PV System Components:Inverters and Charge Controllers

EE 495/695Spring 2011

Maximum Power Point Tracker (MPPT)• Each DC load has its own I-V Curve.• When connecting a DC load directly to a PV system, the

intersection of the load and PV I-V curves dictates the operating point.

• If the intersection point deviates significantly from the maximum power point, then it is desirable to install a Maximum Power Point Tracker (MPPT) between the PV System and Load.

MPPT: Buck Converter (or LCB)

• A buck converter is a DC transformer that reduces voltage and increases current by the same factor.

where D is the duty cycle of the electronic switch, i.e.,

DIIDVV

inout

inout

/==

TTTTTD onoffonon /)/( =+=

http://upload.wikimedia.org/wikipedia/commons/f/f0/Buck_conventions.svghttp://en.wikipedia.org/wiki/File:Buck_chronogram.png

MPPT: Buck-Boost Converter

• A buck converter is a DC transformer that reduces or increase voltage, and reduces or increases current by the same factor.

where D is the duty cycle of the electronic switch, i.e.,

DIDIDDVV

inout

inout

/)1()1/(

−=−=

TTTTTD onoffonon /)/( =+=

http://upload.wikimedia.org/wikipedia/commons/e/e6/Buckboost_conventions.svghttp://en.wikipedia.org/wiki/File:Buckboost_chronogram.svg

DC-AC Inverters

Types of Inverters• Grid-tied inverters are currents sources: the generate a sine

wave current that follows the utility sine voltage.• The off-grid inverters as voltage sources: They generate a

voltage that can be a pure sine wave, a modified square wave, or simply a square wave.

• Different appliances will be affected to greater and lesser degrees by the different forms of AC. – Resistive loads found in incandescent light bulbs and heat

producing appliances such as irons and stoves are unaffected.

– Inductive loads may (such as motors) run with more noise and get warmer.

• Problem with low-order harmonics:– They are hard to filter.– They require a bulky filter that consumes a

significant amount of power.

PWM Waveform:Bi-Polar Switching

ma = Vsin/Vtrianglemf = ftriangle/fsin

• Harmonic components appear at and around the switching frequency

Filtering

• And LC filter is placed on both sides of the inverter to filter out the high frequency components.

• These filters are small and consume minimal power due to their high resonant frequency.

Grid-Tied Inverter Circuit Block Diagram

Electrical Specs of Grid-Tied Inverter

Electrical Specs of off-grid Inverter

Battery Charge Controller

Need for a charge controller• A charge controller, or charge regulator is similar to the

voltage regulator in your car. • The charge controller regulates the voltage and current

coming from the solar panels going to the battery. • Without regulation, the batteries can be damaged from

overcharging. • The charge controller regulates the voltage output of the

panel to what the battery needs at the time.– This voltage typically varies between about 11.5 to 14.5,

depending on the state of charge of the battery, the type of battery, in what mode the controller is in, and temperature.

– Most 12 V batteries need around 14 to 14.5 volts to get fully charged.

Pulse Charging• Some chargers use pulse technology in which a pulse is fed

to the battery. This DC pulse has a strictly controlled pulse width, pulse repetition rate (frequency) and amplitude.

• With pulse charging, high instantaneous voltages can be applied without overheating the battery. In a Lead–acid battery, this breaks down lead-sulfate crystals, thus greatly extending the battery service life.

• The controller constantly checks the state of the battery to determine how fast to send pulses, and how long (wide) the pulses will be.– In a fully charged battery with no load, it may just

“check" every few minutes and send a short pulse to the battery.

– In a discharged battery, the pulses would be very long and almost continuous.

– The controller checks the state of charge on the battery between pulses and adjusts itself each time.

Pulse Charging• Several kinds of pulse charging are patented. Others are

open source hardware.• Some chargers add a "negative pulse charging", also

called "reflex charging" or "burp charging".

IUI Charging• In here, Initially the battery is charged at a constant (I) rate

until the cell voltage reaches a preset value - normally a voltage near to that at which gassing occurs. This first part of the charging cycle is known as the bulk charge phase.

• When the preset voltage has been reached, the charger switches into the constant voltage (U) phase and the current drawn by the battery will gradually drop until it reaches another preset level. This second part of the cycle completes the normal charging of the battery at a slowly diminishing rate.

• Finally the charger switches again into the constant current mode (I) and the voltage continues to rise up to a new higher preset limit when the charger is switched off. This last phase is used to equalize the charge on the individual cells in the battery to maximize battery life.

Charge Controllers with MPPT• Charge Controllers with Maximum power point

tracking (MPPT). These are the ultimate in controllers, with prices to match - but with efficiencies in the 94% to 98% range, they can save considerable money on larger systems since they provide 15% to 30% more power to the battery.

PV System Components:�Inverters and Charge ControllersMaximum Power Point Tracker (MPPT)MPPT: Buck Converter (or LCB)MPPT: Buck-Boost Converter DC-AC InvertersTypes of InvertersPWM Waveform:�Bi-Polar Switching���ma = Vsin/Vtriangle�mf = ftriangle/fsin �FilteringGrid-Tied Inverter Circuit Block DiagramElectrical Specs of Grid-Tied InverterElectrical Specs of off-grid InverterBattery Charge ControllerNeed for a charge controllerPulse ChargingPulse ChargingIUI ChargingCharge Controllers with MPPT