Single phase Electrodynamometer type Wattmeter

The instrument used to measure active power ‘P’ drawn by a load or circuit is called ‘watt-meter’. Three types of watt-meter are in use. They are:

1.       Dynamo-meter type,

2.       Induction type, and

3.       Electrostatic type.

The most commonly used watt-meter and available in labs are the dynamo-meter type. Although digital watt-meter are also in use and are mainly found in industries.

Lets’ have a look into the Electro-dynamo-meter type Watt-meter…

An electro-dynamo-meter type watt-meter has two coils; a fixed coil and a moving coil. The fixed coil is also called the current coil (CC) since it carries the load current or a fraction of it. The current coil, which is connected in series, is made up of thick wires of few turns and is divided into two identical parts (as shown in the figure). The current coil is divided into two to have a uniform magnetic field. The terminals of these fixed or current coils are marked ‘M’ and ‘L’.

The second coil is movable and is called the pressure coil (P.C.). It is located inside the current coil and is made up of large number of turns of very fine wire. A very high resistance is also sometimes added in series with the pressure coil (also called voltage coil) which makes the resistance of pressure coil in kilo-Ohm range; usually 5, 10 or 20 kilo-Ohm. The pressure coil is connected in parallel to the load and carries a definite very low value of current .The terminals of pressure coil are marked ‘COM’ and ‘V’.

Fig 1 and 2: Two different views of Dynamo-meter-type watt-meter.

Working of Electro-dynamo-meter type Watt-meter:

The pressure coil or the moving coil, which is suspended on a spindle, moves in between the two halves of the fixed coil. The movement is due to the interaction of the magnetic fields of the two coils; fixed and the moving. The controlling torque is provided by two fine springs which also serves as leads to pass the current into the pressure coil. A pointer is attached to the moving coil which directly indicates the value of active power recorded by the watt-meter. 

The deflection of the watt-meter is given by:

T = K . V . I. cos(phi)

 where ‘K’ is a constant,

V and I are the r.m.s. value of supply voltage and load current, and

‘phi’ is the phase difference between V and I.  

Multiplying Factor of Electro-dynamo-meter type Watt-meter:

Watt-meters usually have selection facility i.e. one can select the range of voltage as well as current of the watt-meter. Suppose we have a 2.5/5 A watt-meter and by properly connecting the links on the watt-meter we can select either 2.5 A or 5 A capacity range.

Similarly, we can select the voltage range also. Suppose we have a watt-meter with voltage range 75 V, 150 V, and 300 V. One can select any one voltage according to the voltage applied to the circuit. Let’s make you more clear. 

The voltage applied in the short circuit test of a single phase small transformer is very low, usually 10 – 20 V, so in this case we have to select the 75 V range. On the other hand, in the open circuit test of the same transformer normal rated voltage of 230 V is applied, hence we have to select the 300 V range.

Depending on the selection of voltage and current, we have to consider the ‘multiplying factor’ for further calculation. In simple, a ‘multiplying factor’ is a factor which is to be multiplied into the watt-meter reading to obtain the correct value of active power in the circuit.    

An example for ‘multiplying factor’ is given below:

Current selected	Voltage selected
	75 V	150 V	300 V
2.5 A	1	2	4
5 A	2	4	8

The figures in ‘bold’ are the multiplying factors. For  example, when we select (connect to) 150 V and 2.5 A, the ‘multiplying factor’ is 2 and for a selection of (connection to) 150 V and 2.5 A, the ‘multiplying factor’ is 4. Multiplying factor for the same values of current and voltage may vary according to the construction of the watt-meter.

Calculation for Locational Marginal Price

In today’s world all the power utilities are unbundled and de-regulated to a certain extent. The price of electricity is the most important factor to nearly all the market participants. The most basic electricity pricing mechanism is the Market Clearing Price (MCP).

In a power market, after receiving the bids, the System Operator (SO) aggregates the supply bids into a supply curve ‘S’ and aggregates the demand bids into a demand curve ‘D’. The intersection of S and D is the  Market Clearing Price (MCP)

Generally when there is no transmission congestion, MCP is the same for the entire power system, but when there is congestion, the concept of Zonal Market Clearing Price (ZMCP) or Locational Marginal Price (LMP) is used. In other words when there is no congestion, the LMP is the same as the MCP but in the congested state, the marginal cost of each bus is the LMP.

Let’s have a look into the LMP concept using a small example. A small 4 bus system is shown in the figure below.

Fig.  A four bus system.

The system has 4 buses with 2 generators each of capacity 125 MW at bus 1 and 3. A load of 100 MW is connected at bus 4. Suppose that there is no congestion and no losses, then for supplying 100 MW of load at bus 4, the power flows in line –

1-2 is 25 MW,

2-3 is 25 MW,

3-4 is 25 MW, and

1-4 is 75 MW if the lines are identical.

As per the definition, LMP at any node or bus is the cost of supplying add 1 MW at that node. Suppose we have to calculate the LMP at node 4. When there is no congestion and no losses, the power flow in the lines are –

25.25 MW at line 1-2,

25.25 MW at line 2-3,

25.25 MW at line 3-4, and

75.75 MW at line 1-4.

Thus, the additional load of 1 MW at node 4 is supplied by generator 1 at it’s offer price of 300 INR. This generator is the marginal generator and the LMP at node 4 is 300 INR.

LMP when there is Congestion in Lines:

Now suppose that the maximum flow through line 1-4 is limited to 75.2 MW. In this case, to meet the additional 1 MW load at node 4, the generators have to re-scheduled as the old scheduling will overload line 1-4. As per the new scheduling, which can be obtained by running Optimum Power Flow (OPF), the output of generator 1 is to be reduced by 0.1 MW and generator 3 has to supply 1.1 MW. The new line flows are-

24.7 MW in line 1-2,

24.7 MW in line 2-3,

25.8 MW in line 3-4, and

75.2 MW in line 1-4.

Thus, the LMP at node 4 can be calculated as

(1.1 x 350) – (0.1 x 300) = 355 INR

Similarly, the LMP at other buses can be calculated. Now I think that the calculation of LMP is clear to you.  

Let’s know the basics of Arduino Board used for Small Project Applications

Arduino Boards are used commonly in many of the small scale demonstration projects. It has a microprocessor which can be programmed with the help of any of the PCs using the freely available Arduino software. Arduino products i.e. hardware, software etc are based on the concept of open source. The hardware and software developments are freely shared to bring in more new ideas and to further enhance the Arduino concept.

One can implement LED displays and counters, alarm clocks, automatic intensity control of street lights, battery charger, distance sensors and many more demo projects based on Arduino boards. The following paragraphs give the basic idea about Arduino Boards which everyone wishing to get started with Arduino boards will find it interesting.

Arduino Hardware:

The Arduino starter kit essentially consists of an Arduino processing board. It may also have a USB cable to program the Arduino board (from a PC). The board may also be programmed using In System programming (ISP) technique. Other components needed are a breadboard to assemble and check the circuit, jumper wires and elements such as transistors, ICs, resistors, capacitors, LDRs, sensors etc. depending on the application.

Arduino board consists of USB connector to allow programming the processor from any of the PC. It has a USB-to-Serial convertor to establish compatibility between the PC to which it is connected and the ATmega328 processor. The processor is a 28 pin, 8 bit microcontroller arrangement. The processor has a memory system, port system, time system, Analog to Digital Converter (ADC) system, interrupt system and the serial communication system. 

The processor has three main memory sections and they are; 

1. Electrically Erasable Programmable Read Only Memory (EEPROM), 
2. Static Random Access Memory (SRAM) and 
3. Byte Addressable EEPROM.  

The board also has LED indicators to indicate the serial transmission and reception. Analog reference signals, PWM signals, digital Input / Output signals are given to the board through header strips at the top end of the Arduino board. The Output of the board is given to the ADC system and the power supply terminals through another header strips at the bottom end of the board.

Additional features and external hardware may be added to selected Arduino platforms by using Arduino shields or “daughter cards”.

The Arduino board requires power supply. This power may be provided from the USB port or an external DC supply of voltage range 7-12 Volts. The board has an external power supply inlet at the bottom left corner through which external supply is given to the board.

Arduino Software:

The Arduino software is also called "Arduino Development Environment" and is freely available at the Arduino homepage. The detailed instructions regarding the downloading of software, and loading the USB drivers and sample programs are also given in the homepage.

Issues with large scale Renewable Energy integration and the way out

Renewable Energy (RE) particularly Wind and Solar have huge potential and going to be the dominant energy sources in near future. Their large scale integration into the grid is going to cause certain serious issues which need to be addressed. 

Intermittent and Variable in nature:

As we know wind and solar are intermittent and variable in nature, their output depends on the availability of wind and sunlight. Variation in the output may cause significant change in power flow over the transmission and distribution lines, affecting the reliability and security of the power system. 

Most of these RE power plants are located far away from the major load centres and existing transmission lines. Usually these RE plants are connected to the grid at a voltage of 33 kV, 66 kV, 132 kV or 220 kV depending on the capacity of the plant and its location.

Sluggish development of Transmission Infrastructure:

Normally the gestation period of these RE plants are 6 to 12 months, whereas the development of a transmission infrastructure takes some 4 to 5 years depending on the conditions like Right of Way (RoW) requirement, clearances from various government organizations, financial condition of the executing agencies etc.

Limited Reactive Power support:

Currently many of the wind turbines have induction generators which either have no or limited reactive power support, thus causing issues like voltage regulation. Same is the case with line commutated solar PV systems.

Thus, the large scale integration of RE into the grid along with insufficient transmission facility is going to cause serious issues like congestion, voltage regulation, nodal price, supply reliability and security of the system. 

Mitigation methods: 

Some mitigation methods for the above said issues are:

1.   Variation and intermittency in power supply can be better handled by a strong interconnected transmission system.

2. Reactive power support in the form of Static VAr Compensator (SVC) or STATCOM can be provided at the RE power plants or some strategic locations to take care for the reactive compensation and voltage regulation.

3.   Private participation in the transmission sector will help to enhance the execution capability.

4.  Strong weather and hence output forecasting technique along with a strong real time interaction with the System Operator (SO) with help in mitigating certain grid code issues.

5. Some form of storage capacity whether it’s the pumped storage hydro plant or large scale battery storage, will also help to counter a variety of issues discussed earlier.

6.  New energy market structures incorporating special ancillary services such as reactive support services, spinning reserves, flexible generation etc is also going to strengthen the grid operation in the advent of large scale RE penetration.   

7. A separate Renewable Energy Management Centre (REMC), with advanced communication and control techniques, should be planned for the enhanced security and reliability.

So in near future we are going to witness a new and much eco-friendly power system, particularly in the developing countries like India.

Transposition of conductors in Power Transmission Lines

Parameters of Transmission Line:

A transmission line has four parameters, namely resistance, inductance, capacitance and conductance. The resistance ‘R’ of a line is because of conductor resistance, series inductance ‘L’ is due to the magnetic field surrounding the conductors, shunt capacitance ‘C’ is due to the electric field between conductors, and shunt conductance, ‘G’ is because of the leakage current between phases and ground.

What is Transposition of Conductors?

The interchange of conductor positions of a transmission line at regular intervals along the route is known as Transposition of Conductors.

Why transposition is needed?

In the power transmission line when the line conductors are asymmetrically spaced i.e. not equally spaced, the inductance of each phase is different causing voltage drops of different magnitudes in the three phases even if the system is operating under balanced condition (load currents are balanced in the three phases). Also the magnetic field external to the conductors is not zero thereby inducing voltages in adjacent communication lines and causing what is known as “telecommunication interference”. This can be overcome by the interchange of conductor positions at regular intervals along the route and this practice is known as “transposition of conductors”.

How transposition is done?

In a transposed transmission line each of the three conductors occupies all the three positions relative to other conductors (position 1, position 2, and position 3) for one-third of the total length of the transmission line. Transposition also balances out the line capacitance so that electro-statically induced voltages are also balanced. Figure shows the transposition of conductors over a complete cycle.

A complete cycle of transposition of line conductors.

Complications of Conductor Transposition:

Frequent transposition usually leads to complication of support structures (as can be seen by the picture below), increase the cost because of increased number of insulator strings and total weight of supports. 

Transposition on 400 kV, double circuit transmission line, near Bhopal, M.P.  

MATLAB coding for Y Bus partition

Last Updated: Feb 26, 2017

Voltage Stability:

A power system is said to be voltage stable if it is able to maintain steady voltages at all its buses after a disturbance. In other words, one can say that voltage stability is the ability to maintain steady voltages at all the buses in the power system after being exposed to a disturbance. The disturbances may be:

1. Line or Generator outages,
2. Increase in loading,
3. Generators, synchronous condensers and other reactive power sources inching close to their reactive power limits.

A power system is voltage stable if the magnitude of  voltage at a bus increases as the reactive power injection (at the same bus) is increased. At a given operating condition, this is true for every bus in the system.

When the reactive power demand of the load is not fulfilled, voltage collapse occurs. Voltage stability of a power system is on the verge of collapse when a disturbance increases the reactive power demand beyond the available capacity of the system components. Voltage collapse is a usual phenomenon in a heavily loaded power system or a system having shortage of reactive power. The voltage drop in the line impedance during power flow is the main cause of voltage instability. This reduces the power transfer ability of the transmission system and also reduces voltage support ability.

A system is "voltage unstable" if the magnitude of voltage at one or more bus decreases when the reactive power injection at the very bus or buses is increased.

Thus, for a power system, if the V-Q sensitivity is positive for every bus, the system is voltage stable, otherwise for a negative V-Q sensitivity, the system is voltage unstable.

Voltage Stability Index:

Voltage stability analysis of a power system involves determination of an index called the “voltage stability index” which is used as a measure of inclination of Power system towards voltage collapse. These indices are helpful in determining the weak bus so that adequate reactive power allocation can be done.

Methods of determining the Voltage Stability Index:

There are few methods of determining the voltage stability index and “L-index method” is one such method. 

In L-index method, one has to partition the Y bus matrix as Y_GG, Y_GL, Y_LG, and Y_LL, where ‘G’ stands for generator and ‘L’ stands for Load. Matrix Y_LG, and Y_LL are required to calculate the matrix FLG needed for calculation of L-index. Detailed theory can be seen in many research papers.

MATLAB coding for Y bus partition:

The MATLAB coding for Y bus partition is as given below:

 clear; clc;

% File gives the partition of Y bus.

num=6;   % specify the bus system if you to work with many examples.

%  a function file “volt_ang” gives the admittance, magnitude and angle of bus voltage.

%  This file is a part of the NR load flow code  and not given here.

[Y, Vm, Va]= volt_ang(num)

linedt= line_data(num);                     % calling the line data for the system

busdt= bus_data(num);                     % calling the bus data for the system

nb= max(busdt(:,1)) ;                        % gives the total number of buses in the system

type =busdt(:,2) ;                    % identify the type of bus i.e. ref., generator, and load      

pv = find(type==2 l type==1);           % identify the PV bus           

npv = length(pv);                               % gives the number of PV buses
pq = find(type==3);                          % identify the PQ bus 
npq = length(pq);                               % gives the number of PQ buses

for m=1:npq,
for n= 1:npq,
Y_LL (m,n) = Y (pq(m), pq(n));
end
end

for m=1:npq,
for n = 1:npv,
Y_LG(m,n)= Y(pq(m), pv(n));
end
end

FLG = (Y_LL)^-1*Y_LG

Special arrangements for transportation of Large Power Transformer

Large Power Transformers (LPT) are large in dimension, and heavy in weight. They can cost millions of dollars and weigh between 100 to 400 tons. For example a 765 kV, 750 MVA, three phase transformer with size 56 ft (W) x 40 ft (L) x 45 ft (H) can weigh 410 tons  They pose unique requirements to ensure safe and efficient transportation. Hence the weight and dimension of large power transformers need careful planning and the critical transportation aspect should be kept in mind.  

Power transformers can be transported by rail, road, air and sea route. Depending on the size of the transformer unit and on the route and transport conditions, a transformer may be transported completely or partially assembled. LPTs have to be transported with bushings, conservator, cooling arrangements and all other minor accessories removed. If the transformer tank has been drained for transportation, it is necessary that the oil should be replaced by dry air or nitrogen maintained at a slightly positive pressure above the atmosphere. This ensures the dryness of the winding during the entire transportation.

Large power transformers cannot be transported on normal rail cars. The heaviest load a rail-road normally carries is 100 tons whereas the LPT can be 4 times of that weight. A specialized rail-road car called Schnabel car, is used to transport extremely heavy loads. Some LPT are designed and made as an integral part of the Schnabel car. The transformer is designed so that it can be attached to rail car frames with the help of a pinning system. These cars may have 20 or more axles depending on the weight of the transformer to be transported.     

When LPTs are to be transported via road, special permits are also required from various government agencies. Before issuing these special permits, careful inspection of the entire route through which the transformer has to pass, is carried out. Inspection of bridges and their load bearing capacity are of prime importance while issuing such permissions. Hasty permits may lead to serious accidents as one happened in Madhya Pradesh in year 2011, in which a huge trailer carrying a 380 ton power plant equipment on Sagar-Bhopal road was washed away when the bridge through which the consignment was passing collapsed due to the heavy weight killing at least 3 persons and damaging the power plant equipment.