How to determine if there are potential resonance issues in a power system 1

[veritas]

I've just done a harmonic study using PTW. This tells me what the V_THDV% and I_THD%, Vrms, Irms are at several locations in the network, the K factor for trfrs, etc. I was thinking, is there a way of predicting if there are potential resonance issues on the network? The network has several LV pfc capbanks in service. Must admit, not sure where to start.

Thanks.

 

[cuky2000]

In power system in operation, the main symptom of ferroresonance are: overvoltage, over-current, noise and overheating.

For computer simulation, overcurrent and overvoltage can be detected with overvoltage and overcurrent associated with components such as transformer, system inductance that could resonate with capacitance of cables and cap banks.

Often, transient simulation program similar to ATP-EMTP is used to model the resonant effect in power systems. Some mitigation techniques could be applied in the operation mode such as avoid having a transformer with no or light load to avoid core saturation. Another option is to add RC & filters circuits to suppress or reduce the high level of harmonics

 

[jghrist]

If your system is limited to a single inductance, a harmonic source, and capacitor in parallel, then a simple manual calculation can determine the resonant frequency and harmonic voltage distortion. IEEE Std 519 has a section on simple calculations. This could be useful to make a quick check on resonance points for a single capacitor bank on the secondary of a transformer. Most real world systems will require a computer simulation, using something like PTW or EMTP.

 

[dpc]

Frequency scan?

 

[rcw retired EE]

As DPC stated, do a frequency scan at the busses with capacitors and busses with harmonic generating loads like VFD's, UPS systems, Chargers, etc.). Most power system analysis software (ETAP, EasyPower, SKM Hi_Wave), has harmonic analysis modules that can provide a chart of the system impedance versus frequency at a bus and plots of the bus's harmonic voltage and current versus frequency. Comparing the impedance charts with the current and voltage harmonics gives an indication if there is a problem due to resonance.

A high 5th harmonic voltage on a bus with a low 5th harmonic impedance might create high 5th harmonic currents. Or a load drawing high 7th harmonic currents from a system with a high 7th harmonic impedance will generate high harmonic voltages on the bus.

Refer to IEEE Std 3002.8™-2018 "IEEE Recommended Practice for Conducting Harmonic Studies and Analysis of Industrial and Commercial Power Systems." It is a great guide with example plots.

 

[veritas]

Ah yes, I see now in PTW that it has a bus scan impedance magnitude vs harmonic order. It assists in flagging potential resonance issues for a given harmonic(s).