CH-01 Harmonics
Introduction:
- Harmonics is the distortion of the sinusoidal current or voltage due to presence of nonlinear components.
- The basic reason for limitation of harmonics distortion is to avoid or malfunction of other equipment.
- The amount of total harmonics distortion depends on the harmonics current and on the impedance connected to the network.
- Drives-> network harmonic sources
Effects of Harmonics:
- Extra Heating
- Extra Losses
- Interference with sensitive equipment.
- Mechanical Vibration
- Noise
- Harmful torque pulse
Harmonics visualisation
How to reduce Harmonics:
- Increase the network Ssc. (Reduce the voltage harmonics)
- Increase the number of pulses in the converter e.g., (a)12 pulse instead of 6 pulses, (b) two secondary windings of the drive transformer connected in Y-delta., (c) 30-degree phase shift, (d) cancellation of the same harmonics’ components.
- Filter Harmonics
Definition:
Harmonics:
Sinusoidal component of a periodic wave or quantity having a frequency that is a multiple of fundamental frequency.
Characteristics Harmonics:
Harmonics produced by ideally symmetrical semiconductor converter equipment (ie., both converter and supplying network) during steady state operation.
Non-Characteristics Harmonics:
Harmonics produced by non-ideal semiconductor equipment, transformer tolerances and during dynamic operation. Harmonics circulation tool in Drivesmart allows to define how the non-characteristic harmonics are considered in a report.
Characteristic Harmonics:
The order of characteristic harmonic(h) in relation to the rectifier pulse number (n). h=k*n+-1. where k= any integer.
- A 6-pulse rectifier produces 6-1=5th and 6+1=7th,12-1=11th and 12+1=13th harmonic currents.
- A 12-pulse rectifier produces 12-1=11th and 12+1=13th and 24-=23rd and 24+1=25th harmonic currents.
Point of common coupling (PCC):
Point of common coupling(PCC) is the point where the harmonic distortion is specified namely,
- between the plant and the utility network(see PCC1)
- between the non-linear load and other loads within an industrial plant.(see PCC2)
In-plant point coupling (IPC):
The point inside the customer system or installation to be studied.
Distortion Factor (DF):
The ratio of the root mean square of the harmonic content to the root mean square of the fundamental quantity. expressed as a percent of fundamental. is called Distortion Factor (DF)
- The distortion factor is applied for the definition of the Total Harmonics Distortion (THD) for the voltage harmonics and Total Demand Distortion (TDD) for the current harmonics.
- The ratio of Isc/IL is the ratio of the short circuit current available at the point of common coupling(PCC) to the maximum demand load current of the corresponding supply system.
DF= [(sum of square of amplitudes of all harmonics/ square of amplitude of fundamental)^0.5]*100%
Standards:
IEC 61000-2-4, Rev. 2002 (worldwide)
EN 61000-2-4 (Europe)
VDE 0839 Teil 2-4 (Germany)
IEEE 519-1992 (US)
National standards
G5/4 (United Kingdom)
GB/T 14549-93 (China)
etc.
Utility standards
e.g. Electricité de France
Project-specific requirement
Electromagnetic Environment Classes:
Class 1:
The class 1 applies to protected supplies and has compatibility levels lower than public network levels. It relates to the use of equipment very sensitive to disturbances, in the power supply for instance instrumentation of the technological laboratories, some automatization and protection equipment, some computers etc.
Class 2:
This class applies to PCCs and IPCs (in-plant-point coupling) in the industrial environment in general the compatibility levels of the class are identical to those public networks.
Class 3:
This class applies only to IPCs in industrial environments.
It has higher compatibility levels than those of class 2 for some disturbance phenomena.
For instance, this class should be considered when any of the following conditions are met:
- a major part of the load is fed through converters.
- welding machines are present.
- large motors are frequently started;
- loads vary rapidly
Compatibility level for harmonic voltage:
The limits refer to continuous values. For transient harmonics, values up to and including 1.5 times the permanent limits are allowed during a maximum duration of 10% of any observation period of 2.5 min.
Voltage distortion limits for utilities:
The limits should be used as system design values for the “worst case” for normal operation (conditions lasting longer than one hour). For shorter periods, during start-ups or unusual conditions, the limits may be exceeded by 50 %.
Current distortion:
The table is applicable to six-pulse rectifiers. However, when converters with pulse numbers (q) higher than six are used, the limits for the characteristic harmonic orders are increased by a factor equal to (q/6)^0.5 provided that the amplitude of the Non characteristic orders are less than 25% of the limits specified in the table.
Harmonics behaviors of ABB MV Drive:
- ACS 1000 Harmonic current source (12 p, 24 p)
- ACS 2000 Harmonic voltage source ( 6 p)
- ACS 6000 with LSU Harmonic current source (12 p, 24 p)
- ACS 6000 with ARU Harmonic voltage source (6p, 12p, 18p)
- ACS 6000 with ARU/IFU Harmonic voltage source with reduced harmonics (6p)
- ACS 5000 Harmonic current source with specific harmonics
- LCI Harmonic current source (6p, 12p, 24p)
Current source behaviour:
Voltage source behaviour:
Effects of System Paramters:
Drive with both current source and voltage source behaviour:
Pay attention:
- Harmonic calculation is based on a purely inductive network and refers to a single drive
- Cables and power-factor compensation units generate resonance frequencies which can increase harmonic distortion dramatically, if drive-generated harmonic component is of the same frequency
- Addition of filter systems can result in a overcompensation
- Avoid weak networks, a reasonable network looks like
Impact of Cable Resonances:
Resonance effect of long cables:
Top diagram shows harmonic voltage distortions in a pure inductive network (12-pulse LSU drive, ACS1000)
Bottom diagram shows harmonic voltage distortions with the same converter and 6km of parallel 20kV cables on the primary side of the ACS1000 transformer.
Harmonics LSU:
Example with LSU and ARU supply:
Example: Motor Shaft Power 5 MW
- 12-pulse Diode Rectifier (LSU), Voltage Distortion
- 24-pulse Diode Rectifier (LSU), Voltage Distortion
- For comparison:
- 6-pulse Active Rectifier Unit (ARU), Voltage Distortion
- 6-pulse Active Rectifier Unit ARU with Integrated Filter (IFU), Voltage Distortion.
Data for example:
Motor shaft power = 5 MW
Motor efficiency = 0.97
Converter efficiency = 0.98 (0.985 with LSU)
Converter transformer
with LSU: 6.1 MVA, 7.5 %
with ARU: 5.3 MVA, 12 %
Purely inductive network, no cables considered
Ratio network short-circuit power to machine shaft power SCC/P (sh)=10,25,40
Example-Volt THD:
Multiple Converters:
Harmonics add linearly
Absolute values and not percents must be added
For equal converters current distortion stays the same in percent
Converters with same load -> quasi 2xp configurations (Where p is the pulse number of one converter)
Summary:
Harmonics depend not only on the converter, but also on the network the converter is connected to.
At what point on the network do we evaluate harmonics?
One of the most influencing parameters is ISC/IL
Several standards.
Long cables (in the order of km)-> low frequency network resonances.
Harmonics in the vicinity of a network resonance ->Needs careful investigation
More converters on the same point -> Harmonics add linearly