Are DC filters in the HVDC systems necessary? 2

[Bronzeado]

Dear colleagues,

In the past, DC filters were required in HVDC systems to avoid telephone interference, which may be caused by the DC line harmonics.

Nowadays, with all new telephone tecnologies, are these DC filters necessary?

Best regards,

Herivelto S. Bronzeado
Ministério de Minas e Energia - MME, Brasília, Brazil

 

[desrod]

Telephone interference is only part of the problem and a side effect of having harmonics on the power network.

Filters are there to control/maintain harmonic distortion to the fundamental sine wave (60 or 50 hz) at an acceptable level for equipment to function properly. Harmonics presence also translates into more costly network; that's why you want to filter them as close as possible to the emitting device.

Installations in the US and numerous country must meet IEEE 519 harmonics emissions levels. IEC has a series of standard (61000-x-x) for planning level, emissions etc.

New phone technologies don't help. Harmonic interferences are mostly in the audible spectrum meaning that if you remove the filters you will have more humming noise over the lines and if you use the phone to filter that, you will loose part of the conversation.

Daniel

 

[jghrist]

What is the meaning of line harmonics in an HVDC system? The fundamental frequency is zero. Multiples of zero are zero.

 

[cuky2000]

Hi jghrist,

The DC output in the HVDC converter station is not 100% perfectly rectified. Even voltage harmonics (i.e.12th, 24th, 36th etc. ) is required to be reduced with DC filters to minimize the harmonic current in the AC side.

 

[davidbeach]

12th harmonic of what? 12 * 0 = 0.

There may be ac system harmonics riding on the dc, but there are no dc harmonics.

 

[ScottyUK]

DC filter [≡] capacitor?


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If we learn from our mistakes I'm getting a great education!

 

[Bronzeado]

Friends,

I am not talking about AC filters, wich are necessary to supply vars to the DC converters and mitigate the AC distorsions caused by them.

In HVDC sytems, the DC line voltage has a ripple which looks like a waveform with a fundamental frequency of 12x60(or 50) Hz on a DC offset. The DC current follows the same pattern as the voltage.

Due to that, it is usual to put DC filters connected in the DC line to reduce the harmonics in the DC side to avoid telephone interference. We did that in Itaipu Binacional Project, in Foz do Iguaçu, Brazil. All the calculations to specify the filters were based on the TIF (Telephone Influence Factor) obtained 30-50 years ago. The TIF may be different from those values due to new telephone technologies.

We now are constructing another HVDC system and it was especified DC filters, which will be installed in the DC lines.

This new HVDC system (Madeira Project, Brazil) will transmit energy from the Amazom area to São Paulo throught two DC transmission lines of around 2,500 km of extension.

My question is: Are DC filters necessary as the TIF may be different from that in the past.


Best regards,

Herivelto S. Bronzeado
Ministério de Minas e Energia - MME, Brasília, Brazil

 

[rtechn]

The calculation of the TIF takes into consideration weighting curves which are related to the response of the human ear. I believe that if the telephone transmission is analog, interference may occur and should be avoided. This could be done either by using DC filters or shielding the telephone lines.
I suggest that the telephone companies in the area be contacted in order to check their technologies and standards. It seems to me that, at least in the Northern and Central-West regions of Brazil, telephone systems might be somewhat old. On the other hand, population density is not high with a smaller communication network, so that it could be more favourable to shield existing and future telephone lines instead of installing the DC filters.

 

[Bronzeado]

rtechn,

I also think like you.

Any other expert opinion?

Best regards,

Herivelto S. Bronzeado
Ministério de Minas e Energia - MME, Brasília, Brazil

 

[cuky2000]

High Bronzeado,

The DC filters not only are designed to reduce telephone interference but also noise induced in power line carrier (PLC),radio, maritime,military applications,railroad signal, and others communications in a wide frequency bandwidth.

In many countries, the interference of power lines with communication systems is regulated by government agency or independent organizations. For example, In the USA the interference issues are within the jurisdiction of the Federal Communication Commission (FCC). Check in Brazil if the Minister of communication ( Agência Nacional de Telecomunicações/ ANATEL or similar) could help in this matter.

If the converter stations are VSC (voltage Source Converter) or/and interconnected with cables (usually with high capacitance), there is high hope that DC filter may not be required. However, judging for the length of the line (2500 km) most likely this project may be designed in bipolar configuration using LCC (Classic) technology with overhead DC lines. Therefore, this may require more considerations and engineering judgment to justify the DC filters.

In addition with the government regulations and local compliance issues, the following suggestions might help with the DC filter options:

• Check the level of superimposed harmonics in the DC side. (See sample of enclosed graph)
• Analyze the impact on extra losses cause by multiple of the characteristic harmonic (n.12). _{verify if the cost of installed DC filter is higher than the savings}
• Validate the system performance of DC filters with smooth reactor regarding limiting the rapid rise of current flowing into the converter in the event of valve misfiring during commutation.
• Verify DC overvoltage levels particularly in the ground during monopole operating mode.

_{NOTE: Regarding the harmonics superimposed in the DC side, we should keep in mind that the DC rectified process is dealing with non ideal devices. For instance, transformer turn ratios and transformer reactance are not identical for the Y-Y & Y-D large converter transformers and the associated leakage are unbalanced in the AC three phases. As a result, unequal commutation reactance also cause non characteristic voltages on the DC side.}