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:

Substation

Major Components in Electrical Substations and their Workings

The power system is a constituent of power generation, transmission and distribution systems. For all the power system operations, substations are required for their course of action. Substations are congregation of electrical equipment through which consumers get supply of electrical power from generating stations. By varying the voltage levels or frequency or any other aspects, the required electrical quantity can be altered in substations to provide quality power to consumers.

Based on the application of substations, they are classified into different types: Generation substation, Indoor substation, Outdoor substation, Pole mounted substation, Switching substation, Transmission substation, Converter substation and Distribution substation. In rare cases like wind farm power generation system, multiple hydroelectric and thermal power plants one can observe the collector substation which is used for transferring power from multiple turbines into one transmission unit.

Substation

The substation is an assembly of the following major electrical equipments:

• Electrical Power transformers
• Instrument transformers
• Conductors& Insulators
• Isolators
• Bus bars
• Lightning arresters
• Circuit breakers
• Relays
• Capacitor banks and miscellaneous equipment

Electrical Power Transformer

Electrical Power transformer

A static electrical machine used for transforming power from one circuit to another circuit without changing frequency is termed as Power transformer. The transformers are generally used to step down or step up the voltage levels of a system for transmission and generation purpose. These transformers are classified into different types based on their design, utilization purpose, installation methods, and so on.

Instrument Transformers:

Instrument transformers

The current and voltage transformers are together called as the Instrument transformers.

Current Transformer

Current transformer is used for the measurement of the alternating current by taking samples of the higher currents of the system. These reduced samples are in accurate proportions with the actual high currents of the system. These are used for installation and maintenance of the current relays in substations for protection purpose which are normally have low-current ratings for their operation.

Potential Transformer

Potential transformer is quite similar to the current transformer, but it is used for taking samples of high voltages of a system for providing low-voltage to the relays of protection system and also to the low-rating meters for voltage measurement. From this low-voltage measurement, the actual system’s high voltage can be calculated without measuring high voltages directly to avoid the cost of the measurement system.

Conductors

Conductors

The material or object that obeys the electrical property conductance (mostly made of metals such as aluminum and copper) and that allows the flow of electric charge is called conductor. Conductors permit free movement of the flow of electrons through them. These are used for the transmission of power or electrical energy from one place (generating station) to another place (consumer point where power is consumed by the loads) through substations. Conductors are of different types and mostly aluminum conductors are preferred in practical power systems.

 Insulators

Insulators

The metal which does not allow free movement of electrons or electric charge is called as an insulator. Hence, insulators resist electricity with their high resisting property. There are different types of insulators such as suspension type, strain type, stray type, shackle, pin type and so on. A few types of insulators are shown in the above figure. Insulators are used for insulation purpose while erecting electric poles with conductors to avoid short circuit and for other insulation requirements.

Isolators

Isolators

Isolator is a manually operated mechanical switch that isolates the faulty section or the section of a conductor or a part of a circuit of substation meant for repair from a healthy section in order to avoid occurrence of more severe faults. Hence, it is also called as a disconnector or disconnecting switch. There are different types of isolators used for different applications such as single-break isolator, double-break isolator, bus isolator, line isolator, etc.

Bus Bars

Bus bars

The conductor carrying current and having multiple numbers of incoming and outgoing line connections can be called as bus bar, which is commonly used in substations. These are classified into different types like single bus, double bus and ring bus.

Lightening Arresters

Lightening Arresters

The substation equipments such as conductors, transformers, etc., are always erected outdoor. Whenever light surges occur then, a high-voltage pass through these electrical components causing damage to them (either temporary or permanent damage based on the amount of voltage surge). Therefore, to avoid this difficulty, lightening arresters are placed to pass the entire lightening surges to earth. There are other arresters which are used to ground the switching surges called as surge arresters.

Circuit Breakers

Circuit Breakers

For the protection of substation and its components from the over currents or over load due to short circuit or any other fault the faulty section is disconnected from the healthy section either manually or automatically. If once the fault is rectified, then again the original circuit can be rebuilt by manually or automatically. Different types of circuit breakers are designed based on different criteria and usage. But in general mostly used circuit breakers are Oil circuit breaker, Air circuit breaker, SF6 circuit breaker, Vacuum Circuit Breaker, and so on.

Relays

Relays

Relays are used for disconnecting the circuits by manual or automatic operation. Relay consists of the coil which is excited or energized and such that making the contacts of relay closed activates the relay to break or make the circuit connection. There are different types of relays such as over current relays, definite time over current relays, voltage relays, auxiliary relays, reclosing relays, solid state relays, directional relays,inverse time over current relays, microcontroller relays, etc. The above figure shows some basic relays and their operation.

Capacitor banks

A Capacitor bank is a set of many identical capacitors connected in series or parallel within a enclosure and is used for the power factor correction and basic protection of substation.These capacitor banks are acts as a source of reactive power, and thus, the phase difference between voltage and current can be reduced by the capacitor banks. They will increase the ripple current capacity of the supply. It avoids undesirable characteristics in the power system. It is the most economical method for maintaining power factor and of correction of the power lag problems.

Capacitor banks

Emerging trends in technological development have created advancement in the substation installation and maintenance. For example, SCADA, supervisory control and data acquisition technique made it possible to control a substation automatically from a remote location. For more data regarding miscellaneous components and technologies in substations, post your queries in the comments section below.

The Transistor

The transistor is one of two most important inventions of the last century. Without the transistor’s invention, most of the electronic devices on which you’re so hopelessly dependent would not exist. The modern age rely on the transistor – personal computers, televisions, smartphones, tablets, phablets, laptops, routers and foot massagers suffused with billions of them.
Transistors are to electronic devices what cells are to our bodies.