Power Conditioning Unit: A key element of Solar PV System

Power Conditioning Unit (PCU) is a very vital piece of equipment in any solar PV system and is also called solar power conditioning unit. The main components of any solar PV system are PV panels, charge controller, battery bank, inverters, cables, switches and the protection.

"Power Conditioning Unit (PCU) is a combined unit consisting of a solar charge controller, an inverter and a grid charger integrated in a single unit." 

Role of Power Conditioning Unit (PCU)

Power Conditioning Unit (PCU) is usually a DSP based PWM technology using IGBT and MOSFET. It facilitates the charging of battery bank through either solar PV panels or the grid/DG system. All the solar PCU has the ability to continuously monitor the state of battery, solar power output and the load. They not only monitors the state of affairs but even displays vital parameters such as PV voltage and current, load percentage, overload percentage, charging current etc.  

The solar energy is intermittent in nature and therefore a balance has to be made between the solar generation and the demand. When excess energy is generated the extra kWh is to be either fed into the grid, which is possible only in case of grid tied system, or to be stored into the battery bank. Due to over usage of power, if the battery voltage goes below a pre-defined level, the PCU will automatically transfer the load to the utility grid and simultaneously charges the batteries through the grid supply. 

Once the batteries attain a given voltage, the PCU cuts off the grid power to the system and returns back the load on to the solar system. The rest of the battery charging is now done by the solar PV system. In this way the PCU gives priority to the solar power over the grid, and uses the grid power only when the solar power and the battery power of required level is not available.

Fig.1: Back view of an off-grid Solar PCU

Significant advantages of PCU

The significant advantages of PCU are pure sine wave output with low Total Harmonic Distortion (a measure of power quality), higher efficiency, data logging monitoring etc. The commercially available PCU have efficiency more than 85% and nearly 5% Total Harmonic Distortion (THD) for linear loads. These units come with the deep discharge protection and thus ensure the health of batteries. PCUs have nearly 300% overload facility for a few milliseconds which helps during the starting of heavy loads. Thus these units have the inbuilt overload and short-circuit protection and therefore no need to worry about overloads and short circuits. Remote monitoring of the unit can be done with the help of RS232, Ethernet, GSM and GPRS.

Ratings of commercially available single phase PCU

The usual ratings of commercially available single phase off-grid PCU in India are 600 VA/24 V, 1kVA/24 V, 2kVA or 3kVA/48 V, 3/4/ 5/6 kVA/96 V, 7.5 kVA/120 V 

C-rating and Efficiency of a Battery

Batteries are the key elements used in Standalone Roof top solar PV systems and hence one should know a bit about these storage devices. Of the various types of batteries, based on the electrolyte material, the most commonly used battery type is the Lead-Acid type particularly in solar PV applications.

How batteries are rated?

Batteries are rated according to their:

1.   Voltage,

2.   Storage capacity, and

3.   Ability to deliver the stored energy over a given time period.

What is C-rating?

Energy storage capacity is given in Ampere-Hour (Ah) at some nominal voltage and at some specified discharge rate. The storage capacity is never fixed and depends mainly on how fast the energy is extracted from the battery. The manufacturers usually specify the ‘Ah’ capacity at a discharge rate that would drain the battery completely over a specified period of time at a specified temperature. The ability to deliver the stored energy over a given time period is called the ‘C’ rating. Batteries are available in market with different ‘C’ ratings such as C 5, C 10, C 20, and C 100. The batteries usually used for solar application in India are the C 10 type.

Fig.1: C-10 rating battery used in Solar PV application.

Let’s go through an example….

For example, a fully charged 12 V battery that is specified to have a 10 hour, 100 Ah capacity could deliver 10 A for 10 hours, after which the battery will get fully discharged. This ‘Ah’ specification is known as ‘C 10’ rate, where ‘C’ refers to the Ah capacity and the 10 is hours it would take to completely deplete.

Dependency of Storage Capacity.

As mentioned above the ‘Ah’ capacity or storage capacity of a battery is very much dependent on the discharge rate and the temperature. More rapid draining of a battery i.e. higher discharge rate results in lower ‘Ah’ capacity and vice-versa. In simple words, the above 100 Ah, C 10 battery would not last for 1 hour if you drain at a 100 A discharge rate and on the other hand it would last more than 100 hours if the rate is 1 A.

The battery capacity decreases significantly in cold conditions. There is an apparent improvement in battery capacity with the increase in mercury; but this does not mean that batteries are safe in hot climates. Rather the battery life is shortened by approximately 50% for every 10^oC rise above the optimum operating temperature of the battery normally 27^oC.   

Since the rated capacity of the battery is specified at a temperature and discharge rate (specified by the manufacturer), one has to adjust the battery capacity according to the prevailing temperature at the site and the discharge period. 

Does battery connection affect the ‘Ah’ capacity?

The ‘Ah’ capacity of the battery also depends on the connection of the battery. For example, two batteries connected in series will have the same current and hence the ‘Ah’ capacity remains the same. If the same batteries are connected in parallel, their current adds up and so is the ‘Ah’ capacity. But the energy stored in a battery bank, whether the batteries are connected in series or parallel, remains the same. Batteries when connected in series have higher voltage and lower current and hence the voltage drop and power loss are lesser. When batteries are connected in parallel, the weakest battery will bring down the voltage of the entire arrangement. Similarly, in series connected batteries, failure of one battery will completely shut down the system.

Why the manufacturer specifies both ‘Ah’ efficiency and ‘Wh’ efficiency?

Since the voltage varies throughout the discharge period, one cannot calculate the energy delivered by the battery during its discharge by simply multiplying 12 V x 10 A x 10 h = 1200 Wh. Therefore, battery storage capacity is mentioned in ‘Ah’ rather than ‘Wh’.

Manufacturer specifies the efficiency of their battery in terms of ‘Wh’ efficiency and ‘Ah’ efficiency. For example, the ‘Ah’ efficiency and ‘Wh’ efficiency of Luminous make Flooded Lead-Acid tall tubular batteries are greater than 90% and 80% respectively as claimed by the manufacturer.

So let’s have a look into these two efficiencies.

The amount of electrical energy stored in a battery is measured in ‘Wh’  

Energy efficiency in Wh 

= energy discharged in Wh / energy required in Wh to completely recharge

‘Ah’ efficiency 

= 'Ah' discharged / 'Ah' required for complete recharge

The energy or ‘Wh’ efficiency of a battery is always less than the ‘Ah’ efficiency because battery discharges at a lower voltage than they charge at.

A battery should never be discharged more than 80%, even under worst conditions. The more you extract from a battery every day, the more it will wear out. The wear out rate depends on the type of the battery and its cycle life. The life cycle, as mentioned in the Luminous battery catalogue, with 80% Depth of Discharge (DoD) is 1500 cycles, 50% DoD is 3000 cycles, and 20% DoD is 5000 cycles.   With this one can understand what the C rating, ‘Ah’ capacity and ‘Wh’ capacity is and how to interpret the nameplate rating of a battery. 

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.