Power transformers

WHAT????

• Passive device that transfers alternating current (AC) from one circuit through electromagnetic induction
• Normally consist of a ferromagnetic core and two or more coils ( windings)

HOW???

• Changing current in the primary winding creates an alternating magnetic field in the core. The core multiplies this field and couples most of the flux through the secondary windings. This in turn induces an alternating voltage (emf) in each of the secondary coil in accordance to Faraday’s law

TYPES

• Power transformers


•  Laminated core
• Toroidal
• Autotransformer
• Variac
• Induction regulator
• Stray field transformer
• Polyphase transformer
• Resonant transformer
• Constant voltage transformer
• Ferrite core
• Planar transformer
• Oil cooled transformer
• Cast resin transformer
• Isolating transformer

• Instrument transformers
• Current transformer     
• Voltage transformer

• Pulse transformers
• RF transformers
• Air core transformers
• Ferrite-core transformers
• Transmission-line transformer
• Baluns (specifically to connect balanced and unbalanced circuits)
• Audio transformer

etc.

This article’s main concern is power transformers. What is a power transformer? Power transformers are made from high-grade materials, a very special type of electrical steel for the core, high-grade copper for the windings, and cellulose-based paper for insulation.

A power transformer in switch mode power supply (SMPS) is designed to change amplitude of high-frequency pulses by the turns ratio and to provide isolation between circuits. Note that it can't transfer a DC component of the pulse voltage: in a steady state mode net volt-seconds across each winding should be zero, otherwise the core will saturate. DC output voltage is obtained only by using rectifiers. Nevertheless, an average voltage across a real coil's terminals can be non-zero due to non-zero coil's resistance. This DC offset can be used for lossless sensing of an average current across an inductor or a transformer winding. In general, ideal SMPS transformers need to transfer all energy instantaneously from one winding to another while storing no or little energy in the process.

Conversely, a power inductor is used in SMPS as an energy storage device. It accumulates energy in the magnetic field as current flows through it, and then transfers all or a portion of this energy into another circuit during the alternate part of the switching cycle. In power supplies the inductors are also used for filtering out high frequency currents (often called chokes). That is all the information I have for now. Will post more when I get more information.

A little more on generators

So on Friday, the 10^th of June 2011, I was taken to the Genset Room ( generator set room) for the test run that is scheduled weekly. What I got from this experience was pretty interesting as I hadn’t been given this kind of opportunity before. I was briefed on how the generator works and how to use a load bank. On top of that I was allowed to play with the switches and use the load bank for a hands on experience with the machines.

 I had been previously briefed on the basic works of a generator set, and this time a little more was shared. Information such as the synchronization of the generators was explained to me. There are four factors that have to be considered in order for synchronization to take place. These four factors include voltage, frequency, phase, and phase angle.

In the past, synchronization was performed manually using three-lamp method. Nowadays, the process is automatically operated and controlled with the aid of synchronization relays. During installation of a generator, careful checks are made to ensure the generator terminals and all control wiring are correct so that the order of phases (phase sequence) matches the system. Connecting a generator with the wrong phase sequence will result in a short circuit as the system voltages are opposite to those of the generator terminal voltages. 

The sequence of events is similar for manual or automatic synchronization. The generator is brought up to approximate synchronous speed by supplying more energy to its shaft - for example, opening the valves on a steam turbine, opening the gates on a hydraulic turbine, or increasing the fuel rack setting on a diesel engine. The field of the generator is energized and the voltage at the terminals of the generator is observed and compared with the system. The voltage magnitude must be the same as the system voltage.


  Synchronization is essential for running generators on a parallel bus with other generators or the utility. The generator must be synchronized to the bus voltage reference before the paralleling switchgear will close it onto the bus. This is accomplished using a synchronizer unit, which drives the generator’s governor to control its engine speed and output voltage frequency. A generator synchronizer is also used with all closed transition transfer switches.

The synchronizer will sync the generator when a utility reference is presented to the line side of the transfer switch. This accommodates the closed transition from generator to utility. This technology has been used for many years and has evolved to greater speed of action with advancements in electronic synchronization control and governors over mechanical units. A generator synchronizer unit can also be used to hold distributed generators in sync with the utility or each other. This assures the upstream synchronization required in a distributed redundant design that utilizes Delta Conversion Technology. A PLC controller is used to sense each of the generator and utility voltage references. If a valid utility reference is available, the PLC will provide this reference to the synchronizer units that are connected to the distributed generators. If no utility is available, the PLC will pick an operating generator and use its output as the reference for the others. This will enable system synchronization in any operating condition.

               

Attached with this post are some pictures or the generator sets and the load bank. I was allowed to manipulate the testing of these generators with load. I also was introduced to the circuit breakers as well as the way powering up the generator up.I’ve also included a video (very shaky recording) of my experience in the generator room. Enjoy!

Power system harmonics

Power system harmonics is an area that is receiving a great deal of attention recently. This is primarily due to the fact that non-linear (or harmonic producing) loads are comprising an ever-increasing portion of the total load for a typical industrial plant. Incidence of harmonic related problems is low, but awareness of harmonic issues can help to increase plant power system reliability. On the rare occasions that harmonics are a problem, it is either due to the magnitude of the harmonics produced or a power system resonance.

The term harmonic refers to a component of a waveform that occurs at an integer multiple of the fundamental frequency. Fourier theory tells us that any repetitive waveform can be defined in

terms of summing sinusoidal waveforms which are integer multiples (or harmonics) of the fundamental frequency.

It is important to understand that harmonics are a steady state phenomenon and repeat

with every 50 Hz cycle (60Hz in North America). Harmonics should not be confused with spikes, dips, impulses, oscillations or other forms of transients.

A common term that is used in relation to harmonics is Total Harmonic Distortion (THD). Another common term used is Distortion Factor (DF) which is essentially the same as THD.

Now that we have a basic understanding of the term harmonics, we can move into more detailed aspects such as what causes harmonics? Harmonics are caused by non-linear loads, which are loads that draw a non-sinusoidal current from a sinusoidal voltage source. Some examples of harmonic producing loads are electric arc furnaces, static VAR compensators, inverters, DC converters, switch-mode power supplies, and AC or DC motor drives. In the case of a motor drive, the AC current at the input to the rectifier looks more like a square wave than a sine wave.
How do harmonics affect our power system? Power system problems related to harmonics are rare but it is possible for a number of undesirable effects to occur. High levels of harmonic distortion can cause such effects as increased transformer, capacitor, motor or generator heating, incorrect operation of electronic equipment (which relies on voltage zero crossing detection or is sensitive to wave shape), incorrect readings on meters, incorrect operation of protective relays, interference with telephone circuits, etc. The likelihood of such ill effects occurring is greatly increased if a resonant condition occurs. Resonance occurs when a harmonic frequency produced by a non-linear load closely coincides with a power system natural frequency. There are 2 forms of resonance which can occur: parallel resonance and series resonance.

The Chiller

What is a chiller? It is a machine that removes heat from a liquid via vapor-compression or absorption refrigeration cycle. The liquid can be circulated through a heat exchanger to cool air(or equipment) as required. It is maid out of 4 main components. These components include compressor, evaporator, condenser and a metering system.

 The basic theory behind the works of a chiller is that the water and refrigerant sucks up heat. Then it carries it to the condenser. From there it heads to the atmosphere by means of air evaporation or water cooled heat exchanger. It uses the vapor compression cycle to chill water and reject heat collected from the chilled water and heat from the compressor to a second water loop cooled by a cooling tower.

Lets talk about the components that make up a chiller and the basics of how they work. Firstly, we'll look at the evaporator. The heat exchanger that removes the building heat from the chilled water lowering the water temperature in the process. The heat is used to boil the refrigerant, changing it from liquid to gas. 

 

Next, lets look at the compressor. It is made up of prime mover and centrifugal compressor. The centrifugal compressor is a non-positive displacement type device. It raises the pressure and temperature of the refrigerant by converting kinetic energy to pressure energy.

Now, we shall move on to the condenser. It also works as a heat exchanger. It removes heat from the refrigerant causing it to condense from gas to liquid. The heat raises the water temperature. The condenser water then carries the heat to the cooling water then carries the heat to the cooling tower where the heat is rejected to the atmosphere.  

Finally, we can look at the expansion device. This devices comes into the picture after the refrigerant condenses to a liquid, it passes through a pressure reducing device. This is the main function of this device. It can be as simple as an orifice plate or as complicated as an electronic thermal expansion valve ( in the picture to the right ).

Now that we know how the chiller works, we can look at the Air Conditioning & Refrigeration Institute (ARI) standard conditions :

 i) chilled water temperature at 44° F

ii) chilled water flow rate is at 2.4 gpm/tan

iii) entering condenser water temperature at 85°F

iv) condenser water flow rate at 3.0 gpm/tan

v) 0.0001 evaporator fouling factor and 0.00025 condenser fouling factor.

The temperature change is documented by  this formula :  

        Q = WC ΔTF 

where:

Q = quality of heat exchange (btu/hr or kw)

W = flow rate of fluid ( USgpm or l/s)

C = specific heat of fluid ( btu/lb°F /kJ/kg°C)

Δ TF = temperature change of fluid (°F or °C)

Centrifugal compressor theory :

• Aerodynamic of turbine type
• Move gas by converting kinetic energy to pressure energy
• High flow rates capability and good efficiency characteristics     

Circuit Breakers ( ACB and VCB)

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms.

 

Specifically we will look at the air circuit breaker (ACB) and vacuum circuit breaker (VCB). The selection process of the kind of circuit breaker that is required depends on the levels of current in the circuit. Why? Its simply because the higher the current, the farther the arcing distance.

ACBs may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc however, VCBs have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (<2–3 mm). VCBs are frequently used in modern medium-voltage switchgear to 35,000 volts.

VCBs are used to protect medium and high voltage circuits from dangerous electrical situations. Two electrical contacts are enclosed in vacuum. One is fixed, the other is movable. When it detects danger, the movable contact pulls away from the fixed contact thus interrupting the current. Arcing suppressed due to the contacts being in vacuum. This ensures the circuit remains open. As long as it is open, it will not be energized. 

Current Tranformers

 
The term current transformer was alien to me until I witnessed the testing of the UPS system in All Asia Broadcast Centre. They were performing a primary and secondary injection, where I was asked if I knew the difference between these to injection methods. I was clueless and then briefed. The explanation was somewhere along the lines of the main line being the primary and some sort of device that makes it easy to measure as the secondary, in this case a current transformer (CT).
So what is a CT? A current transformer is a type of "instrument transformer" that is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary.
The current transformers used with the Wattnode transducers produce a 333 mV alternating voltage when the rated current is measured (either 30A, or 50A). The OSI power transducers employ CT's that produce 5V output at rated value.
The current transformers used with the Wattnode transducers produce a 333 mV alternating voltage when the rated current is measured (either 30A, or 50A). The OSI power transducers employ CT's that produce 5V output at rated value.
Current transformers are designed to produce either an alternating current or alternating voltage proportional to the current being measured. Current transformers measure power flow and provide electrical inputs to power transformers and instruments. Current transformers produce an alternating current (or voltage) that is proportional to the measured current.
Insulation voltage represents the maximum insulation that current transformers provide when connected to a power source. Accuracy is the degree of certainty with which the measured current agrees with the ideal value. Burden is the maximum load that devices can support while operating within their accuracy ratings. Typically, burden is expressed in volt-amperes (VA), the product of the voltage applied to a circuit and the current.

 

homework 2

      This time the assignment was to figure out a little bit  on how a transfer switch works. A transfer switch is an electrical switch that reconnects electric power source from its primary source to a standby source. Two types of basic transfer switches, open transition (automatic transfer switch) and closed transition (static transfer switch).

      The next step in understanding these switch types was trying to figure out what “break before make” and “make before break” statements mean. An automatic transfer switch (ATS) system is an open transition circuit. It works with a break before make logic. What does this do? It simply means that the switch will break contact with the main source of power before it makes contact with the back up power. It is good in the sense that it prevents backfeeding from the generator into the utility line.

What about make before break? A closed transition circuit is also known as a make before break switch. An example is the static transfer switch. This means that it will make contact with the back up source before breaking contact with the main source. Why is this necessary? Well some loads, are affected by even the slightest loss of power. There are also operational conditions where it may want to transfer loads with 0 interruption of power. The switch will maintain a make before break mode as long as both sources are acceptable and synchronized. Parameters that need to be looked in synchronization are voltage difference ( <5%), frequency difference( <.2Hz) and relative phase angle between the sources of 5 electrical degrees.

It is generally required that the closed transition, or overlap time, be less than 100 milliseconds. If either source is not present or not acceptable, the switch must operate in a break-before-make mode to ensure no backfeeding occurs.
         Closed transition transfer makes code-mandated monthly testing less objectionable because it eliminates the interruption to critical loads, which occur during traditional open transition transfer.

UPS?

Firstly, when one brings up UPS, what comes to mind? The delivery company is what I thought of before being exposed to the idea of UPS in an electrical sense. It is an uninterruptible  power supply (UPS).

What is a UPS system all about?A UPS system provides emergency power to load when input power source, typically the utility mains, fail! Basically its just a temporary power supply mostly used to power data centers and places of that sort to prevent the loss of data and the equipment being damaged.  There are many types of UPS systems, the one we are looking are specifically called the double-conversion ups.
Now another few questions would come to mind, what is a double conversion UPS system and how does it work? The basic principal of double conversion, take the AC source supply and turns it into a DC feed and then converts it back into an AC source.

Why use a double conversion UPS system? It provides the highest level of protection against the weakest spectrum of power. The incoming feed is of AC source and is converted into a regulated DC voltage. From this voltage, a new AC volatege is regenerated, providing continuous, clean and tightly  (between =/- 2-5 degrees celcius). The batteries are then connected to an inverter, so no power transfer switches are required. When power loss occurs, the rectifier cuts the circuit out and the batteries keep the power steady and unchanged. Upon restoration of the power, the rectifier resumes carrying most of the load and begins charging the batteries.

The advantages? One might be wondering why the need to use a double conversion system as opposed to other systems. One of the main advantages of a double conversion system is the ability to provide an electrical firewall between the incoming utility power and sensitive electrical equipment. This is crucial when dealing with the prevention of data loss and other issues of similar concern. 

homework 1

  I was given a homework assignment of finding out what a load bank is for. The picture on the right is a load bank. What is a load bank? It develops an electrical load then applies this load to an electrical power source and converts (or dissipates) the resultant power output of the source. Now that we somewhat know what it is, the next question is what is its purpose? Well, its called a load bank, one would automatically think that its used to store loads? This is not entirely wrong. However, the load it generates is used to accurately mimic the operational ("real") load that the power source will see in actual application. The difference is that the load from a load bank is contained, organized and fully controllable.

first day on the job

The date was May 23^rd 2011 and it was my first day reporting to the Broadcast & Operations Engineers department of Astro.  The first day was all about getting familiar with the areas and equipment that was going to be dealt with.

The first place I was brought to see was the broadcast generator room. I had never been in one of these rooms before so it was a rather thrilling experience. Before today, I hadn’t seen an industrial generator. Astro has 3 generator sets in this room. It works on 2 generators and has the 3^rd in case of problems with either of the other 2.  

Today, I  seen  a couple of things I hadn’t been able to see before. These things include carbon dioxide cylinders, bus bars, battery chargers, load bank, etc. I was introduced to Uninterruptable Power Supplies ( UPS), Main Distribution Supply ( MDS), Emergency Distribution Supply (EDS), Building Management System( BMS)