Power Conditioning System

8/11/2019 Power Conditioning System

1/23

Ukesh Shrestha

069MSR519


2/23

Introduction

Block diagram

Charge controller

DC-DC converter

Inverter

Digital synthesis

Power quality


3/23

Power conditioning units provide a match betweenspecific characteristics of PV generator and connectedbalance of system (BOS) components

for PV to become a significant power supplier, the dcfrom the PV must be converted to utility compatibleac power by a PCS

The characteristic curve of a PV generator varying withsolar radiation and temperature does not matchcharacteristic curve of load

In those cases, power conditioning unit effects atransformation of the loads voltage and current insuch a way that PV generator is operated at itsoptimum operation voltage VMPP even underchanging boundary conditions


4/23


5/23


6/23

charge controllers, DC/DC converters andinverters are the power conditioning unit

In almost every stand-alone system, a charge

controller is required to optimally operate thestorage battery within safe limits as prescribedby the manufacturer

The matching of PV generator and the load can

be achieved by means of DC/DC converterswhich can be integral part of a chargecontroller, an inverter or a DC pump,but canalso be a separate BOS component


7/23

If in stand-alone systems the load requires anAC voltage, inverters are used to convert theDC power supplied by the PV generator or the

storage battery into AC power. Inverters are mandatory in grid-connected PV

systems.

Here, besides high efficiency, reliability andpower quality, safety is an important issue andhas to be dealt with


8/23

The fundamental task of a charge controller is tooperate the battery within the safe limitsdefined withrespect to overcharging and deep discharging by thebattery manufacturer or by the operation mode.

In PV-powered systems, batteries are still thecomponent with the lowest average lifetime.

Compared to solar modules, which in principle havean infinite economic lifetime and in many cases offer aguarantee period of 25 years, the lifetime of batteries ismuch lower.

Furthermore, very high energy efficiency is crucial forall balance-of-system components in PV systems.

Most grid-powered battery chargers offer onlyunsatisfactory efficiency values.


9/23

Series

Shunt


10/23

Both the battery voltage and the PV generator voltage vary over awide range during operation due to the changing state of chargeand boundary conditions such as temperature

and insolation. When directly coupled, this leads to a certain mismatch between

the actual and the optimum operation voltage (MPP voltage) ofthe solar generator and therefore causes energy losses The mismatch can be overcome by introducing a matching

DC/DC converter (MDC) that de-couples the characteristic curvesof the PV generator and the battery.

The power stage of these converters corresponds to the well-known topologies such as buck, boost or inverting converters.

The voltage level can either be fixed (CV mode) or can track theactual MPP by an appropriate searching strategy (MPP tracking,MPPT)


11/23

Strategies Indirect MPP trackers:This type of MPP tracker estimates the MPP voltage

by means of simple assumptions and measurements Direct MPP trackers:In these systems, the optimum operating voltage is

derived from measured currents,voltages or the power of the PVgenerator.

Nevertheless, there are three advantages in using chargecontrollers with matching DC/DC converters: In the case of long wires from the PV generator to the battery, the

generator voltage can be chosen much higher than the batteryvoltage, resulting in lower currents and therefore lower wiringlosses.

In small applications, the PV module can consist of only a few, largecells instead of numerous small cells connected in series. Thisreduces production costs, the impact of

cell mismatch and the sensitivity to partial shading. More complex charging-current profiles can be realised by means of

a DC/DC converter.


12/23

Since the production cost of PV electricity isseveral times more expensive than conventionalelectric energy, conversion efficiency becomespredominant to the economics of the total PV

system. As a consequence extremely highefficiency not only in the nominal power range butalso under a part-load condition is a requirementfor PV inverters in grid-connected as well as in

stand-alone systems. the inverter must allow the PV generator to

operate in the MPP by adjusting the correspondingoperation voltage (MPPT tracking)


13/23

Square-wave-type inverters

Sine wave inverter

By voltage transformation principles

Step down and step up converter Voltage shaping by digital synthesis

Specification of PCS grid tied inverter

Power THD Weight Dimensio

n

Efficienc

y

4 KW 3% 20 KG 540x300x125

92.85%


14/23

Normally used above 5 Kw range and H Bridgeconfiguration

Mostly used for AC pumping

Power outpur and input are almost constant thusno dc capacitor or energy storage is required at dcside

If electric isolation between DC input and AC

output is requested and if the bulky 50-Hztransformer should be avoided, high-frequency(HF) transformer concepts may be used but itscostly again thus hf concept still not widely used.


15/23

Voltage Shaping by digital synthesis In this concept, the desired voltage is obtained by

binary addition of individual voltage sources Depending on the sum of these sources, the sine-

shape can be approximated. Definition of THD: sum of the amplitudes of all

harmonic frequencies compared to the amplitudeof the fundamental signal (the 50- or 60-Hzfrequency).

THD must be less than 5% for high power injection


16/23


17/23


18/23

In a stand-alone operation, the output waveform becomesimportant for many applications and protection must be inbuilt

Square-wave inverters, according to the working principle may beused to power resistive-type loads such as light bulbs or similarobjects.

When feeding power to reactive-type loads such as motors, properoperation might become difficult and losses inside the loadcreated by the square-wave character of the supply might occur .

For these load-types, ideal sinusoidal voltage supply would bebest. In reality, a compromise between this ideal voltage thatresults in high expenses and a lower quality for cheaperinvestments must be found.

very important power-quality element for stand-aloneapplications,the ability to provide and to absorb reactive powershould be mentioned

Bidirectional inverter can utilize surplus energy to charge whensurplus power at theAC side is available.


19/23

Since the output voltage of grid-connected invertershas to correspond with the grids voltage, the quality ofthe current that is to be injected into the grid becomesimportant.

Under ideal circumstances, this current should be inphase with the grids voltage(power factor = 1)

The deviation from this power factor becomestherefore important for the description of the powerquality. All modern transistor-based inverters have a

power factor near unity at nominal load, with atendency towards smaller values under part load

A high-quality grid-connected inverter shows

a THD in the current that is below 5%.


20/23

high-quality inverters will be able to improvethe power quality in the grid by adjusting the power factor,

by reducing the THD and bystabilising power flow through power control. To realisethese functions, appropriate control

and the availability of a communication element becomesnecessary.

A few inverters in the market already show these featurestoday.

An important issue for grid-connected systems is associated

with islanding protection. Islanding may occur, if a part of the local grid is switched

off, for example, for maintenance reasons and if the injectedpower is equal to the actual load in the separated part


21/23

a first measure, frequency and voltage monitoring willidentify by far most situationsin grids turned off since the smallestdeviations inproduction or in consumption will lead to changes infrequency or voltage or in both of them.

In case of heavy loads on the grid, both the voltage andthe frequency may fall below the set pointfurthermethod to identify islanding conditions, monitoring ofthe grids impedance is being performed by injectingpower peaks, which do not correspond with the

fundamental frequency (50 or 60 Hz), by the inverterinto the grid and by monitoring this influence on thegrids voltage shape. This method is currently acceptedby German safety code


22/23


23/23

PV installation guide

Power conditoner research paper

Wikipedia

Gonzalez G, Hill G, Ross Jr. R, PhotovoltaicArray Power Conditioner Interface

Characteristics, DOE/JPL-1012-79, Prepared for U.S.

Department of Energy (1979).

Documents

Power Conditioning System