FEATURE TEMPERATURE, HUMIDITY & MOISTURE CONTROL
TACKLE THE CAUSE OF HUMIDITY, NOT THE PROBLEMS IT BRINGS
Water ingress is well known for its damaging effect on electrical equipment, often resulting in partial discharge and switchgear failure. Rob Cullen, market manager for Power at Roxtec, explains the findings of a White Paper about the effects of humidity and outlines some solutions to mitigate these conditions
W
ater is vital to life, but in the world of cable installation it often causes
technological headaches. Traditional seals of mastic or
compounds are hard to install on HV single core cables in wet or running water conditions. They also don’t provide adequate cable retention. In the long term, these seals can’t cope with the expansion and contraction of cables in operation. Cables are also subject to further forces as the ground settles outside, plus their weight can cause retention issues. An independent report
found that partial discharge on high voltage switchgear occurred over time and interfered with optimum operating conditions. This was due to cable ducts below ground allowing the ingress of water, dust, dirt, pollutants and gas. In particular, water ingress leads to high relative humidity, which in turn results in partial discharge and failure of switchgear. These findings were backed up by a
White Paper report* which Roxtec commissioned from EA Technology on the causes and effects of humidity within indoor substation environments as well as methods of mitigating these factors. Essentially, the White Paper highlights the potential for electrical equipment failure resulting from high relative humidity within high voltage substations, typically operating at 11kV, 33kV, 66kV and 132kV voltages. It further covers relative humidity and the effect it has on the inception or level of partial discharge (PD). The report 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 have wide-reaching implications for the effective technical specification of cable and duct sealing systems. Effective cable sealing systems are
30 NOVEMBER 2016 | PROCESS & CONTROL
important because they can reduce the risk of high voltage electrical equipment failure caused by partial discharge as a result of inadequate specification of cable transit systems. A key finding in the report concludes “cable entry points are one of the main sources of water ingress to substations and they should be effectively sealed.” Additional key findings within the report are highlighted below: Environmental Conditions - Typical
relative humidity 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, a cause
of high relative humidity. Operational Conditions -
Loads or 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 relative humidity. For below ground enclosures and external penetrations, a vermin proof seal would also be advantageous. Partial Discharge (PD) - PDs are
electrical discharges occurring inside or on the surface of insulation materials, caused by high voltage electrical stressing. PD leads to progressive deterioration and plays a role in failures. Partial discharge affects all types of high voltage assets critical to network operation including high voltage switchgear, cables and transformers. High relative humidity and particle contamination (ie. dust) cause and accelerate PD. Large swings in relative humidity also have an adverse effect. Partial discharge is hard to detect unless
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
invasive maintenance is carried out or a failure occurs. The most common cause of high relative humidity 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. 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. Many substations use costly control
Rob Cullen, market manager for Power at Roxtec
UG solutions eliminate flooding and prevent humidity from causing damage
measures, such as sump pumps, heaters and de-humidifiers, to deal with the problem of relative humidity 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. Roxtec provides 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. Electrical substation cabinets for control and switching and 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.
* ’Humidity Effects in Substations’ White Paper
Roxtec
www.roxtec.com
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