Transmission lines

Transmission lines are part of the system that gets the electricity from the power station to your home. The lines that are on poles down your street and that are connected to your house and other premises are referred to as the distribution network and are rarely higher than 11,000 volts. The ones to your house are probably no more than 440 volts. These lines come from a transformer that has an input of 11,000 or 22,000 volts. Some large customers take their electricity direct from the 22,000 network, and others take it at 11,000 volts. These high voltage distribution networks are supplied from transformers that are connected to the transmission system that usually operates at anything between 66,000 and 500,000 volts. These very high voltage lines are usually on large steel towers that run between power stations and large Transformer stations that distribute the power at 11,000 volts, and lower, to the end user.

Antifuse

An Antifuse is an electrical device that performs the opposite function to a fuse. Whereas a fuse starts with a low resistance and is designed to permanently break an electrically conductive path when the current through the circuit exceeds a specified value while an Antifuse starts with a high resistance and is designed to permanently create an electrically conductive path when the voltage across it exceeds a certain value.

It is an electrically programmable two-terminal device with small area and low parasitic resistance and capacitance. Figure below shows the symbolic representation of Untriggered and Controlled Antifuse.

It is a Programmable Chip Technology that creates permanent, conductive paths between transistors.  In contrast to “Blowing Fuses” in the fusible link method, which opens a circuit by breaking the conductive path, the Antifuse method closes the circuit by “growing” a conductive layer via  two metal layers in between a layer of non-conductive, amorphous silicon is sandwiched as shown in figure below. When voltage is applied to this middle layer, the amorphous silicon is turned into poly-silicon, which is conductive. 

A controlled Antifuse can be programmed by using some Chip so that when the Chip issues control command then a high voltage exceeding the limit value of Antifuse is applied and hence it becomes conductive and gives Logical / Boolean 1.

Antifuse – Non conductive when voltage is less than the limit value.

Antifuse – Conductive when voltage across the metal layers is more than the limit value.

For better understanding of Antifuse, I am explaining one use of it.  It is used in Christmas Light / Serial Light.

A Serial Light is connected to the domestic supply voltage. The individual bulbs are not rated for the domestic voltage.  However, as they are connected in series, they are able to withstand and function in the domestic supply voltage.

A series of 48 bulbs of a rating of 2.5 volts can withstand 120 volts. Similarly, a series of 96 lamps can withstand 240 volts.

When one bulb in the series fails, there is a risk of the other lamps not getting the supply as the circuit is open circuited. This is avoided by having an Antifuse below the filament which fuses i.e. becomes conductive when the bulb filament fails.  This happens because the system voltage is applied across the single bulb in this case and hence it becomes conductive.

Once the Antifuse operates and closes the open circuit, the current flows as usual to the remaining bulbs and therefore there is no interruption to the glow of bulbs in Serial Lamps / Christmas Light. It is mostly used to permanently programmed ICs.

HVDC Advantages & Disadvantages

A scheme diagram of HVDC Transmission is shown below for ease in understanding the advantages and disadvantages.

 

Advantages of HVDC:

There is a list of advantages of High Voltage DC Power Transmission, HVDC when compared with High Voltage AC Power Transmission, HVAC. They are listed below with detail while comparing with HVAC.

Line Circuit:

The line construction for HVDC is simpler as compared to HVAC. A single conductor line with ground as return in HVDC can be compared with the 3-phase single circuit HVAC line (Why? Can’t we supply power with two phases in HVAC?). As because when Line to Earth Fault or Line-Line Fault 3-phase system cannot operate. This is why we compared the a single conductor line with ground as return can be compared with the 3-phase single circuit HVAC line. 

Thus HVDC line conductor is comparatively cheaper while having the same reliability as 3-phase HVAC system.

Power Per Conductor:

Power Per conductor in HVDC P_d = V_dI_d

Power Per Conductor in HVAC P_a = V_aI_aCosØ

Where I_d and I_aare the line current in HVAC and HVDC circuit respectively & V_dand V_a are the voltage of line w.r.t ground in HVDC and HVAC respectively.

As crest voltage is same for Insulators of Line, therefore line to ground voltage in HVDC will be root two (1.414) times that of rms value of line to ground voltage in HVAC.

V_d = 1.414V_aand I_d = I_a (assumed for comparison purpose)

Therefore,

P_d / P_a = V_dI_d / V_aI_aCosØ

            = V_dI_d / (V_d/1.414)I_dCosØ

            = 1.414/CosØ

As CosØ <= 1,

P_d /P_a>1

P_d >P_a

Therefore, we see that power per conductor in HVDC is more as compared to HVAC.

Power Per Circuit:

Now, we will compare the power transmission capabilities of 3-phase single circuit line withBipolar HVDC Line. (Bipolar HVDC Line have two conductors one with +ive polarity and another with –ive polarity.)

Therefore, for Bipolar HVDC Line,

P_d = 2×V_dI_d

While for HVAC Line,

P_ac = 3×V_aI_aCosØ

Hence,

P_d / P_ac= 2V_dI_d / 3V_aI_aCosØ

But V_d = 1.414V_aand I_d = I_a

P_d/P_ac= (2×1.414) / 3CosØ

             = 2.828/ 3CosØ ≈ 0.9 (as CosØ <1)

Thus we see that power Transmission Capability of Bipolar HVDC Line is same as 3-phase single circuit HVAC Line. But in case of HVDC, we only need two conductors while in 3-phase HVAC we need three conductors, therefore number of Insulators for supporting conductors on tower will also reduce by 1/3. Hence, HVDC tower is cheaper as compared to HVAC.

Observe the figure below carefully, you will get to know three important points about HVDC

 

No Charging Current:

Unlike HVAC, there is no charging current involved in HVDC which in turn reduces many accessories.

No Skin Effect:

In HVDC Line, the phenomenon of Skin Effect is absent. Therefore current flows through the whole cross section of the conductor in HVDC while in HVAC current only flows on the surface of conductor due to Skin Effect.

No Compensation Required:

Long distance AC power transmission is only feasible with the use of Series and Shunt Compensation applied at intervals along the Transmission Line. For such HVAC line, Shunt Compensation i.e. Shunt Reactor is required to absorb KVARs produced due to the line charging current (because the capacitance of line will dominate during low load / light load condition which is famously known as Farranty Effect.) during light load condition and series compensation for stability purpose.

As HVDC operates at unity power factor and there is no charging current, therefore no compensation is required.

Less Corona Loss and Radio Interference:

As we know that, Corona Loss is directly proportional to (f+25) where f is frequency of supply. Therefore for HVDC Corona Loss will be less as f=0. As Corona Loss is less in HVDC therefore Radio Interference will also be less compared to HVAC.

The interesting thing in HVDC is that, Corona and Radio Interference decreases slightly by foul whether condition like snow, rain or fog whereas they increases Corona and hence  Radio Interference in HVAC.

Higher Operating Voltage:

High Voltage Transmission Lines are designed on the basis of Switching Surges rather than Lightening Surges as Switching Surges is more dangerous compared to Lightening Surges.

As the level of Switching Surges for HVDC is lower as compared with HVAC, therefore the same size of conductors and Insulators can be used for higher voltage for HVDC when compared with HVAC.

No Stability Problem:

As we know that for two Machine system, power transmitted,

P = (E₁E₁Sinδ)/X

Where X is inductive reactance of the line, E₁ & E₂ are the sending and receiving end voltage respectively.

As the length of line increases the value of X increases and hence lower will be the capability of Machine to transmit power from one end to another. Thus, reducing the Steady State Stability Limit. As the Transient Stability Limit is lower than Steady State Stability Limit, thus for longer line Transient Stability Limit becomes very poor.

HVDC do not have any Stability problem in itself as the DC operation is asynchronous operation of Machine.

Now, we will come to disadvantage of HVDC.

Disadvantage of HVDC:

 

Expensive Converters:

The converters used at both end of line in HVDC are very costly as compared to the equipment used in AC. The converters have very little overload capacity and need reactive power which in turn needs to be supplied locally.

Also Filters are required at the AC side of each converter  which also increases the cost.

Voltage Transformation:

Electric Power is used generally at low voltage only. Voltage Transformation is not easier in case of DC.

What is Smart Grid?

What is Smart Grid?

A Smart Grid is an evolved Grid system that manages electricity demand in a sustainable, reliable and economic manner, built on advanced infrastructure and tuned to facilitate the integration of all involved. So tough definition? Let’s make it simple.

Smart Grid is a Power Grid monitored and controlled electronically to maximize efficiency and minimize outages.

This is a recent development in Electric Transmission and Distribution Sector, which enables bidirectional communication between consumer and electricity utility company. Now a day’s one of the fundamental challenges of power system operation is running a true supply-on-demand system that is expected to be absolutely reliable. Historically this challenge led to a power system based on highly controllable supply to match a largely uncontrolled demand. The use of smarter grid operations allows for greater penetration of variable energy sources through the more flexible management of the system.

Basically it is an electric power delivery system that stretches from point of generation to point of consumption. Integrated with advanced communications and information technology, all equipment and devices in a smart grid are connected by sensory elements to form a complete power network. The information is integrated and analyzed to optimize power resources, reduce costs, increase reliability, and enhance electric power efficiency.

A smart grid is an intelligent automated system for monitoring the flow of electricity and making the distribution of electricity more efficient. In a world where protecting the environment is a major concern, it is important to find cost-effective ways of reducing power usage and increasing energy independence.

Smart Grid is a combination of Energy, IT and Telecommunication Technologies.

To summarize,

• Smart Grid provides an interface between consumer appliances and the traditional assets in a power system (generation, transmission and distribution).

• It optimizes the assets of the power system.

• It supports better integration of distributed generation into the conventional centralized power system.

• It possess demand response capacity to help balance electrical consumption with supply.

Smart Grid Ecosystem:

Existing Power Supply Systems implement a Centralized Power Supply that often involves high voltages and large-scale electric power networks. With this type of power supply, failures in the electricity network can have a huge impact on the entire power supply system, and often cause widespread system shutdowns.

Because of this Smart Grid solutions is developed and implementing a Distributed Power Network instead of a centralized network is also considered. Distributed Power Networks are highly integrated and include power generation, power transmission, and power distribution, with power meters and home appliances, such as refrigerators, TV sets, washing machines, personal computers etc. also considered part of the network. A simplified Smart Grid Ecosystem is shown in figure below.

Why Smart Grid?

A Smart Grid solution provides the following benefits:

• Enhances energy usage efficiency

• Increases the proportion of distributed power generation systems and renewable energy solutions

• Enhances the flexibility of the power supply

• Reduces the overall costs of delivering power to end users

• Improves the stability and quality of the power supply

Thank you!

DC SHUNT MOTORS CHARACTERISTICS

Long Shunt and Short Shunt DC Compound Machine:

In short shunt DC Compound Machine, shunt filed winding is connected across the Armature whereas in Long shunt connection it is connected across the line terminal. But there is no difference in operating characteristic in two types of machine.

DC Shunt Generator Characteristics:

There are four basic quantities related to generator namely speed n, Terminal Voltage V_t , Armature Current I_aand Field Current I_f. 

The graphical relationship between two quantities while maintaining other two quantities constant is known as characteristics of Generator. Basically, there are four characteristics of any Generator:

1.    No load Characteristics:– Relationship between Ea and I_f. Ea = f(I_f)

2.    Load Characteristics:– Relationship between V_t and I_f. V_t= f(I_f)

3.   External Characteristics:- Relationship between V_t and I_L. V_t= f(I_L)

4.    Armature / Regulation Characteristics:– Relationship between I_f and I_a. I_f = f(I_a)

1.    No load and Load Characteristics:

2.   External Characteristics:

3. Armature Characteristics:

Typical Utility Pole

Typical North American utility pole, showing hardware for a residential 240/120 V split-phase service drop: (A,B,C) 3-phase primary distribution wires, (D) neutral wire, (E) fuse cutout, (F) lightning arrestor, (G) single phase distribution transformer, (H) ground wire to transformer case, (J) "triplex" service drop cable carries secondary current to customer, (K) telephone and cable television cables.

Power system Design Characteristics

The properly design characteristics of a power system are as shown below: