Basic Theory & ApplicationPhotovoltaic Inverters Different

Dr. Sammy GermanyMarket Director – Renewable Energy

Inverter (Electrical) Commercial Equipment

• Inverters convert Direct Current (DC) to Alternating Current (AC) by various means.

• Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries.

• Static Inverters have no moving parts and can be used in a variety of applications

• Inverters perform opposite function of a rectifier • A rectifier is an electrical device that converts

alternating current (AC), which periodically reverses direction, to direct current (DC), that only flows in one direction

North American PV Systems

• PV Inverter Systems in North America While high power PV Inverter installations certainly vary in configuration,

atypical commercially/Utility installed system consists of the following;

PV Modules stacked in strings and the string voltage is not to exceed 600V(UL) or 1000V for Commercial/Utility Installations

UL 1741 certified inverter whose function in addition to converting direct current functions as varied as optimally extracting energy from the array to assuring NEC compliant operations

An isolating transformer to provide galvanic separation of the PV Facility from the electric utility and voltage ratio changing

Point of common coupling to the utility which may be either a low voltage distribution system or arbitrary size in a 3 phase standard configuration.

Dedicated PV-Utility Connection System

PV DC +/-

Y

>208 480

Y

480 Medium Voltage / 12.4kV

3 points of power match

A

B

C

330 to 600VDC

Conventional PV Distribution Collection System

Y

Dis

trib

uti

on

Sy

ste

m

Y

>208 480330 to 600VDC

PV DC +/- 480 : Medium Voltage

2nd3rd

4th

DCS

Design Power Conversion

• Conventional basic wisdom on inverter design has few major characteristics;– Major design challenge is switching frequency (SF)

Choice of SF permeates almost every aspect of performance; Efficiency, cost, ripple rejection, size control stability, audible noise, and utility connection performance

– Higher the frequency normally the better for all components and performance, “BUT”

Higher power applications have historically kept switching frequencies to a lower value, like < 10kHz

Some of the reasons are; 1) expense of IGBT’s, 2) Cooling methodology of IGBT’s, 3) Design of the breadboard and packaging of IGBT’s, 4) Gate Drive programming C+

In the IGBT direct hard switching without any soft switching lead to high dynamic/thermal losses during “turn on and off”

Design Power Conversion Thermal margins are limited in the packaging of all components, due to nets losses to all cooled

parts or equipment

Beyond the IGBT’s are the line reactors which are inductors, and the purpose is to limit ripple currents being injected into the utility line.

The injection of high frequency currents into the power line is not acceptable and causes power equipment shut down problems.

Inverters that operations <10kHz creates operational issues on the breadboard plane, and transformer magnetic core ripple currents.

Understanding this adds a low-inductance line reactor design, but the thermal issues create deareation if not cooled properly.

One other way of dealing with down stream magnetic ripple is to use shut capacitors on the delta side of the transformer

The usual solution is to go with a relatively low-inductance line reactor design, this could produce high frequency heating which is connected to the cooling plate. The shunt capacitors are connected in a delta. These make up the elements of a Low-capacitors line filter (LCL).

LCL Ripple Rejection Filter

AC to UtilityPV-DC

Inverter Harmonics • On a 60-Hz system, this could include 2nd order harmonics

(120 Hz), 3rd order harmonics (180 Hz), 4th order harmonics (240 Hz), and so on.

• Normally, only odd-order harmonics (3rd, 5th, 7th, 9th) occur on a 3-phase power system.

• This increased heating effect is often noticed in two particular parts of the power system: neutral conductors and transformer windings.

• Harmonics with orders that are odd multiples of the number three (3rd, 9th, 15th, and so on) are particularly troublesome, since they behave like zero-sequence currents.

• These harmonics, called triplen harmonics, are additive due to their zero-sequence-like behavior.

• They flow in the system neutral and circulate in delta-connected transformer windings, generating excessive conductor heating in their wake.

Defining Harmonics Currents

• These third order, zero sequence harmonic currents, unlike positive and negative sequence harmonic currents, do not cancel but add up arithmetically at the neutral bus.

• Harmonics, in an electrical power system, are currents and voltages with frequencies that are integer multiples of the fundamental power frequency.

• Harmonic currents are created by non-linear loads that generate non-sinusoidal currents. • Harmonic currents, acting in an Ohm's Law relationship with the source impedances,

produce harmonic voltages.• The harmonic currents and voltages produced by balanced, three phase, non-linear loads

are positive sequence harmonics (phases displaced by 120 degrees, with the same rotation as the fundamental frequency), and negative sequence harmonics (phases displaced by 120 degrees, with a reversed rotation).

• However, harmonic currents and voltages produced by single phase, non-linear loads, which are connected phase to neutral in a three phase, four wire system, are third order, zero sequence harmonics (the third harmonic and its odd multiples - 3rd, 9th, 15th, 21st, etc., etc., phases displaced by zero degrees).

Inverter Harmonics Commercial Equipment

• Because of the adverse effect of harmonics on power system components, the IEEE developed standard 519-1992 to define recommended practices for harmonic control.

• This standard also stipulates the maximum allowable harmonic distortion allowed in the voltage and current waveforms on various types of systems.

• Two approaches are available for mitigating the effects of excessive heating due to harmonics, and a combination of the two approaches is often implemented.

• One strategy is to reduce the magnitude of the harmonic waveforms, usually by filtering. The other method is to use system components that can handle the harmonics more effectively, such as finely stranded conductors and k-factor transformers. (K-Factor determine how much harmonic current a transformer can handle without exceeding it’s maximum temperature rise level, Scale is 1-50 = 1none and 50 is harsh harmonic back feed. A K factor of 13 is most common.)

Effect of 3rd Order ZSH• Zero Sequence Harmonics (ZSH) Depending upon the capacity and configuration of the

distribution system, the presence of third order, zero sequence currents may include any or all of the following symptoms:

• * High Neutral Current• * High Neutral to Ground Voltage (Common Mode Noise) // High Peak Phase Current• * High Average Phase Current // High Total Harmonic Distortion of the Current• * High Total Harmonic Distortion of the Voltage // High Transformer Losses• * High System Losses // Apparatus Overheating• * Low Power Factor // Electronic Protective Device Malfunction• * High Telephone Interference Factor // Increased Apparatus Vibration• The devices which created the third order, zero sequence harmonics may be the most

sensitive to the problems listed. The performance of the switching frequencies, in particular the charging of its capacitor, is critically dependent on the magnitude of the peak voltage. These voltage harmonics can cause "flat topping" of the voltage waveform or lowering of the peak voltage. In severe cases the control board may reset due to its own power supply's resets.

Impedance Matching• In electronics, impedance matching is the practice of designing

the input impedance of an electrical load or the output impedance of its corresponding signal source in order to maximize the power transfer and minimize reflections from the load.

• In the case of a complex source impedance ZS and load impedance ZL, matching is obtained when, where indicates the complex conjugate.

• The concept of impedance matching was originally developed for electrical power, but can be applied to any other field where a form of energy (not necessarily electrical) is transferred between a source and a load.

• An alternative to impedance matching is impedance bridging, where the load impedance is chosen to be much larger than the source impedance and maximizing voltage transfer, rather than power, is the goal.

Zload = Zline = Zsource, where Zline is the characteristic impedance of the transmission line

Reduced Transformer Dedicated PV Utility Connection

Y

208 Medium Voltage / 12.4kV

2 points of power match

PV DC