SEMICONDUCTORS SWITCH DIODE

The V-I characteristics of the silicon diode and germanium diode. The icon used to represent the diode is drawn in the upper left corner of the figure, together with the polarity markings used in describing the characteristics. The icon 'arrow' itself suggests an intrinsic polarity reflecting the inherent non-linearity of the diode characteristic.
As shown in the figure the diode characteristics have been divided into three ranges of operation for purposes of description. Diodes operate in the forward- and reverse-bias ranges. Forward bias is a range of 'easy' conduction, i.e., after a small threshold voltage level (>> 0.7 volts for silicon) is reached a small voltage change produces a large current change. In this case the diode is forward bias or in "ON" state. The 'breakdown' range on the left side of the figure happened when the reverse applied voltage exceeds the maximum limit that the diode can withstand. At this range the diode destroyed. On the other hand if the polarity of the voltage is reversed the current flows in the reverse direction and the diode operates in 'reverse' bias or in "OFF" state. The theoretical reverse bias current is very small.

Figure The diode V-I characteristics

In practice, while the diode conducts, a small voltage drop appears across its terminals. However, the voltage drop is about 0.7 V for silicon diodes and 0.3 V for germanium diodes, so it can be neglected in most electronic circuits because this voltage drop is small with respect to other circuit voltages. So, a perfect diode behaves like normally closed switch when it is forward bias (as soon as its anode voltage is slightly positive than cathode voltage) and open switch when it is in reverse biased (as soon as its cathode voltage is slightly positive than anode voltage). There are two important characteristics have to be taken into account in choosing diode. These two characteristics are:

• PEAK INVERSE VOLTAGE (PIV): Is the maximum voltage that a diode can withstand only so much voltage before it breaks down. So if the PIV is exceeded than the PIV rated for the diode, then the diode will conduct in both forward and reverse bias and the diode will be immediately destroyed.

• MAXIMUM AVERAGE CURRENT: Is the average current that the diode can carry.

It is convenient for simplicity in discussion and quite useful in making estimates of circuit behavior (rather good estimates if done with care and understanding) to linearize the diode characteristics. Instead of a very small reverse-bias current the idealized model approximates this current as zero. (The practical measure of the appropriateness of this approximation is whether the small reverse bias current causes negligible voltage drops in the circuit in which the diode is embedded. If so the value of the reverse-bias current really does not enter into calculations significantly and can be ignored.) Furthermore the zero current approximation is extended into forward-bias right up to the knee of the curve. Exactly what voltage to cite as the knee voltage is somewhat arguable, although usually the particular value used is not very important.

Comments

Breaker Schemes in Substations

Breaker Schemes in Substations — Types, Design, Advantages, Disadvantages, and Comparison Author: Engr. Aneel Kumar Figure 1: Infographic overview of breaker schemes commonly used in substations. Introduction The breaker scheme or busbar arrangement in a substation defines how incoming feeders, outgoing feeders, and power transformers are connected to the bus. The choice of scheme has a direct impact on system reliability, maintainability, safety, and cost . A simple bus scheme is economical but vulnerable to outages, while advanced schemes such as breaker-and-a-half or double-bus/double-breaker provide very high reliability but at much higher cost and design complexity. Engineers select breaker schemes considering fault tolerance, maintenance needs, space requirements, expansion possibilities, protection coordination, and capital investment . Below, we explain eac...

Advantages of Per Unit System in Power System Analysis | Electrical Engineering

Advantages of Per Unit System in Power System Analysis In electrical power engineering, the per unit (p.u.) system is one of the most widely used techniques for analyzing and modeling power systems. It is a method of expressing electrical quantities — such as voltage, current, power, and impedance — as fractions of chosen base values rather than their actual numerical magnitudes. This normalization technique provides a universal language for system calculations, minimizing errors, simplifying transformer modeling, and enabling consistency across multiple voltage levels. Because of these benefits, the per unit system is essential in fault analysis, load flow studies, transformer testing, and short-circuit calculations . ⚡ What is the Per Unit System? The per unit system is defined as: Q u a n t i t y ( p u ) = A c t u a l V a l u e B a s e V a l u e Quantity_{(pu)} = \dfrac{Actual \ Value}{Base \ Value} Q u an t i t y ( p u ) = B a se ...

PRINCIPLE OF OPERATION OF UNIFIED POWER FLOW CONTROLLER UPFC

UPFC consist of two back to back converters named VSC1 and VSC2, are operated from a DC link provided by a dc storage capacitor. These arrangements operate as an ideal ac to ac converter in which the real power can freely flow either in direction between the ac terminals of the two converts and each converter can independently generate or absorb reactive power as its own ac output terminal. Figure: Basic UPFC scheme One VSC is connected to in shunt to the transmission line via a shunt transformer and other one is connected in series through a series transformer. The DC terminal of two VSCs is coupled and this creates a path for active power exchange between the converters. VSC provide the main function of UPFC by injecting a voltage with controllable magnitude and phase angle in series with the line via an injection transformer. This injected voltage act as a synchronous ac voltage source. The transmission line current flows through this voltage source resulting in reactive an...

PRIMARY SECONDARY AND TERTIARY FREQUENCY CONTROL IN POWER SYSTEMS

Primary, Secondary and Tertiary Frequency Control in Power Systems Author: Engr. Aneel Kumar Keywords: frequency control, primary frequency control, automatic generation control (AGC), tertiary control, load-frequency control, grid stability. Frequency control keeps the power grid stable by balancing generation and load. When generation and demand drift apart, system frequency moves away from its nominal value (50 or 60 Hz). Grids rely on three hierarchical control layers — Primary , Secondary (AGC), and Tertiary — to arrest frequency deviation, restore the set-point and optimize generation dispatch. Related: Power System Stability — causes & mitigation Overview of primary, secondary and tertiary frequency control in power systems. ⚡ Primary Frequency Control (Droop Control) Primary control is a fast, local response implemented by generator governors (dro...

AC Transmission Line and Reactive Power Compensation: A Detailed Overview

Introduction The efficient operation of modern power systems depends significantly on the management of AC transmission lines and reactive power. Reactive power compensation is a vital technique for maintaining voltage stability, improving power transfer capability, and reducing system losses. This article explores the principles of AC transmission lines, the need for reactive power compensation, and its benefits in power systems. Keywords: Reactive Power Compensation Benefits, STATCOM vs SVC Efficiency, Power Transmission Stability Solutions, Voltage Stability in Long-Distance Grids, Dynamic Reactive Power Compensation. Fundamentals of AC Transmission Lines AC transmission lines are the backbone of modern power systems, connecting generation stations to distribution networks. They have distributed electrical parameters such as resistance ( R R R ), inductance ( L L ), capacitance ( C C ), and conductance ( G G ) along their length. These parameters influence ...

CLASSIFICATION OF HVDC LINKS

HVDC links may be broadly classified into the following categories: Monopolar Links Bipolar Links Homopolar Links The basic configuration of a monopolar link is shown in figure. It uses one conductor, usually of negative polarity. The return path is provided by ground or water. Cost considerations often lead to the use of such systems, particularly for cable transmission. This type of configuration may also be the first stage in the development of a bipolar system. Instead of ground return, a metallic return may be used in situation where the earth resistivity is too high or possible interference with underground/ under water metallic structures is objectionable. The conductor forming the metallic return is at low voltage.

MAIN AND PILOT EXCITER

Main Exciter The exciter (sometimes called the main exciter) is a synchronous generator that has its stator and rotor windings inverted. Its field winding is fixed in the stator, and the rotor carries the armature or AC . In addition the rotor carries the semiconductor bridge rectifier that converts the armature voltages to a two-wire DC voltage system. The AC voltages and currents in the armature are often alternating at a higher frequency than those in the main generator, e.g. 400 Hz. The higher frequency improves the speed of response of the exciter. The DC power circuit is coupled to the field of the main generator by the use of insulated conductors that pass coaxially inside the rotor of the exciter and the rotor of the main generator. This eliminates the use of slip rings, which were traditionally used before shaft mounted rectifiers were developed. A slight disadvantage of this technique is that the derivative feedback cannot be taken from the output of the exciter. Howeve...

ENGINEERING ARTICLES

Search This Blog

WRITE FOR US

Calling all authors!