White Paper : Humidity Effects, Partial Discharge & Loss Of Substation Power By EA Technology

By Chris Dodds on 9th February, 2016

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White Paper : Humidity Effects, Partial Discharge & Loss Of Substation Power By EA Technology



Roxtec Cable Transits

White Paper : Humidity Effects in Substations By EA Technology (Download foot of page)

Author Tony Byrne, January 2013

Consequences For The Technical Specification of Cable Sealing Systems

Roxtec are global-market leaders in the innovation, design and manufacture of cable and pipe penetration sealing systems.

Roxtec solutions effectively seal cables, cable ducts and pipes against water and dust ingress into safety critical and sensitive electrical equipment and buildings.

Roxtec Cable Seals

In this White Paper, Roxtec highlight the potential for electrical equipment failure resulting from high relative humidity within high voltage substations, typically operating at 11kV, 33kV, 66kV and 132kV voltages.

Roxtec commissioned EA Technology to produce a report on the causes and effects of high relative humidity within indoor substation environments and methods of mitigation.

This influential report covers Relative Humidity and the effect it has on the inception or level of partial discharge (PD).

The White Paper discusses the factors affecting the environment within a high voltage substation and the best practice for the design of substations and internal environment control.

The findings in this White Paper have wide-reaching implications for the effective technical specification of cable and duct sealing systems.

Cable sealing systems can reduce the risk of high voltage electrical equipment failure caused by partial discharge as a result of inadequate specification of cable transit systems.

The report concludes, "cable entry points are one of the main sources of water ingress to substations and they should be effectively sealed."  

  • Who Should Read This ?

We summarise several of the Key Points below, however a thorough and careful complete read is recommended to electrical industry professionals, substation design engineers, project managers and installers. 

This White Paper should be mandatory reading for Lead Electrical Engineers, Electrical Project Engineers, Principal Electrical Engineers, Plant Engineers, LV HV EHV Cable Jointers, ATEX Inspectors, Substation Commissioning Managers, LV HV EHV Project Managers, Buyers & Procurement Managers, SAP's.

  • Environmental Conditions

Typical relative humidity (RH%) in the UK is between 65% - 95% in an internal substation environment.

Where there are cable trenches and basement ducts adequate cable sealing must be adopted to prevent water ingress in “normal” and “flood” conditions.

Even raised substations should seal all cable entries to prevent moisture ingress, the cause of high RH%.

  • Operational Conditions

Loads/faults can cause considerable cable expansion and contraction – which may lead to traditional cable seal failure as mastic sealants are often not able to provide sufficient cable retention. 

Due to lower ambient temperature and their location below ground, trenches and basements will experience a higher level of RH.

For below ground enclosures and external penetrations, a vermin proof seal would also be advantageous.

Pictured : Onshore Wind Power. Transformer Station & Shelter.

Using Roxtec R type cable transit frame and seals to provide a fireproof, watertight, gas-tight and anti-rodent seal. 

Roxtec Cable Transit R Seal

PD’s are electrical discharges occurring inside or on the surface of insulation materials, caused by HV electrical stressing of the insulation.

PD leads to progressive deterioration and plays a significant part in failures. Partial discharge affects all types of HV assets critical to network operation including high voltage switchgear, cables and transformers.

High RH% and particle contamination (typically dust ) cause and accelerate PD. Large swings in RH% also have an adverse effect.

Partial discharge is hard to detect unless invasive maintenance is carried out or a failure occures.

The most common cause of high RH in electricity substations is water ingress through the structure and below ground cable ducts.

Most common dust ingress points include below ground cable ducts and cable entries in walls and roofs.

  • Partial Discharge Effects

PD can ultimately lead to a catastrophic equipment failure.

This typically cannot be rectified through cleaning or refurbishment and can incur significant costs through the need to replace substation equipment, maintenance downtime and loss of supply.

Pictured : Offshore Wind Power. Switchgear Sealing.

Using Roxtec RS round cable transit frames to seal large and rigid power cables with connectors in offshore substations.

Switchgear Sealing

Switchgear Specification & Design

  • IEC 62271-1:2007 & BS EN62271-1: 2008 Ambient air not polluted with gas, smoke, corrosive / flammable gases, vapours
  • Average RH% over 24hrs not to exceed 95% 
  • Average RH% over 1 month does not to exceed 90% 
  • Design notes – highlights the use of “control measures” to manage RH% 
  • All tests are short timescale, long term effects over several years are unknown

Environmental Requirements – Manufacturers Literature

Manufacturers recognise the design notes in the standards and use typical qualifying statements in equipment manuals.

Example 1 : “RH% must not exceed 80% and the combination of temp and humidity must be such that condensation, in or on the equipment, will not occur. Where RH% exceeds 80% take special precautions to prevent.”

Example 2 : “If the average air humidity exceeds 75% we recommend appropriate remedies are adopted.”

Example 3 : “Ideal conditions – Humidity below 40% and no dripping water. Standard conditions – Humidity below 60%”

Ideal conditions can be as low as 40% normal conditions less than 60%

Logical Conclusions

  • RH% in UK between 65-95% as standard - manufacturers recommended operating levels may be below this
  • Water / dust ingress – mainly below ground cable ducts – but also through walls
  • Cable retention is a key factor as movement will occur in normal operation due to expansion and contraction of the cables, the natural weight of HV power cabling and the disturbance caused by routine maintenance and engineering works
  • Beneficial to have vermin proof sealing

Many substations use costly control measures, such as sump pumps, heaters and de-humidifiers, to deal with the problem of RH without tackling the cause.

The sealing of service penetrations is not highlighted in many standards, although these are the number one ingress point for water and dust.

It makes sense to reduce operating costs and reliance on control measures to provide duct and service penetration sealing, which not only seals but remain tight under load and fault.

To repeat, "cable entry points are one of the main sources of water ingress to substations and they should be effectively sealed."

Pictured : Sealing Cabinets, Panels & Electrical Equipment In Underground Substations.

Electrical substation cabinets for control and switching and any other cabinets used inside or outside the substation should be sealed against environmental hazards that may disturb the functionality of sensitive electrical and electronic equipment.

Sealing Cabinets Sealing Cabinets     

Roxtec Seals HV Electricity Substations, Transmission & Distribution

Roxtec have a track record in providing reliable Substation Cable Sealing to UK DNO's and overseas electricity authorities.

Typical applications include sealing cables into control rooms, service rooms and control boxes at the high or medium voltage electricity substation.

Roxtec H3 Triplex Seals prevent water ingress generating increased humidity leading to partial discharge and eventual loss of power supply  into 11kV, 33kV and 132kV substations.  

Roxtec cable seals provide sealing solutions, with spare capacity for future cables, to prevent water ingress, gas migration and rodent infestation from damaging substation electrical equipment and causing loss of power supply.

Pictured : Roxtec H3 Cable Seal For Medium Voltage 11kV 33kV Trefoil Cables

Cable Seal 11kV 33kV

Video Playlist: Roxtec Cable Transits - Seal Cable Ducts & Pipe Entries

  • Further reading:



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White Paper : Humidity Effects, Partial Discharge & Loss Of Substation Power By EA Technology

Category:  Cable Transits & Duct Sealing

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