Factors influencing substation design, part 1 ambient temperature. Factors influencing substation equipment design are. Ambient temperature, relative humidity, altitude, snowfall, rainfall and wind pressure, pollution level, corrosion due to soil and ambient atmosphere. Hazardous areas, seismic zone, avoidance of use of materials known to impact environment, selection of dielectric strength and testing. We'll be discussing each of these influencing factors one after the other. Ambient temperature is the temperature in the environment where an equipment is stored or operated. It is an important factor governing substation design. This topic covers the effect of ambient temperature on substation design. The rated capacity of electrical equipment is primarily dependent upon the temperature withstand capability of insulating materials used in the product. The International Electro Technical Commission, IEC 60085 provides the classification of insulation materials. The table identifies various insulating materials based on insulation class, and they're applicable maximum permissible withstand temperature. Temperatures mentioned against these materials are the maximum permissible temperature limit. However, the temperature limit may further be reduced by manufacturers considering other relevant factors. Cable manufacturers limit continuous operating temperatures for PVC insulated cables at 70°Celsius as against 90°Celsius in the table considering a lifespan of 25 to 30 years. Similarly, for XLPE insulated cables, the continuous operating temperature is limited to 90°Celsius as against 120°Celsius in the table. If cables exceed this temperature continuously, then dielectric strength of PVC and XLPE insulation will deteriorate and effectively reduce the cable lifespan. In general transformer winding conductors are wrapped in a craft paper and immersed in mineral oil. Looking from the table, you can say that it is class A. Also from the table, you can understand that the maximum operating temperature of class A insulation material is limited to 105°Celsius. Therefore, oil-filled transformers can be operated safely only up to a maximum oil temperature of 105°Celsius to enjoy its utility for its intended design life. What happens if oil-filled transformers are operated beyond 105°Celsius. For every 10° rise beyond this permissible temperature limit the life of the insulation material reduces by 50%. The thermal stress and electrical stress along with dirt and other contaminants accelerate the failure of insulation, leading to a fault within the transformer. Depending on the severity of the fault, this may lead to fire or explosion. Transformers are a cube with suitable failsafe protection devices to prevent transformer oil temperature from rising beyond 105°Celsius. Any electrical equipment is at ambient temperature when not in operation for an extended duration. During its operation, owing to the current flow I in the motor winding conductor of resistance R. Power laws I square R within the winding results in heating of the motor winding insulation and equipment in that order. As current our load is increased, the equipment temperature also increases. All parts of the equipment withstand the heat generated during this normal operation. The enclosure and parts of the equipment handled during normal operation must not exceed the safe temperature to prevent any injury or burn to the operator. So, the normal operating regimen of any electrical equipment lies between the maximum ambient temperature and the final limiting temperature of the equipment. All electrical equipment are designed for operating at their rated capacities at specified design parameters without exceeding the final limiting temperature. This equipment might also have certain inherent overloading capability for a short duration without exceeding final limiting temperature. Remember, frequent overload operations or operations closer to the maximum withstand temperature of the insulation material will reduce overall expected design life of the equipment. However, allowable overloading of electrical equipment is a complex phenomenon. And it depends on several factors such as ambient temperature, equipment temperature and percentage loading prior to overloading. Load up to 125% for 20 to 30 minutes. Frequent overloading beyond permissible limits reduces life. Allowable overloading depends on ambient temperature, equipment temperature and percentage of load before overloading. Load up to 110% for 1 hour in every 12 hours of continuous operation. 25 instances per year, frequent overloading beyond permissible limits reduces life. Allowable overloading depends on ambient temperature, equipment temperature, percentage of load before overloading. From the examples discussed, you have now understood how equipment temperature rises over ambient temperature during continuous operation. And it never exceeds final limiting design temperature during normal operation. The rise in temperature of the equipment during its operation is termed as temperature rise. The allowable temperature rise over maximum ambient temperature is the designing criteria for rating electrical equipment. Temperature rise tests are an essential part of performance testing of all major electrical equipment, such as transformers, generators, motors and HV LV switchgear. Standard IEEE 1 general principles for temperature limits in the rating of electrical equipment and for the evaluation of electrical insulation. Provides principles of temperature limits and the methodology to evaluate electrical insulation that shall be applied in other IEEE standards towards design, manufacturing. And testing of various electrical equipment. With varying ambient temperatures, the equipment can handle different ratings considering the final limiting temperature. IEC standardizes the rating of all electrical equipment based on the standard ambient temperature of 40°Celsius. Ambient temperature varies, depends on time, season, location and geographical region. Very low or extreme weather conditions may impact the performance of electrical equipment. Hence it is essential to ascertain the suitability of electrical equipment for satisfactory operation. At minimum ambient temperature certain materials may become brittle if used in extreme cold conditions. The performance of a certain lubricating materials may deteriorate. Plastics may get hardened and lose flexibility. Space heating shall be resorted to where necessary. An electrical equipment has an IEC standard reading of 1000 Amperes. If this equipment is used in a location where the ambient temperature is 30°Celsius, how will the rating be affected? Will it be technically higher than 1000 Amperes or less than 1000 Amperes? No change in rating, creating increases, creating decreases. If the same electrical equipment is used in a location where the ambient temperature is 50°Celsius, what will be the effect on its rating? Will it be higher than 1000 Amperes or less than 1000 Amperes? No change in rating, rating increases, rating decreases, click the answer button for solution. Any equipment designed as per IEC is rated at 40°Celsius. The equipment is required to be derated if it is used At an ambient temperature of more than 40°Celsius. Conversely, if the same equipment is used at reduced ambient temperatures, it will not be operated as there may be other constraints. The constraints may not allow the flow of increased load current. Unless the manufacturer provides the capability diagram of equipment over a range of operating temperatures in sub 40°Celsius range. Here is a quick recap of what you have learned thus far. Ambient temperature, environmental temperature where an equipment is stored or operated. IEC standard defines insulation class for insulating materials and final limiting operating temperatures. All electrical equipment designed as per IEC standards are rated at 40°Celsius. If equipment are used at ambient temperature about 40°Celsius, rated capacity has to be reduced. Equipment rating may not be operated if operated below ambient temperature of 40°Celsius, as it depends on various constraints. Equipment have inherent overloading capability without exceeding final limiting temperature. Temperature rise test is important to verify thermal capability of insulating material. Extreme weather conditions impact performance of electrical equipment. Equipment shall be suitably selected considering prevalent weather conditions.
