Reducing The High Voltage In Transmission Lines

[Dexer]

Dear Engineers & Experts ,

We have an area in our power network (380 kV) which confronts a high voltage profile exceeding the operating limits. The only option we have to reduce that voltage level is by de-energizing some 380 kV lines (although we have shunt reactors but not enough) which really bothers us for the reliability of the network point-of-view.

My Question is : Is there any new technique that reduce this voltage? I have heard of something called "Mobile Shunt Reactor" and I tried to google it but I haven't found anything related to it, may be it has a different name.

Please share with me your opinions.


Regards,

 

[Bill West]

Hello Dexer
I tried Google and found some financial reports for a mobile shunt reactor at WAPA (US Western Area Power Administration). The dollar amounts were similar to those for 115 or 69kv 10MVA mobile transformers in the same accounts. Mounting a reactor of that size on a road trailer is easy but of course is too small for your case.

I assume you have already considered:
-borrowing from neighbouring utilities that might have spare reactors
-using the tertiaries of system transformers as an alternate connection point and thus an alternate voltage choice
-seeing if any large industries might be able to reduce their power factor

Bill

 

[FPelec]

Hello Dexer,
Shunt reactors are the most efficient and effective solution to cope with high voltages in EHV grids.
However the rated power of the shunt reactors used in EHV is too large to foresee a "mobile" solution.
Take into account that, for a given short circuit power Scc at a connection node, the per unit voltage drop (DV) you can attain connecting a shunt reactor with a Qn rated power is DV = Qn/Scc.

Since the beginning of the deregulation process, we installed more than 8000 Mvar of shunt reactors: the need for new shunt reactors is going to further increase due to the shut down of conventional power plants.
My suggestion is to develop a long term plan for reactive power sources (mainly, but not only, reactors), to cope with the needs of your grid.
Mobile solution can be a solution for emergencies, but not for the long term evolution of the system.
Furthermore, it seems to me difficult to build a mobile solution with such large units: the "standard" units we use on the 400 kV network are single phase units with a 258/3 Mvar rating. The weight of such single phase units is about 100 t, which is not really something you can define as "mobile".

As a last resort, you could use transformer tap staggering; although it is an inefficient solution, you could get some tens of Mvar from a couple of EHV/HV autotransformers.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[Bill West]

One other thought Dexer. Are there any shut down power plant generators that could be adapted to synchronous condenser service? If they are in the wrong place or on a neighbouring system could they displace existing reactors for relocation?

This would not be cheap but I assume you are looking ahead 1 to 3 years and are trying to circumvent long lead times for new reactors.

Bill

 

[bacon4life]

A recent "smart grid" trend seems to be implementing Volt/VAR Optimization at the distribution level. However, distribution VVO seems to be operated independently from the transmission system. As a very long term solution, consider incentivizing customers, distribution system operators, and distributed generators to remove shunt power factor correction capacitors during light load periods. Retail tariffs can be structured to incentivize either fixed capacitor banks or switchable shunt capacitor banks, with the former contributing to excessive transmission voltages.

 

[cuky2000]

It appears that the excessive voltage above up to 5% is due to the Ferranti effect of a long line during light loading conditions. Perhaps this is the reason that the system operator allows shut down the line if voltage exceed the acceptable max. operating voltage (> 400kV ?)

Not sure if there is any new technique that can compete with a mechanical switching shunt reactor in terms of cost and faster to be deployed to solve the voltage rise issues on the 380 kV section (s).

Possible cost-effective options to explore are:
a) Installing another reactor of the same size in parallel
b) Install the existing reactor or add a second reactor in parallel If it is available a tertiary transformer.
c) See if a dry-type reactor could be used in lieu of oil-filled reactor

A system study is recommended to size the reactor and determine an acceptable range to control the voltage below the maximum operating voltage of the existing equipment in the substation.

For a 380 kV OH line the surge impedance loading, SIL = 430 MW approx. For illustration purposes, the enclosed link shows a general line loading characteristics as a function of the SIL and acceptable voltage rise limits for an overhead T. Line voltage regulation.

 

[Dexer]

Thank you guys for all the replies and I'm sorry for the late reply. The thing is I even communicated with manfucatureres about it but no good feedback.

 

[prc]

1) What is required is not more capacitive banks or better power factor load. Need is more lagging load or poor power factor load so that capacitive MVA of line can be compensated.
2) It always better to go for line EHV reactors than putting reactors on tertiaries of transformers. Of course before reliable EHV reactors were developed, that was the only option.
3) One expert in India has been suggesting to run old generators ( became redundant due to renewable energy addition)as synchronous machines absorbing reactive power from grid.

 

[RRaghunath]

In a power system with long EHV transmission lines that are not loaded much, overvoltage is a problem. While there are many long term solutions like adding more shunt reactors, SVC etc. to the system, short term solutions are only two:
1) Run the generators on VAR absorption mode. Especially those machines which are not supplying much active power can absorb substantial reactive power without adverse impact on the machine.
2) Disconnect selected transmission lines to reduce floating excess VARs in the power system and thus reduce the voltage. This is what you are already practicing.
Peruvian grid also practiced the second method and you may find the attached article interesting.

 

[bacon4life]

Agree, no additional capacitor banks are needed. Instead, existing fixed capacitor banks on the distribution system or owned by customers could be retrofitted with power factor controllers in order to reduce the amount of leading power factor during light load periods. On my local system, switching all of the 15 kV pole mounted capacitors would add about 20% to existing controllable reactive capability. Reducing capacitance on the distribution system does not eliminate the need for reactors, but it could certainly reduce the number/size needed.

The new standards for solar inverters require active voltage support and reactive power output while the panels are generating real power. Perhaps future inverters need to also provide voltage control features even when no real power is being produced. Seems like distributed generation should actually distributed ALL the services generation provides rather than distributing real power production and require new centralized reactive power devices.

 

[Bill West]

Bacon, last sentence -very good observation. Now to get the solar supportive public to understand. So many of our technology advances only do 90% of the job. We all know what quality of a job you get when 90% work is compounded 7 times in a technologically short 1-2 decades.

Bill