Basics of Active Harmonic Filter

Basics of Active Harmonic Filter:

Modern industries are defined by widespread use of non-linear loads such as DC drives, VFDs etc. which introduce large content of harmonics into the system and its subsequent effects. There arises the need for intelligent solutions for harmonic mitigation. Active harmonic filters have developed as the most ideal solution for harmonic filtering in both industrial and commercial facilities. Added advantage is that they also offer solutions for phase balancing and power factor improvement. This article briefs about its basic principle of operation, need for active compensation and sizing.

Principle of Operation

An active harmonic filter is based on the following principle:

I filter = I main - I load

It detects the difference between the ideal current sine wave (I main) and the actual current which has been deformed by harmonics (I filter). It, then, injects this difference (I load), which is the negative of the harmonic currents present in the load current, into the system on a real-time basis. This cancels out the high frequency harmonics and results in almost pure sine wave.

Benefits of using LT Capacitors over HT Capacitors

Benefits of using LT Capacitors over HT Capacitors:

Power factor compensation can be provided on either LT or HT side of the distribution transformer. Often, compensation is done on the HT side as the electricity board measures power factor on HT side for penalty calculation. Also, HT capacitors involve low initial investment as compared to LT capacitors. However, compensation achieved by HT capacitors does not provide the benefits offered by the use of LT capacitors, as discussed in this article.

Consider two cases with compensation provided on HT and LT side respectively as shown in figure 1 and 2.

As seen in fig 1, with the capacitor connected on the HT side, the compensated reactive power flow through the transformer does not reduce and hence there is no change in current flow. Although the HT side power factor is improved, the LT side power factor remains same. However, as seen in fig 2, connecting capacitor on LT side reduces the reactive power flow through the transformer and we get improved power factor and reduced

current flow on both LT and HT sides.

1. Reduction in transformer copper losses:

Consider a load of 1200 kW connected to a transformer of 2000 kVA. The typical full-load copper losses in a 2000 kVA transformer are 25000 W.

When compensation is provided on HT side, operating power factor of the transformer is 0.75 (same as uncompensated), denoted by cosØ1.

When compensation is provided on LT side, operating power factor of the transformer is 0.98 (compensated),

denoted by cosØ2.

Power Saving = Wf * k * (1/cosØ1 - 1/cosØ2)

Where,

Wf = Full Load Copper Losses = 25000 W

k= Load in kW/kVA rating of Transformer = 0.6

Power Saving (in W) = 25000*0.6*(1/.75-1/.98) = 4694 W

Monthly Energy Savings (in kWh) = 4694 *24*30/1000 = 3380 kWh

Typical Energy Charge (in Rs/kWh) = Rs. 6 per kWh

Monthly Cost Saving (in Rs) = 6*3380 = Rs. 20278/-

Yearly Cost Savings (in Rs) = 20278*12 = Rs. 243330/-

Thus, LT compensation provides monthly savings of Rs. 20,278/- for a 2000 kVA transformer. Additionally, the operating temperature of the transformer is relatively less because of reduced copper losses. Hence, apart from monetary benefits, LT compensation also ensures longer life of the transformer.

2. Capacity release in transformer:

Consider a 2000 kVA transformer connected to a load. When compensation is provided on HT side, operating PF of transformer = 0.75

Case 1: Maximum load that can be connected = 2000*0.75 = 1500 kW

When compensation is provided on LT side, operating PF of transformer = 0.98

Cast 2: Maximum Load that can be connected = 2000*0.98 = 1960 kW

Additional load that can be connected under the same transformer = 460 kW

LT compensation allows release of capacity of 460 kW with the same transformer. Thus, additional load can be easily connected to the system without any additional investments in new transformers.

3. Optimized main incomer switchgear rating:

LT side capacitor, when connected after the main incomer reduces the current drawn by the same set of loads. Hence incomer switchgear rating can be optimized and the investment cost of the main incomer can be reduced.

Apart from those mentioned above, LT compensation also offers other advantages, such as

 Maintenance of LT capacitors and panels is easier and does not require complex safety measures.

 Spares and accessories for the same are easily available and relatively cheaper.

Thus, for a factory with all LT loads, power factor compensation with LT capacitors proves to be a better option, with its relatively lesser payback period.

Current Transformer

Current Transformer (CT):

CT's are categorized as Protection CT, Special Protection.

CT and Measuring CT. Based on this, the CT's are classified. Here is the meaning ofthe CT classes:

Class 5P20:

The letter 'P' indicates it is a protection CT.

The number 5 indicates the accuracy of the CT. Most common accuracy numbers are 5 and 10.

The number 20 (called accuracy limit factor) indicates that the CT will sense the current with the specified accuracy even with 20 times of its secondary current flows in the secondary. This is required for protection CT, because the fault current is high and the CT should be able to sense the high fault current accurately to protect the system.

The common numbers are 10, 15, 20 and 30.

Class PS:

PS is for 'Protection Special'. This class of CT's are used for special protection such as differential protection, distance protection etc.

Class 1M:

The letter 'M' indicates it is a measuring CT.

The number 1 indicated the accuracy of the CT. The measuring CT's should be more accurate than the protection CT. The most common accuracy numbers are 0.5 and 1.

Brief on IEC 61439

IEC 60439 : Present standard

• The IEC 60439 standard applies to enclosures for which the rated voltage is under or equal to 1000 V AC, (at frequencies not exceeding 1000 Hz) or 1500 V DC.
• The standard makes a distinction between type-tested assemblies (TTA) and partially type-tested assemblies (PTTA).
• All parts mentioned under standard have equal weightage. Formal hierarchy is not maintained. Each part is a complete entity and can be used on an individual basis

IEC 60439 - 1 Type-tested and partially type-tested assemblies 

IEC 60439 – 2 Requirements for busbar trunking systems (busways) 

IEC 60439 – 3 Requirements for low-voltage switchgear and controlgear assemblies which are to be installed in locations where unskilled persons have access for their use.

IEC 60439 – 4 Requirements for assemblies for construction sites (ACS) 

IEC 60439 – 5 Requirements for assemblies intended to be installed outdoors in public places – Cable distribution cabinets (CDCs) for power distribution in networks

The standard remains valid until 2014 and will be replaced by IEC 61439

IEC 61439 : New Standard

Why this change :

• Over a period market needs for assemblies have changed drastically and new designs have evolved based on different applications. The present standard IEC 60439-1 no longer encompasses many commonly used arrangements.

For example, modular systems are not effectively covered with respect to temperature rise performance.

• It is not practical to fully type test every possible configuration of assembly produced. In some of the cases, type testing is not feasible and hence does not have alternative ways of ensuring an assembly that meets the minimum required safety and performance criteria.
• The methods for proving the design of a 'partially type tested assembly' in accordance with IEC 60439-1 are weak and rely entirely on the capability and integrity of assembly designer.
• There is no standard for assemblies that do not fit within the categories of type tested or partially type tested assemblies.
• It is essential that the Standard shall aligned with safety requirements and safety performance is rigorous, consistent and equally applied to all assemblies.

IEC 61439 :

Purpose : To harmonize and define all general requirements for low-voltage electrical Assemblies.

Scope : The new IEC 61439 standard applies to enclosures for which the rated voltage is under 1000 V AC (at frequencies not exceeding 1000 Hz) or 1500 V DC.

New standard has brought considerable clarity in technical interpretation. The new standard follows the philosophy of IEC 60947 series. Released in Jan 2009

IEC 61439 - 0 Guide for specifying assemblies 

IEC 61439 - 1 General rules 

IEC 61439 – 2 Power switchgear and controlgear assemblies 

IEC 61439 – 3 Distribution boards 

IEC 61439 – 4 Assemblies for construction sites 

IEC 61439 - 5 Assemblies for power distribution 

IEC 61439 - 6 Busbar trunking systems 

IEC 61439 - 7 Electric Vehicles

Main Changes in IEC 61439 :

The standard defines the ‘ Design Verified Assemblies ‘ and eliminates completely the categories TTA and PTTA.

Type tests have been replaced by a design verification which can be carried out by the three equivalent and alternative methods i.e. Testing, Calculation / Measurement or Application of design rules.

Technical changes in areas such as:

• Verification of mechanical operations
• Verification of temperature rise
• Rated diversity factor
• Clearance verification
• Neutral cross section
• Addition of few verification test such as
• Verification of resistance to UV radiation for outdoor plastic enclosures
• Verification of corrosion resistance
• Mandatory declaration and confirmation of an impulse rating
• Lifting, mechanical impact and marking
• New terms introduced as ‘ Original manufacturer ‘ and ‘Assembly manufacturer ‘
• Split in product responsibility between the ‘ Original manufacturer ‘ and ‘ Assembly manufacturer ‘

Earth Leakage Relay

Earth Leakage Relay

Introduction:

Majority of us have experienced electrical shock while using electrical equipment at some point of our lives. Though momentary, it is quite dangerous.

Earth leakage occurs due to reasons like normal wear and tear of equipment or moisture around terminals which can result in partial breakdown of insulation between supply and earth. Earth leakage currents are dangerous as it can lead to cable heat generation and insulation failure. This can result in a major catastrophe thus leading to significant loss of property and human lives.

Difference between Earth Fault and Earth Leakage:

According to IEC 60947-2, Annex B, Earth fault current is the current flowing to earth due to

insulation fault and Earth leakage current is the current flowing from the live parts of the installation to earth in the absence of an insulation fault.

Conventional SCPD are not designed to detect earth leakage currents. Earth Leakage Circuit breaker (ELCB or RCCB) has integral current breaking device. It detects as well as protects the system by opening the protected circuit when the residual current exceeds the set value. ELR is a relay that send a signal to the shunt coil of a circuit breaker (MCB/MCCB or ACB) whenever the leakage current exceeds the set level.

Effect of earth leakage on human body:

Earth Leakage current beyond 30mA can be lethal leading to death. 30mA sensitivity is required for protection in domestic installations where the person may come in direct contact with electric equipment in locations for eg labs, schools, workshops, etc

100mA and 300mA protection is required where there is indirect contact or due to insulation failure in the cables.

ELR with CBCT:

The Earth Leakage relay with Core Balanced Current Transformer provides protection from earth leakage with advanced intimation of impending occurrence of the event. The user can proactively take action to avoid occurrence of any mishaps.

ELR with Type class ‘A’ true RMS measurement (as per IEC 60947-2 Annexure M) provides

the user with benefits that go the extra mile. Earth Leakage relay is a microcontroller based device meant to measure low level of leakage current and isolate the faulty circuit from the system. Leakage current is sensed through core balanced current transformer. Definite Time Trip occurs when Earth Leakage Current exceeds the trip time which is adjustable by means of a front mounted potentiometer. The user can set the threshold level ranging from 30mA to 30A. In case of earth leakage then the LED indicators will glow depending upon the percentage of set threshold value. 

For eg: If the set level is 30mA and the leakage current is around 23mA then 75% LED indicator will glow which will provide a visual alert to the user. This empowers the user to take corrective actions before any accident.

Output 1C/O can be given to shunt trip of MCB/MCCB or ACB and 1 NO output for alarm indication. The relay has Fail-safe feature inbuilt in it. Core Balanced Current Transformer (CBCT) uses the technology of residual magnetic flux. All conductors to be protected shall pass through the core balance current transformer. The vector sum of all the currents should be equal to zero.

Īr + Īy + Īb= 0 for 3 phase 3 wire system.

Īr + Īy + Īb + Īn =0 for 3 phase 4 wire system

The CT wires should be placed adequately away from high current carrying conductors or source of strong magnetic field to avoid noise pickup.

Applications:

1. Motor Control Panels and Switchboards:

The relay combined with the CBCT capable of monitoring the supply conductors can be separately mounted within the confines of the switchboard. ELR’s features such as test / reset, alarm output and continuous digital indication for the residual current value are more suitable to industrial applications.

ELR is used along with shunt release of MCCB in motor applications. Leakage of current from the motor body to earth causes the CBCT to sense the earth leakage current and hence provides protection to human personnel working in the near vicinity.

2. Earth Mines

In Earth mines any leakage current above the allowed level is lethal. Workers operating in mines with various instruments face a severe danger of fires. The ELR must be used in conjunction with circuit breaker of appropriate rating.

Typical usage areas for ELR:

• Steel Plants

• Generators and Transformers

• Cement plants

• Oil Refineries

• Buildings

• Mobile Operating equipment

• Control Panels

• Switchboards

MPCBs for Motor Protection

Are you still using MCB for your motor applications?

MCB is commonly used for motor applications. However, such a solution is not reliable as MCB when used for short circuit protection in a feeder does not offer complete protection. Hence there is always a possibility of damage to relay or contactor because of incorrect selection.

The correct solution is to go for recommended Type 2 selection with MPCB and contactor. This ensures complete safety to the personnel along with reliability of switching devices.

So are you still using MCB for your motor applications? If yes, read further to know the benefits of MPCB and how it offers a safe, reliable and technically correct solution for all your motor applications.

What is an MPCB?

MPCB stands for Motor Protection circuit breaker. As the name suggests, MPCB is specially designed to offer reliable protection for your motor applications. MPCB has the benefit of having both overload and short circuit protection in a single compact unit. Hence no separate overload relay is required when used in DOL motor starters. This makes the solution compact and economical. Another important advantage one gets with using MPCB in motor feeder applications is protection as per Type 2 Coordination.

Besides, various different accessories like bus bars, Trip-Alarm contacts can be used with MPCB for ease of wiring and indication and interlocking purposes.

Problem of using MCB in Motor Feeder Applications

An MCB is a peak sensing device. The starting transients of the motor are usually up to 12 times the motor rated current (Peak Value). Hence when a MCB is used, it must be ensured that it does not trip during motor starting. If a C-Curve MCB is used it must be ensured that 12 times the rated motor current must be lesser than 5 times the MCB’s nominal rating to prevent nuisance tripping of MCB at start. (Since magnetic trip setting of C-Curve MCB is 5 – 10 times the rated current)

Let us understand with the help of an example,

Consider an 11KW motor with DOL starting,

Rated motor full load current = 21A

Peak current at starting = 12 x 21 = 252A

Peak starting current must be less than 5 times the MCB nominal rating.

Hence, MCB rating must be greater than 252A/5 i.e. greater than 51A.

In this case we have to select a C-Curve 63A MCB.

If we select a 25A contactor with relay range of 14 – 23A and considering the worst case scenario, MCB will trip at 10 times i.e. 630A. Just below this value it may happen that the MCB wont trip and the thermal overload relay has to give tripping signal to the contactor to break this current

As per IS breaking capacity of a contactor is 8 times its AC3 current. Considering this, a 25A contactor which would have breaking capacity of about 200A, it may get damaged in such a scenario. This can be addressed by using higher rating contactor. The contactor selected must have a breaking capacity almost equivalent to 630A. Hence, we would need a contactor with AC3 rating of 630 / 8 i.e 80A

However, even than the reliability is not fully ensured as the relay is still susceptible to damage if the current exceeds it’s withstand capacity. Such kind of damage is common when MCB is used for motor applications.

Hence complete reliability of contactors and relays is not ensured when MCB is used. This is the main reason why MCB is not recommended in Type 2 co-ordination.

The correct solution is to use MPCB with contactor as per the recommended type 2 co-ordination. This will ensure both safety and reliability of your entire motor feeder application.

Control Transformer sizing

Control Transformer sizing for contactor actuation:

Introduction

A contactor is an electromagnetic device consisting of a coil and magnet system along with fixed and moving contacts. When the coil is energized, it produces a magnetic field thereby attracting the moving magnet. This causes the fixed and moving contacts to connect and the contactor is said to be actuated. The energization of contactor coil is usually done through a control transformer.

This is mainly done because voltage requirements vary with control systems and with an intermediary control transformer the desired voltage can be obtained.

When a contactor coil is energized, it draws in a high inrush current momentarily. Apart from contactor coils, relays and solenoids are some other devices which draw inrush current when energized. The control transformer selected must be able to accommodate this momentary high inrush current for a satisfactory operation.

Selection of a control transformer

For a proper selection of control transformer, three parameters of the load circuit must be determined in addition to the minimum voltage required to operate the circuit. These are Hold on VA, Pick-Up VA, and Inrush load power factor.

Hold-On VA: Hold-On VA is the product of load voltage (V) multiplied by the current that is required to operate the circuit after initial start up or under normal operating conditions. It is calculated by adding the hold-on VA requirements of all the electrical devices of the circuit that will be energized at any given time. Hold-On VA is also sometimes referred as steady state VA.

Pick-Up VA: Pick-Up VA is the product of load voltage (V) multiplied by the current (A) that is required during start up. It is calculated by adding the pick-up VA requirements of all devices (contactors, timers, relays, solenoids, etc) which will be energized together. Energization of electromagnetic devices takes 20-50 milliseconds. During this inrush period, the electromagnetic devices draw 3 to 10 times the normal current.

Inrush Load power factor: Inrush load power factor is difficult to determine without a detailed vector analysis of all the load components. Generally such analysis is not feasible; hence a safe assumption would be 40% power factor. Until recently 20% power factor was commonly used for transformer calculations; however tests conducted on major brands of control devices indicate that 40% power factor is a same assumption.

It is recommended that a control transformer be sized at 40% power factor. Some electromagnetic devices typically operate at that level due to their inherently low power factor. Selecting a control transformer at 40% power factor will be more than the adequate size for all the various loads in the circuit.

Besides the above parameters there are two parameters of primary and secondary voltage. Primary voltage is the voltage available from electrical distribution system which is connected to the transformer supply terminals. Secondary voltage is the voltage required for load operation which is connected to the transformer load voltage terminals.

Steps for selection of control transformer

1) Determine the supply and load voltages as per requirement. The supply voltage is the voltage available to control transformer and load voltage is the operating voltage of all the devices connected to the transformer output.

2) Determine the hold-on and pick-up VA of each coil in the control circuit. This data is provided by the product manufacturer in the datasheet.

3) Calculate the hold-on VA by adding the VA requirements of all the equipment that will be energized together (timers, contactors, relays, solenoids, pilot lamps etc).

4) Calculate the Pick Up VA of all the coils that will be energized together. Be sure to include the hold-on VA of components that don’t have inrush (lamps, timers) as they present load to the transformer during maximum inrush.

5) Calculate the application Inrush VA by using the following industry accepted formula,

Application Inrush VA= Sq.root (Sq.(pickup VA)+Sq.(Hold on VA))

Let us further understand this with the help of an example,

Consider MNX 110 contactor,

Pick Up VA = 550VA

Hold On VA = 36VA

Application Inrush = √ (5502 + 362) = 552 VA

STANDARD COIL VOLTAGES AND THEIR APPLICATIONS

Standard Coil Voltage Ratings used in India

240V

Coils with rated voltage of 240V are the most widely used coils in Industrial and commercial applications. 240V single phase-neutral supply can be easily derived from a 415V Three Phase Four Wire system by connecting across one phase and neutral point (415/sqrt(3)=240). Since this distribution system is prevalent across many industrial applications, 240V coils find their application in majority of contactor applications. Common applications are industrial motor feeder systems. Also, in most of the industrial installations voltage values are quite stable and variations are limited. Hence in such systems with very less voltage fluctuations, it is viable to go for 240V coil with a standard coil band of 80% to 110% of rated coil voltage.

220V

220V coils are generally preferred in applications where the available supply is slightly less than the rated voltage of 240V. In such applications it is advisable to go for a 220V coil because one gets a lower value of pick up voltage as compared to 240V. For example for a 240V coil the coil band would be 156 – 288V. If one goes for a 220V coil then the available coil band is 143 – 264V. This takes care of the slight fluctuation in voltage which is below the band specified for 240V or a consistent low voltage.

415V

415V coils are used when there is a possibility of neutral floating condition affecting contactor operation. Neutral floating arises when the neutral is not properly grounded or ground connection is completely broken. Conventional distribution systems are three phase four wire systems in which individual single phase systems are derived from a three phase supply. In such cases the neutral is grounded and ideally must be at zero potential. In a perfectly balanced three phase four wire systems, loss of neutral conductor will not cause any abnormal voltage variation on connected single phase loads. However this condition is extremely rare and there is always some current flowing through the neutral owing to imbalances in the single phase loads. In such a scenario a loss of neutral will lead to abnormal voltage variations across the connected single phase loads. The extent of voltage variation will depend on the extent of unbalance in the single phase loads. However the imbalance in voltages will not affect the line voltages and they will continue to be at 415V.

In such a scenario if one used 240V coils then they may get damaged due to overvoltage condition arising out of neutral floating. This problem can be efficiently eliminated by going for 415V coils as neutral floating condition does not affect the line voltages. Hence the issue of coil burning due to neutral floating is completely eliminated. Improper neutral grounding can lead to voltage rise and hence going for 415V coils is advisable.

Hence for all changeover application involving four Pole contactors, it is recommended to go for 415V coils. But, it should be noted that the allowable control cable length due to cable capacitance is lowest at 415V.

360 or 380V

These coil voltages are mainly used in agricultural applications. In agriculture applications even though the rated secondary of transformer is 415V, because of simultaneous running of loads leading to sustained voltage drop and absence of voltage stablilizers, many of the users get voltages in the range of 360-380V. Since this voltage levels are much lower than 415V special coils of 360 or 380 volts have to be designed specifically for agricultural applications. These coils are restricted to applications where it is known that reduced voltage is available. These coils don’t find their applications in industrial applications where voltage supply is as per rated and stable. The choice of 360V and 380V coils can be based on how low the supply voltage can dip to in that particular installation. It is also to be noted

that in such installation Phase to neutral voltage connection is not preferred for coil voltages, due to the possible problem of neutral floating.

440V

These coil voltages are mainly used in Industrial applications, and there are chances of failure of coils due to sustained high voltages These coils are restricted to applications where it is known that higher voltage is available. These coils don’t find their applications in industrial applications where voltage supply is rated and stable.

110V

110V coils are generally used in applications where one wants to prevent any unauthorized start of the contactor. For example in many applications, operating personnel tend to override the contactor drop command given by a Distributed control system (DCS). This is mainly done by using easily available 240V single phase supply to on the contactor. However if one uses 110V coils, it acts as an efficient deterrent against overriding DCS commands as 240V supply to an 110V coil will damage the coil beyond repair. This acts as an efficient safety feature in the system. It also efficiently isolates the coil supply from the main supply through a control transformer. 110V 60Hz supply is also used mainly in western countries as 110V is much safer to operating personnel as compared to 240V. Also it should be noted that the allowable control cable length due to cable capacitance is highest at 110V. 

24V DC

24V DC coils are mainly used in automation applications and in contactors which are used along with backup supplies. In many process industries having the entire control through PLC one finds applications of 24V DC coil contactors as 24V DC is predominantly required for PLC. Some of the contactors have low coil consumption coils and can be directly actuated by the PLC without the use of an interface relay. PLC output, generally being 24VDC, DC coil voltage is required. 24V DC Coils are also largely used in battery backed up systems and UPS applications. For example, in power plant a lot of critical equipment is kept on backup supply where actuation is done through a DC coil contactor, 24VDC being the most widely used.

To Summarize

240 VAC: Most commonly used coil voltage, Limitation where pickup at low voltage is required

220 VAC: Used where voltage fluctuation on lower side. Can pick up at lower voltage, Overvoltage withstand will be limited as band shifts to lower side

415 VAC: Ideal for DG applications, there is a chance of neutral floating, Allowable control cable length reduces

360 or 380 VAC: To be used in agricultural applications , where undervoltage is prevalent Overvoltage withstand will be limited as band shifts to lower side

440 VAC:

Used where voltage fluctuation on higher side. Better withstand at sustained high voltage Pick up at lower side gets limited as band shifts to higher side

110 VAC

Provides separation between control voltage and common available single phase supply. Separate control transformer is needed which makes it expensive

24 VDC

Used in PLC applications or Automation systems, Eliminates need for interposing relay

Expensive due to high cost of DC Coils and limitations of NC contacts

AUTO POWER TRANSFER SOLUTIONS IN LV SYSTEM

Continuity of power supply is extremely important in any critical installations. In order to avoid any power outage, users often employ alternate sources such as DG set, UPS or integrated power generation units. This also demand a reliable power transfer scheme that switches from a preferred to an alternate source in the event of a power disruption & return back to the preferred supply when the faults are taken care of.

There are two types of transfer systems. They are:

• Manual Transfer system: 

These are generally toggle / knob operated switches or circuit breakers which need to be manually switched on so that the load circuit gets transferred from one power source to the other. The manual transfer switches can be used where power outage happens quite rarely and loss of power does not cause any loss to the appliances or systems used with the electric power supply.

• Automatic Transfer system: 

These automatically transfer the power to the load circuit from one power source to the other. Thus, these are more convenient to use as one does not have to manually operate to switch the power source. During normal power interruption, these switching devices will automatically transfer the load circuits to the emergency power source. Once normal power has been restored, the process is automatically reversed. Automatic transfer systems are useful where even a small loss of power can cause a lot of losses in the system. Automatic transfer systems have therefore found their popularity and utility in several industrial and commercial applications where a constant source of power is necessary.

Automatic transfer systems operate in two different methods i.e. open transition and close

transition.

1. Open Transition Transfer

• Break before make switching action. In this, the connection to one power source is opened before the connection to the other source is made and during this process of power transfer, the flow of electricity is interrupted. This change-over time can be adjusted by using different time setting in any voltage sensing controller.

• This is the most popular method used in many installations for automatic power transfer. This system is widely used in applications which can accept a small interruption of power from few msec to few seconds.

• It does not require alternate hot source (like a continuous running DG set or an UPS).

2. Closed Transition Transfer

• Make before break switching action for uninterrupted power transfer. This facilitate a seamless transfer of power supply from one source to other by momentarily paralleling both the sources (<100 msec) during the transfer period. The transfer switch monitors the phase angle difference between the two sources and when it approaches zero degree, the switch operates.

• This system is used primarily in critical installations like Hospitals, Data Centre etc where even momentary power interruption is not acceptable.

• However, this system necessarily requires alternate hot source (like a continuous running DG set or an UPS) all the time.

While the closed transition method is the best to ensure no interruption of power at all, open

transition method is more popularly used due to following reasons:

1. Most power transfer application accepts a momentary interruption in the order of 60              msec  to 5 seconds.

2. Non-availability of hot sources in most applications.

3. Very high prices of close transition auto transfer switches.

4. Multiple choices available to the user for open transition power transfer & protection with      a combinations of conventional switching, sensing & control devices

5. Ease of maintenance

A typical open transition auto transfer system involves:

1. Two 4 pole, mechanically and/or electrically interlocked power switching devices which can be remotely operated.

2. Voltage and/or frequency sensing accessories or controller.

3. Back up protection devices like circuit breakers or fuses in case the power switching devices have only switching capability.

As mentioned earlier, the key elements in any source transfer systems are:

1. Sensing & control

2. Switching & protection

3. Interlocking

Sensing & Control

For any ASTS, it is important to monitor the source voltage to decide on which source needs to be in service & a control system to ensure the correct logic is in place to get the most optimized power.

The different options used for this are:

• Use of Under voltage release in circuit breaker to monitor the source voltages & enable a control logic with auxiliary & trip alarm contacts

• Simple controller with separate voltage sensor, contactors, timers, logic & interlocking control circuit

• High end digital auto transfer controller with in-built voltage, frequency sensor & a complete logic controller for all power transfer control, interlocking features, multiple setting for voltage & time, digital display, communication etc.

Switching & Protection

ASTS necessarily needs two separate 4 pole switching devices suitable to offer complete isolation in OFF state. Depending on the application & installation requirement, they must have on-load or offload switching duty. In addition to the switching device, it must have the necessary protections available against any abnormal condition. The switching & protection functions can be combined into one device e.g. Air circuit breakers & Moulded Case circuit breakers. In case the switching devices like contactors, switch-disconnectors etc, separate upstream protection devices like circuit breakers or HRC fuses must be provided.

Interlocking

One of the key and a must safety feature for any open transition ASTS is to ensure that under no circumstances, both the sources will get switched on together even momentarily. Hence, reliable and failsafe mechanisms must be incorporated to ensure that the two switching devices are fully interlocked so that only one device can be closed at any point of time.

Interlocking of the two switching device can be done by following means:

1. Mechanical interlock - This is the most reliable method of interlocking. This can be done through suitable interlocking mechanisms like base plate, clutch wire or see-saw toggle interlocks.

2. Electrical Interlocking - This is generally used in addition to the mechanical interlocks. It electrically interlocks the two switching devices like circuit breakers, contactors etc and can be logically programmed to operating sequence, time delay etc. This can be done by using:

a. a combination of under voltage release with Auxiliary contacts for circuit breakers

b. Using an external controller & suitably wiring it

c. Using the NO & NC contacts with the coil in case of contactors

3. Self interlocked mechanism – This is generally adopted in the change-over SDs or Auto Transfer switches. The basic mechanism of SDs will not permit closure of both switches together.

Keeping all the above requirements of ASTS, there can be multiple combinations which can be selected. The selection of transfer system for specific installations can be optimized by keeping following parameters in mind:

1. Feeder Ratings

2. Application need in terms of maximum acceptable change-over time

3. Desired features in terms of sensing & interlocking

4. Specific safety considerations

5. Panel space

6. Life expectancy

7. Cost