Low Volatge Electrical Power System Overview - Revised

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.

Introduction to Machinery Principles . . . . .

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 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.

MAGNETIC FIELD . . . . 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.

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.

THREE – PHASE CIRCUITS Int roducti on 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.

Analysis of three-phase balanced circuits . . . . Wye connection

Delta connection

Voltage magnitudes

V LL = 3VPh

V LL = V Ph

Current magnitudes

I L = I Ph

I L = 3I Ph

ABC phase sequence ACB phase sequence

VAB leads VA by 30ο VAB lags VA by 30ο

IA lags IAB by 30ο IA leads IAB by 30ο

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, PT = 3PPh = 3VPh × I Ph × cos θ = 3 × VLL × I L × cos θ where, θ is the phase angle between the voltage and the current or, 2 V 2 PT = 3I Ph × RPh = 3 Ph RPh Similarly, the total reactive power is, QT = 3QPh = 3VPh × I Ph × sin θ = 3 × V LL × I L × sin θ or QT = 3I × X Ph 2 Ph

V Ph2 =3 X Ph

P1

P1

P1

P1

Total apparent power is, * ST = PT ± jQT = 3VPh × I Ph

P2

= 3 × VLL × I L*

P2

2 = 3 × I Ph × Z Ph

V Ph2 or, S T = 3 Z Ph

P3

P3

PT = P1 + P 2 + P 3

P2 P2 PT = P1 + P2