a predetermined output voltage.
12. A direct current (DC) power system, comprising:
a solar PV array directly connected to a DC bus for powering an inverter;
a battery connected to a controller, the controller directly connected to the DC bus;
the controller comprising:
a pair of input terminals for receiving a battery DC voltage from the battery;
a switching module for controlling a bi-directional current flow, the switching module having a pair of output terminals and a voltage sensor connected across the output terminals;
a control path for controlling the switching module, the control path comprising a current generator to generate a reference current in response to a reference voltage detected by the voltage sensor; and
a comparator connected at the output of the current generator, the comparator configured to compare the reference current with a battery current and output a signal to a controller module; the controller module configured to generate a reference value; and a pulse width modulating generator configured to receive the reference value and modulate the switch module to control controller output voltage, current flow and energy flow between the DC bus and the battery so the controller output voltage matches DC bus voltage.
13. The system of claim 12 , wherein the inverter is a conventional off-the-shelf (COTS) inverter that is controlled by a droop function.
14. The system of claim 12 , wherein the inverter input is a conventional off-the-shelf (COTS) inverter that is controlled by a maximum power point tracking algorithm.
15. The system of claim 12 , wherein the controller further comprises a micro-controller, the micro-controller configured to:
a. Set type of COTS Inverter
b. Set Operating Mode
c. Set Desired Battery Current Value
d. Adaptively control bidirectional DC/DC Converter voltage and current to present appropriate Current-Voltage characteristic at inverter input for desired Battery current and generate reference value for DC/DC converter
e. Modulate the switch module and maintain desired Battery current direction
f. Modulate the switch module and maintain desired battery current direction.
16. The system of claim 12 , wherein the controller is configured to control the switching module to switch a power flow to charge the battery from the DC bus in response to sensing a utility shutdown or fault on a connected alternating current (AC) grid, with the inverter isolated from the AC grid.
17. The system of claim 12 , wherein the battery being connected in parallel with a first capacitor, and in series with an inductor; and the switching module having a first directional switch connected between the inductor and a first converter output terminal, and a second directional switch connected between the inductor and a second converter input terminal.
18. The system of claim 12 , further comprising a second capacitor connected in parallel across the first directional switch and the second directional switch; and between the first converter output terminal and the second converter output terminal.
19. The system of claim 12 , further comprising the switching module is controlled by the control path for distributing a bi-directional flow of power to charge battery in response to the PV array generating energy without an inverter connected load; and support the DC bus voltage when DC bus voltage drops below a predetermined output voltage.