Inverter Design and applicaitons

1. Basic Theory & Application
Photovoltaic Inverters Different
Dr. Sammy Germany
Market Director – Renewable Energy

2. 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

3. 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.

4. 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

5. Conventional PV
Distribution Collection System
Y
DistributionSystem
Y
>208 480330 to 600VDC
PV DC +/- 480 : Medium Voltage
2nd
3rd
4th
DCS

6. 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”

7. 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).

8. LCL Ripple Rejection Filter
AC to UtilityPV-DC

9. 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.

10. 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).

11. 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.)

12. 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.

13. 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

14. Reduced Transformer
Dedicated PV Utility Connection
Y
208 Medium Voltage / 12.4kV
2 points of power match
PV DC