Mad with Power: An Introduction to Power Processing

The core concept of power processing can be explained together with several significant applications of power electronics. Power electronics is the technology of processing and controlling the flow of electric power by modifying and supplying the voltage and current in a form that is best suitable to the customer at the load end.

Introduction to Power Processing

Power electronics is the technology of processing and controlling the flow of electric power by modifying and supplying the voltage and current in a form that is best suitable to the customer at the load end. A characteristic block diagram is shown in Fig.1.

Power electronics is more concerned with the electronics principles in the situations that are related at a power level than at the signal level.  This area of electronics originated from the silicon-controlled Rectifier Evolution in 1956 by Bell Laboratory. SCR technology led the breakthrough in power-handling capability of electronic devices by making minor changes in the structure of signal electronics devices. This led to high-performance semiconductor devices with better voltage and current-handling capability which are currently used in different applications such as in blowers, fans, AC, battery chargers, refrigerators, dryers, HVDC transmissions, pf correction with static compensators, static circuit breakers, relays, UPS, etc.Power electronics amalgamates three major areas of electrical engineering: power, electronics, and control, as shown in Fig. 1.

Fig.1 Representation of Major Components in Power Electronics System

The power source to the  PE system may be a DC or AC supply, depending upon the application or condition of that location. The output of the PE system may be a variable AC or DC voltage or may be a variable voltage and frequency. Thus, the power needs to be processed inbetween the source and load. This is done with the help of a converter.

A failure in the converter affects the users, and the loss in the converter is the loss in the overall system. A converter must be reliable and must be made of components having a negligible loss such as semiconductor switches.

The feedback component measures the parameters of the load and compares it with a command signal. The difference of the two serves as a basis for the turn-on and turn-off commands of the power electronics device.

Thus, power electronics processors can be majorly categorized according to their operation:

● AC to DC Converter (Rectifier)

● DC to AC Converter (Inverter)

● DC to DC Converter (Chopper)

● AC to AC Converter (Cycloconverter)

They can also be categorized based on the turn-on and turn-off characteristics as well as the gate signal requirement and the degree of controllability.

(a) Uncontrolled rectifying devices (such as diodes): Their on and off state are dependent on the power supply.

(b) Semi-controlled devices (such as thyristor): They can be turned on by a gate signal but its turn-off condition is dependent on the power circuit.

(c) Fully-controlled switches (such as BJT, MOSFET, GTO and IGBT): These devices are turned on and turned off by the application of control signals.

Applications of Power Electronics

Power electronics have revolutionized innumberable control techniques in both motor and non-motor applications, like the following:

(1) Induction motor speed control. The speed of induction motor can be varied by AC voltage control, voltage/frequency control, rotor resistance variation for control, slip power recovery system control, etc. The cheapest method of controlling the speed of the induction motor is to vary the applied voltage by using an AC regulator in each line, consisting of the thyristors. This is a voltage control method. V/f control is implemented with the combination of rectifier/converter with an inverter. This inverter may be a voltage-source inverter (VSI), a current-source inverter, (CSI) or an impedance-source inverter (ZSI). Rotor resistance variation is a traditional method employed for the speed control of induction motor, but this causes sparking and other maintenance problems.

With the advancement of power electronics, a static variation in the rotor resistance is implemented with the help of a chopper in the rotor. Although a great improvement is achieved with the help of a static variation of the rotor resistance, the better method for speed control is the slip power recovery scheme, which is of course a power electronics application. As in the rotor resistance controlled drive, the efficiency is low as the slip power is dissipated in the external resistance. This is more drastic at low speed or high-slip value. Closed loop systems named as speed loop and current loop are usually used for better control and current protection purposes.

(2) Induction motor braking. There are three types of braking that employ the power electronics circuitry to stop the running induction motor. These are called the plugging, dynamic and regenerative braking. Aside from this, phase sequence can also be reversed with the help of semiconductor switches.

(3) DC motor speed control. The prime method for the speed control of DC motors is armature control and field control. In armature control, armature voltage is varied while keeping the field voltage constant. In the case of the field control, the field voltage is varied while keeping the armature voltage constant. Speed control below the base speed is obtained with the help of armature control method while the speed control above the base speed is done with the help of the field control method. Multi-quadrant operations including forward motoring and generation, and reverse motoring and generation can be obtained with the help of different schemes of converters. Speed can be varied by varying the firing angle of the converter.  Closed loop control drive can also be used for a more precise and detailed control.

(4) DC motor braking. Similar to the induction motor, DC machine can also be stopped with the help of plugging, dynamic, and regenerative braking.

(5) Power Supply Regulator. There are two types of DC power supply regulators: conventional regulated power supply and the regulated power supply. These are usually used in several applications such as in laboratories, electronics circuits of medical equipment, etc. In the case of the conventional regulated power supply, the AC supply available is stepped down to a lower level with the help of a transformer and then rectified it into DC by a rectifier. It then also needs to be stabilized by the help of a zener diode.

The main disadvantages of a conventional voltage regulator are its bulky transformer and bulky inductor. These disadvantages can be fixed with the help of a more advanced version of the regulator called the switch-mode power supply (SMPS). Here, AC supply is directly fed into the rectifier without a transformer. The DC output of the rectifier is the input to the high-frequency inverter. The output of the inverter is then stepped down to a low-voltage AC with the help of a transformer. Now, this high-frequency low-voltage AC is rectified by a rectifier and filtered by the use of L and C components. As the frequency in the transformer increases, based on the basic e.m.f. equation, the flux required decreases. Thus, the transformer size is reduced.

(6) Welding. Welding is the process of fusing metal pieces together by passing short-lived and high-value currents through the area of contact. The power required for welding is

P=I2R.P=I2R.

 Current is increased with the help of a step-down transformer. This welding power is controlled by an AC chopper.  Integral number of cycles can be applied by firing the thyristors for a particular time interval and subsequently turning it off for a particular time.

(7) Heaters. There are two types of heating that are done with the help of the power electronics equipments. They are resistance heating and induction heating.

Resistance heating is done with the help of a metallic conductor, non-metallic conductor e.g. carbon tubes, liquids and the power electronics equipment i.e. AC chopper. Heating resistor is made up of alloys such as nickel, chromium and etc. Its principle is similar to welding i.e. integral cycle control of AC chopper. This type of heating is used to heat insulated furnaces for production or laboratory purposes.

Induction heating is used in several applications such as melting, forging, brazing, soldering, annealing, forging, surface hardening and etc. It is also known as the eddy current heating as the heat generated in it is due to the eddy current. When an alternating voltage is applied to the job coil, an alternating current will flow. This current induces an alternating magnetic field. The metallic job in this field cuts the alternating magnetic flux and an emf is induced in it. This induced emf causes the circulation of the eddy current and heat is produced.  If the supply frequency is increased with the help of semiconductor devices such as cycloconverter, the eddy current will also increase and more heat is produced.

The induction heating is also currently used for electronic heaters employing a rectifier, filter and an inverter in sequence. When an alternating field is applied to the non-conducting material, heat is generated. This heat generated is due to the dielectric loss and this process is called as the dielectric heating. The amount of heat depends on the dielectric strength of the job. This dielectric heating method is used in plastic, wood, textile, rubber, food and chemical industries and etc.

(8) Static on load tap changing gear. To reduce the voltage variation obtained from the load variation, a tap changing gear transformer is required. Changing the tap setting actually means changing the turn ratio of the transformer in order to vary the voltage. It requires the making and braking of the electrical contacts, which is usually done manually. However, there may be loads where load disconnection is not acceptable, where an on-load tap changer is required for such purpose. The making and braking of electrical contacts may cause sparking and pitting or erosion of electrical contacts. Also, it may contaminate the oil of the transformer. Moreover, on-load tap changers have high-cost, high-maintenance requirements, slow responses, and voltage fluctuation problems.  These problems can be overcome with the aid of a voltage controller that has two anti-parallel controlled switches. It reduces the transient voltage dips and continuous control of voltage is obtained.

(9) Static VAR compensators. These are power electronics devices which supply and compensate the lagging reactive power consumed by  the inductive loads. They help in maintaining the constant supply voltage with power factor improvement. There are various power electronics equipments such as thyristor-switched capacitors (TSC), thyristor-controlled reactors (TCR), STATCOM, UPSC, UPFC and etc.

(10) Uninterrupted power supply (UPS): A simple UPS consists of a rectifier, an inverter, and a battery. The battery provides the supply to the inverter whenever there is no supply power from the conventional source. However, if any UPS component fails, it stops working. It is actually not very reliable. A more reliable system can be obtained by using more components in parallel. In the case of parallel configuration, if there is any fault in any of the inverters, that part can be isolated with the help of solid-state interrupter. UPS must also minimize voltage transients on the bus and supply the power to the critical load during the long power outage.

(11) HVDC transmission. Generation of high-voltage DC is limited due to the conditions/limitations of DC generators. Thus, it is converted into DC at the sending end and then transmitted. At the receiving end, it is converted back into an AC with the help of an inverter. Monopolar and bipolar schemes are employed for the HVDC. Since the series reactance in the case of HVDC system is nil, there is no stability problem. Hence, higher operating voltage of HVDC is possible.

(12) Static switches. These are switches that have no moving parts. Thyristors are used as static switches for high-power applications while power transistors are used for low-power applications. Static switches have very high switching speed and long operational life. Since they have no moving components, maintenance is almost not required.

Static switches have two types depending upon the supply i.e. AC static switch and DC static switch. These switches act as relays for AC and DC circuits. If the input is AC, then AC static switches are used; and when the input is DC, the DC static switches are used. The switching speed of AC switches depends upon the supply frequency while the switching speed of DC switches depends upon the commutation circuitry used.

(13) Static Circuit Breakers. These are semiconductor devices which provide a rapid and reliable interruption of a continuous current. There are two types of static circuit breakers, the AC and the DC circuit breaker.