Welcome to this module on station AC/DC, auxiliary power system. Categorization of auxiliary loads. This topic describes need for station auxiliary power. Categorization of auxiliary loads. Auxiliary loads for individual equipment. Identification of type of source of power. A substitution rated as say, 33 by 11 kV, receives power at 33 kV steps down to 11 kV, and distributes power at 11 kV in the nearby areas. It does not consume any power at 33 kV nor 11 kV within the substation. Several substitution equipment are electrically operated. Hence, electrical power is required for satisfactory functioning of the substation. Also, there are several ancillary equipment and building services which also operate on electrical power. The electrical power which is required for the satisfactory control, monitoring, operation, and maintenance of the substitution equipment and allied services is termed as auxiliary power. Auxiliary power is usually at low voltage and can be both AC and DC. AC voltage is rated at 415 by 240 volts, or 400 by 230 volts, as appropriate, based on the standard adopted in the geographical region. DC voltage is rated at 110 volts and 48 volts. Typically auxiliary loads in a substation are all control, protection or trip circuits, status monitoring, interlock circuit for all switch gear. Motor loads, open or close of isolators, transformer cooler fans, or OLTC mechanism, spring charging model for CBs. Building service loads, such as HVAC loads are heating, ventilation, and air conditioning, fire detection, and firefighting loads like fire water pumps, lighting, indoor and outdoor, and small power loads. Water lift pumps, anti-condensation, heater loads. Power consumption of digital control system, supervisory control, and data acquisition. Computer systems, human-machine interface, closed circuit television, public address general alarm systems, power consumption of telecom systems. What happens if a substation auxiliaries are deprived of electrical power supply? Let us now see what happens if a substation auxiliary loads lose power supply. Control, prediction, or trip circuits, status monitoring, interlock circuits. Control, monitoring, and protection systems in any substation are regarded as the heart and brain of a substation. These systems require auxiliary power for their proper functioning. Failure or interruption, even for a brief moment of auxiliary power could result in inability to detect electrical faults in operation of protective relays or circuit breakers. Failure of safety interlocks, which might lead to mal-operation, including that of manually operated equipment, which would, in turn, lead to damage to asset or injury to operating personnel. Unavailability of indications and alarms, inability to control devices or equipment from remote locations. Motor loads will not operate. Opening or closing of isolators is not possible electrically. Transformer cooler fans will stop if they were running, which could lead to temperature rise of the transformer. Oil DC mechanism will not operate, leading to operation at incorrect tap. Spring charging for CBs do not take place jeopardizing further CB open or close operations. HVAC system will not function, leading to rise in ambient temperature inside substation and loss of air circulation. Fire detection and firefighting systems, like fire water pumps do not operate. There will not be lighting as substations are not normally provided with windows, indoor area will become dark. Any remedial or restoration work inside substation will be difficult to carry out in darkness. Moving out to safety is also difficult. DCS or skater or computer systems will fail. CCTV or PAGA systems do not function. Telecom systems will fail and all communication of data or voice which would, in turn, result in loss of a remote control, monitoring, and inter-tripping. In short, there will be total chaos in the substation leading to unavailability of the substation, to say the least. Some of the failures discussed above may result in injury or loss of life or damage to assets. We need to assess and scale the relative importance of keeping such auxiliary loads, live. Vital loads are those loads in the absence of power supply could result in injury or loss of life or assets. Vital loads shall receive power from uninterrupted power supplies. Vital loads are also termed as critical loads. Essentially loads are those loads in the absence of power supply could result in loss of revenue or production or impact quality of the product. Essential loads shall receive power supply from redundant sources so that if power from one source fails, the other source will take over. Momentary loss or even few minutes of loss of a power supply during changeovers from one source to another may be tolerated without any major adverse impact. Non-essential loads are such loads which do not fall under the category of both vital and essential loads. Non-essential loads usually receive power supply from only one source. Interruption of power for longer duration is generally tolerated without any major adverse impact in substation operations. Generally, all loads are classified as vital, essential. Total power consumption of both essential and non-essential loads are relatively less. Both types of loads may be fed from a switch gear or DB connected to two sources. Criteria for the definition of vital, essential, and non-essential, and the power supply distribution standard for these categories of loads could vary from industry to industry, are based on applications. About definition and power distribution arrangement hold good for most of the sectors. Continued in the next part.
