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.