ET201 - ELECTRICAL CIRCUIT (THREE PHASE PART 6)

1.1.2        Interconnection of three phase.

If the three armature coils of the 3-phase alternator are not interconnected but are kept separate  as shown in Figure 1.1.3 (a), then each phase or circuit would need two conductors, the total number of conductors, in that case, being six. It means that each transmission cable would contain six conductors which will make the whole system complicated and expensive.

 

Hence, the three phases are generally interconnected which results in substantial saving of copper. The general methods of interconnection are;

(a)                Star or wye connection

(b)               Delta or mesh connection

(a)        Star or wye (Y) connection

In this method of interconnection, the similar* ends say, ‘start’ ends of three coils (it could be ‘finishing’ ends also) are joined together at point N as shown in Figure 1.1.3(b). The point N is known as star point or neutral point. The three conductors meeting at point N are replaced by a single conductor known as neutral conductor. Such an interconnected system is known as 4-wire, 3-phase system as shown in Figure 1.1.3(c).  

 

         Figure 1.1.3 (b) and (c)  Interconnected system for Star connection

b)         Delta or mesh (D) connection

In this form of interconnection, the dissimilar* ends of three phase windings are joined together i.e. the ‘starting’ end of one phase is joined to the ‘finishing’ end of the other phase and so on as shown in Figure 1.1.3(d). In other words, the three windings are joined in series to form a closed mesh as shown in Figure 1.1.3(e).  

 

         Figure 1.1.3 (d) and (e)  Interconnected system for Delta connection

ET201:ELECTRICAL CIRCUIT (THREE PHASE PART 5)

The complete emf waveform for three phase system is shown below with corresponds to the angle and magnitude.

Three Phase Voltages (emf)

               

 

a)                  Three phase vector diagram

Alternating voltages may be represented by revolving vectors which indicate their maximum value(or r.m.s values if desired).The actual values of these voltages vary from peak positive to zero and peak negative values in one revolution of the vectors. In Figure 1.1.2 (g) are shown the three vectors representing the voltages of the three phases e_R, e_Y and e_B. Remember, R is the reference; therefore its vector is placed on the 0⁰ plane of x-axis. This vector diagram is rotated to the position where R is on the vertical y-axis to make better understanding as in Figure 1.1.2 (h). The phase different still 120⁰ apart.

  (g) geometri arrangemenT

(h)  conventional arrangement

 

                                

ET201: THREE PHASE SYSTEMS (ANSWER FOR DISCUSSION EXPERIMENT 4)

(ii)       Current

Figure 1.1.4 (b) shows the current in star connection. The value of line current (I_R, I_Y , I_B ) and phase current (I_r , I_y , I_b) are the same.

     

Line current,      I_L:  I_R , I_Y , I_B

Phase current, I_PH :  I_r , I_y , I_b


So, current element in star connection:

              I_L = I_PH


Neutral current (I_N) is the sum of the three current i.e. I_R, I_Y, and I_{B ;} having same magnitude but 120^o difference in phase. Therefore mathematically, the sum of these vectors will make neutral current I_N equals zero.

       I_N = I_R + I_Y + I_B = 0 

ET201: THREE PHASE SYSTEMS (PART 4)

a)                  Three phase emf generation

Let us consider Red (R) phase taken as reference and the coil rotates clockwise as indicated in Figure 1.1.2 (c) to (e), the induced voltage (emf) waveform for R (e_R), Y(e_Y)  and B(e_B)  can be derived separately. Figure 1.1.2 (f) shows the combination of the RYB waveform; therefore it is called the three phase waveform.

Figure 1.1.2. Emf waveform; (c) R phase  (d) Y phase (e) B phase (f) Three phase

ET201: THREE PHASE SYSTEMS (PART 3)

1.1.2        Generation of three phase voltages (emf).

In a single phase AC system, alternating voltage may be generated by rotating a coil in magnetic field or by rotating a magnetic field within a stationary coil. The value of the voltage generated depends, in each case, upon the number of turns in the coil. Consider a case of a single phase alternator with one armature winding only. If the number of armature windings is increased, then it becomes polyphase alternator and it produces as many independent voltage waves as the number of windings or phases. These windings are displaced from one another by equal angles, the values of these angles being determined by the number of phases or windings. In fact, the word “polyphase” means poly(many) and phase(winding or circuit).

A three phase alternator, as the name shows, has three independent armature windings which are 120 electrical degrees apart. Hence, the voltages induced in the three windings are 120⁰ apart in time phase. In Figure 1.1.2(a) is shown a two pole, stationary-armature, rotating-field type three phase alternator. It has three armature coils RR’, YY’ and BB’ displaced 120⁰ apart from one another. Figure 1.1.2(b) is shown a pole, stationary-magnet, rotating coils type.

Figure 1.1.2. (a) Rotating-field, stationary coils   (b)Rotating- coils, stationary field

ET201: THREE PHASE SYSTEMS (PART 2)

1.1.1    Three phase system advantages

There are several reasons why three phase system is superior to single phase system for transmission and distribution of electricity;

a)   In a balanced three phase system, the conductors need be only about 75% the size of conductors for a single phase two-wire system of the same power (KVA) rating. This helps to reduce the cost because less copper (or aluminium) is needed.

b)    The power delivered by a single-phase system pulsates and falls to zero three times during each cycle. The power delivered by a three-phase circuit pulsates also, but it never falls to zero

Figure 1.1.1. Three phase power never falls to zero

a)     The horsepower rating of three phase motors and the KVA (kilo-volt-amp) rating of three phase transformers is about 150% greater than for single-phase motors or transformers with a similar frame size.

b)   The efficiency and power factor of three phase motors much better than single phase motors for the same power transferred.

c)   Three phase motors have the ability to “self-start”; caused by the phase difference between three phase coils, but not in single phase motors.

d)  Three phase transformers are lighter, more efficient and cheaper compared to single phase transformer for the same size.

e)     When connected in parallel, single phase generator present difficulties which do not occur with three phase generators.

h)  The instantaneous three phase power flow is constant.  This is advantageous for both the end user and the generator, which does not have to supply a pulsating torque.

ET201: THREE PHASE SYSTEMS (PART 1)

1.0    Introduction

Three Phase Systems

Most domestic supplies are single-phase alternating current (AC). As you are aware, to transmit power with single phase AC, we need two wires (live wire, L and neutral wire, N). However, you would have seen that distribution lines usually have 4 wires. This is because distribution is done using three phase and the 4^th wire is the neutral. (4-wire supplies are normally used to distribute domestic supplies since they can provide an earthed neutral. 3-wire systems are commonly used for transmission of high voltage supplies between substations because money is saved by not providing a neutral wire).

Therefore, most of the electrical power generated and transmitted in the world today is three phase. Industrial supplies are three phase, whilst domestic supplies receive only one of the phases. 

Three phase system contains three AC voltages. The three phases usually designated R, Y, B corresponding to Red (R), yellow (Y) and Blue (B), which are phase displaced from one another. We can obtained single phase from three phase system by connecting any phase (R or Y or B) and neural (N) to form R-N, Y-N or B-N single-phase system. The single phase load of domestic supplies is shared out across the three phases by supplying different groups of users with different phase voltages.

Three phase supply voltages and load systems have two basic configurations, the star (wye) connection and the delta connection.

Figure 1.0 shows the three phase RYB system with star and delta connections and also single phase systems obtained from the three phase system as discussed earlier.

ET201: ELECTRICAL CIRCUITS (TRANSFORMER)

Transformers

A transformer is a device for stepping-up, or stepping-down, the voltage of an alternating electric signal. Without efficient transformers, the transmission and distribution of AC electric power over long distances would be impossible. There are two circuits. Namely, the primary circuit, and the secondary circuit. There is no direct electrical connection between the two circuits, but each circuit contains a coil which links it inductively to the other circuit. In real transformers, the two coils are wound onto the same iron core. The purpose of the iron core is to channel the magnetic flux generated by the current flowing around the primary coil, so that as much of it as possible also links the secondary coil. The common magnetic flux linking the two coils is conventionally denoted in circuit diagrams by a number of parallel straight lines drawn between the coils.