Many of the circuit components may fail due to the over voltage, overheating, over current or unnecessary change in voltage & current. For the over-current situations, the components can be protected by using fuses at specific positions. Similarly, for overheating, heat sinks, and fans are used to push the excess heat away from the unwanted area. Although switching the load on and off is a common practice in the large-scale power distribution facilities however in some situations, the switching off/on may result in generating a transient condition leading to disturbance in the system’s stability.
This disturbance causes the exceed in the insulation levels of the equipment i.e. demeaning their insulation effect over time. This is where the snubber circuits come in to play. A snubber circuit is a recognized approach to lessen the effect of the switching transients to maintain the reliability of the power system’s gear and keep them functional for the longer run.
A snubber is a circuit that acts as a protection by limiting or stopping (snub) voltage spikes i.e. switching voltage amplitude and its rate of rise, therefore reducing the total power dissipation. It operates by clamping the voltage spikes but does not alter the ring frequency of the system.
When this circuit consisting of a resistor, capacitor, fuse, and a surge arrestor is made to work with a desired device, it absorbs the voltage rise and normalizes the disturbance in its waveform. Such circuits are best in removing transient voltages and in smoothing resonances and reflections even prior to their chance of building up.
The abnormalities or disturbance in the voltages also known as the Transient Recovery Voltages (TRV), have many reasons including:
Before jumping right into the snubber circuit’s design, it is essential to understand the waveform of the power circuits. These waveforms provide us with motivation and more importantly a pattern to understand the use of snubbers and information regarding its design.
Many different devices of the daily use have different circuits. Fortunately, all of them have a common class or common waveform associated with them. This classification allows us the ease to design a snubber circuit for a group of devises that holds the same characteristics rather than collecting data from every device.
Although a snubber circuit basically consists of a resistor and capacitor but designing it is one of the most complex tasks in circuit design given the requirement of a deep and firm knowledge of the circuit’s foundation. However, after reading this, you may have a slight idea of how to design a good snubber circuit of a desired device.
The design of a snubber circuit consists of a capacitor in series along with a resistor connected parallel with a thyristor. When the Voltage is applied initially the snubber circuit behaves as a short circuit therefore the voltage across the device in nil.
The voltage across the capacitor increases gradually but at a slow rate so that dv/dt (rate of voltage change) across the thyristor stays in the desired range. Prior to switching on the thyristor, the capacitor is charged to the fullest and as soon as the thyristor is turned on it discharges through the silicon-controlled diode (SCR).
This discharge current is then limited with the Resistor (R) connected in series with the capacitor (C) in order to keep current’s value and its rate of change (di/dt) in the safe boundary.
The Capacitors of the snubber circuit are usually subjected to high peaks of currents and rate of change of voltages (dv/dt). A conventional snubber circuit has to meet two major requirements.
For the resistor of the snubber circuit to be effective in this ringing frequency, the capacitor is used to minimize the dissipation at frequency switching. The perfect design is to select the impedance of the capacitor same as that of the resistor at ringing frequency.
The analysis of the snubber circuit study is done in 3 major steps:
The Concerned area and the equipment connected downstream of it is thoroughly studied, analyzed and then modelled in an EMTP (Electromagnetic Transient Programs). In this program the key parameters of TRV including its magnitude, rate of rise, frequency and energy are extracted from the proposed simulations. Many simulations are done for different scenarios including the worst-case scenario and the probability of equipment damage is estimated.
The very next step is the analysis of the frequency response of the equipment of the system downstream of the concerned area in order to find any possibility of resonance between the natural occurring frequency of the system and the transient. This step is repeated at every possible point of any possibility to avoid further occurrence of such an event.
The final step of this process is to recommend the suitable parameters for the design of a snubber circuit that reduces the system’s surges and switching transients to a desired level. The primary importance is given to synchronize the frequency response of the connected snubber circuit with the that of the system and deviation in its response is critically recorded and documented.
As discussed previously in the same blog that immediate tripping of the equipment results in the sudden change of impedance to a high value, this situation allows a small current to flow through the device which induces a high voltage across it. The faster rate of change in current (di/dt) has the very similar effect too. Such situations require an element like the snubber circuit to perform in the long run.
Snubber Circuits are responsible for the following functions:
The concerned protective devices or any circuit breaker and even the downstream system is crucially evaluated for “ The unwanted worst case scenario “ condition of transients and is made more inert towards tackling such situations through a counteractive recommended action plan by the Snubber Circuit analysis.
Summing up the entire discussion, these major points are to be expected from a snubber circuit analysis:
After going through this technical blog, you should have an idea regarding the importance of this snubber circuit study in power systems.
How to detect worst case transients through simulations?
Why to reduce switching transients to a minimum level?
How to reduce equipment damage and improve system reliability?
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