Construction And Working:
The static network is so designed that it gives an output signal in the tripping direction whenever a threshold condition is reached.
The output signal, in turn, operates a tripping device, which may be electronic or electromagnetic.
The need for static relays arose because of the requirement of fast and reliable protective schemes for modern power systems, which are growing both in complexity and fault levels.
The scheme should be fast so as to preserve the dynamic stability of the system.
Transistors are most widely used in static relays. In fact, when we talk of static relays we generally mean transistor relays.
As transistors can be used both as an amplifying device and as a switching device, makes this component is suitable for achieving any functional characteristics.
The transistors circuits cannot only perform the essential functions of a relay such as comparison of inputs, summation, and integrating them; but they also provide the necessary flexibility to suit the various relay requirements.
The output of CTs of PTs or transducers is rectified in the rectifier.
The rectified output is fed to the measuring unit; the measuring unit comprises comparators, level detectors, filters, and logic circuits.
The output is initiated when the input reaches the threshold value.
The amplifier amplifies the output of the measuring unit.
The amplifier output is given to the output unit, which energizes the trip coil only when the relay operates.
Advantages of static relays:
1. Low power consumption :
Static relays provide less burden on CTs and PTs as compared to conventional relays. The consumption of 1 milliwatt is quite common in static overcurrent relay. Whereas, an equivalent electromechanical relay can have a consumption of about two watts. This reduced consumption has the following merits:
a) CTs and PTs of less VA rating.
b) The accuracy of CTs and PTs increased.
c) Air-gapped CTs can be used.
d) Saturation problems of CTs are avoided.
e) Overall reduction in the cost of CTs and PTs.
2. Resetting time and overshoot :
By using special circuits, the resetting times and overshoot time can be reduced thereby the selectivity can be improved.
3. No moving contacts :
As there are no moving contacts, associated problems of arcing, contact bouncing, erosion, replacement of contacts are avoided.
4. Gravity effect :
There is no effect of gravity on the operation of static relays. The relay can be installed at any location and at any position.
5. Single relays for several functions :
By combining various functional circuits, a single static relay can replace several conventional relays. A single static relay can provide over current, under voltage, single phasing, short circuit protection by incorporating respective functional blocks. This is not possible in electromechanical relays.
6. Compactness :
The space required for installing protective relay and control relays etc. is reduced. A single panel can incorporate a protection and control system for several functions.
7. Superior characteristics and accuracy:
The characteristics of static relays are accurate and superior. They can be altered within a certain range as per the requirement of protection. Static relays of superior speed (1/2 cycle, 1 cycle) are available.
8. Static relays can think :
Complex protection schemes employ logic circuits. Logic means the process of reasoning, induction or deduction. Suppose several conditions are imposed on a protective system such that for certain conditions, the relay should operate, and for some other condition, the relay should remain stable, in such cases, logic gates can be operated.
9. Programmable operation :
The characteristic of programmable relays can be altered by changing the program. Program means sequential instructions that direct the microprocessor in the relay to perform specific functions.
10. Online computation and functions:
The characteristics and functions of programmable relays can be altered on the basis of the online computation of various variables. For E.g. which backup breaker to operate with minimum outage can be decided prevailing network configuration and online relay time data.
11. Repeated operations possible :
Static relays can be designed for repeated operations, as there are no moving parts in the measuring circuit.
12. Effect of vibration and shocks :
Most of the components in the static relays, including the auxiliary relays in the output stage, are relatively unaffected by vibrations and shocks. The risk of unwanted tripping is less with static relays as compared to electromagnetic relays.
13. Self-supervision and monitoring of the relay :
Complex static relays have a facility of continuous and comprehensive self-monitoring by special hardware called 'watchdog and test software. Any fault, which occurs within the relay, is detected at once. Thus, periodic testing of the relay can be minimized.
14 Simplified testing and servicing:
Static relays are provided with integrated features for self-monitoring, easy testing, and servicing. Defective modules can be replaced quickly.
Limitations of static relays:
1. Auxiliary voltage requirement:
Static relays required d.c. supply for their operations. But this disadvantage is now not of any importance as auxiliary voltage can be obtained from the station battery supply.
2. Electrostatic discharges (ESD) :
Semiconductor components are sensitive to electrostatic discharges. Even a small discharge can damage the components, which would normally withstand 100V. Precautions are necessary for the manufacturing static relays to avoid ESD-caused component failure.
3. Voltage transients:
The static relays are sensitive to voltage spikes or voltage transients. Such voltage transients are caused by the operation of the breaker and isolator. Such voltage spikes of small duration can damage semiconductor components and can also cause mal-operation of relays. Special measures are taken in static relays to overcome this difficulty. This includes the use of filter circuits in relays.
4. Temperature dependence of static relays:
The characteristics of semiconductors are influenced by ambient temperature. Accurate measurement of relay should not be affected by temperature (-10°C to +50°C).
5. Cost :
For simple, single-function relays the price of static relays is higher than the equivalent electromagnetic types.