THERMAL PLANT AUXILIARY SYSTEM

SELECTION OF AUXILIARY SYSTEM VOLTAGES

The most common plant auxiliary system voltages are 13,800 V, 6900 V, 4160 V, 2400 V, and 480 V. The highest voltage is determined by the largest motor. If motors of 4000 hp or larger are required, one should consider using 13,800 V. If the largest motor required is less than 4000 hp, then 4160 V should be satisfactory.

AUXILIARY SYSTEM LOADS

Auxiliary load consists of motors and transformers. Transformers supply lower level buses which supply smaller motors and transformers which supply lower voltage buses. Generation plants built before 1950 may have an auxiliary generator that is connected to the main generator shaft. The auxiliary generator will supply plant loads when the plant is up and running.

AUXILIARY SYSTEM POWER SOURCES

The power sources for a generating plant consist of one or more off-site sources and one or more onsite sources. The on-site sources are the generator and, in some cases, a black start diesel generator or a gas turbine generator which may be used as a peaker.

AUXILIARY SYSTEM VOLTAGE REGULATION REQUIREMENTS

Most plants will not require voltage regulation. A load flow study will indicate if voltage regulation is required. Transformers with tap changers, static VAR compensators, or induction regulators may be used to keep plant bus voltages within acceptable limits. Switched capacitor banks and overexcited synchronous motors may also be used to regulate bus voltage.

AUTO TRANSFORMER TAP CHANGING

Figure shows diagrammatically auto-transformer tap changing. Here, a mid-tapped auto-transformer or reactor is used. One of the lines is connected to its mid-tapping. One end, say a of this transformer is connected to a series of switches across the odd tappings and the other end b is connected to switches across even tappings. A short-circuiting switch S is connected across the auto-transformer and remains in the closed position under normal operation. In the normal operation, there is no inductive voltage drop across the auto-transformer. Referring to Figure, it is clear that with switch 5 closed, minimum secondary turns are in the circuit and hence the output voltage will be the lowest. On the other hand, the output voltage will be maximum when switch 1 is closed. Suppose now it is desired to alter the tapping point from position 5 to position 4 in order to raise the output voltage. For this purpose, short-circuiting switch S is opened, switch 4 is closed, then switch 5 is op...

COMPONENTS OF HIGH VOLTAGE DC TRANSMISSION SYSTEM

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

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...

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...

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 ...

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...

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