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What are you witnessing in you region with regard to the use of EHV and UHV Transmission Lines for Voltage Control?

Leo St. Hilaire's picture
Technical Advisor GCC eLab

Over 40 years of utility experience with twenty five (25) years’ experience in Supervisory and Managerial roles in power system operations. I have advance technical knowledge in Operations and...

  • Member since 2021
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  • Nov 24, 2021

In your region, are you experiencing increased removal of EHV and/or UHV Transmission Lines/cables for voltage control? Is it a result of increased renewables or transmission expansion?

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There is no UHV in the New England region. However, for decades it was common practice to take EHV circuits out of service to help manage high voltage conditions. This was especially true in the Boston area where you have an extensive 345 and 115KV cable system. As time moved forward, many of the transmission owners became hesitant to allow use of switching circuits to manage high voltage conditions. There was concern over wear and tear on switching equipment and loss of life on substation gear. Also, there is an inherent risk to overall system reliability as you take more and more facilities out, you decrease the system ability to recover from contingencies. In summary, while it may be a short term solution, the reality is the system should really consider adding either static or dynamic reactive devices.

Leo St. Hilaire's picture
Leo St. Hilaire on Nov 29, 2021

Thanks John

This is a contradicting question. Increasing RE utilization necessitates transmission network expansion not removal. However, removal may be due to upgrading the transmission voltage to ease transferring more power on the same corridors.

Leo St. Hilaire's picture
Leo St. Hilaire on Nov 29, 2021

Thanks Dr. Amal Khashab

Removal of EHV transmission for voltage control is very common in the PNW and not a new practice. Lightly loaded 500kV lines produce ~2MVAR per mile. Multiplied by several hundred miles of line the impact can be rather large. 

At a minimum renewable resources that lack VAR control won't help the problem. Coupled with the loss of dispatchable resources that have VAR control the problem gets worse. Adding transmission to access the renewables or for any other reason will create more VAR production in light load conditions adding to the high voltage problem.

Leo St. Hilaire's picture
Leo St. Hilaire on Nov 29, 2021

Thanks Chris for the response. 

EHV and HV are related to transmission in decreasing the current in the line which happens according to a certain set procedure.  Ultra-high voltage level above 800KV is mostly porcelain insulators made up of disk types and each disk withstands up to 12 kV.


EHV substations are designed through adoption and layout of equipment in the switchyard.  Shielding control cables and special problems in the design of such substations demand a lot of care.  Voltage limits in India are: Low Voltage (not exceeding 250 V); medium Voltage (Not exceeding 650 V) and High Voltage (not exceeding 33000 V.


In addition to this, there are Rated Voltage (maximum standard voltage that can be produced by generating substation); Nominal Voltage (Predetermined system voltage) and Extra Low Voltage (below 70 Volts) which does not harm humans.


In India, mostly porcelain insulators made up of 8 disks have the ability to withstand up to 12kV – each of 12 kV capacities would serve 96 kV.  This is calculated based on line voltage plus 15% tolerance plus one additional disk safety.  Accordingly, 66kV plus 9.9 kV will account for 87.9 kV and therefore we need 9 disks as per calculations.


In view of the advantages of UHVAC generating large capacity charging MVAR and to limit over voltages under highly loaded conditions and limit switching over voltages, many countries have opted for it.   Overhead line insulation guaranteed by the air clearance and the insulation withstand capability if the insular strings withstand various over voltage due to power-frequency. Being in the atmosphere, wind and rain do influence as also pollution and even snow. 


The Impulse Laboratory has been set up at UHV Rl, Hyderabad to undertake R & D for breakdown characteristics of air insulation, optimization of window clearance and transmission lines in EHV/UHV range.  This can generate 4.4 MV standard lightning impulse of 1.2/50 us under dry condition and 3.2 switching impulse of 250/2500 us with the standard load of 2nF.


The transmission system needs to be compared based on not only technical basis (taking into adaptability) and most modern and extremely fast control features (modulation control, swing control, frequency control, power direction control etc.).  In this regard, UHV ac and 400 or 765 kv ac transmission systems can be compared because of the same philosophy in both.

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