Standardized Drawing Symbols
The American National Standards Institute or ANSI for short, in
cooperation with the Institute of Electrical & Electronics Engineers has
developed standardized drawing symbols and nomenclature to represent common
devices represented on one-lines, control schematics and other electrical
drawings. The existing Standard for North America (including the Canadian Standard
CSA Z99) is IEEE 315-1975 (Reaffirmed 1993)/ANSI Y32.2.
This version recognizes
that “Electrical diagrams are a factor in international trade: the use of one
common symbol language ensures a clear presentation and economical diagram
preparation for a variety of users.” Consequently, the Standards Coordinating
Committee has added various International Electrotechnical Commission (IEC) symbols that are in use
worldwide.
Item A4.1.1 of IEEE 315 defines a Single-Line or
(One-Line) Diagram as: “A diagram which shows,
by means of single lines and graphic symbols, the course of an electric circuit
or system of circuits and the component devices or parts used therein.”
Components
such as those representing circuit protective devices like fuses and circuit
breakers are indicated in their most basic form. Device representations can be
created by adding other componentsand nomenclature to the base component drawing.
Low-voltage <1000 V circuit breakers are represented by the first two of the
following symbols shown in Figure 1.1-3.
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| Figure 1.1-3. Circuit Breaker Symbols |
Medium-voltage
circuit breakers shown on a one-line typically incorporate the Basic Square Breaker
symbol with the ANSI Device Number 52 inside. Medium-voltage breakers may be either
fixed mount (square with device number inside) or drawout as shown in Figure
1.1-3 as well as the system one-line.
It is important to develop a naming convention so personnel working on or responding to an event on the power system
can readily identify the equipment experiencing any problems. This naming convention
is also useful for those doing preventative maintenance in documenting which specific
switchgear, breaker, transformer or protective relay they need to address.
Transformers are common components of a power system and are used on both
medium-voltage and low-voltage applications to step a voltage up or down to a
desired level. They are available in a variety of winding configurations as detailed
in the “Typical Components of a Power System” in this document.)
Because there
are many types and configurations of transformers available, it is necessary to
properly document the specific requirements on the One-Line. Primary unit substation
transformers are used to convert a medium voltage to another medium voltage.
Secondary
Unit Substation Transformers transform a Medium Voltage to a Low Voltage Level,
generally under 1000 Vac. They are available in Fluid-Filled and Dry-Type
styles.
Both types of unit substation transformers can be supplied with fans to
increase the transformer’s kVA ratings. Figure 1.1-4 from the
mediumvoltage half of the system one-line.
The
transformer’s kVA ratings are indicated at the KNAN, (Natural Air Cooled by
Convection Over 300C Fire Point Fluid Filled) and KNAF (Forced Air Cooled Over
300C Fire Point Fluid Filled) ratings.
If the transformer is described as “Delta”
Primary, “Wye” Secondary configuration in the text as well as further depicted
by the relationship of the “H1, H2 and H3” connections to the X1, X2, X3 and X0
symbols adjacent to it. Similarly, the verbiage in the text calls for surge and
lightning protection. Symbols for the arrester and the capacitor are shown connected
to the incoming terminations. Their actual ratings should be defined on the
drawing or in the specifications.
Both the transformer’s primary and secondary
amps are included as a reference for sizing the conductors. This is useful to
determine the quantity and size of the MV cables per NEC Article 310.60. While
medium-voltage conductors are available in 90C (MV90) or 105C (MV105) ratings,
the actual terminations in the transformer or switchgear cable compartments are
limited to 90C. When sizing the MV cables, the NEC derating factors must also
be applied depending on the type of raceway or duct bank that will be required.
Where higher transformer secondary currents are involved, a busway flange and
non-segregated busway can be supplied to connect it to the downstream MV
switchgear (as shown in Figure 1.1-4). Proper selection and application
of the busway requires that the rated short time and short circuit withstand
current values be specified.
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Figure 1.1-4. Transformer Information and Symbols
Short-circuit
values are critical in the design and specification of all electrical equipment
in a power system. The transformer’s Impedance, (often abbreviated as %Z) must
be shown on the One-Line in order to calculate the required ratings of
downstream equipment as indicated in Figure 1.1-5.
It is important to
remember that all transformers designed to ANSI standards have a plus and minus
7.5% tolerance for impedance. If a transformer requires an absolute minimum impedance
to ensure the secondary short-circuit level does not exceed a critical value,
it must be noted on the One-Line and in the accompanying project
specifications.
Consideration must also be given to the types of cable terminations
based on the available short-circuit ratings. Where the available short-circuit
exceeds 12.5 kA, medium-voltage molded rubber deadfront terminations are
generally not an option. In these cases, the type of terminations must be
specified. Stress Cone cable terminations are available in either Hot Shrink or
Cold Shrink configurations. Porcelain terminators or potheads are a more
expensive option, but often have higher short circuit ratings.
Current
transformers are used in both low- and medium-voltage applications as sensing
devices for protective relays and meters. They are available in “donut” style,
which encircle the conductor, as well as bar style, which is bolted in series
with the load conductors. Both styles work on the principal of electromagnetic
coupling; a current flowing through the conductor they surround induces a
proportional isolated low level signal (either 1 A or 5 A) that can be measured
by an electromechanical or electronic device.
Current transformers may be shown
in several formats as indicated in Figure 1.1-6.
The dots, X’s or boxes
are used to denote the instantaneous polarity orientation of the CT. The
polarity marks on the conductor generally face toward the source of the current
flow. The polarity mark on the CT winding represents the relationship of the
CT’s X1 secondary terminal to the H1 medium-voltage terminal on bar type CTs or
its input orientation for donut style CTs.
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Figure 1.1-5. Incoming Service Calculation
NOTE: CALCULATION DOES NOT INCLUDE DOWNSTREAM MOTOR CONTRIBUTION
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Figure 1.1-6. Current Transformer Symbols
In the case of Differential Protection circuits
such as the 87-T1 Transformer Differential or the 87-B1 Bus Differential, the
CTs are oriented in opposing directions as illustrated in Figure 1.1-7.
This permits the Differential Relays to measure the current going into a transformer
or bus bar and deduct the current flowing out of it. When more current is
flowing into the zone of protection than is proportionally flowing out, the
relay senses the “differential” and trips the circuit breakers at high speed to
protect against a fault anywhere in the zone. Note the “Y” symbol, as well as
the quantity “(3)” next to the CTs. This represents three CTs configured in a three-phase
grounded wye arrangement. While most of the CTs on the system one-line on Page
1.1-8 are shown this way, the CTs on the output side of the 2000 A breaker
S1A are not grounded. This is done to
indicate to the equipment manufacturer or installing contractor that the CT
inputs to the relay should not be grounded in more than one location. CTs
generally are wired to shorting terminal blocks as indicated by the “SB” in the
box shown in Figure 1.1-7. These are used to short out the secondary of
the CTs prior to equipment installation or when servicing them.
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Figure 1.1-7. Example of Differential Circuit with
To be Continued.......
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