Friday 15 March 2019

ABT Meter Functions & Its Mechanism!

The ABT Energy Meters are specifically designed Three Phase, Four Wire static meters having accuracy class of 0.2S. Function of ABT meters is to measure the net energy transmitted in Wh in each 15-minute time block of real time along with the average frequency. This meters are designed to record reactive energy in VARh under the predefined high /low voltage condition.  

How does the mechanism work   

The process starts with the Central generating stations in the region declaring their expected output capability for the next day to the Regional Load Dispatch Centre (RLDC). The RLDC breaks up and tabulates these output capability declarations as per the beneficiaries' plant-wise shares and conveys their entitlements to State Load Dispatch Centres (SLDCs). The latter then carry out an exercise to see how best they can meet the load of their consumers over the day, from their own generating stations, along with their entitlement in the Central stations. They also take into account the irrigation release requirements and load curtailment etc. that they propose in their respective areas. The SLDCs then convey to the RLDC their schedule of power drawal from the Central stations (limited to their entitlement for the day). The RLDC aggregates these requisitions and determines the dispatch schedules for the Central generating stations and the drawal schedules for the beneficiaries duly incorporating any bilateral agreements and adjusting for transmission losses. These schedules are then issued by the RLDC to all concerned and become the operational as well as commercial datum. However, in case of contingencies, Central stations can prospectively revise the output capability declaration, beneficiaries can prospectively revise requisitions, and the schedules are correspondingly revised by RLDC.

     While the schedules so finalized become the operational datum, and the regional constituents are expected to regulate their generation and consumer load in a way that the actual generation and drawls generally follow these schedules, deviations are allowed as long as they do not endanger the system security. The schedules are also used for determination of the amounts payable as energy charges, as described earlier. Deviations from schedules are determined in 15-minute time blocks through special metering, and these deviations are priced depending on frequency. As long as the actual generation/drawal is equal to the given schedule, payment on account of the third component of Availability Tariff is zero. In case of under-drawal, a beneficiary is paid back to that extent according to the frequency dependent rate specified for deviations from schedule.  

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Why Availability Tariff is necessary ? & Its Benefits

The regional grids had been operating in a very undisciplined and haphazard manner. There were large deviations in frequency from the rated frequency of 50.0 cycles per second (Hz). Low frequency situations result when the total generation available in the grid is less than the total consumer load. These can be curtailed by enhancing generation and/or curtailing consumer load. High frequency is a result of insufficient backing down of generation when the total consumer load has fallen during off-peak hours. The earlier tariff mechanisms did not provide any incentive for either backing down generation during off-peak hours or for reducing consumer load / enhancing generation during peak-load hours. In fact, it was profitable to go on generating at a high level even when the consumer demand had come down. In other words, the earlier tariff mechanisms encouraged grid indiscipline.

The Availability Tariff directly addresses these issues. Firstly, by giving incentives for enhancing output capability of power plants, it enables more consumer load to be met during peak load hours. Secondly, backing down during off-peak hours no longer results in financial loss to generating stations, and the earlier incentive for not backing down is neutralized. Thirdly, the shares of beneficiaries in the Central generating stations acquire a meaning, which was previously missing. The beneficiaries now have well-defined entitlements, and are able to draw power up to the specified limits at normal rates of the respective power plants. In case of over-drawal, they have to pay at a higher rate during peak load hours, which discourages them from over-drawing further. This payment then goes to beneficiaries who received less energy than was scheduled, and acts as an incentive/compensation for them.

How does it benefit everyone 

The mechanism has dramatically streamlined the operation of regional grids in India. Firstly, through the system and procedure in place, constituents’ schedules get determined as per their shares in Central stations, and they clearly know the implications of deviating from these schedules. Any constituent which helps others by under-drawal from the regional grid in a deficit situation, gets compensated at a good price for the quantum of energy under-drawn. Secondly, the grid parameters, i.e., frequency and voltage, have improved, and equipment damage correspondingly reduced. During peak load hours, the frequency can be improved only by reducing drawls, and necessary incentives are provided in the mechanism for the same. High frequency situation on the other hand, is being checked by encouraging reduction in generation during off-peak hours. Thirdly, because of clear separation between fixed and variable charges, generation according to merit-order is encouraged and pithead stations do not have to back down normally. The overall generation cost accordingly comes down. Fourthly, a mechanism is established for harnessing captive and co-generation and for bilateral trading between the constituents. Lastly, Availability Tariff, by rewarding plant availability, enables more consumer load to be catered at any point of time.

How do the beneficiaries share the payments  

The Central generating stations in different regions of the country have various States of the Region as their specified beneficiaries or bulk consumers. The latter have shares in these plants calculated according to Gadgil formula, and duly notified by the Ministry of Power. The beneficiaries have to pay the capacity charge for these plants in proportion to their share in the respective plants. This payment is dependent on the declared output capability of the plant for the day and the beneficiary's percentage share in that plant, and not on power / energy intended to be drawn or actually drawn by the beneficiary from the Central station.

The energy charge to be paid by a beneficiary to a Central station for a particular day would be the fuel cost for the energy scheduled to be supplied from the power plant to the beneficiary during the day. In addition, if a beneficiary draws more power from the regional grid than what is totally scheduled to be supplied to him from the various Central generating stations at a particular time, he has to pay for the excess drawal at a rate dependent on the system conditions, the rate being lower if the frequency is high, and being higher if the frequency is low.

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Thursday 14 March 2019

Overview of Availability Based Tariff (ABT) Meters

Availability Based Tariff (ABT) is a frequency based pricing mechanism applicable in India for unscheduled electric power transactions.

ABT Mechanism in Electricity sector in India is adopted since the year 2000 and in a few other countries for pricing bulk power across various stakeholders. ABT concerns itself with the tariff structure for bulk power and is aimed at bringing about more responsibility and accountability in power generation and consumption through a scheme of incentives and disincentives. As per the notification, ABT was initially made applicable to only central generating stations having more than one SEB/State/Union Territory as its beneficiary. Through this scheme, the Central Electricity Regulatory Commission (CERC) looks forward to improve the quality of power and curtail the following disruptive trends in power sector:

Unacceptably rapid and high frequency deviations (from 50 Hz) causing damage and disruption to large scale industrial consumers Frequent grid disturbances resulting in generators tripping, power outages and power grid disintegration.

The ABT scheme has now been expanded to cover the Intrastate systems as well. The power generation or grid capacity has increased substantially in last fifteen years particularly after the Electricity Act 2003 by introduction of competition and un-bundling of vertically integrated utilities (SEBs) into separate entities in charge of electricity generation, electricity transmission, and electricity distribution. Deregulation and competition has facilitated participation of private sector on large scale in electricity generation, transmission and distribution. Of late, Indian electricity sector is transforming from perennial deficit to surplus electricity availability. The volume of purchased electricity that could not be transmitted to the buyers due to transmission lines congestion is only 0.3% of the total electricity consumed in the financial year 2013-14. It means that the actual power deficit in India is less than 1% excluding under priced electricity demand. ABT/DSM mechanism needs improvements to address the requirements of all stake holders (including final electricity consumers) for encouraging least cost electricity generation / tariff based on demand verses availability in the grid. There is a need of well represented Electric Reliability Organization to involve all the grid participants for framing guidelines for power system operation and accreditation which is presently looked after by the CEA

Bulk power purchasers can buy electricity on daily basis for short, medium and long term duration from reverse e-auction facility. In reverse e-auction, availability based tariff /Deviation Settlement Mechanism (DSM) is applied to settle the failed commitments by the electricity sellers or buyers The electricity prices transacted under reverse e-auction facility are far less than the prices agreed under bilateral agreements.

For those power generators who have made power purchase agreements (PPA) with Discoms and need not participate in day ahead market (DAM) trading on daily basis, the pecking order among the power generators in a state is called merit order power generation where the lesser variable generation cost electricity producer is selected out of the available generators to maintain the normal grid frequency.

 What is Availability Tariff ?

     The term Availability Tariff, particularly in the Indian context, stands for a rational tariff structure for power supply from generating stations, on a contracted basis.

 The power plants have fixed and variable costs:

               I.      The fixed cost elements are interest on loan, return on equity, depreciation, O&M expenses, insurance, taxes and interest on working capital.

             II.      The variable cost comprises of the fuel cost, i.e., coal and oil in case of thermal plants and nuclear fuel in case of nuclear plants. In the Availability Tariff mechanism,

The fixed and variable cost components are treated separately. The payment of fixed cost to the generating company is linked to availability of the plant, that is, its capability to deliver MWs on a day-by-day basis. The total amount payable to the generating company over a year towards the fixed cost depends on the average availability (MW delivering capability) of the plant over the year. In case the average actually achieved over the year is higher than the specified norm for plant availability, the generating company gets a higher payment. In case the average availability achieved is lower, the payment is also lower. Hence the name ‘Availability Tariff’. This is the first component of Availability Tariff, and is termed ‘capacity charge’.

The second component of Availability Tariff is the ‘energy charge’, which comprises of the variable cost (i.e., fuel cost) of the power plant for generating energy as per the given schedule for the day. It may specifically be noted that energy charge (at the specified plant-specific rate) is not based on actual generation and plant output, but on scheduled generation. In case there are deviations from the schedule (e.g., if a power plant delivers 600 MW while it was scheduled to supply only 500 MW), the energy charge payment would still be for the scheduled generation (500 MW), and the excess generation (100 MW) would get paid for at a rate dependent on the system conditions prevailing at the time. If the grid has surplus power at the time and frequency is above 50.0 cycles, the rate would be lower. If the excess generation takes place at the time of generation shortage in the system (in which condition the frequency would be below 50.0 cycles), the payment for extra generation would be at a higher rate.

To recapitulate, the Indian version of Availability Tariff comprises of three components: (a) capacity charge, towards reimbursement of the fixed cost of the plant, linked to the plant's declared capacity to supply MWs, (b) energy charge, to reimburse the fuel cost for scheduled generation, and (c) a payment for deviations from schedule, at a rate dependent on system conditions. The last component would be negative (indicating a payment by the generator for the deviation) in case the power plant is delivering less power than scheduled.

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Tuesday 5 March 2019

Interesting Facts - Voice Controls for Lighting

A virtual digital assistant is a device with software that can perform tasks for consumers such as looking up information on the internet, playing media and buying products online. It uses speech recognition technology to respond to voice commands.

Amazon Alexa, Apple’s Siri, Google Assistant and Microsoft’s Cortana are the most popular virtual assistant platforms. Market research firm Tractica estimated 390 million people used virtual assistants in 2015 and projected this number to grow to 1 billion in 2018 and 1.8 billion in 2021.

An increasing number of lighting control and home automation manufacturers now offer voice recognition as an input for their systems. Until now, smart lighting in the home was controlled using apps and devices such as keypads, dimmers, switches and sensors. With these methods, homeowners can operate and schedule their lighting. By using a system with a compatible virtual assistant app/device, users can now also control their lighting, shades, thermostats, audio/video, and other smart devices using voice commands.

The benefit, of course, is convenience and enhanced lifestyle. The same benefits of automated home lighting control but with a simple and familiar additional control input. The leading markets are tech-savvy homeowners, people with disabilities and the elderly, who have limited freedom of movement. 

Voice control boosts the cool factor for smart lighting and may even serve as a discussion point—did you know you could use Siri to control your house? For contractors, voice control provides a selling feature for home automation, with lighting control a strong first step that provides benefits that are immediately seen.

Recommending lighting and home automation always begins with the home’s users, their lifestyle and their needs. Many homeowners are likely to start with just a few devices and grow from there based on gaining comfort with devices that are providing value. Exterior lighting is a likely first candidate along with indoor spaces where occupants spend most of their time, such as the kitchen, dining room, living room and bedrooms. As the customer gets comfortable, they can work their way up to more systems and ultimately a home-automation hub that ties everything together.

For voice-controlled lighting, what’s needed is a lighting or home control system, compatible virtual assistant device (phone or speaker) and app, and a robust Wi-Fi connection. These assistants are control-manufacturer-agnostic, meaning they are compatible with a range of products. Examples of lighting and home control systems compatible with various virtual assistants include Home Works QS; and Philips Lighting’s Hue system.


Using commands such as “Alexa, turn lights to 50 percent,” users can switch, dim, select tailored scenes, or in some cases, change the color of their lighting. If extended to other systems, user commands for scenes such as “home” and “relax” could create custom combinations of lighting, window shades, temperature, and media such as music.

User voice commands pass through the virtual assistant’s external cloud-based service to the control provider’s cloud-based service, which inter operate using an application programming interface. The control provider then sends the appropriate control signal to the controller in the home to execute the command. Control response should be almost immediate.

Many manufacturers offer wireless software updates to simplify delivery of upgrades and security patches. An interesting possibility for home control is If This, Then That integration, which enables systems to respond to each other without the need for user intervention; for example, if a surveillance camera detects motion, the porch lights could automatically turn on.

Smart lighting is typically plug and play and relatively simple to install. For one solution, the contractor swaps out light switches with smart switches then uses an app to connect all devices. Manufacturers offer training on how to install and set up these systems. One way to get comfortable is to try it yourself with a few control points.

While installation is somewhat simple, different systems offer varying levels of size, scalability, hardware, and simplicity. This requires appropriate matching to user needs, installer’s skill level and application characteristics.

Basic Knowledge is necessary to learn manufacturers’ systems, how devices communicate, what they’re inter-operable with, and how they’re set up, as many owners will want assurance everything will work properly. For voice control, the contractor will also need to gain familiarity with how voice commands must be phrased, as there is currently no standardized phrasing for these commands.

Virtual assistants and voice control use is expected to continue to grow, which will impact and add more value to home automation. Electrical contractors who invest in getting to know these systems will likely become more competitive with customers seeking automation’s benefits.

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Electrical Drive - Advantages & Disadvantages

The system which is used for controlling the motion of an electrical machine, such type of system is called an electrical drive. In other words, the drive which uses the electric motor is called electrical drive. The electrical drive uses any of the prime movers like diesel or a petrol engine, gas or steam turbines, steam engines, hydraulic motors and electrical motors as a primary source of energy. This prime mover supplies the mechanical energy to the drive for motion control.

The block diagram of the electrical drive is shown in the figure below. The electrical load like fans, pumps, trains, etc., consists the electrical motor. The requirement of an electrical load is determined regarding speed and torque. The motor which suited the capabilities of the load is chosen for the load drive.

Parts of Electrical Drive

The main parts of the electrical drives are power modulator, motor, controlling unit and sensing units.Their parts are explained below in details.

Power Modulator – The power modulator regulates the output power of the source. It controls the power from the source to the motor in such a manner that motor transmits the speed-torque characteristic required by the load. During the transient operations like starting, braking and speed reversing the excessive current drawn from the source. This excessive current drawn from the source may overload it or may cause a voltage drop. Hence the power modulator restricts the source and motor current.

The power modulator converts the energy according to the requirement of the motor e.g. if the source is DC and an induction motor is used then power modulator convert DC into AC. It also selects the mode of operation of the motor, i.e., motoring or braking.

Control Unit – The control unit controls the power modulator which operates at small voltage and power levels. The control unit also operates the power modulator as desired. It also generates the commands for the protection of power modulator and motor. An input command signal which adjusts the operating point of the drive, from an input to the control unit.

Sensing Unit – It senses the certain drive parameter like motor current and speed. It mainly required either for protection or for closed loop operation.
Advantages of Electrical Drive

Application of Electric Drive

It is used in a large number of industrial and domestic applications like transportation systems, rolling mills, paper machines, textile mills, machine tools, fans, pumps, robots and washing, etc.

The following are the advantages of electrical drive.
  1. The electric drive has very large range of torque, speed and power.
  2. Their working is independent of the environmental condition.
  3. The electric drives are free from pollution.
  4. The electric drives operate on all the quadrants of speed torque plane.
  5. The drive can easily be started and it does not require any refueling.
  6. The efficiency of the drives is high because fewer losses occur on it.

The electric drives have many advantages shown above. 

Because of the following advantages, the mechanical energy already available from a non-electrical prime mover is sometimes first converted into electrical energy by a generator and back to a mechanical energy of an electrical motor. Electrical link thus provides between the non-electrical prime mover and the load impact to the drive flexible control characteristic.

For example – The diesel locomotive produces the diesel energy by the help of the diesel engine. The mechanical energy is converted into an electrical energy by the help of the generator. This electrical energy is used for driving the other locomotive.

Disadvantages of Electrical Drive

The power failure completely disabled the whole of the system.
  1. The application of the drive is limited because it cannot use in a place where the power supply is not available.
  2. It can cause noise pollution.
  3. The initial cost of the system is high.
  4. It has a poor dynamic response.
  5. The output power obtained from the drive is low.
  6. During the breakdown of conductors or short circuit, the system may get damaged due to which several problems occur.
  7. Harmonics 

The only main disadvantage of the drive is that sometimes the mechanical energy produced by the prime mover is first converted into electrical energy and then into a mechanical work by the help of the motor. This can be done by the help of the electrical link which is associated with the prime mover and the load.

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Sunday 3 March 2019

Electrical Rescue Techniques

Electric shock accidents are caused by an electric current passing through the body. The effects from a shock can be anything from a tingling to instant death. Knowing what to do in the event of an electrical shock could save a life.

Approaching the accident:

Take a moment to assess the scene and look for any obvious dangers.
Check for the source of the electrical shock. 
Look to see if the victim is still in contact with the source. 

Remember that electricity can flow through the victim and into you. 
  • Call emergency number 108/112 as soon as possible.
  • Never use water, even if there is a fire, as water can conduct electricity.
  • Never enter an area where electrical equipment is used if the floor is wet.
  • Use a fire extinguisher made for electrical fires. Fire extinguishers for use on electrical fires will be labelled as a C, BC, or ABC extinguisher.
  • Never rush into an accident situation.
  • Get the aid of trained electrical personnel if possible.
  • Approach the accident scene cautiously. 

Examining the scene:

Visually examine victims to determine if they are in contact with energized conductors.

Metal surfaces, objects near the victim or the earth itself may be energized.
You may become a victim if you touch an energized victim or conductive surface.

Do not touch the victim or conductive surfaces while they are energized.

De-energize electrical circuits if at all possible. 

Methods to de-energize:

An extension or power cord probably powers portable electrical equipment.
Unplug portable electrical equipment to remove power.
Open a disconnecting device or circuit breaker to de-energize fixed electrical equipment. 

Hazards and solutions:

Be alert for hazards such as stored energy, heated surfaces and fire.
If you can’t de-energize the power source use extreme care:
Ensure that your hands and feet are dry.
Wear protective equipment such as low voltage gloves and overshoes if available.
Stand on a clean dry surface.
Use non-conductive material to remove a victim from the conductor. 

High voltage rescue:

Special training is required for rescues if high voltage is present.
Protective equipment such as high voltage gloves and overshoes must be worn.
Special insulated tools should be used 

Insulated tools:

Insulated tools, with high voltage ratings, are a lifesaver!
Use devices such as hot sticks or shotgun sticks to remove a victim from energized conductors.
In some cases, non-conductive rope or cord may be used to remove a victim from a conductor. High voltage rescue:

Rescuing the victim:

Stand on a dry rubber blanket or other insulating material if possible.
Do not touch the victim or conductive material near the victim until the power is off. 
Once power is off, examine the victim to determine if they should be moved.
Give “First Aid.” 

First Aid:

A victim may require Cardio-Pulmonary Resuscitation (CPR).
If the victim is breathing and has a heartbeat, give first aid for injuries and treat for shock.
Ensure the victim gets medical care as soon as possible.
Provide medical personnel with information on voltage level, shock duration & entry/exit points.  The treating/attending physician must have detailed specific information to properly diagnose and care for the victim.  
The physician must determine whether the victim should be sent to a “Trauma or Burn Center.” 

Basics about Electrical Shock

Basically, electrical hazards can be categorized into three types. The first and most commonly recognized hazard is electrical shock. The second type of hazard is electrical burns and the third is the effects of blasts which include pressure impact, flying particles from vaporized conductors and first breath

Electric shock occurs when the body becomes part of an electrical circuit. Shocks can happen in three ways.

• A person may come in contact with both conductors in a circuit.
• A person may provide a path between an ungrounded conductor and the ground.
• A person may provide a path between the ground and a conducting material that is in contact with an ungrounded conductor.

The terms high voltage and low voltage are relative terms. In transmission-line terminology, "low voltage" is much higher than the 600 volts. At home, you would not think of 600 volts as being low voltage.

Even when applied to 120-volt circuits, the term low voltage is deceiving. To some people low voltage means low hazard. Actually, low voltage does not necessarily mean low hazard, because potential difference is only one factor making up the dangerous effects of electricity. For purposes of this Lesson,you can think of "low voltage" as being a potential difference of 24-600 volts. 

The extent of injury accompanying electric shock depends on three factors.
• The amount of current conducted through the body.
• The path of the current through the body.
• The length of time a person is subjected to the current.

The amount of the current depends on the potential difference and the resistance. The effects of low current on the human body range from a temporary mild tingling sensation to death. An electric shock can injure you in either or both of the following.

• A severe shock can stop the heart or the breathing muscles, or both.
• The heating effects of the current can cause severe burns, especially at points where the electricity enters and leaves the body.

Other effects include severe bleeding, breathing difficulty, and ventricular fibrillation. In addition, you may strike something, or have some other accident as a result of your response to the shock. The effects of electric current are listed in below. 

The extent of injury accompanying electric shock depends on three factors.
• The amount of current conducted through the body.
• The path of the current through the body.
• The length of time a person is subjected to the current.

The amount of the current depends on the potential difference and the resistance. The effects of low current on the human body range from a temporary mild tingling sensation to death. An electric shock can injure you in either or both of the following.

• A severe shock can stop the heart or the breathing muscles, or both.
• The heating effects of the current can cause severe burns, especially at points where the electricity enters and leaves the body.

Other effects include severe bleeding, breathing difficulty, and ventricular fibrillation. In addition, you may strike something, or have some other accident as a result of your response to the shock. 

Current is the killing factor in electrical shock.  Voltage is important only in that it determines how much current will flow through a given body resistance.  The current necessary to operate a 10 watt light bulb is eight to ten times more current than the amount that would kill a person.  A pressure of 120 volts is enough to cause a current to flow which is many times greater than that necessary to kill. 

The following values are given for human resistance to electrical current in Figure 1: 
Figure 1: Resistance Values
With 120 volts and a skin resistance plus internal resistance totaling 1200 Ohms, we would have 1/10 ampere electric current, that is 100 milli amperes.  If skin contact in the circuit is maintained while the current flows through the skin, the skin resistance gradually decreases.  During this time, proper first aid
can mean the difference between life and death. Sufficient circulation can sometimes be maintained by heart compression, which should always be supported with mouth-to-mouth resuscitation.  This combination of treatments is commonly known as CPR. 

Friday 1 March 2019


AC motors are abundant in the most of the industry. Every fan and pump application requires an AC motor. For efficient and reliable control, adjustable-speed drives are becoming a common companion for these motors. However, as drives are boosting production, they might also be contributing to electrical noise problems, such as line current harmonics.

Pumping and mixing motors are becoming abundant in industrial wastewater facilities as systems expand to keep up with population growth and demand for more water, distributed over larger areas. To maintain water pressure and control flow, adjustable speed drives are a valuable companion for these AC motors.

Drives are an efficient alternative to throttling values, as they match power needs and motor speed to the demands of varying flow. Drives can save energy and electrical costs, increase pressure reliability, reduce cumbersome pump changes and lessen damage to mechanical equipment. However, as drives are boosting efficiency and reliability, they might also be contributing to power quality problems, such as line current and voltage harmonics.

Harmonics are deviations from the sinusoidal fundamental AC line voltage and current. Most electrical power in North America operates at a frequency of 50 hertz. A harmonic frequency is an integer multiple of this fundamental frequency. So in a 50-hertz system, the second harmonic would be 100-hertz, the third would be 150-hertz, and so on. The addition of any harmonic to the sinusoidal fundamental current or voltage will create distortion. The greater the amplitudes of the harmonics present, the greater the distortion in the electrical waveform. Very simply, whenever a voltage or current does not look like a perfect sinusoidal waveform, it contains harmonics.

Compared to DC motor drives, AC motor drives cause very few problems. However, poorly designed applications can result in power line voltage distortions. These voltage distortions can cause problems for other equipment connected to the same power lines – resulting in erratic operation of controls, dimming of lights, audible noise on telephone systems, and static on AM and FM radios. The distribution transformers and cables feeding these drives will also experience additional heating, which reduces the power utilization of those components.

What Causes Harmonics?

Unlike an AC motor operating across the power line, the current drawn from a distribution transformer feeding a typical AC drive is far from a sinusoidal waveform. This occurs because the drive is taking current from the transformer only during certain times of the cycle to convert the AC line voltage to a fixed DC voltage within the drive. The drive then pulse-width-modulates this fixed DC voltage into variable frequency, variable voltage for the motor. The AC-to-DC conversion is what causes the harmonics. Current flows only during part of the cycle and is off during other parts of the cycle, creating the odd-looking current waveform. It is this distorted current that creates the voltage distortion.

As the number of drives employed in automation systems grows, so too does the misconception that drives are the only piece of the puzzle to harmonic problems. While they may comprise a sizable portion of non-linear loads, they are only one element in the harmonic equation. That's why it's important to analyze all electrical loads that could potentially cause problems for a system before making any final conclusions.

What Affects Harmonic Current?

The harmonic current created by the drive can be affected by the presence of a DC link choke, the DC bus capacitance and the line inductance between the drive input and its source of voltage; such as transformer inductance, cable inductance and additional line reactors.

The closer you are to the drive, the greater the voltage harmonic distortion. This is why the voltage distortion is greater at the drive than at the transformer. If other equipment needs to be connected to the same transformer as the drive, connect the other equipment as close to the transformer as possible. This will minimize the effect of the current harmonics produced by the drive on the voltage waveform, which can cause problems with the other equipment.

Reducing Harmonics

There are several methods and products to reduce the line current harmonics created by drives. Cost is not the only factor that distinguishes one solution from another. Even though the addition of line reactors or passive filters can help reduce the current harmonics, they will also reduce the DC bus voltage within the drive at full speed, full load conditions. This will prevent the drive from being able to provide full power to the motor, limiting the power out of the motor to about 95% of its nameplate rating.

Why would you install a 100 hp motor only to have its capability limited to 95 hp, especially under peak demand conditions? This is why multi-pulse solutions are a better fit for most situations, since no derating is necessary and it is less expensive than other mitigation methods.

Harmonic Mitigation Methods
For a 250 hp drive, the chart above details the typical harmonic current, I(THD), and relative installed costs associated with various harmonic mitigation methods.

(For simplicity, we will say that the 250hp drive is connected to a supply with a short circuit capacity of 60,000A, will draw 260Arms of fundamental current for its load, and will have a cost of 100%.)

The goal for those specifying harmonic mitigation solutions is to combine the most cost effective solution for the particular drive and power distribution system. Since several factors come into play in determining the harmonics in the drive system, Emerich Power Quality Analysis  needs to consider all factors before arriving at a final recommendation.

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