History of Relay
The static relay is the next generation relay
after electromechanical type.The Solid Static relays was first introduced in
1960’s. The term ‘static’ implies that the relay has no moving
mechanical parts in it.
Compared to the Electromechanical
Relay, the Solid Static relay has longer life-span, decreased noise
when operates and faster respond speed.
However, it is not as robust as the
Electromechanical Relay.
Static relays were manufactured as
semiconductor devices which incorporate transistors, ICs, capacitors, small
microprocessors etc.
The static relays have been designed
to replace almost all the functions which were being achieved earlier by electromechanical relays.
Measuring principles
The working principle of the Solid Static relays is
similar to that of the Electromechanical Relay which means the Solid Static
relays can perform tasks that the Electromechanical Relay can perform.
The Solid Static relays use analogue
electronic devices instead of magnetic coils and mechanical components to
create the relay characteristics. The measurement is carried out by static
circuits consisting of comparators, level detectors, filter etc while in a
conventional electromagnetic relay it is done by comparing operating torque (or
force) with restraining torque (or force). The relaying quantity such as
voltage/current is rectified and measured.
When the quantity
under measurement attains certain well-defined value, the output device is
triggered and thereby the circuit breaker trip circuit is energized.
In a solid state relay, the incoming
voltage and current waveforms are monitored by analog circuits, not
recorded or digitized. The analog values are compared to settings made by the
user via potentiometers in the relay, and in some case, taps on transformers.
In some solid state relays, a simple
microprocessor does some of the relay logic, but the logic is fixed and simple.
For instance, in some time over
current solid state relays, the incoming AC current is first converted into a
small signal AC value, and then the AC is fed into a rectifier and filter that
converts the AC to a DC value proportionate to the AC waveform. An op-amp and
comparator is used to create a DC that rises when a trip point is reached. Then
a relatively simple microprocessor does a slow speed A/D conversion of the DC
signal, integrates the results to create the time-over current curve response,
and trips when the integration rises above a set point.
Though this relay has a
microprocessor, it lacks the attributes of a digital/numeric relay, and hence
the term “microprocessor relay” is not a clear term.
Function of Relay
Early versions used discrete devices
such as transistors and diodes in conjunction with resistors, capacitors,
inductors, etc., but advances in electronics enabled the use of linear and
digital integrated circuits in later versions for signal processing and
implementation of logic functions.
While basic circuits may be common
to a number of relays, the packaging was still essentially restricted to a
single protection function per case, while complex functions required several
cases of hardware suitably interconnected.
Basler
Electric BE1-27 Solid State Protective Relay, Over/Under Voltage
User programming was restricted to
the basic functions of adjustment of relay characteristic curves.
Therefore it can be viewed in simple
terms as an analogue electronic replacement for electromechanical relays, with
some additional flexibility in settings and some saving in space requirements.
In some cases, relay burden is
reduced, making for reduced CT/VT output requirements. In a static relay there
is no armature or other moving element and response is developed by electronic,
magnetic or other components without mechanical motion.
A relay using
combination of both static and electromagnetic units is also called
a static relay provided that static units accomplish the
response. Additional electromechanical relay units may be employed in
output stage as auxiliary relays. A protective system is formed by static
relays and electromechanical auxiliary relays.
The
performance of static relay is better than electromagnetic
relays as they are fast acting and accuracy of measurement is better than
electromagnetic relay.
The constraint in static relay is
limited function/features.
In the last decade, some
microprocessors were introduced in this relay to achieve the functions like:
- Fuse failure features
- Self check feature
- Dead Pole detection and
- Carrier aided protection features
Operation of Relay
The essential components of static
relays are shown in figure below. The output of CT and PT are not suitable for
static components so they are brought down to suitable level by auxiliary CT
and PT. Then auxiliary CT output is given to rectifier.
Rectifier rectifies the relaying
quantity i.e., the output from a CT or PT or a Transducer.
Solid
state relay - Operation
The rectified output is supplied to
a measuring unit comprising of comparators, level detectors, filters, logic
circuits.
The output is actuated when the
dynamic input (i.e., the relaying quantity) attains the threshold value. This
output of the measuring unit is amplified by amplifier and fed to the output
unit device, which is usually an electromagnetic one.
The output unit energizes the trip
coil only when relay operates.
Advantages of
Solid State Relay
- Static Relay burden is less than Electromagnetic type
of relays. Hence error is less.
- Low Weight
- Required Less Space which results in panel space
saving.
- Arc less switching
- No acoustical noise.
- Multi-function integration.
- Fast response.
- Long life (High Reliability): more than 109 operations
- High Range of Setting compared to electromechanical
Relay
- More Accurate compared to electromechanical Relay
- Low Electromagnetic Interference.
- Less power consumption.
- Shock and vibration resistant
- No contact bounce
- Microprocessor compatible.
- Isolation of Voltage
No moving parts: There are no moving parts to wear out or arcing contacts to
deteriorate that are often the primary cause of failure with an Electro Mechanical
Relay.
No mechanical contact bounce or
arcing: A solid-state relay doesn’t depend
on mechanical forces or moving contacts for its operation but performs
electronically. Thus, timing is very accurate even for currents as low as the
pickup value. There is no mechanical contact bounce or arcing, and reset times
are extremely short.
Low input signal levels: Ideal for Telecommunication or microprocessor control
industries. Solid state relays are fast becoming the better choice in many
applications, especially throughout the telecommunication and microprocessor
control industries.
Cost Issues: In the past, there has been a rather large gap between the
price of an electromechanical relay and the price of a solid state relay. With
continual advancement in manufacturing technology, this gap has been reduced
dramatically making the advantages of solid state technology accessible to a
growing number of design engineers.
Limitations of static
relays
- Auxiliary voltage requirement
for Relay Operation.
- Static relays are sensitive to
voltage transients which are caused by operation of breaker and isolator
in the primary circuit of CTs and PTs.
- Serious over voltage is also
caused by breaking of control circuit, relay contacts etc. Such voltage
spikes of small duration can damage the semiconductor components and also
cause mal operation of relays.
- Temperature dependence of
static relays: The characteristics of semiconductor devices are affected
by ambient temperature.
- Highly sophisticated isolation
and filter circuits are required to be built into the relay design to take
care of electromagnetic interference and transient switching disturbances
in the power system.
- Highly reliable power supply
circuits are required.
- Effect of environmental
conditions like humidity, high ambient temperature, dust accumulation on
PCB leading to tracking.
- The component failure.
- Non availability of fault data.
- Characteristic variations with
passage of time.
References
- Handbook of Switchgear –Bhel
- Digital/Numerical Relays -T.S.M. Rao
