For series connected SCRs, static equalization is obtained with the help of
Resistors only
When multiple Silicon Controlled Rectifiers (SCRs) are connected in series, the primary reason is to achieve a higher voltage blocking capability than a single SCR can provide. However, due to inherent variations in manufacturing processes, individual SCRs even from the same batch will have slightly different characteristics. These differences can lead to unequal voltage distribution across the SCRs, especially when they are in the OFF (blocking) state. This unequal distribution can cause one or more SCRs to experience an overvoltage, potentially leading to their breakdown and failure. To prevent this, equalization circuits are used, which are broadly categorized into static and dynamic equalization.
SCR Static Equalization Explained
Static equalization refers to the method used to ensure uniform voltage sharing among series-connected SCRs when they are in the OFF (blocking) state. In the OFF state, a small leakage current flows through each SCR. Due to variations in their reverse blocking characteristics, the leakage currents are not identical. If no equalization circuit is used, the SCR with the lowest leakage current would block the largest voltage, potentially exceeding its breakdown voltage.
Resistors for Static Equalization
To achieve static equalization, resistors are connected in parallel with each SCR. These are often referred to as "equalizing resistors" or "sharing resistors".
| Component | Role in Static Equalization | Explanation |
|---|---|---|
| Resistors only | Primary component for static equalization |
When a resistor (\(R\)) is connected in parallel with each SCR, the total leakage current through each parallel combination is primarily determined by the resistor rather than the SCR's highly variable leakage current. Since the parallel resistors are typically chosen to have a much lower resistance than the SCR's OFF-state resistance, they provide a dominant current path. The total series current flowing through the string of parallel combinations will distribute itself such that the voltage across each parallel combination is nearly equal (\(V = I_R \cdot R\), where \(I_R\) is the current through the resistor). This ensures that the voltage shared by each SCR in the OFF state is almost uniform, preventing any single SCR from being overstressed. The resistor current is significantly larger than the SCR leakage current, effectively swamping out the differences in leakage currents. |
Distinguishing Static and Dynamic Equalization
It is important to differentiate static equalization from dynamic equalization:
- Dynamic equalization: This deals with voltage transients that occur during the turn-on and turn-off processes of the SCRs. Due to variations in turn-on and turn-off times, some SCRs might turn on or off faster than others, leading to temporary overvoltages across the slower devices. Capacitors are used in parallel with each SCR to provide dynamic equalization. These capacitors (often part of a snubber circuit) help to absorb or limit the rate of rise of voltage (\(\text{dv}/\text{dt}\)) and distribute the voltage during switching transients.
- Diodes: Diodes are not typically used for the primary purpose of voltage equalization in series SCR strings. They might be part of snubber circuits in conjunction with resistors and capacitors to provide a discharge path for the capacitor or to prevent reverse voltage, but they do not directly contribute to balancing static OFF-state voltages.
Therefore, when the question specifically asks for "static equalization," the correct component is resistors only.
The final answer is Resistors only.
Important Questions from Thyristor Protection
Which of the following is a safety device connected in series to the circuit?
What is the term for the minimum anode current below which a thyristor will turn OFF and stay OFF, even if a gate pulse was previously applied?
The Snubber circuit is used to thyristor circuits for -
Which of the following is the function of an R-C snubber circuit connected in parallel to an SCR?
Match List-I with List-II and select the correct answer using the given lists:
List – I
(SCR rating)
List – II
(Protective element)
A. di/dt limit
1. Snubber
B. dV/dt limit
2. Heat sink
C. i2t limit
3. Series reactor
D. Junction temperature limit
4. Avoid runway speed on no load
