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High Voltage Technologies and Condition Assessment

Brief Description:

Part A: –    Insulation Coordination for a MV/HV Substation (50%)

Prepare a report based on a literature survey on insulation coordination for Insulation design of an electrical substation with AC Transmission and Distribution plant items. The report should address the following:

  • Lists the major plant items that need to be considered                    [5 marks]
  • What do you understand by insulation coordination?                       [7 marks]
  • Standards being followed in insulation coordination                        [5 marks]
  • Methods of insulation coordination                                                           [8 marks]
  • Factors to be considered in insulation coordination                         [10 marks]
  • Role, Types and selection criterion of surge arresters  in insulation coordination                                                                              [15 marks]

Open literature sources, such as journal papers, text books, company websites, and any other source, should be employed. The report should have the sources suitably identified and embedded within the text.

Part B: –    : Condition assessment of an insulator string(50%)

  1. Explain the two different techniques that can be used for condition monitoring of insulators for overhear power transmission lines.  (max 500 words) [20 marks]

2.BS EN 60507 (1993) provides guidance on testing high voltage insulators. Explain why the test should be carried out under wet condition? (max 200 words)[5 marks]

3. Consider a polluted insulator collected from a transmission line. The deposit was removed and carefully collected from the surface of the insulator excluding metal parts. It was dissolved in V=500 cm3 demineralised water. The resulting slurry was kept stirred for at least 2 minutes before the measurement of its volume conductivity σ20 (S/m) at the temperature 20 °C.


                                    Part A

  1. According to the IEEE std.1313 act, the major plant items that need to be considered in insulation coordination of MN/HV subsystems are;

-Power transformers

  • Rating and capacity
  • Cooling methods
  • Insulation classes
  • Load tap changers (LTC) or on-load tap changers (OLTC)

-Circuit breakers

  • Arc quenching methods
  • Circuit configurations (e.g., single-line, ring main, etc.)
  • Protective relays

-Surge arresters

  • Type of surge arrestors (e.g., gapless, gapped)
  • Installation locations (substations, equipment)
  • Coordination with protective devices

-Disconnect switches

  • Instrument transformers (CTs and PTs)
  • Lightning arresters
  • Grounding systems


  • Configuration and layout
  • Materials and construction

-Instrument Transformers (Current and voltage transformers)

  • Ratio and burden specifications
  • Applications in protection and metering
  • Testing and calibration

-Capacitor banks

  • Power factor correction
  • Capacitor ratings and switching mechanisms
  • Harmonic filtering


  • Types of reactors (e.g., shunt, series)
  • Application in system stability and compensation
  • Ratings and impedance characteristics

 b) Insulation coordination definition:

Insulation coordination is an important aspect of electrical engineering that focuses on the integrity and dependability of power systems. It entails the careful selection and placement of insulating materials and components in order to reduce the risks caused by overvoltages and surges. The ultimate goal is to avoid disruptive breakdowns and maintain uninterrupted operation in the face of transient voltage events.

Insulation coordination includes elements such as determining insulation level, applying surge arresters, and coordinating with protective equipment. It aims to find a compromise between suitable insulation for regular voltage strains and the ability to survive abnormal occurrences such as lightning strikes and switching surges.

Successful insulation coordination improves power system dependability, decreases downtime, and increases equipment service life. It is required in a wide range of applications, including substations and transmission lines, as well as industrial and residential installations. Engineers improve insulation designs for robustness and durability using advanced simulation tools and industry standards, assuring the resilience of modern electrical systems (IEEE Std. 1313.2-1999) .

Insulation coordination involves understanding the insulation capabilities of power system components. It entails determining the equipment’s insulation strength, both for internal and external insulation, under regular and abnormal voltage conditions.

The insulating design ensures that the equipment can tolerate the greatest power frequency system voltage, brief power frequency overvoltages, and occasional lightning surges. Each power system equipment is assigned a rated insulation level, which can be verified through various types of tests to confirm its capability. (Archana T. 2016)

According to Powertech labs (2023), insulation coordination is the process of deliberately selecting the most matched instrument dielectric strength and implementing it to prevent insulating damage and preserve continuous service.  A reliable substation design involves studying different over voltages that might impact the insulation, such as switching transients, fault conditions, power frequency resonance, or lightning impulses. Analyzing these factors during the planning stage allows for cost-saving opportunities by selecting appropriate equipment ratings. By identifying the locations and levels of maximum overvoltage stresses, designers can ensure minimum clearances, allocate space for overvoltage protection equipment, and maintain safe operations during both normal and contingency scenarios.

c) Standards being followed in insulation coordination.

(I) Table 1.0 The insulation ratings for substation equipment based on the Substation Standard for Substation Primary Plant Rating (STNW3015).

Nominal Voltage kVHighest Voltage kVRated power frequency tolerate voltage for short duration (PFWV) kVVoltage required to sustain a lightning strike (LIWV) kVp
1 11 22 8 9  5 (7  5)
2 22 45 01 5 0 (1 2 5)
3 33 67 02 0 0 (1 7 0)
6 67 2 .51 4 03 2 5
1 1 01 2 32 3 05 5 0
1 3 21 4 52 7 5 (2 3 0)6 5 0 (5 5 0)
2 2 02 4 54 6 0 (3 9 5)1 0 5 0 (9 5 0)

 Fig 1.0 m The origin and classification of voltage stress Voltage stressors on insulation (IEC 60071.1, 2019) are classified according to the structure of the voltage waveform which specifies their influence on insulating and safety equipment, rather than the source of the overvoltage.

The Cause of Temporary OvervoltageThe magnitude of the overvoltage (p.u.)Time of failure
Overvoltage at the fault (Effective Earthing)1.3In less than one seconds
Overvoltage at the fault (Impedance Earthed)1.3 – 1.73In less than three seconds
Load Elimination (Moderately Extensive)<1.27 minutes maximum
Load Elimination (Extended system)1.5Several seconds
Load Elimination (Resonance & Ferro resonance)3Until the situation is resolved
Activation of the Transformer1.5-2.0It could last a few seconds.
The longitudinal approach Overvoltage (synchronization)2.0Several seconds to several hours
Slow Front Overvoltage (Line Energization Ph-E)2.8-3.0
Slow Front Overvoltage (Line Energization Ph-Ph)1.55 x Line Energization Ph-E Fault level
Slow Front Overvoltage (Fault initiation max.)2(k-1) k = earth fault factor –
Slow front overvoltage (maximum fault clearance)2.0
Slow Front Overvoltage (Capacitive or inductive current switching)< 2.0 (Ph-Ph) < 3.0 (Ph-E)
Switching (Lightning Type)2.0 (Without restrike) 3.0 (With restrike)
Case / EquipmentRequirement for Surge Arrester
Equipment linked to an overhead power lineLocation at feeder bay entrance/exit.
Transition point from overhead line to cableHigh voltages are used in distribution, sub-transmission, and transmission.
Power transformerAll high-voltage exposed bushings must be mounted on an additional structure that is no more than 5m away from the bushings, or on a bracket that is connected to the transformer. Arresters are not required.: transformer-equipped cable bundlesOn plug-in bushings When the LIWV of the winding exceeds the inductive transmission voltage and the initial spike voltage.  When directly connected to cables and completely shielded by an earthed enclosure, the subsequent transitions to overhead conductors are protected by surge arresters. Direct earthing of the neutral  Neutral bushings with the impedance grounded on uniformly isolated windings
Gas insulated switchgear (GIS) and MV switchboardsFor switchgear with a lower LIWV rating that is connected by a mixed overhead/underground feeder, surge arresters must be installed at the overhead transition pole and the next pole back.
Capacitor bank and shunt reactorNot more than five meters from the line terminals
ReclosersArresters are strategically placed at high-voltage bushings to reduce Lightning Impulse Withstand Voltage (LIWV), where appropriate. Sheaths are also used for underground cables of 33kV and higher voltage levels. This integrated technique ensures effective protection against voltage spikes and improves the dependability of power distribution networks.
Underground cable sheaths (33kV cables and above)Moreover, to ensure comprehensive protection, shielding of the external cable cover or over sheath may be necessary apart from the primary insulation. Sheath Voltage Limiters (SVLs) might be essential for curtailing transient and steady-state voltages on the cable screen or sheath. This precaution helps maintain voltage levels within acceptable thresholds, preventing damage to the cable jacket or over sheath due to puncture. When a cable-specific insulation coordination study is not available, the following SVLs can be employed:   For 33kV, an SVL of 3kV is recommended. For 66kV, an SVL of 4.5kV is suggested. For 110/132kV, an SVL of 6.0kV is advised.

d) Methods of insulation coordination

 Here’s an explanation of each method with a space for Harvard in-text citations:

Clearances and Creepage Distances: This method involves maintaining appropriate spacing between conductors and insulators to prevent electrical breakdown and ensure safe operation within the substation (Petrov, 2018).

Surge Arresters: Installing surge arresters at strategic points within the substation is crucial to protect equipment from transient over voltages that may occur due to lightning strikes or switching operations (Chowdhury et al., 2016).

Shielding: Utilizing shielding devices or metallic enclosures around sensitive equipment helps protect them from external electrical influences, such as electromagnetic interference (EMI) and radio frequency interference (RFI) (Wang & Khodaei, 2019).

Insulating Materials: The use of high-quality insulating materials with appropriate dielectric properties is vital to reduce the risk of insulation failures and breakdowns in the substation (Pilli & Maheshwari, 2017).

Grounding: Implementing a proper grounding system is essential to minimize the risk of voltage surges and to ensure fault currents are safely discharged to the ground (Wu et al., 2020).

Insulation Monitoring: Installation of insulation monitoring devices allows continuous monitoring of the insulation condition in the substation, enabling early detection of potential insulation faults (Mujacic et al., 2018).

Overvoltage Protection: Protective relays and voltage regulators are used to prevent overvoltage situations in the substation, safeguarding equipment from damage (Malavasi et al., 2015).

Surge Suppression: Surge suppressors are utilized to mitigate transient voltage spikes and protect sensitive equipment from voltage surges (Peña-Patarroyo et al., 2017).

Dielectric Strength Testing: Periodically testing the dielectric strength of insulation materials and equipment helps identify potential weaknesses and ensures the reliability of the substation (Gao et al., 2019).

Coordination Studies: Conducting coordination studies helps analyze the interaction of protection devices and ensures proper coordination between them, enhancing the overall protection system in the substation (Göksu et al., 2022).

e) Factors to be considered in insulation coordination

In insulation coordination, several factors need to be considered to ensure the reliability and safety of electrical systems (IEEE Std. 1313.2-1999):

System voltage level: The operating voltage of the electrical system is crucial in determining the required insulation strength and coordination.

Equipment insulation level: Understanding the insulation capabilities of individual components, such as transformers, circuit breakers, and cables, is essential for proper coordination.

Overvoltage sources: Identifying potential overvoltage sources like lightning, switching operations, and transient events helps in designing appropriate insulation measures.

Insulation coordination levels: Different equipment may have varying insulation capabilities. Coordinating these levels ensures that the entire system operates effectively during faults and transient conditions.

Protective devices: Coordination of protective devices like surge arresters and fuses is necessary to mitigate overvoltage effects and protect the equipment.

Clearances and creepage distances: Ensuring adequate clearance and creepage distances between conductive parts prevents flashovers and reduces the risk of insulation failure.

Environmental conditions: Factors such as temperature, humidity, and pollution levels can affect the performance of insulation materials.

Redundancy and reliability: Implementing redundant insulation in critical areas enhances system reliability and minimizes the impact of insulation failure.

Maintenance practices: Regular inspection, testing, and maintenance of insulation systems are crucial to ensure long-term reliability. Standards and regulations: Compliance with industry standards and safety regulations helps in achieving reliable insulation coordination.

Industry Standards: Adherence to pertinent rules and regulations, such as IEEE and IEC standards, aids in ensuring effective insulation coordination procedures.

Economic considerations: For cost-effective insulation coordination, it is essential to strike a balance between the price of protective gear and insulation materials and the potential implications of an insulation failure.

Operating Conditions: Changing electric stress on insulation can be caused by changes in load and system dynamics. Variations in operational conditions should be taken into account when coordinating insulation.

Type and Size of Conductors: The distribution of the electric field and the requirements for insulation depend on the type (solid, stranded, etc.) and size of the conductors. Certain insulating materials and designs may be necessary for different conductor materials.

Ageing and Degradation: Thermal, electrical, or environmental stresses can cause insulating materials to deteriorate over time. The peculiarities of insulation’s ageing should be taken into account.

These factors collectively contribute to effective insulation coordination, ensuring a resilient and reliable electrical power system.

f) A surge arrester is a safety feature that is parallel-connected to the system equipment that needs to be protected. The arrester, which carries surge energy to ground and safeguards the device, limits overvoltages at the protected equipment. An arrester’s highly non-linear properties enable it to limit the voltage across its terminal to almost a constant value over a broad range of arrester current.

Surge arresters play a vital role in insulation coordination as they protect electrical systems from over voltages caused by lightning strikes, switching operations, and transient events (IEEE Power & Energy Society Insulated Conductors Committee, 2019). By providing a low-impedance path to ground for the surge energy, they prevent insulation breakdown and safeguard sensitive electrical equipment like transformers, cables, and switchgear (IEEE Power & Energy Society Insulated Conductors Committee, 2019).

In order to shield electrical systems from the damaging effects of voltage surges or transients, surge arresters are essential. These devices serve as a first line of defence, rerouting high transient currents away from delicate machinery and avoiding insulation damage and breakdown. Surge arresters protect the integrity, dependability, and longevity of electrical components by swiftly reducing voltage spikes brought on by lightning strikes or switching activities. By preventing disruptions, equipment breakdowns, and possible risks, they improve system and employee safety. The continuous operation of modern electrical networks depends on this vital protective function. In insulation coordination, surge arresters are strategically placed to mitigate the effects of high-energy transients. They help maintain appropriate insulation levels and avoid electrical breakdown, ensuring the integrity and longevity of the system by regulating the voltage across crucial components. Surge arresters are a key component of insulation coordination techniques because they greatly improve the performance, safety, and reliability of electrical networks. (IEEE Std C62.11-2012, “IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits).

Types of Surge Arresters:

There are three main types of surge arresters used in insulation coordination.

  1. Metal-Oxide Surge Arresters (MOAs): It is common practise in insulation coordination to utilise metal-oxide arresters with metal-oxide varistors (MOVs). They are ideal for medium and high-voltage applications because they effectively defend against high-energy transients. It utilizes metal-oxide blocks with nonlinear characteristics, providing high energy absorption capability and fast response times (IEEE Power & Energy Society Insulated Conductors Committee, 2019).
  2. Silicon Carbide Surge Arresters: Silicon Carbide (SiC) surge arresters are a specialized type of surge protection device used in insulation coordination to guard against transient overvoltages. High Energy Handling, Fast Response, Temperature Stability, Long Service Life, and Harsh Environment Suitability are just a few of the distinctive qualities that SiC surge arresters provide, making them ideal for particular electrical system applications.

Depending on the unique requirements and difficulties of an electrical system, SiC surge arresters may be used in insulation coordination. They serve as important tools for preserving insulation integrity and assuring system reliability due to their capacity for managing high energy. (IEEE Std C62.11-2012, “IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits.”).

3.Traditional Gapped Surge Arresters: This type of surge arresters uses spark gaps for conducting the surge energy to ground but have limited energy-handling capacity and slower response times compared to MOAs. These arresters utilize a physical air gap and nonlinear resistors to divert transient currents. Gap type arresters are commonly used in low-voltage applications where the energy of the transient is relatively low. (IEEE Power & Energy Society Insulated Conductors Committee, 2019).

Selection Criteria for Surge Arresters in Insulation Coordination:

Several important variables should be considered while selecting surge arresters for insulation coordination.

To begin, the surge arrester’s nominal voltage must be compatible with the system voltage it is designed to protect.

The surge arrester must have the energy handling capability to absorb and dissipate energy associated with worst-case transient events without exceeding thermal or voltage stress limits.

The response time of the surge arrester should also be fast to minimize the overvoltage duration experienced by the protected equipment. The surge arrester’s response time should align with the protection coordination of the system to avoid conflicts with other protective devices.

Furthermore, proper coordination with other insulation components, such as transformer and cable insulation levels, is essential for effective insulation coordination.

Surge arresters should withstand environmental conditions, including temperature, humidity, and pollution levels, should be considered during surge arrester selection.

To ensure consistent performance and protection levels over the course of their operating life, choose surge arresters with minimum ageing effects.

Consider the surge arresters’ cost-effectiveness while offering the best possible protection. In the selecting process, it’s critical to strike a balance between the required level of protection and financial limits.

Finally, the reliability and ease of maintenance should be evaluated to ensure a robust insulation system. (IEEE Power & Energy Society Insulated Conductors Committee, 2019).

In order to coordinate insulation effectively, prevent insulation breakdown, safeguard equipment, and preserve the electrical system’s dependability, surge arresters should be chosen based on these requirements.


Q: (i)


The process of continuously monitoring vibration and temperature states in machinery to detect changes that may indicate a developing defect is known as condition monitoring. It is an essential component of preventative maintenance since combining condition monitoring allows maintenance to be scheduled and preventive efforts to be taken to avoid additional failure and unexpected downtime.

Overhead power transmission line insulator condition monitoring is crucial to preventing equipment failure and service interruptions to ensure that the power system operates safely and reliably (Aggarwal, Johns, Jayasinghe, and Su, 2000). Visual inspection and non-destructive testing are two often utilized procedures. To guarantee that the power system functions safely and reliably, the health of power equipment must be monitored and evaluated both online and offline.

Visual Inspection: This is the simplest, yet one of the most effective techniques for condition monitoring. In a visual inspection, trained personnel visually check the insulators for any apparent physical signs of deterioration, such as cracks, erosion, discoloration, or contamination (Farzaneh & Chisholm, 2022). This method also includes observation of insulator strings for any tilting or irregularity in alignment. Inspection can be conducted from the ground using binoculars or from close range during maintenance shutdowns. With the advent of drone technology, aerial inspections have become more common, providing a safer and more efficient means to visually inspect power lines and insulators. Drones equipped with high-resolution cameras can capture detailed images of insulators from different angles, allowing for precise evaluation of their condition.

Non-destructive Testing (NDT): This technique involves the use of advanced technologies to assess the condition of insulators without causing any damage (Zhang, et al., 2022). Several NDT methods are available, but two popular ones are:

a) Infrared Thermography: This technique uses infrared cameras to capture heat images of insulators under normal operating conditions. It’s based on the principle that an increase in temperature is often indicative of a fault. Hot spots can be indicative of increased resistive losses due to surface contamination or internal defects.

                                                    Infrared thermography is known as thermal imaging and it is the collection and display of infrared radiation generated by objects in order to detect temperature fluctuations. It reveals hidden flaws by viewing temperature variations, assisting in predictive maintenance and problem detection. It is widely utilized in varied industries such as building inspections, electrical diagnostics, and medical imaging.

b) Ultrasonic Testing: This technique is used to detect internal defects that are not visible on the surface. Ultrasonic waves are sent into the insulator, and the reflected waves are analyzed. Any variation in the wave pattern could indicate potential flaws or defects in the insulator material.

                                                       Ultrasonic Testing (UT) is a non-destructive testing procedure that inspects materials for faults, cracks, and thickness measurements using high-frequency sound waves. UT finds structural flaws in different industries, including manufacturing, aerospace, and medical, by detecting the echoes of these waves, ensuring quality and safety while inflicting no damage.

These techniques provide utilities with an effective way of monitoring the condition of insulators and scheduling predictive maintenance, ultimately increasing the reliability and efficiency of the power transmission system. While visual inspections provide a good general overview of the insulator’s condition, NDT offers a more in-depth analysis of potential internal and non-visible external defects. The combined use of these techniques can help prevent unexpected insulator failures and reduce outage incidences.

Q: (ii).  

Answer: The BS EN 60507 standard, also known as IEC 60507, gives instructions for testing the performance of electrical insulating materials under certain conditions. This standard, which was published in 1993, describes numerous test methods and processes for assessing the behavior and qualities of insulating materials when subjected to electrical stress.

The test for high voltage insulators, as specified in BS EN 60507 (1993), is conducted under wet conditions to simulate real-world environmental conditions that the insulators may encounter during their operational life. High voltage insulators are typically used in outdoor settings, such as power transmission and distribution systems, where they are exposed to various weather conditions, including rain, dew, and fog.

Testing under wet conditions is crucial for several reasons:

Realistic simulation: Wet conditions replicate the insulator’s exposure to moisture and surface contaminants, which can affect its performance in terms of leakage current, flashover voltage, and overall insulation resistance. Testing under wet conditions ensures that the insulator can withstand the stresses imposed by moisture and contaminants.

Performance evaluation: Wet testing allows engineers to assess the insulator’s ability to maintain its insulating properties and prevent leakage current or flashovers under moist conditions. This is essential for guaranteeing the reliable and safe operation of the equipment.

Safety assurance: Insulators must maintain their electrical integrity to ensure the safety of personnel and equipment. By testing under wet conditions, potential issues related to tracking, arcing, or electrical breakdown can be identified and addressed, reducing the risk of accidents and equipment failure.

Conducting high voltage insulator tests under wet conditions is a fundamental step in evaluating their performance and safety in real-world scenarios, ultimately contributing to the reliability and longevity of power transmission and distribution systems.

Q:(iii) (a)


The density of salt deposits left on the surface of insulators is referred to as ESDD, or Equivalent Salt Deposit Density. It is commonly expressed in milligrams per square centimeter (mg/cm2). The ESDD is a critical measure for assessing the contamination level of insulators in outdoor situations, particularly in locations with high pollution levels. Contaminants such as salt and dust can build up on insulator surfaces, reducing their performance and electrical properties.

The density of non-soluble deposits on the insulator’s surface, known as NSDD, is often stated in the same unit: milligrams per square centimeter (mg/cm2). Materials that do not dissolve in water and can contribute to insulator pollution are included (Miao, X., Zhiqiang W., & Xiliang G. (2017)).

The following formulas will be used to compute the ESDD (Equivalent Salt Deposit Density) and NSDD (Non-Soluble Material Deposit Density):

Q: (b)

To determine the severity of pollution on the insulator using the calculated NSDD (Non-Soluble Material Deposit Density), we can compare the NSDD value with established guidelines or standards. These guidelines provide thresholds that indicate different levels of pollution severity. Generally, higher NSDD values correspond to higher pollution levels, which can affect the insulator’s performance and safety (Sathya, S., & Ilamparithi, R. (2016)).

There are guidelines that categorize pollution severity levels as follows:

The ESDD value of  indicates the amount of salt deposits on the surface of the insulator. Higher ESDD values ​​indicate greater salt accumulation, which may be due to environmental factors such as proximity to the coast or industrial pollutants. This increased salt content can create surface conductivity, which increases the likelihood of leakage currents and consequent flashover, thereby threatening insulating integrity.


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IEEE Std. 1313.2-1999. (1999). IEEE Guide for the Application of Insulation Coordination. IEEE.

Gao, L., Sun, C., & Hu, Y. (2019). Dielectric Strength Test of Power Transformers Based on the Thermal Effect of Oil Paper Insulation. In 2019 IEEE Electrical Insulation Conference (EIC) (pp. 1-4).

Göksu, O., Çelik, Y., & Güzel, E. (2022). Coordination of Overcurrent Relays Using Genetic Algorithm. Journal of Energy Engineering, 148(5), 04022019.

Malavasi, D., Cianetti, F., & Nucci, C. A. (2015). Overvoltage Protection of Power Transformers Based on a New Active Voltage Compensation Method. IEEE Transactions on Power Delivery, 31(2), 566-574.

Mujacic, S., Hoornweg, M., & Johnson, C. M. (2018). Online Condition Monitoring of Insulation for High-Voltage Substation Equipment Using Novel RF Sensors. IEEE Transactions on Dielectrics and Electrical Insulation, 25(4), 1429-1437.

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Wang, M., & Khodaei, A. (2019). Shielding the Substations Against a High-altitude Electromagnetic Pulse (HEMP) Attack: An Overview. In 2019 North American Power Symposium (NAPS) (pp. 1-6).

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Aggarwal, R.K., Johns, A.T., Jayasinghe, J.A.S.B. and Su, W., 2000. An overview of the condition monitoring of overhead lines. Electric Power systems research, 53(1), pp.15-22.

Farzaneh, M. and Chisholm, W.A., 2022. Protective Coatings for Overhead Lines in Winter Conditions. In Techniques for Protecting Overhead Lines in Winter Conditions: Dimensioning, Icephobic Surfaces, De-Icing Strategies (pp. 195-309). Cham: Springer International Publishing.

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