ANSI C37.121-1989 standard covers the requirements for three-phase unit substations for step-down operation in the range of 112.5 kVA through 10 000 kVA at primary voltages of 601 volts through 38 kilovolts.
The standard is intended for use as the basis for the coordination of equipment by assisting in the selection of
components. A variety of designs for unit substations are possible using various combinations of incoming sections, transformer sections, outgoing sections, and transition (throat) sections. Since the transformer section determines the kVA and voltage capabilities of a unit substation, the ratings of the various types of unit substations are described in terms of transformer capability.
Below are the unit substation requirement standard ratings as per ANSI C37.121-1989
The rating of each section of a unit substation shall comply with the applicable standards for its components and shall be equal to or greater than the rating of the unit substation. The kVA, high-voltage, and low-voltage ratings of the transformer section shall be the basis for those ratings of the unit substation. Other sections shall be coordinated with those ratings. The unit substation shall have the following ratings:
Rated Frequency
The rated frequency of a unit substation shall be the frequency of the circuit for which it is designed.
Rated kVA
The rated kVA of a unit substation shall be the rated kVA of the three-phase power transformer. The kVA rating of a double-ended unit substation shall be the total kVA of the two transformers.
Rated High Voltage (or Primary Voltage) and Rated Low Voltage (or Secondary Voltage)
In combination with the rated kVA of a unit substation, the rated voltages.
Rated Continuous Current
The rated continuous current of a unit substation shall be the maximum current in root-mean-square (rms) amperes, at rated frequency, which is intended to be carried continuously by the circuit components, including buses and connections, without raising temperatures above the limits specified in applicable standards.
The rated continuous current for high-voltage and low-voltage equipment of a unit substation shall be equal to, or greater than, the respective high-voltage and low-voltage full-load c urrents of the transformer section.
Rated Short-Circuit Current (Carrying)
The rated short-circuit current (carrying) of a unit substation is the rms short-circuit current that is intended to be carried for a specified period of time without causing electrical, thermal, or mechanical damage. The current shall be the rms value determined over the specified period of time.
The rated short-circuit current (carrying) rating of a unit substation shall be the rated short-circuit current (carrying) rating of the high-voltage interrupting device. If no high-voltage interrupting device is present, the rated short-circuit current (carrying) rating of the unit substation shall be the let-through short-circuit current of the transformer, in terms of primary amperes.
Rated Short-Circuit Current Withstand (Momentary)
The rated short-circuit current withstand of a unit substation is the maximum rms total current that is intended to be carried momentarily without causing electrical, thermal, or mechanical damage or permanent deformation. The current shall be the rms value, including the direct-current component, at the major peak of the maximum cycle, as determined from the envelope of the current wave during the test period.
The rated short-circuit current-withstand rating of a unit substation shall be the rated short-circuit current-withstand rating of the high-voltage interrupting device. If no high-voltage interrupting device is present, the rated short-circuit current-withstand rating of the unit substation shall be the let-through current of the transformer, in terms of primary amperes.
Rated Low-Frequency, One-Minute Withstand Voltage
The rated low-frequency, one-minute withstand voltage of a unit substation is the maximum alternating-current voltage that it is intended to withstand for one minute. The alternating-current voltage shall have a crest value equal to 1.41 times the rms value, shall be as close to a sine wave as practical, and shall have a frequency not less than the rated frequency.
The rated low-frequency, one-minute withstand voltage of the unit substation, on its high-voltage end, shall be the lesser rating of adjacent high-voltage sections. On its low voltage end, the rated low-frequency one-minute withstand voltage of the unit substation shall be the lesser rating of adjacent low-voltage sections.
Rated Impulse-Withstand Voltage (BIL)
The rated impulse-withstand voltage of a unit substation is the maximum impulse voltage that it can withstand. The impulse voltage shall be a 1.2 ´ 50 microsecond full wave with a wave shape as defined in ANSI/IEEE 4-1978.
The rated impulse-withstand voltage of the unit substation on its high-voltage end shall be the lesser rating of adjacent high-voltage sections. The rated impulse-withstand voltage of the unit substation on its low-voltage end shall be the lesser rating of adjacent low-voltage sections. If the unit substation is to operate in an environment that requires a greater impulse-withstand capability, surge arrestors should be used to ensure that the equipment is properly protected at the required impulse level. Impulse levels are not applicable to low-voltage equipment below 1000 V.
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