Instrument Transformer and Power Management

Instrument Transformer and Power Management (P1) Course

Contents :

1) Introduction

2) Safety basic

3) Power in AC Circuit

4) Power Measurement in 1-ph Circuits

5) Power Measurement in 3-ph Circuits

6) Current Transformers

7)Voltage Transformers

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Chapter (1) ; Introduction

1.1 Introduction to electrical power system

In order to understand the protection of electrical power system, it is necessary to know how it is monitored and how it is controlled. This course provides an introduction to the power system and the measurement of power flow. Some of the topics that will be discussed in this course are :

Elements of power system.

- Operating diagram

- Safety concerns while working in a substation.

- The use of meters to measure electrical quantities.

- Single phase power

- Current transformer

- Voltage transformer

- Three phase power

- Phasor diagram

The electrical power system :

The electrical power system is used for the generation and distribution of electrical energy. It consists of devices (normally called system components) that are all interconnected to provide a reliable source of electrical energy.

Power is created by devices called generators. These are essentially electric motors that are "pushed" a mechanical system. Usually steam or water is used to turn a turbine that it attached to the generator shaft. The output of a generator is generally in the range of 10 to 20 thousand volts (kv), with current that are often in the thousands of amperes.

To transient the generator output over long distances. Transmission lines are used. These are often conductors placed on large steel towers, but may also be cables that are buried underground. Their purpose is to provide a path for the transmission of power by providing an electrical connection from the generators to the load.

This would be all that is required for an electrical system, if it were not for the resistance of the transmission lines, that all equipment requires maintenance , and that system elements may fail

Due to the resistance of the transmission lines, a voltage drop occurs on them, when current flows. From Ohm's law, voltage dropped is proportional to the current flowing and the resistance that it flows through. For a long line , this drop may be so great that the voltage is too low at the end of lines to be of any use. Clearly something must be done to reduce the current that flows in order to reduce the voltage drops.

Transformers are used to reduce voltages drops, and the energy that is wasted because of them. A transformer can change voltage levels while consuming very little power. By using a transformer to increase the voltage on the transmission lines, the current will be proportionally reduced resulting in smaller voltage drops in the system. Often a generator output is "stepped up" in voltage by a factor of ten, resulting in a ten fold decrease in current. At the receiving end of the transmission line the voltage is "stepped down" to useable levels.

Having generators, transmission lines and transformers is still not enough. As system demand for power changes, generators will have to be "brought on line" and off also, if a piece of equipment fails, or require maintenance, it will have to isolated. So meet this need, switches and circuit breakers are used.

Switches (disconnectors isolators) are used to reconfigure the system in order to isolate equipment or provide alternate paths for current to flow. Generally switches are not build rugged enough to interrupt load currents and are not used for this. (we will discuss this further in P2).

To interrupt load currents, and the very large fault currents found in the power system, circuit breaker are used. These devices are essentially very rugged high speed switches that are capable of interrupting fault currents, and switching the forces that this current produce. Circuit breakers are usually classified by the method used to extinguish the arc that result when a current is interrupted. Commons types air air blast and SF6.

Another "system element" commonly found in a substation is the bus-bar . This is a section of conductor that is used to join a group of circuits (or feeders) in a station. By connecting circuits together at several substation, many parallel paths are formed between the generators and the load. This helps to reduce voltage drops further ( resistance is deceased), and also provide alternate paths for current to flow if equipment is taken out of service.

Operating diagrams :

In order to operate the high voltage system schematic diagrams (commonly called operating diagram) are often used this diagrams provide a pictorial diagram of the system in a simplified fashion.

Unlike civil drawing that are often drawn to scale, operating diagrams are generally not drawn to any scale. As with electrical schematic, no purpose would be served be providing a scale. A 100 km transmission line can be represented by an impedance, just as a 10 km line can, further, no purpose would be served by providing a scale drawing of a generator, as this provides no useable electrical information.

Operating diagrams are very much like electronic schematics in that they are constructed by connecting a group of a symbols together with lines. Each line represents a conductor on an electronic schematics while a line represents a conductor on an electronic schematic, while a line on an operating diagram generally refers to a circuit (three or four conductors in a three phase system). The fact is a circuit is represented by a single line on the diagrams to the other name that this diagrams are, often to as single line diagrams.

On the next pages are operating diagrams that shows some of the typical symbols that are found. The large number of manufacturers that have built stations for the EEA has resulted in a large number of different systems is presenting this information. Following the operating diagrams are tables of some of the more common symbols that can be found.