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FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS
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INTRODUCTION
Protective relays are the "tools" of the protection engineer. As in any craft, an intimate
knowledge of the characteristics and capabilities of the available tools is essential to their
most effective use. Therefore, we shall spend some time learning about these tools without
too much regard to their eventual use.
GENERAL CONSIDERATIONS
All the relays that we shall consider operate in response to one or more electrical quantities
either to close or to open contacts. We shall not bother with the details of actual
mechanical construction except where it may be necessary for a clear understanding of the
operation. One of the things that tend to dismay the novice is the great variation in
appearance and types of relays, but actually there are surprisingly few fundamental
differences. Our attention will be directed to the response of the few basic types to the
electrical quantities that actuate them.
OPERATING PRINCIPLES
There are really only two fundamentally different operating principles: (1) electro-
magnetic attraction, and (2) electromagnetic induction. Electromagnetic attraction relays
operate by virtue of a plunger being drawn into a solenoid, or an armature being attracted
to the poles of an electromagnet. Such relays may be actuated by d-c or by a-c quantities.
Electromagnetic-induction relays use the principle of the induction motor whereby torque
is developed by induction in a rotor; this operating principle applies only to relays actuated
by alternating current, and in dealing with those relays we shall call them simply
"induction-type" relays.
DEFINITIONS OF OPERATION
Mechanical movement of the operating mechanism is imparted to a contact structure to
close or to open contacts. When we say that a relay "operates," we mean that it either closes
or opens its contacts-whichever is the required action under the circumstances. Most
relays have a "control spring," or are restrained by gravity, so that they assume a given
position when completely de-energized; a contact that is closed under this condition is
called a "closed" contact, and one that is open is called and "open" contact. This is
standardized nomenclature,
OPERATION INDICATORS
Generally, a protective relay is provided with an indicator that shows when the relay has
operated to trip a circuit breaker. Such "operation indicators" or "targets" are distinctively
colored elements that are actuated either mechanically by movement of the relay's
operating mechanism, or electrically by the flow of contact current, and come into view
when the relay operates. They are arranged to be reset manually after their indication has
been noted, so as to be ready for the next operation. One type of indicator is shown in
Fig. 2. Electrically operated targets are generally preferred because they give definite
assurance that there was a current flow in the contact circuit. Mechanically operated
targets may be used when the closing of a relay contact always completes the trip circuit
where tripping is not dependent on the closing of some other series contact.
SEAL-IN AND HOLDING COILS, AND SEAL-IN RELAYS
In order to protect the contacts against damage resulting from a possible inadvertent
attempt to interrupt the flow of the circuit tripcoil current, some relays are provided with
a holding mechanism comprising a small coil in series with the contacts; this coil is on a
small electromagnet that acts on a small armature on the moving contact assembly to hold
the contacts tightly closed once they have established the flow of trip-coil current. This coil
is called a "seal-in" or "holding" coil. Figure 2 shows such a structure. Other relays use a
small auxiliary relay whose contacts by-pass the protective-relay contacts and seal the circuit
closed while tripping current flows. This seal-in relay may also display the target. In either
case, the circuit is arranged so that, once the trip-coil current starts to flow, it can be
interrupted only by a circuit-breaker auxiliary switch that is connected in series with the
trip-coil circuit and that opens when the breaker opens. This auxiliary switch is defined as
an " a " contact. The circuits of both alternatives are shown in Fig. 3.
TIME DELAY AND ITS DEFINITIONS
Some relays have adjustable time delay, and others are "instantaneous" or "high speed."
The term "instantaneous" means "having no intentional time delay" and is applied to
relays that operate in a minimum time of approximately 0.1 second. The term "high
speed" connotes operation in less than approximately 0.1 second and usually in 0.05
second or less. The operating time of high-speed relays is usually expressed in cycles based
on the power-system frequency; for example, "one cycle" would be 1/60 second in a 60-cycle
system. Originally, only the term "instantaneous" was used, but, as relay speed was
increased, the term "high speed" was felt to be necessary in order to differentiate such
relays from the earlier, slower types. This book will use the term "instantaneous" for
general reference to either instantaneous or high-speed relays, reserving the term "high-
speed" for use only when the terminology is significant.
RATIO OF RESET TO PICKUP
One characteristic that affects the application of some of these relays is the relatively large
difference between their pickup and reset values. As such a relay picks up, it shortens its air
gap, which permits a smaller magnitude of coil current to keep the relay picked up than
was required to pick it up. This effect is less pronounced in a-c than in d-c relays. By special
design, the reset can be made as high as 90% to 95% of pickup for a-c relays, and 60% to
90% of pickup for d-c relays. Where the pickup is adjusted by adjusting the initial air gap,
a higher pickup calibration will have a lower ratio of reset to pickup. For overcurrent
applications where such relays are often used, the relay trips a circuit breaker which
reduces the current to zero, and hence the reset value is of no consequence. However, if a
low-reset relay is used in conjuction with other relays in such a way that a breaker is not
always tripped when the low-reset relay operates, the application should be carefully
examined. When the reset value is a low percentage of the pickup value, there is the
possibility that an abnormal condition might cause the relay to pick up (or to reset), but
that a return to normal conditions might not return the relay to its normal operating
position, and undesired operation might result.
EFFECT OF TRANSIENTS
Because these relays operate so quickly and with almost equal current facility on either
alternating current or direct current, they are affected by transients, and particularly by
d-c offset in a-c waves. This tendency must be taken into consideration when the proper
adjustment for any application is being determined. Even though the steady-state value of
an offset wave is less than the relay's pickup value, the relay may pick up during such a
transient, depending on the amount of offset, its time constant, and the operating speed
of the relay. This tendency is called "overreach" for reasons that will be given later.
TIME CHARACTERISTICS
This type of relay is inherently fast and is used generally where time delay is not required.
Time delay can be obtained, as previously stated, by delaying mechanisms such as bellows,
dash pots, or escapements. Very short time delays are obtainable in d-c relays by encircling
the magnetic circuit with a low-resistance ring, or "slug" as it is sometimes called. This ring
delays changes in flux, and it can be positioned either to have more effect on air increase
if time-delay pickup is desired, or to have more effect on air-gap-flux decrease if time-delay
reset is required.
SINGLE-QUANTITY INDUCTION RELAYS
A single-quantity relay is actuated from a single current or voltage source. Any of the
induction-relay actuating structures may be used. The shaded-pole structure is used only
for single-quantity relays. When any of the other structures is used, its two actuating circuits
are connected in series or in parallel; and the required phase angle between the two fluxes
is obtained by arranging the two circuits to have different X/R (reactance-to-resistance)
ratios by the use of auxiliary resistance and/or capacitance in combination with one of the
circuits.
THE POLARIZING QUANTITY OF A DIRECTIONAL RELAY
The quantity that produces one of the fluxes is called the "polarizing" quantity. It is the
reference against which the phase angle of the other quantity is compared. Consequently,
the phase angle of the polarizing quantity must remain more or less fixed when the other
quantity suffers wide changes in phase angle. The choice of a suitable polarizing quantity
will be discussed later, since it does not affect our present considerations