Explain Series and Parallel operation of SCR or Tyristor

Explain series and parallel operation of SCR's with suitable circuit diagrams

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Discuss series and parallel operation of thyristor

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Explain series and parallel operation of SCR in detail

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What is Series Parallel operation of SCR

SCR are connected in series for h.v demand and in parallel for fulfilling high current demand. Sting efficiency can be defined as measure of the degree of utilization on SCRs in a string.

String efficiency < 1.

Derating factor (DRF) 1 – string efficiency.

If DRF more then no. of SCRs will more, so string is more reliable.

Series and parallel operation of SCR or Tyristor

• SCR ratings have improved considerably since its introduction in 1957. Presently, SCRS with voltage and current ratings of 10 kV and 3 kA are available. 
• SCRs are connected in series in order to meet the h.v. demand and in parallel for fulfilling the high current demand. For series or parallel connected SCRs, it should be ensured that each SCR rating is fully utilized and the system operation is satisfactory. 
• String efficiency is a term that is used for measuring the degree of utilization of SCRs in a String efficiency of SCRs connected in series/parallel is defined string efficiency = [Actual voltage/current rating of the whole string ] /[(Individual voltage/current rating of one SCR) (Number of SCRS in the string)]
• This ratio is less than one. For obtaining highest possible string efficiency, the SCRS connected in series/parallel string must have identical V-I characteristics.
• The string efficiency can never be equal to one. However, unequal voltage/current sharing by the SCRS in a string can be minimised to a great extent by using external equalizing circuits.
• The string efficiency is less than unity. In case one extra unit is added to the series/parallel string, the voltage/current shared by each device would become lower than its normal rating. The use of this extra unit will certainly improve the reliability of the string though at an increased cost.
• A measure of the reliability of string is given by a factor called derating factor DRF defined as under : DRF = 1-string efficiency
• If the value of recommended DRF is more, the number of devices used in series/parallel string will be more. This will certainly improve the reliability of the string for a given rating of the string.
• It is also possible to combine both series and parallel connections, depending on the application's requirements. In general, careful consideration should be given to the circuit design and the ratings of the thyristors when using them in series or parallel connections to ensure safe and reliable operation.

Series Operation 

• The series connection of thyristors is also known as a thyristor string or a thyristor stack. The number of thyristors in the string depends on the desired voltage and power rating. The basic principle of operation of a thyristor string is that the voltage across each thyristor is evenly distributed, and each thyristor carries a proportionate share of the total current.
• The thyristor string is connected in series with the load, and the control circuit provides a gate signal to trigger the thyristors. When the thyristors are triggered, they conduct current, and the load receives power. The thyristors remain in the conducting state until the current falls below the holding current or the supply voltage is removed.
• The series connection increases the overall voltage rating of the circuit. However, it is important to note that if one thyristor fails, the entire circuit will fail, since the current cannot flow through the faulty thyristor.
• two SCRs can support a maximum voltage of V₁ + V₂ and not the rated blocking voltage  2V₁ The string efficiency for two series connected SCRS.
• Therefore,(V₁ + V₂)/(2V₁) = 1/2 * (1 + V₂/V₁)
• This shows that even though SCRs have identical ratings, voltage shared by each is not the same and string efficiency is therefore less than one.

Static-voltage equalization for series-connected string or static equalizing circuits

• A uniform voltage distribution in steady state can be achieved by connecting a suitable resistance across each SCR such that each parallel combination has the same resistance. This will require different value of resistance for each SCR which is a difficult proposition. A more practical way of obtaining a reasonably uniform voltage distribution during steady state working of series-connected SCR is to connect the same value of shunt resistance R across each SCR as shown fig. This shunt resistance R is called the static equalizing circuit.
• Static-voltage equalization is a method that balances the voltage of each component in the series-connected string. This can be achieved by using a circuit that allows a small amount of current to flow through each component until they are all at the same voltage.
• There are different types of static-voltage equalization circuits available, but they all typically work by connecting a resistor in parallel with each component in the string. The resistors allow a small current to flow through each component, which helps to balance the voltage across the string.
• Static-voltage equalization is a passive method, meaning that it doesn't require any active components or control circuitry. It is a relatively simple and inexpensive way to ensure that all components in a series-connected string are operating at the same voltage and can help to improve the performance and lifespan of the system.
• It is important to note that static-voltage equalization is not a perfect solution, as it can lead to power dissipation in the resistors and may not be effective in all cases. In some situations, dynamic voltage equalization may be a more effective technique, which uses active control circuitry to dynamically adjust the voltage of each element in real-time.
• When two SCR have connected in series in forwarding blocking mode, then voltage distribution across each SCR is unequal.
• In order to make voltage distribution uniform, the static equalizing circuit is used in which a resistance is connected in parallel with each SCR.
• The value of resistance 'R' is given by: R = (nVbm-Vs)/(n-1)∆Ib 
• where, n = No. of SCR connected in series;Vbm = Maximum blocking voltage;∆lb = Blocking state current

Unequal voltage distribution for two series connected SCRS during turn-on and turn-off

• When two SCRs are connected in series, their voltage distribution during turn-on and turn-off can be unequal. This is because SCRs have a nonlinear voltage-current characteristic, and small differences in the device parameters or external circuit conditions can result in unequal sharing of the voltage across the two SCRs.
• During turn-on, the SCR with the lower threshold voltage will start conducting first, and will carry a larger share of the current. This will result in a voltage drop across that SCR, which can cause the other SCR to experience a higher voltage than expected. This voltage imbalance can lead to an overvoltage condition that can damage the SCR.
• During turn-off, the SCR with the higher holding current will turn off last, and will experience a higher reverse voltage than the other SCR. This can lead to reverse breakdown and damage to the SCR. In addition, any voltage imbalances during turn-on can carry over to turn-off and exacerbate the problem.
• Unbalanced voltage distribution can lead to several problems, including reduced reliability, decreased efficiency, and increased heat dissipation. To avoid these issues, it is important to design the circuit carefully and ensure that the SCRs are matched as closely as possible in terms of their electrical characteristics. Additionally, proper triggering and commutation circuits should be used to ensure that the SCRs turn on and off at the same time and share the current evenly.

Dynamic and static equalizing circuits for series-connected SCRs

• In series-connected SCRs (Silicon Controlled Rectifiers), dynamic and static equalizing circuits are used to balance the voltage across each SCR in the circuit. This is important because if the voltage across any one SCR becomes too high, it may turn on and conduct before the other SCRs in the series, leading to uneven current sharing and potential damage to the SCR.
• A dynamic equalizing circuit is a circuit that is designed to balance the voltage across each SCR dynamically, as the load current changes. This type of circuit typically uses a resistor and a capacitor in parallel with each SCR, which forms an RC circuit. The resistor provides a path for current to flow through the SCR, while the capacitor stores charge and releases it when the voltage across the SCR exceeds a certain threshold. This helps to balance the voltage across each SCR in real-time, ensuring that they turn on and off at the same time.
• A static equalizing circuit is a circuit that is designed to balance the voltage across each SCR statically, regardless of the load current. This type of circuit typically uses a series of resistors and diodes in parallel with each SCR. The resistors provide a path for current to flow through the SCR, while the diodes ensure that the voltage across each SCR is the same. This helps to balance the voltage across each SCR even when the load current is constant, ensuring that they turn on and off at the same time.
• Both dynamic and static equalizing circuits are important for ensuring the reliable operation of series-connected SCRs. The choice of circuit will depend on the specific requirements of the application and the characteristics of the load.

Dynamic equalization circuit

• The factor which is responsible for unequal distribution of voltage in the reverse biased condition is the junction capacitance of the SCR. This junction capacitance is also known as the self-capacitance of the SCR.
• Since the capacitance is different for different SCR, so the voltage across each SCR also differs.
• Therefore, the value of the discharging current may be very high. Therefore, to limit the discharge current, a limiting resistance Rc is connected in series with the shunt capacitor.
• A combination of Rc and capacitor (C) is called a dynamic equalizing circuit.

Flow of reverse recovery current if SCR1 recovers first and Variation of reverse recovery characteristics for two SCRs 

When two SCRs are connected in series, the overall operation of the circuit can be affected by the reverse recovery characteristics of each SCR.

Assuming that the two SCRs are identical, the flow of reverse recovery current when SCR1 recovers first can be described as follows:

• During forward conduction, both SCRs are in the ON state, and current flows through both devices.
• When the current through SCR1 drops below its holding current, it turns OFF. However, current continues to flow through SCR2.
• As the current through SCR2 approaches zero, it too turns OFF.
• During the reverse recovery period of SCR1, a reverse recovery current flows through SCR2 in the reverse direction.
• Once the reverse recovery current through SCR2 has decayed, the circuit returns to its initial state.

The variation of reverse recovery characteristics for two SCRs in series can have an impact on the overall performance of the circuit. If the reverse recovery time of one SCR is significantly longer than the other, it can cause uneven sharing of the current between the two devices. This can lead to thermal stress on the SCR with the longer reverse recovery time, potentially causing it to fail prematurely.

Parallel Operation 

Parallel operation of thyristors involves connecting multiple thyristors in parallel to share the current and voltage across the devices. This configuration is known as a thyristor bank or a parallel thyristor circuit.

When thyristors are connected in parallel, it is essential to ensure that they share the load equally. This is typically achieved by using a balancing circuit that equalizes the voltage across each thyristor in the bank. In addition, it is important to ensure that the turn-on and turn-off characteristics of each thyristor are matched, as any mismatch can lead to uneven current sharing and potential device failure.

The parallel connection increases the overall current rating of the circuit. However, it is important to ensure that each thyristor shares an equal amount of current. If one thyristor carries more current than the others, it may fail due to overheating.

When current required by the load is more than the rated current of a single thyristor, SCRS are connected in parallel in a string. For equal sharing of currents, V-I characteristics of SCRs during forward conduction must be identical as far as possible.

For parallel-connected SCRs, voltage VT across them must be equal. Figure (b) show that for a same voltage drop VT, SCR1 shares a rated current I₁ whereas SCR2 carries current I₂ much less than rated current I₁ . The total current carried by the unit is I₁ + I₂ and not the rated current 2I₁ as required. Therefore string efficiency is given by:

(I₁ + I₂)/2I₁ = 0.5(1 + I₂/I₁)

• When SCRs are connected in parallel, all units must operate at the same temperature as far as possible. This is done by having a common heat sink.
• Unequal current distribution in a parallel unit is also caused by the inductive carrying effect of conductors. current This unequal current distribution can be avoided by mounting the SCRs symmetrically on the heat sink.
• When three or more SCRs are connected in parallel, reactors can be arranged accordingly so as to minimise the current unbalance.
• Parallel operation of thyristors can provide several benefits, including increased power handling capacity, improved reliability, and redundancy.

VI Characteristics of Parallel Connected SCRs

• The VI characteristics must be identical as far as possible for the SCRs to be connected in parallel.For proper operation of these parallel connected SCRs, they should get turned on at the same moment.
• We can understand this with the help of following discussion. Consider n parallel connected SCRs.For satisfactory operation of these SCRs, they should get turned on at the same time. Consider that SCR1 has large turn-on time whereas the remaining (n-1) SCRs have low turn-on time.
• Under this assumption, (n-1) SCRs will turn on first but one SCR1 with longer turn-on time is to remain off.The voltage drop across (n-1) SCRs falls to a low value and SCR1 is now subjected to this low voltage.
• If the voltage across SCR1 goes below finger voltage, then this SCR will not turn on.So the remaining (n-1) SCRs will have to share the entire load current. Consequently these SCRs may be overloaded and damaged because of heating caused by overcurrent.

Conclusion 

The series-parallel operation of SCR (Silicon Controlled Rectifier) is a technique that allows for the control of high power loads in industrial applications. This technique involves connecting multiple SCRs in series or parallel to achieve the desired voltage and current ratings.

When SCR's are connected in series, their voltage ratings add up, and the resulting voltage rating is the sum of the individual SCR's voltage rating. However, the current rating remains the same. This technique is commonly used in high-voltage applications.

On the other hand, when SCRs are connected in parallel, their current ratings add up, and the resulting current rating is the sum of the individual SCR's current rating. However, the voltage rating remains the same. This technique is commonly used in high-current applications.

Series-parallel operation of SCR offers several advantages, including increased reliability, improved efficiency, and the ability to control high-power loads. However, it requires careful consideration of factors such as voltage and current ratings, gate triggering, and thermal management.

The series-parallel operation of SCR is an important technique that enables the control of high power loads in industrial applications, and it can offer several advantages when implemented correctly.Click here to download the PDF version of this entire article.

FAQ

Q1:Discuss the conditions which must be satisfied for turning-on an SCR with a gate signal.

Conditions which must be satisfied for turning-on SCR with a gate signal are as under:

1. An SCR must be forward-biased. It means that anode must be positive with respect to cathode.
2. Gate pulse width must be more than the turn-on time of an SCR. This will ensure that anode current exceeds the latching current before gate signal is removed.
3. Anode to cathode voltage must be more than finger voltage. A finger voltage is that voltage below which an SCR cannot be turned on with a gate signal. 
4. Magnitude of gate current must be more than the minimum gate current required to turn-on a thyristor, otherwise the thyristor turn-on will not be reliable. 
5. Magnitude of gate current must be less than the maximum gate current allowed,otherwise gate circuit may be damaged. 
6. The gate triggering must synchronize with the ac supply.

Q2:What is the series and parallel operation of thyristor?

When single SCR is not sufficient to provide required voltage or current rating, then more SCR's, are connected in series (to meet higher voltage demand) or in parallel (to meet higher current demand).

Q3:Why is series and parallel operation of thyristor needed?

SCR are connected in series for h.v demand and in parallel for fulfilling high current demand. Sting efficiency can be defined as measure of the degree of utilization on SCRs in a string. SCR are connected in series for h.v demand and in parallel for fulfilling high current demand.

Q4:What do you mean by series operation of thyristors?

What is series and parallel operation?

In a series circuit, the same amount of current flows through all the components placed in it. On the other hand, in parallel circuits, the components are placed in parallel with each other due to which the circuit splits the current flow.

Q5:Why SCR is connected in parallel?

When the load current exceeds the rating of a single SCR, SCR's are connected in parallel to increase their common current capability. If SCR's are not perfectly matched, this results in an unequal sharing of current between them.

Q6:What is the necessity of connecting SCRs in parallel?

The necessity of connecting SCRs (Silicon Controlled Rectifiers) in parallel is to increase the current-carrying capacity of the circuit beyond what a single SCR can handle.

Q7:What's the use of scr's connecting in series ?

SCRs (Silicon Controlled Rectifiers) are commonly connected in series to increase the overall voltage rating of the circuit. When connected in series, each SCR can handle a portion of the total voltage, allowing for higher voltage applications. Additionally, connecting SCR's in series can improve system reliability by providing redundancy, as a failure in one SCR will not necessarily lead to a system failure.

Q8:What is the purpose of using diodes in parallel with SCRs?

The purpose of using diodes in parallel with SCRs is to provide a path for reverse current during AC power applications and protect the SCR and circuit from damage caused by voltage spikes.

Q9:What are the members of the thyristor family?

• Thyristors include: silicon controlled rectifier (SCR), TRIAC, gate turn off switch (GTO), silicon controlled switch (SCS), AC diode (DIAC), unijunction transistor (UJT), programmable unijunction transistor (PUT). Only the SCR is examined in this section; though the GTO is mentioned.
• OTHER MEMBERS OF THE THYRISTOR FAMILY:The term thyristor includes all four-layer p-n-p-n devices used for the control of power in ac and dc systems. The silicon controlled rectifier is the most popular member of thyristor family. There are several other members of thyristor family like PUT, SUS, SCS, triac, diac etc. All these devices, except triac, are low power devices. Several new devices have been developed and added to the thyristor family. These recently developed thyristor devices are asymmetric thyristor (ASCR), reverse conducting thyristor (RCT), static induction thyristor (SITH), gate-assisted turn-off thyristor and gate turn-off (GTO) thyristor. The latest addition to the thyristor family is the MOS-controlled thyristor (MCT).

Q10:Proved that derating factor(DRF) = 1- String efficiency 

The derating factor (DRF) of a solar PV system is a measure of how much the system's power output is reduced compared to its theoretical maximum output, under certain operating conditions. The string efficiency of a solar PV system refers to the ratio of the actual power output of the system to the theoretical maximum output of the system, under the same operating conditions.

To prove that DRF = 1 - string efficiency, we can start with the definition of the derating factor:

DRF = (Pmax - Pmp) / Pmax

where Pmax is the theoretical maximum power output of the system and Pmp is the maximum power output of the system under the operating conditions.

Next, we can use the definition of the string efficiency:

string efficiency = Pmp / Pmax

We can rearrange this equation to solve for Pmp:

Pmp = string efficiency x Pmax

We can substitute this expression for Pmp into the equation for the derating factor:

DRF = (Pmax - (string efficiency x Pmax)) / Pmax

Simplifying this expression, we get:

DRF = 1 - string efficiency

Therefore, we have prove that DRF = 1 - string efficiency.

Q11:What is String Efficiency of SCR?

String Efficiency of SCR is the degree of capacity utilization of individual SCRS in a string of series / parallel connected SCRs. String efficiency is always less than 1.

String Efficiency = V/V₁N = I/I₁N

Where,

• V = Actual voltage of whole string
•  I = Actual current of whole string
• V₁ = Voltage rating of one SCR
• I₁ = Current rating of one SCR 
• N = Total number of SCRs in a string

Q12:What is Derating Factor(DRF) of SCR?

Derating Factor (DFR) of SCR: Derating Factor is the amount by which the string efficiency deviates from unity or 100%.

DFR = [1 - String Efficiency] 

DFR = [100-% String Efficiency]

• Derating Factor gives an idea of unused capacity available in a sting of SCRS.
• That's why it is a measure of reliability of string.
• The lesser the value of sting efficiency, the lesser will be voltage / current sharing by the individual SCRs.
• It means, more unused capacity will be available with the string and hence more DFR.
• But lesser string efficiency increases the cost of string.
• So that a compromise is made in between economy and reliability by properly designing a value of string efficiency.

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#Series configuration #parallel configuration #the thyristors #SCR #Silicon Controlled Rectifier #DRF #String Efficiency # Thyristor Family #Derating Factor #Static and Dynamic