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Harmonics Management

Harmonics are demanding more attention today because more and more of power distribution networks are infested with harmonics.

Growing use of non-linear equipments such as power rectifiers, UPS & Inverters, VFDs, workstations is responsible for harmonic pollution. Other common sources of harmonics are arc welders, fluorescent lighting, magnetic amplifiers, etc.

Example of Linear equipments are motors or resistance heaters where electric current obeys Ohm's Law, i.e. current is proportional to voltage.

Not so in case of non-linear equipments such as SMPS of a PC or a rectifier; where current is not in proportion to the voltage applied but by adjusting firing angle of a static switch or blocking cycle of an AC waveform. Such applications invariably alter sine wave of AC in the process and as a result generate harmonics.

Harmonics are identical to power frequency currents except for they have higher frequencies that are integral multiple of power supply frequency.

Harmonic order
1
2
3
4
5
6
7
8
9
Frequency Hz
50
100
150
200
250
300
350
400
450
Frequency Hz
60
120
180
240
300
360
420
480
540

The first order is called the fundamental. A typical commercial installation comprising of workstations, UPS, inverters and fluorescent lighting shall have 3'rd, 5'th, 7'th and 11'th harmonics that are mix of 1 phase & 3 phase non-linear load. A typical industrial automated plant shall have 5'th, 7'th, 11'th, 13'th harmonic orders which are generated by three phase non-linear load such as VFDs, DC drives, etc.

It is important to note that harmonics are nature of non-linear equipments and like reactive power; could only be compensated but no avoided.

Harmonic compensation becomes an essential aspect of power quality management and energy conservation in the view of most common ill effects of harmonics studied so far

  1. Increase in copper and iron losses.
  2. Increase in electrical loading.
  3. Braking effect in motor drives leading to overload.
  4. Rapid aging and abrupt failure of power factor correction due to effect of series and parallel resonance.
  5. Increase in electrical noise resulting in EMI effects that could affect data transmission and interfere with communication network.
  6. Increase in neutral currents leading of higher N-G voltages and weakening or loss of neutral leading to serious equipment damages.
  7. Harmonics are manifested in waveform distortion of current and voltage. A severely distorted voltage waveforms could affect reliable operation of sensitive electronic equipments.
  8. Inadvertent operation of micro-processor based protection relaying resulting in undesired shutdowns.

Field image of burnout of contactor in a PF correction system due to harmonic resonance causing short circuit so severe that HRC fuses could not check the damage.

Harmonic Solutions

Harmonic solutions are offered in a step-by-step process

  • Harmonic studies with advanced power quality analyzer
  • Load flow studies
  • Design of harmonic mitigation system
  • Vendor sourcing
  • Supervision of installation and post installation monitoring
  • Continued support

Harmonic mitigation employing harmonic filters
Harmonic filters are broadly classified into passive & active.

Passive harmonic filters
A passive harmonic filter is built using basic AC components namely capacitors, inductors and resistors. It can take the form of a simple line reactor or may use a series of parallel resonant filters to eliminate harmonics. Passive harmonic filters are also divided based on the way they are connected with the load.
Series filter:
Here the filter is placed in series with the load and utilizes inductors and capacitors connected in parallel. This filter is a current rejector.
Parallel filter :
The filter is placed in parallel with the load but its components are built in series. This filter is a current acceptor.
Design of filter based on frequency response

  • Band-pass filter is a common passive filter that is built using a capacitor connected in series with a resistor.
  • High-pass filter has a resistor connected in parallel with a reactor. This helps in reducing the "Q" value of the filter, which will in turn help reduce the higher frequencies. A High-pass filter used in combination with a band-pass filter mitigates moderate levels found in medium voltage and sub-transmission voltage networks.
  • C-type filter is used for complex loads, cyclic converters, welders and electric arc furnaces and is a special design of high pass filter. This filter will provides the load with leading and lagging reactive power and avoids condition of parallel resonance with the load.

Active Harmonic Filter
An active harmonic filter is active in that it compensates the harmonic distortions produced by the non-linear load dynamically and does not utilize passive components of R, L, C.

  • It measures harmonic demand from load in terms of current components.
  • Generates and injects identical current components using power electronics with inverse phase rotation.

Type of connection.
Series active filter is connected in series with the AC distribution network. It mitigates harmonic distortions caused by the load as well as that present in the AC system from upstream.

Parallel active filter is connected in parallel with the AC distribution network and mitigates harmonic caused by non-linear load alone.

Hybrid filter is a combination of an active and a passive filter and could be of a series or a parallel configuration.

Other simple designs of harmonic filters
Line choke or block reactor:
It is basically an inductor that resists the flow of high frequency harmonics and damps the harmonic currents during switching. Line reactors serve to suppress current spikes and limit peak currents because of their reactance. Typically applications : VFDs, DC drives, and power rectifier & inverters.

Selection of Harmonic Filter

  1. Harmonic levels and effect on the installation.
  2. Economic considerations.
  3. Power systems consideration such as loading patterns, source impedance, harmonic data, harmonic orders, neutral currents, etc.
  4. Power quality requirement

Benefits of harmonic mitigation

  1. Voltage stability and control of power quality within acceptable limits ensuring reliable performance of equipments.
  2. Reduction of losses resulting in energy conservation.
  3. Release of spare capacity of power distribution components offering a better utilization factor.
  4. Reduced downtime, aging and electrical faults.

Severe distortion of voltage & current waveforms and bar graphs mixed 1 & 3 phase non-linear load
De-tuned harmonic filter
Tuned harmonic filter

 

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