Technical and basic information for the study of prevention (trends, polarities and vehicles)
1- Introduction
In addition to the basic information of electrical power
systems, relays practitioners must have the ability to deal with and understand
the directions, polarities and basic components that contain important
information of current and directional voltage in fault conditions.
2- Vector Phasors
A vector is a complex number
used to represent variable quantities in the form of a sine curve. In relay
systems, vectors are used to design relays and perform a complete analysis of
their performance during failures. A
faasor diagram must be accompanied by the circle it represents, unless the
circle is quite clear. The vector diagram shows both the value and relative
angles of all currents and potential differences, while the circuit only shows
the locations, direction, polarity of currents and potential differences, and
this difference should be obvious because most of the time confusion or
confusion occurs when the direction drawing and the circuit drawing are
combined or the circuit drawing is omitted altogether.
2-1 Symbols for drawing the circuit of the current:
The
current or overflow is represented in two ways the first is the representation
of the current with a uniform arrow on the circuit, Figure (1). The second method is represented by lower
binary symbols that express the terminal from which the current begins to move
to the terminal it ends (IAB example), Figure (2).
Figure (1)
Figure (2)
2.2 Alternating voltage codes
The polarity of the voltage is represented in two
ways, one is to add a sign (+) at the end of the arrow representing the
potential difference, which expresses the highest point of voltage, and the
second method is to use lower symbols, for example: (Vab) expresses the voltage
difference between two voltages (a&b) where a is higher than b, and the symbols that are used
are as follows:
1.
The symbol (V) is used
to express voltage, and will be used to express the voltage difference. (E) is
used to represent the voltage generated. The symbol U is also used in some
countries.
2.
The use of (+) at the
end of the arrow on the circuit expresses the positive party with respect to
the other end in half of one cycle.
3.
In the method of the
lower symbols of the current or the potential difference, it expresses the real
or assumed direction of the decrease in voltage when the voltage is at half the
positive cycle. An example of the voltage difference between the two ends is a,
b, and the voltage
is positive if the voltage of the other end (a) is higher than the voltage of
the other end (b) in the positive alternating wave, and during the negative
half of the wave, the situation is reversed.
2.3 Vector Symbols:
The most common type of axes consists of the axis of real
quantities (X) and the axis of imaginary quantities (Y) as shown in Figure 3.
These two axes are fixed in plane, while vectors move because they represent
sine curves (the positive direction of rotation is counterclockwise) and thus
the position of the vector is different at any moment in time. The length of
the vector is proportional to its maximum value. Projecting this vector onto
the real and imaginary axes, the real and imaginary compounds are represented
at this moment. The vector can be represented by the effective value of R.M.S ., which is often used. Point A can be
represented in Figure (3) as follows:
1.
Phasor
form
2.
Polar form
3.
Complex form
4.
Exponential
form
Figure (3): Fixed axes and directional
labeling
2.3.1 The Law of Multiplication
The absolute value of directional
multiplication is the product of the absolute values of the vectors and the
angle is the sum of the angles for each.
2.3.2 The Law of Partition
The absolute value of the division is the product of
the absolute value of the vectors and the angle is the product of the
subtraction of the angles for each.
2-4 Vector diagram
The
vectors are plotted from the origin point as shown in Figure (4) while in
Figure (5) the directional drawing of the voltage has been shifted to show the
voltage of the successive points (ABCD) of the connection respectively. In the
United States the vectors of the three-phase system are called (a,b,c) or (a,b,c) or (1,2,3), and
in some other countries they are called (r, s,t), the symbol (n) of the neutral
point, and the symbol (g) of the earth, are generally inconsistent, as they
each have a different meaning, and the voltage difference between them is
generally not equal to zero. The ground resistance (Rg) is shown in the ground
potential difference of the relays.
Figure (4): Directional
drawing of the open type of basic elements (resistor - reactor - capacitor)
connected in sequence
Figure (5) Drawing the closed
vector of the circle in Figure (4)
1.
Rotation of Faces and
Rotation of Vector
Phase rotation(phase sequence) vs. phasor rotation
Rotation
of faces or sequence of faces is the order in which successive vectors are
given when they reach a maximum positive value, and the rotation of a vector by
a universal definition is in a counterclockwise direction. All standard relays
have rotations of the three axes (a, b, c) and the conductivity can change from
one rotation to another and the conductivity changes between (b, c) .
3- Polarity in the circles of relays:
3-1 Transformer Polarity
Figures
(6 and 7) show the polarity of both the current and voltage transformers and
the sum or subtraction. The polarity is denoted by X or
by a black box as in Figures (6 and 7). The passage of current outside the
terminal marked polarity at the secondary coil of the transformer is observed
and it must be in the same direction as the current entry at the side with the
polarity symbol at the primary coil.
The
voltage difference from the pole to the non-polarity mark in the first coil of
the transformer must be in the same direction as the potential difference from
the point that bears the polarity to the non-polarity mark at the secondary
coil and the expression (it is necessary that they be in the same direction)
allows a small error in the phase angle.
Figure (6) Polarity and Circuit Drawing of Transformers
.jpg)
0 Comments