VOLTAGE STABILITY AND UNDERVOLTAGE LOAD SHEDDING

Voltage stability is defined by the System Dynamic Performance Subcommittee of the IEEE Power System Engineering Committee as being the ability of a system to maintain voltage such that when load admittance is increased, load power will increase, and so that both power and voltage are controllable. Also, voltage collapse is defined as being the process by which voltage instability leads to a very low voltage profile in a significant part of the system. It is accepted that this instability is caused by the load characteristics, as opposed to the angular instability that is caused by the rotor dynamics of generators.

The risk of voltage instability increases as the transmission system becomes more heavily loaded. The typical scenario of these instabilities starts with a high system loading, followed by a relay action due to either a fault, a line overload, or hitting an excitation limit.

Voltage instability can be alleviated by a combination of the following remedial measures: adding reactive compensation near load centers, strengthening the transmission lines, varying the operating conditions such as voltage profile and generation dispatch, coordinating relays and controls, and load shedding. Most utilities rely on planning and operation studies to guard against voltage instability. Many utilities utilize localized voltage measurements in order to achieve load shedding as a measure against incipient voltage instability. The efficiency of the load shedding depends on the selected voltage thresholds, locations of pilot points in which the voltages are monitored, locations and sizes of the blocks of load to be shed, as well as the operating conditions that may activate the shedding. The wide variety of conditions that may lead to voltage instability suggests that the most accurate decisions should imply the adaptive relay settings, but such applications are still in the stage of early development.

Comments

Auto Transformer Tap Changing

Auto Transformer Tap Changing: Working Principle, Switching Sequence and Applications Auto transformer tap changing is a practical method of adjusting transformer output voltage without unnecessarily interrupting the electrical supply. In power transmission and distribution networks, the load does not remain constant throughout the day. As load current changes, voltage drops across transformers, cables, feeders, and transmission lines also change. A tap-changing arrangement compensates for these variations by modifying the effective number of turns in the transformer winding. A reactor-type on-load tap changer uses a center-tapped reactor, selector switches, and a bypass or short-circuiting switch to transfer the load from one tap to the next. The reactor limits the circulating current during the transition, allowing the tap position to change while the transformer continues supplying the connected load. This article explains the auto transformer tap-changing working principle , switch...

PLUG REVERSING OF CAPACITOR START MOTORS

The capacitor start motors suffer from the disadvantage that they are not easily reversible due to the centrifugal switch connected in the auxiliary winding. The motor cannot be instantaneously reversed by simple control as the auxiliary winding remains disconnected till the motor comes near to zero speed. However, by proper design of the control circuit the motor can be made instantly reversible. This is accomplished by using an electromagnetic relay along with a special two contact centrifugal switch as shown in Figure. The circuit shown in Figure is for a small hoist using a capacitor start motor. The upper and lower limits of travel are controlled by two limit switches viz. LSU and LSD. When UP-push button is pressed, contactor U gets energized. Its contacts U 1 and U 2 energies the main winding. Closing of its contact U 3 causes relay R to get energized through the centrifugal switch contacts A—B. After the energization of relay R auxiliary winding gets ene...

COMPONENTS OF HIGH VOLTAGE DC TRANSMISSION SYSTEM

Figure: A schematic of a bipolar HVDC system identifying main components

BENEFITS OF UTILIZING FACTS DEVICES

The advantages of using FACTS devices in electrical transmission systems are described below. 1. MORE UTILIZATION OF EXISTING TRANSMISSION SYSTEM In all the countries, the power demand is increasing day by day to transfer the electrical power and controlling the load flow of the transmission system is very necessary this can be achieved by more load centers which can change frequently. Addition of new transmission line is very costly to take the increased load on the system; in that case FACTS devices are much economical to meet the increased load on the same transmission lines. 2. MORE INCREASED TRANSIENT AND DYNAMIC STABILITY OF THE SYSTEM The Long transmission lines are inter-connected with grids to absorb the changing the loading of the transmission line and it is also seen that there should be no line fault creates in the line / transmission system. By doing this the power flow is reduced and transmission line can be trip. By the use of FACTS devices high power t...

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 ...

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...

Reversing the Direction of a Universal Motor: Methods and Circuit Diagram

How to Reverse the Direction of Rotation of a Universal Motor Introduction A universal motor is a high-speed electric motor that can operate on either an alternating-current or direct-current supply when designed for the applicable voltage and frequency. It is widely used in portable electric drills, grinders, vacuum cleaners, mixers, sewing machines, small machine tools, and other equipment requiring high starting torque and compact construction. The direction of rotation of a universal motor can be changed by reversing the direction of current through either the armature winding or the field winding relative to the other. The current must be reversed in only one of these windings. If the connections of both windings are reversed simultaneously, the relative direction of the field flux and armature current remains unchanged, and the motor continues rotating in the same direction. Before studying the reversing methods, readers may review the Universal Series Motors Electric Motors and...

ENGINEERING ARTICLES

Search This Blog