Analysis and Design of a Solar Charge Controller BY using MPPT

Analysis and Design of a Solar Charge Controller BY using MPPT

Introduction

This paper presents a new solar/battery charge controller that combines both MPPT and over-voltage controls as single control function. The converter is configured in parallel power transfer mode (PPT) to achieve higher efficiency. PWM control with maximumPower point tracking algorithm is used to extract maximum power from the PV modules In Solar Home Systems and inverter systems as well as in some PV hybrid systems solar charge controllers are the central control unit regulating the overall energy flow within the system. Therefore it is a central and critical component which has to be selected carefully. It is important to extract maximum power from PV under varying temperature and solar radiation levels. A maximum power point tracking (MPPT) device is used between the PV array and the load (typically batteries) to optimize the power transfer from the PV array to the load. Controlling the power flow by providing two paths using a parallel power transfer (PPT) technique results in reduced power loss is in the converter. The parallel power transfer approach is to split the power flowing into the load into two paths as: (1) power flowing directly to the load without the converter (Pp) and (2) power flowing to the load through the converter (Pm)

FUNCTIONS OF SOLAR CHARGE CONTROLLERS

Ø  It has to control the whole energy flow in the system

Ø  low voltage disconnection (LVD) to protect the battery from deep discharge

Ø  high voltage disconnection (HVD) to protect the battery from overcharging.

Ø  It should have a good battery state of charge calculation (SOC) in order to be able to monitor the battery status

CONTROL LAW FOR CHARGE CONTROLLER

To find a control law that regulates the battery voltage in the face of a current disturbance using classical control tools, a small signal model governing how a small change in battery current dynamically affects the battery voltage should be analyzed. For this purpose a small signal model of a lead acid battery with battery current i as control input and battery voltage Vbatt as the controlled state variable is derived. The following assumptions were made while deriving the small signal model and subsequent transfer functions:

1. Operating point is near full battery charge corresponding to a maximum allowed battery thresh-hold voltage VTH since the over-voltage controller is supposed to work close to this point as will be pointed out later.

2. Capacity change due to change in current amplitude is assumed to be small.

3. Any variations in resistance and capacitance due to SOC near operating point can be neglected.

4. Variation in load current is an external disturbance.

5. Battery current has positive polarity during charging.

SHUNT CONTROLLERS

Good solar charge controllers have a very low self consumption  (< 4 mA) and come within a robust case with big connection terminals. In addition to this good controllers have

a user friendly display indicating all system values.

Schematic of Shunt controller:

In case T1 is open, current flows through T2 to the battery. If T1 is closed the short circuit current of the module flows through T1 and no current flows to the battery. In the night T2 is open and prevents a current from the battery back to the module. T3 controls the load

 Characteristics

Shunt controllers have a very good EMC behaviour as the current changes in switching mode just between the charging current Im and the short circuit current Isc. Such  controllers have the highest charging efficiency as the current just flows through one part T2. This topology allows good protections against wrong polarity (battery and load side) and protection features against high temperature and error currents

Hot Spots

In case of partial shading of the solar module a local hot spot might appear. The voltages of the irradiated cells add up, while the shaded cell is driven in reverse voltage mode. If this cell is driven at less than -20V a pn break through could destroy this cell and the module. To prevent this all TÜV, IEC or ISPRA certified modules have suitable bypass diodes which protect the cell in case of partial shading. Normally 18 cells are protected by one diode to keep the reverse voltage below -10V. In case of partial shading the current then flows through the bypass diode. It has been reported that the use of shunt controllers effects a hot spot in solar modules. Technically this is only possible if solar modules without bypass diodes are used. Nowadays such modules do no longer exit. This means there is no limitation in using shunt controllers due to hot spot risk. Beside this more than 1 Mio shunt controllers dominate the world market and prove day by day a good compatibility with all types of solar modules

THE PROPOSED CHARGE CONTROLLER

 A schematic of the proposed charge controller where the current reference generated by the voltage control loop is dynamically limited to have an upper value equal to maximum power point current. The DC/DC converter can be thought of as a controlled current source that injects a given amount of current i into the battery depending on the amount of deviation of the battery voltage from a set thresh-hold value Vbatt ,ref. If a voltage source type load is connected at the output of a DC/DC converter, the output power can be maximized by increasing the output current [9, 10]. In this case since the load seen by the DC/DC converter is battery which is a voltage source type load, so long as there is an error between the battery voltage and thresh hold voltage set point Vbatt, ref, the reference current generated will increase significantly as a result of the over-voltage control action. The output current will, however, never exceed the maximum current due to the dynamic limitation. The PV array, therefore, will always work at MPP at battery voltages away from the over voltage thresh hold point and will automatically start shifting the PV operating point to limit the PV power produced as the voltage nears the thresh hold point (i.e. over-voltage control). The maximum power point current referred to the output (inductor) side of the DC/DC converter is dynamically calculated by the MPPT algorithm as function of the instantaneous irradiance, temperature and battery voltage. This imposes a dynamic upper limit on the current going into the battery and enables a seamless change between MPPT and overvoltage control operations realized in a single block without the need for switching between different modes or separate units. Finally, it is important to note that the voltage control loop will produce a large reference current due to the accumulation of error at normal operation under MPPT due to its integral action. To prevent wind up effect as the over-voltage control action starts, an anti-wind up is implemented to reset the integral output. It is important to point out also that as the over-voltage controller’s operating regime is only near the threshold point where the battery resistance and capacitance are not expected to change, our original assumption to neglect the independence on state of charge will not entail any error on the choice of the controller parameters.

CONCLUSION

A simplified solar/battery charge controller which combines both MPPT and over-voltage controls as single control function is proposed. A small signal model of lead acid battery, not available in literature, is also derived in detail to enable accurate design of the developed charge controller. Two case studies are conducted first to evaluate the transient and voltage overshoot response of the designed controller. Secondly, a comparative study is made based on realistic irradiance data to evaluate the performance of the proposed charge controller in terms of energy utilization factor and overvoltage compared to the conventional series hysteretic on/off controller. The designed controller is shown to have very fast transient response and very small transitory voltage overshoot. It is also found that the proposed charge controller shows better PV energy capture than the on/off controller.