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