How to convert your existing battery inverter system into a solar PV /battery inverter system?

Renewable energy such as solar energy is fast becoming a reality in the Indian domestic scenario. People are keenly interested in installing the roof top solar PV system:

• to reduce their dependence on the grid electricity, 
• to reduce their bill on electricity and 
• to contribute from their end towards the greener environment (In India more than 70% of the electricity is produced through thermal power plants). 

In such a situation, people who already have a battery and inverter system in their house and office are in a dilemma as they want to switch to the newer and emerging trend at the same time concerned towards the investment already made in the battery/ inverter system. The answer to them is the hybrid solar charge controllers available in the market.
  

How a solar charge controller works?

A solar charge controller can be connected to any existing home UPS /inverter to make the simple battery inverter system into a solar PV/ battery inverter system.  

When the battery or battery bank is fully charged and the solar power is still available, it automatically disconnects the mains supply from the inverter and the power to your electrical load is fed from the battery and solar PV system. The inverter is now working in the back-up mode and thereby forcing energy conservation; which is the much serious agenda now a days. This automatic operation of hybrid solar charge controller helps in utilizing the solar power to the most.

When the battery is discharged below a specific limit, the electrical load is automatically transferred to the mains supply i.e. grid supply, and the battery is now connected to the mains through the inverter for the charging process. The limit to which the battery energy is to be used is also user defined i.e. user can set the battery discharge limit to one of the available limits.

There are several hybrid solar charge controllers available in the Indian market, for example Solarcon by Su-Kam, which converts any existing domestic or commercial home UPS/ inverter into an intelligent solar PV system inverter.

Fig.1: Su-Kam Solarcon charge controller

Charging of the battery:

The charging of the battery is carried out by these controllers in 3 modes and the modes are:

1)      Boost mode,

2)      Absorption mode and

3)      Float mode.

Boost mode is the first stage charging where the battery is charged with maximum available charging current and that too at the highest voltage. This voltage is usually 14.2 V for Flat Plate Lead-acid battery and 14.6 V for a Tubular Lead-acid battery.

Once the battery is charged to its 80% charge level, the controller shifts to absorption mode. At this stage most controllers will maintain a steady voltage and a reduced current. This lower current charging increases the charge on the battery till it reaches 100% level. This stage takes comparatively more time as the charging current is low. The controller remains in this state for a pre-defined time after which the operation is shifted to float mode.

In the float mode, the charging voltage is reduced to the float level, about 13.7 V, and this level is maintained. The current is also reduced to a low value, called the "trickle". Charge is going into the battery all the time but only at a rate safe enough to ensure full state of charge.

Some charge controllers even have three additional stages of charging which enhances the performance of the battery, and hence the overall PV system. The six stages are:

1. De-sulphation stage,
2. Soft start,
3. Bulk or Boost charging,
4. Absorption charging,
5. Float charging,  
6. Equalization stage.

In the De-sulphation stage, a low value of current but at a high voltage is given to the battery so as to detach the sulphate crystals from the battery plates. De-sulphation helps the battery in producing optimum power.

During soft start, as the name suggests, charging is done at a low voltage and current until the battery attains 10-15% state of charge. Equalization happens once in a month or so as programmed by the manufacturer, during which a high voltage is applied to the battery which shakes the battery electrolyte. This enhances the charging efficiency.

How they improve the battery life:

Thus, these intelligent controller prevents overcharging and overheating of batteries and hence enhances the life to a greater extent. Different types of batteries such as lead-acid, tubular etc. are used with the inverter system and these controllers have the ability to easily adapt to the type of battery used and changes the charging profile and thereby enhancing the battery performance.

Several controllers come with the monitoring feature i.e. it continuously monitors and displays the generation through solar PV system. It also displays other essential parameters of the system such as battery voltage, PV voltage etc.

The Su-Kam Solar Charge Controllers  are available in 12 V, 24 V and 48 V option with the current range of 10 A to 45 A. Normally these controllers are equipped with short circuit protection. A 12/ 24 V 20 A charge controller cost about 3,300 INR whereas a 45 A is available at 4,400 INR (as on Dec 2016 in Bhopal.)

   

Initiation of Arc in Electric Circuit Breakers and it’s Interruption theory

Initiation of Arc in an Electric Circuit Breaker:

The electric arc is a type of electric discharge between the contacts of the circuit breaker. Arc plays an important role in the behavior of an electric circuit breaker. A circuit breaker should be capable of extinguishing the arc without getting damaged.

As the contacts of a circuit breaker begin to separate, the voltage is appreciable and the distance of separation is very small. Therefore, a large voltage gradient occurs at the contact surface. When the voltage gradient attains a sufficiently high value (10⁶ V/cm) electrons are dragged out of the surface causing ionization of the particles between the contacts. The emission of electrons because of the high value of voltage gradient is known as field emission.

Although this high voltage gradient exist only for a fraction of micro-seconds, but a large number of electrons are liberated from the cathode because of this. These electrons move towards the positive contact i.e. anode at a very rapid pace. On their way to anode, these electrons collide with the atoms and molecules of the gases and vapour existing between the contacts. Hence, each liberated electron tends to create other electrons. If the current is high, which is certainly in case of an electric fault, the discharge attains the form of an arc.  

The temperature of arc is high enough and causes thermal ionization. The liberation of electrons because of high temperature is called thermal emission. Thus, in an electric circuit breaker, an arc is initiated because of field emission but is maintained due to thermal ionization.

Arc Extinction in Circuit Breakers:

As per the above discussion, in every electric circuit breaker the arc is there to be formed. So there should be ways and methods to extinct or quench the arc. There are two methods of arc extinction. They are:

1.       High resistance method, and

2.       Low resistance or Current zero Interruption method.

High Resistance Method of Arc Extinction:

In this method, the arc resistance is increased with time to such a high value that the current is reduced to a value insufficient to maintain the arc. The rate at which the resistance is increased or the current is decreased should be such that no harmful voltages are induced in the system.

The resistance of an arc can be increased by any or all of the following de-ionization methods:

1.       Lengthening the arc by increasing the gap between the contacts.

2.    Constraining the arc; the arc is constrained to a very narrow channel. The arc resistance increases with the decrease in the cross section of the arc.

3.    Cooling the arc; the voltage required to maintain ionization increases with the decrease in temperature.

4.     Splitting the arc; the arc resistance can be increased by splitting the arc into a number of smaller arcs in series.

Now the question arises where, I mean, in which circuit breaker, this method of arc extinction is used? Since the energy dissipated in the arc is high and therefore, the high resistance method of arc extinction is employed only in low and medium power AC circuit breakers and in DC circuit breakers.

Low Resistance or Current Zero Method of Arc Extinction:

In AC circuits there is a natural zero of current present in the system. This property of AC circuits is used for the interruption purpose and the current is not allowed to rise again after a zero occurs. It is undesirable to cut-off the current at any other point, other than natural zero, because this may induce a high voltage in the system. 

The current zero method of circuit interruption is adopted only in AC circuits and is employed in all modern AC circuit breakers.

The two main theories explaining current zero interruption are:

1.       Energy Balance or Cassie theory: According to this theory, the interruption or re-establishment of the arc is an energy balance process. If the rate at which the heat is generated between the breaker contacts is lower than the rate at which heat is dissipated the arc will be extinguished or else it will re-strike. The amount of heat generated is variable and depends on the separation of contacts. Initially when the contacts are about to open the heat generated is zero as the re-striking voltage is zero. The heat generated is again zero when the contacts are fully open as the space between the breaker contacts has become de-ionized and hence the resistance is very high. The heat generation rises to a maximum in between the two states of the breaker contacts. This theory is also known as Cassie theory.

2.       Slepian theory: Current zero is the stage where the degree of ionization is minimum. The ionization at current zero depends upon the voltage appearing across the breaker contacts. This voltage is known as re-striking voltage. If at this current zero, the electrons and ions between the contact space can be removed either by recombining them with neutral molecules or by sweeping them away by forcing the insulation at a faster rate than the rate of ionization, the arc will be interrupted. The recombination can be accelerated by cooling and by increasing the pressure in the arc space. This theory was given by Dr. J. Slepian and therefore called the Slepian theory.

What are HRC Fuses and how they are superior to ordinary fuse?

A fuse is a small piece of wire connected in between two terminals mounted on an insulated base. It is the cheapest and simplest form of protection and is used for protecting low voltage equipment against overloads and short circuits. Although fuses such as drop-out fuses are employed for the protection of high voltage equipments also. For example; protecting a 33/11 kV transformer using drop-out fuse at the HV side.

A fuse is connected in series with the electrical circuit and is supposed to carry the normal working current of the circuit safely. During overloads or short-circuits, the current in the circuit and through the fuse increases, depending on the type of the fault. This increase in current causes the fuse element to heat up to the melting point. 

Thus, one can say that a fuse is a protective element in the power circuit which essentially consists of a fusible element in the form of metal conductor of specially selected small cross-sectional area.     

How the normal fuse works?

The heat developed in the fuse element during normal operation is readily dissipated and therefore, the fuse element remains at a temperature below its melting point. However during abnormal conditions, the heat generated due to the heavy currents cannot be dissipated fast enough and the fusible element melts and eventually breaks and protects the circuit from further damage.

The time needed for the blowing out of fuse depends on the magnitude of the abnormal current. The fuse has inverse time-current characteristics i.e. larger the current, smaller is the time required to operate.

What are the materials used in a normal fuse element?

The materials used for the fuse element must be of low melting point, high conductivity, low ohmic losses, low cost and free from deterioration. The materials normally used as fuse element are tin, lead, silver, zinc, aluminum, copper etc. For small values of current alloy of lead and tin (37% lead and 63% tin) is used. When the circuit current is expected to be more than 15 A, the diameter of the fuse wire will be large and after fusing the metal released will be excessive if alloy of lead and tin is used as fuse element and hence copper or silver is used.

What is HRC Fuse and why they are superior to ordinary fuse?

The protective capacity of simple re-wirable fuse is uncertain. The fuse wire is subjected to deterioration due to oxidation through the continuous heating up of the fuse element. In a modern distribution system, it is essential that fuses should have a definite known breaking capacity and this breaking capacity should have a high value. The answer is High Rupturing Capacity (HRC) fuses.

An HRC fuse consists of a ceramic body and brass end caps to which are welded fusible silver (or bimetallic) current carrying elements. The space within the cartridge surrounding the fuse element is completely packed with a filling power usually Quartz. Quartz has excellent arc extinguishing properties.

The operation of HRC fuse consists of the folowing steps:

1.       Overcurrent or short circuit in the network causes melting and vaporisation of the fusing (silver) element.  

2.       During arcing, the fusion of silver vapor and the quartz filling powder takes place.

3.       A high resistance is created as a result of this fusion.

Characteristics of HRC fuse:

The HRC fuse elements are accurately shaped and designed for consistency and reliability. HRC fuse link operates within very close band to the time-current values given in Indian and International standards and with a very low let through energy and cut-off current at short circuit. When an HRC fuse interrupts a heavy fault, it reveals an ability to limit the short circuit current. This ability is referred as a “cut-off” and has the effect of reducing the magnetic and thermal stresses both in the system and within the fuse itself under fault conditions. Because of the “cut-off”, the operating time of an HRC fuse is as low as 1/4^th of a cycle. The fuse gives satisfactory performance under both long term cyclic overload conditions and repeated transient overload conditions such as motor starting surges or inrush currents of transformers or capacitors. These fuse links are designed to very low power loss for energy saving and cool running.

All the characteristics of HRC fuse are maintained throughout its life. An HRC fuse do not falsely operate when carrying normal currents, which commonly happens in a re-wirable fuse because of oxidation and the reduction of the cross-sectional area. This is avoided in HRC fuses by hermetically sealing the fuse element within the ceramic body.

Applications of HRC fuses:

HRC fuses are ideal for low and medium voltage circuits of industrial and general nature for protection of motors and other equipments. They are also used for the protection of distribution transformers. A circuit breaker of low rupturing capacity may sometimes be used along with an HRC fuse.   These fuse links come in the capacity range from 6 A to 800 A in different sizes with a breaking capacity of more than 80 kA at 415 V, 50 Hz AC.     

TESTING OF PROTECTIVE RELAYS

"Protective equipments including relays are in-operative most of their useful life." 

They operate only under fault conditions. Testing of protective gears thus pose a problem. Testing of protective relays under normal operating condition normally does not give realistic results. Hence defects may easily pass undetected until revealed by incorrect operation or no operation on occurrence of actual fault.

Question arises why the relays or any other protective devices or gears are tested? 

Testing of relays is usually done to ensure that:

1)      The relay will operate correctly so as to clear a fault, and

2)   The relay will remain in-operative on faults outside its specified zone.

Now let’s talk about the various tests that are necessary for a protective gear and relay. Broadly the tests that are conducted on a relay are:

1)      Factory Test: For testing of a relay, the operational condition of the system during faults must be closely reproduced. The plant supplying the current for these tests particularly for high speed protection must be of sufficient capacity and design. Relay test benches are used for light current tests, and heavy current tests are carried out with the relays and CTs connected operationally.

Factory test also include many other tests such as material investigation, effects of impacts and vibration, resistance to atmospheric corrosion, effects of temperature and dust, tightness of relay case etc.

2)      Installation or Commissioning Test: The main purpose of commissioning tests is to prove that the relay has been properly installed and connected and it functions correctly and is ready for service. Comprehensive tests at sites of many other elements associated with the protective relays and which may have been supplied by different suppliers, are of great importance (Eg. Ratio and polarity test of CTs and PTs).

These tests include the general inspection of the equipment, checking of all connections etc. Insulation resistance of all the circuits is measured and the relay is tested by secondary injection. Further these tests at the site provide a check over the components which have passed factory tests and have been transported safely.

3)      Periodic Maintenance Test: Moisture and dust tend to produce deterioration of insulation, increase resistance of contacts, stickiness of relay bearings and pivots. A general inspection of the physical condition of all equipments, a check of accessible connections including fuses and links, secondary injection tests of relays, functioning test of logic and trip circuits are included in this test. The operation of indicating and alarm circuits should also be checked during this test.