Unit 2 Thyristor - Copy PDF

1.6. Rating. 1. Voltage Rating (Vdrm or Vrrm): Maximum repetitive peak reverse voltage that the thyristor can withstand. 2. Current Rating (Idrm or Irrm): Maximum repetitive peak reverse current. 3. Gate Trigger Voltage (Vgt): Minimum voltage r...

1.6. Rating. 1. Voltage Rating (Vdrm or Vrrm): Maximum repetitive peak reverse voltage that the thyristor can withstand. 2. Current Rating (Idrm or Irrm): Maximum repetitive peak reverse current. 3. Gate Trigger Voltage (Vgt): Minimum voltage required to turn the thyristor on. 4. Gate Trigger Current (Igt): Minimum current required to turn the thyristor on. 5. Holding Current (Ih): Minimum current required to keep the thyristor in the on-state after triggering. 6. Surge Current (Itsm): Maximum allowable surge current for a short duration. These ratings are crucial for selecting and using thyristors in various applications, ensuring they operate within their specified limits for reliable performance. B] Thyristor turn ON/Triggering methods: - Thyristor Turn off (Commutation)Process The process of turning OFF SCR is defined as “Commutation”. In all commutation techniques, a reverse voltage is applied across the thyristor during the turn OFF process. By turning OFF a thyristor we bring it from forward conducting to the forward blocking mode. The condition to be satisfied in order to turn OFF an SCR are: 1. IA < IH ( Anode current must be less than holding current) 2. A reverse voltage is applied to SCR for sufficient time enabling it to recover its blocking state. There are two methods by which a thyristor can be turned OFF. 1. Natural Commutation 2. Forced Commutation Natural Commutation In natural commutation, the source of commutation voltage is the supply source itself. If the SCR is connected to an AC supply, at every end of the positive half cycle, the anode current naturally becomes zero (due to the alternating nature of the AC Supply). As the current in the circuit goes through the natural zero, a reverse voltage is applied immediately across the SCR (due to the negative half cycle). These conditions turn OFF the SCR. Forced Commutation  In Class F- AC line commutation, the source of commutation voltage is the supply source itself. If the SCR is connected to an AC supply, at every end of the positive half cycle, the anode current naturally becomes zero (due to the alternating nature of the AC Supply). As the current in the circuit goes through the natural zero, a reverse voltage is applied immediately across the SCR (due to the negative half cycle). These conditions turn OFF the SCR. It is also known as natural commutation. This commutation is possible with line commutated inverters, controlled rectifiers, cyclo converters and AC voltage regulators because the supply is the AC source in all these converters. Thyristor Protections Basics of Thyristor Protection We have said in the beginning that the device must operate within limits. But what happens when the device exceeds the specified rating? We all are aware of the fact that no device is ideal and on a practical basis, overvoltage or overcurrent conditions may get introduced during operation. Some of the conditions are: 1. When SCR is turned on then di/dt can be quite large that cannot be handled by the device. 2. A high value of dv/dt leads to cause the unexpected triggering of the device (i.e., without the presence of gate pulse). 3. Sometimes the existence of an unwanted signal at the gate cathode terminal also triggers the device. All the above-discussed conditions are not suitable for the reliable operation of thyristor thus it must be protected against such rare and dangerous conditions. The various thyristor protection schemes are as follows: di/dt Protection  dv/dt Protection Overvoltage Protection Overcurrent Protection Gate Protection Let us now understand each one separately. di/dt Protection dv/dt Protection Overvoltage Protection Protection of thyristors from overvoltage is one of the major concerns. When these devices are subjected to high voltages, then there are chances of maloperation such as unusual turning on of the device or permanent damage because of reverse breakdown. In the case of thyristors, the overvoltage can be of two types: 1. Internal Overvoltage: Sometimes a high internal voltage is generated when commutation of thyristors occurs. Even when anode current becomes zero, the stored charges reverse the anode current. Once the peak of this reverse recovery is attained the SCR starts blocking. After attaining the peak, the current further starts to decrease with possessing a large change in current. The presence of large inductance within the circuit will produce a large transient voltage which is quite higher than the breakover voltage of the device and this may damage the device permanently. 2. External Overvoltage: External overvoltage is associated with the interrupt in the flow of current through the inductive circuit. Also, the lines that provide the supply to the thyristor when subjected to lighting leads to overvoltage. During the turn-on of the thyristor, large overvoltage appears across the load because of which a high amount of fault current flows. Such a high voltage damages the thyristor by causing inverse breakdown. Thus, must be avoided or suppressed to prevent damage. Overcurrent Protection SCRs possess low value of thermal time constants. Thus, the faulty conditions lead to overcurrent due to which the temperature at the junction becomes higher than the specified value. Thereby destroying the device. Hence, overcurrent protection is quite necessary. So, circuit breakers or fuses are required to resists the flow of current more than the rated value. However, the type of system where the measures have to be applied is to be properly considered. For a weak supply network, the fault current is limited, by keeping the source impedance below the surge current rating of the SCR. This means conventional fuses and circuit breakers are employed to deal with overcurrent. It is to be noted here that the operation must occur in a coordinated way by ensuring that before the overcurrent could damage the device, it must be controlled and the branches that are faulty must be isolated. Gate Protection Protection of the gate circuit from overvoltage and overcurrent is quite an important aspect when we deal with thyristor protection. We have already discussed that when overvoltage exists then it leads to false triggering of the thyristor. While due to overcurrent, the junction temperature may rise by which the device gets damaged. Besides this, spurious signals appear at the gate terminal when there exist transients in the power circuit. Due to this, the SCR gets on by the unwanted gate triggering. Thus, in order to protect the gate terminal from such actions, shielded cables are used for gate protection. The presence of such cables lowers the chances of inducing emf thus, the unwanted triggering of the thyristors is minimized to large extent Over temperature protection. Over-temperature protection for thyristors is critical to prevent damage caused by excessive heat. Elevated temperatures can degrade the performance of thyristors, reduce their lifespan, or lead to catastrophic failure. Various methods are employed to protect thyristors from over- temperature conditions. Here's an explanation of the key aspects of over-temperature protection for thyristors: 1. Temperature Sensing Devices: Thermistors or Temperature Sensors: These devices are placed in close proximity to the thyristor or directly on its surface. They measure the temperature of the thyristor or the surrounding environment. 2. Temperature Monitoring Circuit: Analog or Digital Monitoring Circuit: The output from the temperature sensor is fed into a monitoring circuit. This circuit continuously monitors the temperature and compares it to a pre-set threshold or a temperature limit. 3. Threshold Setting: Trip Point: The monitoring circuit is configured with a trip point, which is the temperature level at which protective actions are initiated. This trip point is set based on the maximum safe operating temperature for the thyristor. 4. Protective Actions: Reduced Power or Current: When the monitored temperature exceeds the trip point, the system can respond by reducing the power supplied to the thyristor or limiting the current flowing through it. Shutdown Mechanism: In extreme cases, where the temperature continues to rise despite reduced power, the protective circuit may initiate a shutdown of the thyristor or the entire system. This is a preventive measure to avoid further temperature escalation. 5. Cooling Systems: Fans, Heat Sinks, or Liquid Cooling: Depending on the application, additional cooling mechanisms may be employed. Fans, heat sinks, or liquid cooling systems help dissipate excess heat and maintain the temperature within acceptable limits. 6. Thermal Modeling: Predictive Algorithms: Advanced systems may use thermal modeling based on the current operating conditions. Predictive algorithms estimate the temperature of the thyristor, allowing the system to take proactive measures before reaching critical temperatures. 7. Feedback Control: Closed-Loop Control: In systems with active cooling, a closed-loop control system may be implemented. Temperature readings are continuously monitored, and the cooling system is adjusted in real-time to maintain the temperature within safe limits. 8. Warning Alarms: Visual or Audible Alarms: In addition to taking protective actions, the system may provide a warning signal when the temperature is approaching the critical threshold. This alerts operators or maintenance personnel to potential issues. 9. Shutdown and Restart: Protective Shutdown and Restart: In cases where the temperature exceeds safe limits, a protective shutdown is initiated. After the thyristor has cooled down, the system can be safely restarted. Implementing an effective over-temperature protection system is essential to ensure the reliable and safe operation of thyristors, particularly in applications where temperature variations and transient loads are common. The specific protection measures chosen depend on factors such as the thyristor's characteristics, the application requirements, and the desired level of reliability. Firing circuit of Thyristors:- The firing circuit of thyristors is a crucial component in power electronics applications as it controls the triggering of the thyristor, initiating the conduction state. The firing circuit provides the necessary gate voltage or gate current to turn on the thyristor at the desired point in the AC cycle. Types of Firing Circuits: The most commonly used firing or firing circuits for thyristor or SCR are, Resistance Firing Circuit (R-Firing), Resistance-Capacitance Firing(RC-Firing), UJT-Firing Circuit. Resistance Firing Circuit (R-Firing), This firing circuit is the simplest method of controlling the firing angle of SCR. In this firing circuit, the firing angle can vary over a limited range of 0° to 90°. Instead of giving gate pulses to the thyristor, an ac supply is given to the gate terminal for firing. Working of Resistance Firing Circuit (R-Firing) : The working of the resistance firing circuit is as follows, During the positive half-cycle of the voltage source VS, thyristor, T is forward-biased, but it doesn’t conduct because of insufficient gate current. Hence, load voltage VL is zero. As voltage source VS increases, thyristor and diode both are forward-biased, and gate current IG flows in the circuit. When gate current IG reaches to value equal to IG(min), the thyristor is turned-ON and load voltage follows source voltage, and the voltage drop across the thyristor is equal to the on-state drop. During the negative half cycle of the supply voltage, the thyristor is reverse-biased, and hence it is turned OFF. Thus load voltage VL becomes zero and voltage across the thyristor VT will be equal to source voltage VS. The diode in the gate circuit prevents the reverse voltage of the thyristor during the negative half-cycle from exceeding peak reverse voltage. The limiting resistance RL placed between anode and gate of thyristor limits the gate current not to exceed peak gate current IG(max). From the waveforms above, the firing angle and the output voltage can be controlled by varying the variable resistance RV. If RV is large, then the current will be small, and hence firing angle (α) increases and vice versa. Advantages of Resistance Firing Circuit : The firing circuit is very easy and simple to operate. The firing angle can be varied from 0° to 90°. By using a capacitor and a diode, the limited firing angle issue is resolved. Disadvantages of Resistance Firing Circuit : Limited firing angle i.e., up to 90° only. The firing angle is totally dependent on the minimum gate current of thyristors. The value of minimum gate current changes between the thyristors. It is a temperature-dependent circuit. RC Half-Wave Firing Circuit : The above figure illustrates the RC half-wave firing circuit. The capacitor charges to the negative peak of the ac voltage in every negative half-cycle through the diode D2. During the positive half-cycle, it begins to charge through the resistance RV. When the voltage across the capacitor reaches the required positive value, the thyristor is fired and the capacitor voltage remains almost constant. The diode D1 prevents the breakdown of the gate-cathode junction during the negative half-cycle. For power frequencies, the value of RV C for zero output voltage is empirically given by, The maximum value of variable resistance RV is given by, If the value of RV is high, then the capacitor takes more time to charge. Hence the firing angle is more but the average output is low and vice-versa. In order to have more output, the value of RV should be less. UJT Firing Circuit : The above discussed R-firing and RC-firing circuits produce continuous gate pulses due to which there will be quite high power dissipation at the gate circuit of the thyristor. This power dissipation in the gate circuit can be reduced by using a UJT in the firing circuit. The UJT will work as a relaxation oscillator that produces sharp repetitive pulses with good rise time, and it also has good frequency stability under voltage fluctuations and temperature variations. In the above UJT firing circuit, the UJT starts conducting once the value of the voltage across the capacitor is equal to or more than the peak voltage VP value of the UJT. Now the capacitor starts discharging, and once its voltage decreases to the valley voltage VV of UJT, the UJT will be turned OFF. The capacitor again gets charged to supply voltage and the above process repeats. Thus whenever UJT conducts it produces pulses at the gate terminal of the thyristor as shown in the waveform. The capacitor charging time depends upon the value of variable resistance RV. Therefore, by varying the value of RV the firing angle can be varied.