General power system

objective

• Review general energy and power system

• Application of fundamental knowledge of principal machines , transformer and other power device to large electrical systems.

• To present general picture of typical electrical for High voltage generation , transformation and distribution

• Difference between shipboard and land based HV power generation

RECAP

• Principle of electrical machines • Power generation, transformation and distribution • Electromagnetism• Induction and transformer • Losses• Single and 3 phase circuit• Active and reactive power• Electric machine and power • Ancillaries

General Power Systems• Majority of merchant ships have a 3-phase 3 wire, 440 V insulated neutral

earth power systems• This power system falls in the category of LV and meets the power

demands of medium capacity motors up to 200 kW• When large loads are connected to the LV system the magnitude of current

flow becomes too large resulting in overheating due to high iron and copper losses

• P = VI Cos • Copper loss = I2 R [kW]

• Losses in Electrical Systems• Copper Losses in electrical cables and machine windings• Hysteresis Loss in magnetic cores• Eddy current loss in conductors and cores• All these losses are current dependent and rise to very high levels in LV

machines for large power ratings

Introduction to Machinery Principles

An electric machine:

is a device that can convert either mechanical energy to electric energy or vice versa.

A generator :

converts mechanical energy to electrical energy.

A motor:

converts electrical energy to mechanical energy.

Almost all practical motors and generators convert energy from one form to another through the action of a magnetic field.

Another closely related device is the transformer.

A transformer:

is a device that converts ac electrical energy at one voltage level to ac electrical energy at another voltage level, but with the same frequency.In general, transformers operate on the same principles as generators and motors, and are usually studied together with generators and motors. These 3 types of devices are inevitable in modern daily life.

*Motors find applications in several home appliances. *In the work place, motors provide the motive power for almost all tools.

*Generators are essential to supply the power used by all these motors.

Introduction to Machinery Principles . . . . .

INTRODUCTION

The electromagnetic system is a necessary element of all rotating and static electric machinery and electromechanical devices.

The role of electromagnetic system is to establish and control electromagnetic fields for carrying out conversion of energy, and transfer.

Practically all motors and generators, depend upon the magnetic field as the coupling medium allowing interchange of energy in either direction between electrical and mechanical systems.

A transformer though not an electromechanical conversion device, provides a means of transferring electrical energy between two electrical ports via the medium of a magnetic field.

3. A current-carrying wire in the presence of a magnetic field has a force induced on it. This is the basis of motor action.

4. A moving wire in the presence of a magnetic field has a voltage induced in it. This is the basis of the generator action.

2. A time-changing magnetic field induces a voltage in a coil of wire if it passes through that coil. This is the basis of transformer action.

MAGNETIC FIELD . . . .

THREE – PHASE CIRCUITSIntroduction

Three-phase systems differ from single-phase systems in that they use a set of three voltages instead of one.

In general, a three-phase system is merely a combination of three single phase systems of which the three voltages differ in phase by 120 electrical degrees from each other in a particular sequence.

There are two basic three-phase connections, the wye (star) and the delta connections.

Wye connection Delta connection

Voltage magnitudes

Current magnitudes

ABC phase sequence

VAB leads VA by

30

IA lags IAB by 30

ACB phase sequence

VAB lags VA by

30

IA leads IAB by

30

PhL II 3PhL II

PhLL VV PhLL VV 3

Analysis of three-phase balanced circuits . . . .

Active power and reactive power calculations

The total average power absorbed by a three phase balanced load delivered by a three-phase generator, is equal to the sum of the powers in each phase. The voltage and current in each phase are equal.

The total 3-phase real power is, cos3cos33 LLLPhPhPhT IVIVPPwhere, is the phase angle between the voltage and the current or,

Ph

PhPhPhT R

VRIP

22 33

Similarly, the total reactive power is, sin3sin33 LLLPhPhPhT IVIVQQ

or Ph

PhPhPhT X

VXIQ

22 33

or,

Total apparent power is,

PhPh

LLL

PhPhTTT

ZI

IV

IVjQPS

2

*

*

3

3

3

Ph

PhT Z

VS

2

3

P1

P3

P2

P1

P2

P1

P3

P2

P1

P2

PT = P1 + P2 + P3 PT = P1 + P2