The Advantage and the disadvantage of radial and ring distribution systems

Advantages of the radial distribution system 

• Simplest as fed at only end.
• The initial cost is low.
• It is useful when the generating is at low voltage.
• Preferred when the station is located at the center of the load.
• More economical for some areas which have a low load requirement
• Require less amount of cables
• It has a low maintenance

Disadvantages of the radial distribution system 

• The end of distributor near to the substation gets heavily loaded.
• When load on the distributor changes, the clients at the distant end of the    distributor face serious voltage fluctuations.
• As users are dependent on single feeder and distributor, a fault on any of these two causes interruption in supply to all the users connected to that distributor

Advantages of the ring distribution system

• In ring power is supplied from both ends as compared to radial
• In case of a fault in the radial circuit the entire system goes off unlike in ring where by incase one end gets a fault the other end still keeps on supplying power
• Compared to the radial system, the voltage drop is less along the distribution line
• More subscribers can be installed to the system than the radial system
• Less voltage fluctuations can be seen at client’s terminals. Voltage  fluctuations in high loaded areas can be reduced using a tie line

Disadvantages of the ring distribution system

• Ring is very expensive n requires more materials than radial
• Radial circuit is more economical
• High maintenance cost
• It is not usable when the client is located at the center of the load

Methods that can be used to improve voltage profile in a distribution system

• By installing high power distribution transformers
• Balancing of the loads on the primary feeders
• By increasing the feeder conductor size
• Establishing new stations and primary feeders
• Increase the load factor and the voltage profile by using capacitor banks
• By installing more single phase transformers and establishing ring distribution systems where possible, can get rid of voltage fluctuations

Comparison and contrast between the underground cables and overhead lines in distribution systems

• Maintenance costs:
The present worth of the maintenance costs associated with underground lines is difficult to assess. Many variables are involved, and many assumptions are required to arrive at what would be a guess at best. Predicting the performance of an underground line is difficult, yet the maintenance costs associated with an underground line are significant and one of the major impediments to the more extensive use of underground construction. Major factors that impact the maintenance costs for underground transmission lines include.

• Cable repairs:
Underground lines are better protected against weather and other conditions that can impact overhead lines, but they are susceptible to insulation deterioration because of the loading cycles the lines undergo during their lifetimes. As time passes, the cables' insulation weakens, which increases the potential for a line fault. If the cables are installed properly, this debilitating process can take years and might be avoided. If and when a fault occurs, however, the cost of finding its location, trenching, cable splicing, and re-embedment is sometimes five to 10 times more expensive than repairing a fault in an overhead line where the conductors are visible, readily accessible and easier to repair.

• Line modifications:
Overhead power lines are easily tapped, rerouted or modified to serve customers; underground lines are more difficult to modify after the cables have been installed. Such modifications to underground power lines are more expensive because of the inability to readily access lines or relocate sections of lines.

• Appearance:
The general appearance of the underground system is better since all the distribution lines are invisible. The conductors are visible and easily accessible in overhead system.

• Flexibility:
Underground cables are less flexible since manholes, duct lines etc. but overhead line are more flexible due to poles, wires, transformers etc. Those can be easily shifted to meet the chances in load conditions.

Allowable limits for distribution voltage and frequency in Sri Lanka

Normally the distribution voltage is 230V in Sri Lanka. But a variation of 6% of distribution voltage is allowed. 

Then;

The minimum voltage allowed is 216.2 V.

The maximum voltage allowed is 243.8 V.

Normally the distribution frequency is around 50 Hz. But a variation of 1% of distribution frequency is allowed.

Then;

The minimum frequency allowed is 49.5 Hz.

The maximum frequency allowed is 50.5 Hz. 

Other method of braking a DC motor

Basically, there are three types of electrical breaking done in a DC Motor

Regenerative Braking

It is a form of braking in which the kinetic energy of the motor is returned to the power supply system. This type of braking is possible when the driven load forces the motor to run at a speed higher than its no-load speed with a constant excitation.

The motor back emf Eb is greater than the supply voltage V, which reverses the direction of the motor armature current. The motor begins to operate as an electric generator. It is very interesting to note that regenerative braking cannot be used to stop a motor but to control its speed above the no-load speed of the motor driving the descending loads.

Dynamic Braking

It is also known as Rheostat braking. In this type of braking, the dc motor is disconnected from the supply and a braking resistor Rb is immediately connected across the armature. The motor will now work as a generator, and produces the braking torque. During electric braking when the motor works as a generator, the kinetic energy stored in the rotating parts of the motor and connected load is converted into electrical energy. It is dissipated as heat in the braking resistance Rb and armature circuit resistance Ra. Dynamic Braking is an inefficient method of braking as all the generated energy is dissipated as heat in resistances.

Plugging

It is also known as reverse current braking. The armature terminals or supply polarity of a separately excited dc motor or shunt dc motor when running are reversed. Therefore, the supply Voltage V and the induced voltage Eb i.e. back emf will act in the same direction. The effective voltage across the armature will be V + Eb which is almost twice the supply voltage. Thus, the armature current is reversed and a high braking torque is produced. Plugging is a highly inefficient method of braking because in addition to the power supplied by the load, power supplied by the source is wasted in resistances.

The importance of the parallel operation of transformers

• Reducing the total capacity of electrical transformers (as compared to separate their work). The decrease of total installed capacity is reached:

1. By lowering the overall demand load to the diversity of loads connected to different transformers.
2. By using a higher load rate of parallel transformers 
3. Less required backup in case of electrical transformer failure 

• Reduction of electricity losses in electrical transformers due to a possible disconnection of unloaded transformers

• Improving the power quality due to the stable level of short circuit current throughout the network

• Increasing the reliability of operation of protective devices in the case of phase-to-earth short circuits in the network.

• Possibility of placing electrical transformers in operation phase-by-phase

Importance of transformer in an electrical power system

Transformers (sometimes called "voltage transformers") are devices used in electrical circuits to change the voltage of electricity flowing in the circuit. Transformers can be used either to increase the voltage (called "stepping up") or decrease the voltage ("step down").

Energy is lost in the process of transmitting electricity long distances, such as during the journey from a power plant to your home. Less energy is lost if the voltage is very high, so electrical utilities use high voltage in long-distance transmission wires. However, this high voltage is too dangerous for home use. Electrical utilities use transformers to change the voltage of electricity as it travels from the power plant to you. First, the voltage of electricity coming from the power plant is "stepped up" using transformers to the right level for long-distance transmission. Later, the voltage is stepped down before it enters your home - once again using transformers.

In order for the electrical power distribution network to function, voltages must be stepped up before power is transmitted great distances over power lines. One major problem is that power is lost between the power plant and the consumers because currents use some of the power to heat the transmission lines. The power transmitted along the line is equal to the voltage times the current. The higher the voltage the lower the current that must flow within the transmission lines to deliver the same power. Lower currents produce much less heating and much less power loss. Of course, the high voltages (needed to drive the low currents) must be stepped back down before power is supplied to our homes. Transformers are the critical elements that step up and down the voltages at each end of the line

Speed control of shunt motor

Flux Control Method

It is seen that speed of the motor is inversely proportional to flux. Thus by decreasing flux speed can be increased and vice versa.

To control the flux, a rheostat is added in series with the field winding, as shown in the circuit diagram. Adding more resistance in series with field winding will increase the speed, as it will decrease the flux. Field current is relatively small and hence I2R loss is small, hence this method is quiet efficient. Though speed can be increased by reducing flux with this method, it puts a limit to maximum speed as weakening of flux beyond the limit will adversely affect the commutation.

Armature Control Method

Speed of the motor is directly proportional to the back emf Eb and Eb = V- IaRa. That is when supply voltage V and armature resistance Ra are kept constant, speed is directly proportional to armature current Ia. Thus if we add resistance in series with armature, Ia decreases and hence speed decreases.

Greater the resistance in series with armature, greater the decrease in speed.


Voltage Control Method

• Multiple voltage control:

In this method the, shunt filed is connected to a fixed exciting voltage, and armature is supplied with different voltages. Voltage across armature is changed with the help of a suitable switchgear. The speed is approximately proportional to the voltage across the armature.


• Ward-Leonard System:

This system is used where very sensitive speed control of motor is required (e.g. electric excavators, elevators etc.) The arrangement of this system is as required in the figure beside.

M2 is the motor whose speed control is required. M1 may be any AC motor or DC motor with constant speed. G is the generator directly coupled to M1. In this method the output from the generator G is fed to the armature of the motor M2 whose speed is to be controlled. The output voltage of the generator G can be varied from zero to its maximum value, and hence the armature voltage of the motor M2 is varied very smoothly. Hence very smooth speed control of motor can be obtained by this method.

DC series motor starter

Construction of DC series motor starters is very basic as shown in the figure. A start arm is simply moved towards right to start the motor. Thus at first maximum resistance is connected in series with the armature and then gradually decreased as the start arm moves towards right. The no load release coil holds the start arm to the run position and leaves it at no load.

3 Point Starter

When motor is to be started, the lever is turned gradually to the right. When lever touches point 1, the field winding gets directly connected across the supply, and the armature winding gets connected with resistances R1 to R5 in series. Hence at starting full resistance is added in series with armature. Then as the lever is moved further, the resistance is gradually is cut out from the armature circuit. Now, as the lever reaches to position 6, all the resistance is cut out from the armature circuit and armature gets directly connected across the supply. The electromagnet E (no voltage coil) holds the lever at this position. This electromagnet releases the lever when there is no (or low) supply voltage.
When the motor is overloaded beyond a predefined value, overcurrent release electromagnet D gets activated, which short circuits electromagnet E, and hence releases the lever and motor is turned off.

Important of starting method of a DC motor

Thus, to avoid the above dangers while starting a DC motor, it is necessary to limit the starting current. For that purpose, starters are used to start a DC motor. There are various starters like, 3 point starter, 4 point starter, No load release coil starter, thyristor starter etc.

The main concept behind every DC motor starter is, adding external resistance to the armature winding at starting.

4 Point Starter

In four point starters, the hold on coil is connected directly across the supply line through a protective resistance R. when the armature touches stud no 1.the line current divides into three parts 

i) Armature starting resistance and overload release. 

ii) A variable resistance and shunt field winding. 

iii) Holding coil and current limiting resistance.

The field gets directly connected to the supply, as the lever moves touching the brass arc. The no voltage coil (or Hold on coil) is connected with a current limiting resistance Rh. This arrangement ensures that any change of current in the shunt field does not affect the current through hold on coil at all. This means that electromagnet pull of the hold-on coil will always be sufficient so that the spring does not unnecessarily restore the lever to the off position. This starter is used where field current is to be adjusted by means of a field rheostat.

The basic difference between three point and four point starters is the manner in which the hold on coil is connected. The unnecessary tripping of starter can be stopped by connecting separately or parallel both magnetizing and field coil. They are connected in such a way that both should carry their individual current. Thus voltage drop in one coil will not affect the voltage in other coil.

Disadvantages of four coil starter:

The only limitation of the four point starter is that it does not provide high speed protection to the motor. If under running condition field gets opened; the field current reduces to zero. As there is some residual flux present and speed (N) is directly proportional to flux (ø) the motor will tries to run with dangerously high speed .this is called high speed action of motor. In three point starter as no volt coil is directly connected to across the supply; its current is maintained irrespective of the current through the field winding .hence it always maintain the handle in run position as long as supply is there .and thus it doesn’t protect the motor from field failure conditions which returns into high speeding of the motor