LDHT Engineering, power system engineering consultancy

Study

Harmonics

Distortion levels, resonance risk and filtering on networks that include inverters, drives and capacitor banks.

A capacitor bank and the network inductance form a resonant circuit. The resonance order is roughly the square root of Ssc / Qc. With 250 MVA and 5 Mvar this gives order 7, which happens to be an order at which inverters and drives inject current. The same harmonic current then produces a much higher harmonic voltage.

We carry out a frequency impedance scan for each configuration (one more capacitor bank, one less transformer, weak or strong upstream network), then calculate the harmonic voltages from the source spectra. The results are compared with the network operator's limits and IEC 61000-3-6, or IEEE 519 on international projects.

Long MV cables, common in PV plants and wind farms, add their own capacitance, which shifts the resonance down to lower orders.

01What we do

Calculations

  • Frequency impedance scan
  • Individual and total distortion levels
  • Several operating scenarios
  • Transformer derating

Solutions

  • Detuning reactors
  • Passive filters
  • Setting of the compensation steps

02Examples

Harmonic impedance seen from the 20 kV busbar, with and without capacitor bank135791113151719212325010203040h5h7h11h13resonance h ≈ 7.1with 5 Mvar capacitorswithout capacitors20 kV · Ssc 250 MVA · damping load 10 MWHarmonic order|Z| (Ω)
Fig. 015 Mvar capacitor bank on a 20 kV, 250 MVA network. The resonance lies at order 7.1, on the 7th harmonic, and the harmonic voltage is amplified when there is no reactor.Worked example
Harmonic voltages at the 20 kV busbar, with and without capacitor bank0 %1 %2 %3 %4 %5 %Uh / Unh5h7h11h13h17h19h23h25with capacitors: THD ≈ 4.7 %without capacitors: THD ≈ 2.9 %6-pulse rectifier 3 MW, Ih = I1 / h
Fig. 023 MW rectifier on a 20 kV, 250 MVA network with a 5 Mvar capacitor bank. The 7th harmonic is amplified and THD rises from 2.9 to 4.7 %.Worked example

03Our offer

Usual content of an offer for this type of assignment. The exact scope, deliverables and price are set in the quotation.

  1. Package 1Model

    Network, cables, capacitors, upstream equivalent and its variations.

  2. Package 2Impedance scan

    Resonances identified per configuration.

  3. Package 3Harmonic voltages

    Per order and THD, compared with the applicable limits.

  4. Package 4Solutions

    Tuning reactor, filter, revised steps, with their quantified effect.

04How we work

  1. 01Model

    Network, cables, capacitors, upstream equivalent and its variations.

  2. 02Impedance scan

    Per configuration, with resonances identified.

  3. 03Harmonic voltages

    Per order and THD, compared with the limits.

  4. 04Solutions

    Tuning reactor, filter, revised compensation steps.

05References

Power producer, PV plant

Harmonic study on an industrial MV ring

Distortion levels across several scenarios, resonance risk and transformer derating.

Generating substation

Capacitor bank

Analysis of SCADA logs, identification of control hunting and projection of circuit-breaker wear.

All references

06Frequently asked questions

Are measurements needed?

On an existing site, yes, preferably. Power quality measurements are carried out with Volt Expertise.

Which software?

PowerFactory (harmonics module) or ETAP.

07See also

For a project, give us the voltage, power, location and deadline in a few lines.
Reply within 48 working hours.

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