About the Solar Charge Controller

A solar PV system has several components. Apart from the solar PV panels, it has charge controller, inverter, battery bank, supporting structure, protective fuses, breakers & surge protectors, cables etc. All the components other than solar PV panel are collectively called Balance of System (BoS). A well designed system is required for the smooth and reliable operation of the Solar PV plant.

What is Solar Charge Controller and how it works?

A Solar Charge Controllers (S.C.C.) or simply Charge Controller monitors and controls the power output from the solar PV panels and the battery. The Controller controls the flow of charge into the battery while charging from the solar PV system, thus prevents overcharging of batteries. It detects the status of battery charge by measuring the terminal voltage of the battery.

In overcharge condition, the battery terminal voltage increases above a certain level. When the battery reaches overcharge status, as shown by the measured voltage, the charge controller cuts off the solar PV supply to the battery.

As the connected load continues to use the battery energy, the terminal voltage of the battery drops down. This drop in voltage is also detected by the charge controller as it is continuously monitoring the activities. Whenever the battery voltage reaches the normal operating range, the battery is connected back to the solar PV system, by the charge controller, for regular charging.    

Similarly, the charge controller also controls the discharge from the battery. It prevents deep discharge of the battery. In deep discharge condition, the battery terminal voltage decreases below a certain level. This happens because of excessive drainage of charge from the battery, probably due to prolonged use of significant load. As the battery gets into deep discharge state, the charge controller detects it from the measured voltage and disconnects the battery from the circuit, so that no current can be further drawn from the battery. The life of a Lead-Acid battery very much depends on the Depth of Discharge (DoD).

"For example, as per Luminous, a renowned solar battery manufacturer in India, solar Flooded Lead-acid Tubular batteries have a life of 1500 cycles at 80% DoD, 3000 cycles at 50% DoD and 5000 cycles at 20% DoD."

Thus, both overcharging and deep discharging of battery or battery bank must be avoided to enhance the battery life.

Features available in Charge Controllers:

A Charge Controller usually/ may have the following features:

1.       LED or LCD display of various parameters or functions,

2. Automatic Temperature Compensation feature; automatically adjusts the battery charging according to the ambient temperature,

3.  User defined setting of battery voltage and type,

4.  Protection for low voltage, overvoltage and reverse connection,

5.  Automatic priority selection feature,

6.  Equalization feature.

Omission or addition of certain feature may happen, and it depends on the manufacturer.

Types of Charge Controllers:

The two commonly available types of Charge Controllers are:

     Pulse Width Modulation (PWM) Charge Controller, and

2.  Maximum Power Point Tracking (MPPT) Charge Controller.

Pulse Width Modulation (PWM) Charge Controller:

PWM Charge Controller is a solid state controller that usually works on three step charging algorithm. It has a semi-conductor switch which is switched ON and off by PWM at a variable frequency to maintain the battery voltage. When the battery voltage reaches the pre-specified value, the charging current is reduced as per the charging algorithm to avoid heating and gassing of the battery.

The PWM charge controller adjusts the charging according to the battery condition and requirement by controlling the speed of the switching element, which breaks the PV output current into pulses at some constant frequency and varies the width and time of pulses to regulate the amount of charge flowing into the battery.

Pulses of current helps the battery as it mixes the electrolyte, clears the lead electrode and prevents sulphation.

PWM charge controller maintains the battery capacities to 90-95% and has the ability to recover lost battery capacity. This helps in equalizing drifting of battery cells, automatically adjusts battery aging, increase the charge acceptance of the battery and self regulates the voltage drops and temperature effects of the solar PV system. These charge controllers are cheap and available in a wider range of capacities.

PWM charge controller is a good low cost option for small roof top solar PV systems where the ambient temperature is moderate or high.

Drawbacks of PWM charge controller:

1. The controller voltage must match the battery bank voltage, and

2. Usually the maximum current capacity of PWM charge controller is limited to 60 A.

Maximum Power Point Tracking (MPPT) controller:

Maximum Power Point Tracking (MPPT) controller is a charge controller with an additional device called the Maximum Power Point Tracker. These controllers provide a digital tracking of the PV panel output and compare it with the battery voltage. It then works out the maximum power that the panel can flush out to charge the battery or to the load. It tracks the optimum voltage so as to get the maximum amperage to charge the battery. Actually it is the amperage which makes sense for the battery.

MPPT charge controllers have higher efficiency, thus higher output power and overall better battery management than PWM charge controllers. These charge controllers continuously adjust the load on the solar PV system under varying operating conditions and keeps it operating at the maximum power point. As mentioned earlier, the controller checks the output of the PV array and compares to the battery voltage. It then calculates the maximum power that the PV array can produce. Accordingly, the controller converts the PV output voltage and converts it to the optimum value that fetches the maximum current into the battery or the load.

MPPT power varies according to the weather conditions, i.e. solar radiation, ambient temperature, and cell temperature. The voltage point at which the PV system produces maximum power is called Maximum Power Point (M.P.P.). Thus, the MPPT charge controller acts as a DC voltage converter which converts the PV array voltage into a voltage that fetches maximum power. The converter converts the DC input from the PV into a AC voltage and converts it back to DC matching the battery voltage. A Buck converter is used to step down the voltage, whereas a Boost converter is used to step up the voltage. They are used in PV systems of higher capacity, although MPPT charge controllers with smaller capacity say 17 A, is also available in the market.

For a PWM charge controller, the output current is the same as the input current, whereas for a MPPT charge controller, the output power is the difference of the input power and controller losses. With an MPPT charge controller employed, the system can deliver 20 to 45% more power in winter and 10 to 15% more in summer. The actual gain may vary depending on the weather temperature, state of the battery charge etc.

Figure 1 shows a 12 V, 17 A MPPT solar charge controller of Su-Kam make which is available in approximately 2,300 INR (in Bhopal, MP).

Fig.1: 12 V, 17 A MPPT solar charge controller of Su-Kam make 

    

MPPT charge controller has the following advantages:

1.  More effective in low temperature and cloudy days,

2. Have better efficiency in the range 93-97%. ( much better than PWM controller).

The drawbacks are complex circuit and comparatively higher cost.

In some solar PV systems, the charge controller and the inverter are collectively housed in a single unit and are called the Power Conditioning Unit (P.C.U.). Batteries are also among the key elements used in off-grid solar PV systems and hence one should know a bit about these storage devices.

Also read:

Ø  PowerConditioning Unit: A key element of Solar PV System

Ø  C-ratingand Efficiency of a battery
Ø  Knowyour Solar Batteries

Net Metering arrangements for Roof top Solar PV Systems

Most of the Indian cities, towns and villages have nearly 250 to 300 days of sunny days and thus roof top solar PV system can be a very attractive option. The Jawaharlal Nehru National Solar Mission (JNNSM) with the goal to encourage roof top solar PV systems in India has implemented a separate scheme known as “Roof top PV and Small Scale Solar Generation Program”. 

In my opinion, Roof top solar PV systems are going to witness appreciable capacity expansion particularly on the residential and commercial buildings in India and can be seen as a significant business opportunity.

Types and Models of Roof top PV systems:

The roof top solar PV system can be:
1. A grid interactive system or,
2. A stand-alone system. 

In grid interactive system, the DC power produced is converted to AC power, and after due conditioning this power is fed to the captive loads of the premise. The excess power is injected to the utility grid through 11 kV or LT lines, depending on the size of the solar PV system. When sufficient solar PV power is not available, the loads are fed from the grid supply. 

Although the grid interactive solar PV system is not supposed to have a battery pack, but to enhance the reliability a minimum battery backup of 1 hour is technically recommended. There is some minimum capacity constraints ( 1 kWp) below which the system can not be grid interactive.    

In the stand-alone Roof top Solar PV system, the electrical energy produced by the solar modules are fed to the battery bank through a charge controller. A suitable inverter is connected to the battery which converts the DC output of the battery (or the Solar PV) into AC of specified voltage and frequency to run the electrical appliances of the premises. This system is not connected to the utility grid and hence excess electrical energy produced by the solar PV cannot be exported to the grid and similarly if extra energy, in excess of solar PV output, is required cannot be imported from the utility mains. Thus, such a system requires fine balancing of generation, demand and storage capacity.  

Different arrangements of Roof top solar PV system:

Two different arrangements of Roof top solar PV system do exist. They are:

1.      Self owned roof top solar PV system, and

2.      Third party owned roof top solar PV system.

In the self owned arrangement, the roof top owner or the premise owner installs the solar PV system, either of its own or with the aid of a system supplier. The power produced is first used to feed the captive loads within the premise and the excess power is fed to the grid.

The third party roof top solar PV system can be an attractive and viable model particularly for residential sector in India and seems to be a good business and job provider. In this model a developer or intermediate agency called the “third party” design, install and lease out the solar PV system to interested roof top owners who in turn pay them a monthly rent. 

This model offers some great advantages; 
(i) the roof top owner does not have to invest for the PV system, and
(ii) does not have to bear the brunt of the technological risks involved in a fast changing solar PV business.

Net Metering arrangement:

In both the above mentioned roof top solar PV models, the feeding of excess energy to the grid or the drawal from the grid in case of insufficient solar power generation is through a net metering arrangement. 

Two meters have to be installed at the premise and will replace the existing meter. One, the solar energy meter to record the energy produced by the PV system and other the net meter, a bi-directional meter, to record the import/export of energy by the consumer from/to grid. The point of solar power injection may be in between the load and the bi-directional meter. These meters are supposed to have the MRI and AMR facility and should adhere to the standards specified by the CEA regulations. Figure 1 shows a net meter installed for a 2 kW grid tie Roof Top Solar PV system in Bhopal, India.

Fig. 1: A Net Meter installed for a 2 kW Grid-tie Roof Top Solar PV system in Bhopal, India.

The net energy (kWh) injected to the grid during a billing month/period is supposed to be carried forward. At the end of the financial year any net energy credits which is yet not adjusted is paid to the owner of the premise/consumer by the distribution company. To promote the scheme, certain states have waived off the wheeling charge, cross subsidy surcharge and other similar charges/levy for a period of 5 years.  

If the cost of solar energy and the grid energy is kept the same, there may not be any significant financial benefit to the residential consumers, although the commercial consumer paying at a much higher grid tariff may get benefited. To get more, the residential premise owners have to install a much larger PV system. 

Policy on roof top solar PV systems of certain states in India has put a cap on the solar generation which is about 90 % of the annual energy drawal. In the net metered solar PV arrangement the roof top owner  can also avail the available subsidy (currently 30 %) from the Ministry of new and Renewable Energy (MNRE) for which he or she has to approach the state nodal agency.

Safety in Power Plants, Stations and Electrical Networks: A Collective responsibility?

Indian electrical power system consists of many generating stations, sub-stations, and transmission & distribution lines of voltage range 1200/ 765/ 400/ 220/ 132/ 66/ 33/ 11 kV. All the associated equipments of power system are monitored, attended, operated and maintained round the clock for un-interrupted and reliable flow of electricity throughout the country. New construction or capacity addition activities are also carried out simultaneously. Most of the operation and maintenance activities are done by regular engineers and technicians of the concerned organization, whereas bulk of the construction works is carried out through outsourcing.

The electrical and electronic equipments and systems used in any power system network are designed in such a way that they are safe during normal operation, but in case of mal-operation or faults they can be very dangerous.

This article is supposed to throw some light on safety aspects, and to grab the attention of our young engineers and technicians.

What is safety?

What the word safety means as far as electrical power stations and networks are concerned?

“Safety may be interpreted as proper planning of task, proper usage of safety tools and equipments, following safety procedures, exercising good judgement and intelligent supervision.”

Fig 1: A combination of unsafe working conditions,and unsafe activities.

 Analysis show that majority of accidents were preventable.

Safety: A Collective responsibility

With a massive growth and expansion of generating stations, substations, lines and associated systems, safety at work place is of prime importance. Accident do not “just happen”, but they are the outcome of unsafe working conditions, unsafe activities or a combination of both. 

Then who is responsible for the “safe working environment” in electrical plants and networks?

It is our, i.e. of electrical engineers, technicians, and contractors, collective responsibility to implement safety standards and maintain a safe working environment in the best possible way.

We must learn from past mistakes, evolve gradually and move towards an accident free atmosphere. The presently followed safety standards should be enhanced to minimize, if not possible to eliminate, accidents and injuries.

Rules, Act and Code to be followed:

One should follow the safety policy, statutory provisions pertaining to safety, responsibility assignment, hazard identification, and personnel protective equipments and tools.

All the electrical engineers/ technicians working on electrical power projects in India, must ensure compliance with the requirements of Indian Electricity (I.E.) Rule 1956, Indian Electricity Act 2003, Indian Electricity Grid Code and Central Electricity Authority (C.E.A.) Regulation 2010.

Role of Electrical Engineers:

As an electrical engineer it must be our policy to perform each task in the safest possible manner, together with good practice. The health, safety and well-being of our workers and all those who are likely to be affected, are our responsibility. Safety aspect should be kept at par with our business objectives. Not only the engineers, it is the duty of each worker engaged in power system construction, operation and maintenance activities to exercise due care and caution for his/ her own safety, of fellow workers, and the concerned equipment and system.

As an engineer, we must:

Ø  Provide safety information, instruction and training to the team,

Ø  Ensure that tasks are allocated to competent workers,

Ø  Ensure safe and proper handling and use of equipments,

Ø  Provide and maintain a safe plant and Equipment, and

Ø Consult with our fellow engineers, sub-ordinates, and technicians on matters affecting the safety.

Remember:

Prevention of accidents or mis-happenings requires a whole hearted co-operation of all the concerned. Usually it’s the capable and mentally alert worker who avoids the accident. Good luck.

String size calculation in Grid tied Solar PV system

Solar modules are connected in series or parallel or series-parallel combination to get the required voltage, current and power. Series connection will increase the string voltage while keeping the current constant. On the other hand parallel connection will increase the current whereas the voltage remains the same. Now what about the voltage, current and power in the series-parallel connection? Yes  you are correct, all the three will increase.

The capacity of the solar PV plant depends on the roof space available and the individual requirement. Once the PV module capacity is decided we can size the inverter. Normally the inverter size is smaller than the rated output of the PV array at Standard Test Conditions (STC). This is due to the losses or de-rating factors such as panel dissimilarity, dust, losses in cables etc. An inverter of 80 % size of the PV array is quite normal.  

What is a string in the solar PV system?

Number of modules connected in series is called the “string”. Size of the string determines the voltage input to the inverter. The maximum and minimum number of modules in the string depends on the maximum and minimum voltage of the inverter.

The output voltage generated from the PV should never damage the inverter that’s why we have to calculate the maximum system voltage. The maximum system voltage should not be more than the highest acceptable inverter voltage.

Similarly we have to calculate the minimum number of modules in the string so that in worst case scenario the PV system’s output voltage is sufficient enough to turn the inverter ‘ON’.

Relationship between Ambient Temperature and String Output:

As expected, a certain relation exists between the ambient temperature and the string voltage, which is to be considered while calculating or designing the string size.  The PV output is inverse to the ambient temperature i.e. with the decrease in ambient temperature; there is a certain increase in string voltage and power. The vice-versa happens during summer season. With a correctly sized PV array, the DC output will remain within the optimum operating range of the inverter under different working and ambient conditions.

What should be the minimum string size in the solar PV system?

Softwares are also used in the design, construction, operation and maintenance of Solar PV plants. These softwares help to optimize the design configuration and system layouts.

In the following paragraphs, manual calculations of string size is given for the easy understanding of concepts. They are easy enough to be done manually.

For the calculation we need some data from the data-sheet of the module. Suppose the;

Voltage at Open Circuit, V_OC = 43.4 V,

Temperature at Standard Test Condition (STC) = 25^oC,

Temperature Coefficient at V_OC = (-) 0.15 V/ ^oC,

Voltage at Maximum Power, V_mp = 35.4 V,

Temperature Coefficient at V_mp = (-) 0.17 V/ ^oC,

Now from the inverter data sheet, we have to get the maximum input DC voltage and the start ( or strike) voltage of the inverter. From the environmental data we have to collect the hottest day-time temperature and coldest day-time temperature of the location where the PV system is to be installed.

Suppose that the effective cell temperature during the hottest day is 70 ^oC, which is 45 ^oC above the temperature at Standard Test Condition (STC) of 25^oC.

Therefore, the V_mp voltage would be changed (reduced) by 45 x (-) 0.17 = (-) 7.65 V

Hence the V_mp at 70 ^oC cell temperature would be 35.4 + (– 7.65) = 27.75 V.

Next we should consider the de-rating factor which is due to the earlier mentioned factors such as module mismatch, dust & dirt, cable loss etc. Let it be 0.88.

So the effective minimum V_mp for each module at the inverter input would be 27.75 x 0.88 = 24.4 V.

From the above results we can calculate the minimum permissible number of modules in the string.

Assume that the minimum start (or strike) voltage for the inverter is 140 V. Taking a safety margin of 10%, i.e. 140 x 1.1 = 154 V. This means that the string size should be so selected that a minimum of 154 V is maintained at the inverter input terminals in the worst case scenario.

Hence the minimum number of modules in the string to surpass the start voltage is

154/ 24.4 =  6.31 rounded up to 7 modules.

What should be the maximum string size in the solar PV system?

As mentioned earlier, the output voltage of a solar PV module increases as the ambient temperature drops below the temperature at STC.  At the coldest day-time temperature the V_OC of the array shall never be greater than the maximum allowed input voltage of the inverter. Now we have to calculate the maximum number of modules in the string. Oversizing the string can damage the inverter, cancel warranties and violate the safety codes.

Let’s suppose that during the coldest day, the effective cell temperature is 15 ^oC, which is 10 ^oC below the temperature at STC of 25^oC. 

Therefore the V_OC should be increased by (15 – 25) x (-) 0.15 = 1.5 V. You can say that the Voc at 15 ^oC is 43.4 + 1.5 = 44.9 V.

Assume that the maximum safe working voltage allowed by the inverter is 400 V. Then the maximum number of modules permissible in the string is 400/ 44.9 = 8.9 rounded down to 8 modules.

The goal of this article was to convey the basic process for sizing the PV string for a grid connected system. Let’s promote Solar PV system in our vicinity.