How To Use an Arbiter 928A Power System Multimeter

    The Arbiter 928A power system multimeter, mentioned in the blog post In-Service Load Checks, is an excellent device for measuring electrical power during commissioning checks.  It is a 2-channel device that measures and compares two AC signals coming from the secondary side of instrument transformers.  It is important though to set it up properly to meet your personal preference and to use it successfully for the first time.

    In this example, the 928A was set up as follows in the phase preference settings.  Channel A was chosen as the reference.  Voltage was applied to Channel A and current was applied to Channel B.  The polarity was set for positive.  The degree range was set for 0-360.  The 928A was used to measure the electrical power flowing at one end of a 69kV sub-transmission line.  The instrument transformers used was a bus, potential transformer (PT) and a line breaker, bus-side current transformer (CT).  The secondary voltage of the A phase PT was applied to Channel A and the secondary current of the A phase CT was applied to channel B.  The amp probe arrow was facing away from the CT polarity mark and looking towards the relaying and metering.  The 928A readings of 65.222V, .506A and 197.48 angular difference in Picture 1 revealed that the current lagged the voltage by 197.48 degrees and that 13.6 megawatts and 4.3 megavars were both flowing into the bus from the line.  By comparing this power flow to the power flow at the other end of the line, this proved to be an accurate reading.

    The 928A is able to make this analysis by looking for similar zero crossings of the two AC signals and determining the angular difference between the two crossings.  In picture 2, the voltage and current for this example is graphed on an amplitude vs. time graph.  Because voltage was selected as the reference, the voltage sine wave rises and crosses zero amplitude at time = 0 seconds.  About ten milliseconds later, the current rises and crosses its zero amplitude.  The 928A determines from this time difference that the current lags the voltage by 197.48 degrees.

    Two common mistakes made when using the 928A is incorrect placement of the amp probe arrow and incorrect interpretation of the angular difference reading, not to be confused with phase angle.  Regardless of which way power is flowing, the amp probe arrow should always be placed so that the arrow faces away from the CT polarity mark and towards the relaying and metering, which is connected in the CT circuit. This is done because it is standard for the direction of CT current flowing away from its polarity mark and into the relaying and metering to be compared to the phase to ground voltage signal.  Picture 2 shows how secondary CT current flows in relation to primary current flow.

    When you want to draw current and voltage vectors on a power graph, the 928A set-up dictates how you should interpret the angular difference angle that is given.  In this example, because the voltage is the reference, its vector is drawn on the zero degree mark on far right.  Because the polarity in the phase preference settings of the 928A is positive, the current vector is drawn 197.48 degrees clockwise from the reference to show that the current is lagging the voltage by 197.48 degrees.  Technicians might want the angle shown to direct the current vector to be drawn counter-clockwise from reference.  In this case, if the polarity in the 928A set-up was changed to negative, the 928A would give an angle of 360 minus the angular difference reading, being 162.52.  Now, by drawing the current vector 162.52 degrees counter-clockwise from the reference, it would put you in the same spot.  It is important to note that the Arbiter always looks for a lagging value of the non-reference channel to the reference channel.  If current was selected as the reference, then the 928A would look to see how the voltage lags the current.  It is also important to notice that CT secondary current is flowing opposite to the direction of the amp probe arrow.  If the megawatts would have been flowing in the other direction, flowing from bus out on the line, the angular difference would have been 17.48 degrees, which is the phase angle.  Opposite current direction in this example causes a 180 degree phase shift and is what indicates to the technician that megawatts is actually flowing into the station, rather than out of the station.

    Article by Dan Scrobe III

How To Prove CT Connections Using Vectors

     On a Delta - Wye transformer, CT's on the Delta side are wired in Wye and CT's on the Wye side are wired in Delta in order to correct a phase rotation and to filter out zero sequence current. However, how would a commissioning engineer go about proving that the delta CT is wired correctly on the Wye side?  In other words, would polarity of the CT on A phase connect to non-polarity of B or C?  A Linked In forum member shared with me that it is typical that the CT wiring configuration on the Wye side of the transformer should mirror the Delta side winding connections of the transformer.  For taking it an extra step, this could be proven using vectors.

     Dan Scrobe III

In-Service Load Checks

photo: www.energy-parts.com

     Before new substation circuit breakers are released for normal service to transmission operators, it is imperative to ensure that its current transformers are providing the proper secondary current inputs to protective relaying and metering, as called for by protection engineers.  This is done by performing in-service load checks when the breaker is carrying load for the first time.  A technician will measure secondary currents and voltages, and the phase angle shift between the two of them, that go to relaying and metering.  The secondary voltages are taken from line/bus potential transformers and are used as a reference to measure the phase angle.  The engineer will calculate the primary and secondary currents, along with the phase angle, based off of what is happening on equipment that is already in service.  Once the technician and engineer agree with their findings, they will verify that these secondary values are properly telemetered to transmission control centers.

     In this example, Cat-Trac has a 115kV bus with two transmission lines connected to it.  Each line breaker has bushing current transformers (CT) and the bus has a potential three phase transformer (PT), both of which provide secondary inputs to relaying and metering.  Breaker 98712 was replaced, therefore, breaker 97112 was used as the reference, for purpose of calculating current magnitudes and phase angle for breaker 98712.  Work was scheduled in accordance with PJM.  This system utilizes A-B-C rotation.  A balanced three phase system was assumed.  Voltage and current magnitudes are in RMS.  

     The phase angle is the angular difference between voltage and current and it is based off of the real and reactive power flows.  Since breaker 97112 showed .7 megawatts (MW) and 8.3 megavars (MX) flowing into the bus, breaker 98712 should have the same readings but both flowing away from the bus.  Using basic power formulas, the phase angle was shown to be 85 degrees.  The angle is considered lagging because the MW's and MX's are in the same direction.  On a distribution and transmission system, it is accepted that inductive reactive power will flow in the same direction as real power.  Capactive reactive power would flow opposite to real power.

     Using basic power formulas and a reference line-to-line voltage reading, taken from a voltage meter from the bus potential transformer, primary amps was calculated to be 41.5 line amps (A).  CT secondary current was calculated to be .17A, using the CT's ratio of 240, which is a ratio number of primary amps to secondary amps.
     It helps to draw current vectors on a power quadrant graph. You simply pick the quadrant you need to be in, based on what the real and reactive power flows are. Because current vectors rotate counter clockwise, it is important to draw the graph so that negative MX's are on the positive vertical axis and positive MX's are on the negative vertical axis.  First Energy standard is that power that flows away from the substation is considered positive and vice versa for flows that come into a substation.  Therefore, since both real and reactive power flowed away from the substation in this example, the first current vector is drawn in quadrant two with an angular difference from the horizontal axis.  If positive real power and negative reactive power flow was observed, then the current vector would have been drawn in quadrant one.  Negative real and positive reactive would have been quadrant three.  Negative real and negative reactive would have been quadrant four.
     Because a three phase potential transformer was used for the voltage readings, A phase is considered the reference.  A reference vector is typically drawn at zero degrees.  Therefore, you can imagine this voltage reference located at zero degrees on the power quadrant graph.  The first vector drawn in quadrant two in the picture shown means that A phase current vector lags A phase voltage vector by 85 degrees.  Once this first current vector is drawn, then it easy to draw the other vectors to show 120 degree displacement and A-B-C counter-clockwise rotation.
     When the breaker was put into service, current vectors were read from a SEL-411 relay and drawn on a graph.  A phase current was shown as 86 degrees lagging, which is very close to the calculation. You will also notice an imbalance in the current magnitudes.  There will always be some imbalance but it is very important to verify the correct phase angle and phase rotation, so that when readings are telemetered to transmission control centers, the operators there know the correct direction that real and reactive power is flowing.
     Article by Dan Scrobe III

What Is An Arc Fault Circuit Interrupter?

     For homeowners of new home construction, you might find some unusual looking yellow circuit breakers in your circuit breaker panel.  According to an online journal report from Lansing, Michigan, that was put out last October, more than 50 percent of the electrical fires that occur every year in the US could have been prevented with the installation of arc fault circuit interrupters or AFCI's, which immediately shut off power when a fire hazard - or an arc fault - is recognized.  Typical household fuses and circuit breakers do not respond to early arcing and sparking conditions in home wiring.  By the time a fuse or circuit breaker opens a circuit to defuse these conditions, a fire may already have begun. AFCI's defend against damaged electrical cords and sparking wires, is currently a residential code requirement in 49 states (Indiana being the exception) and runs about $40 for each device.  As a result, families should be better protected from the threat of electrical fires in the home. So, how does an AFCI work?

     Unlike a standard circuit breaker, which detects overloads and short circuits, an AFCI is a breaker that utilizes advanced electronics to sense certain current waveform characteristics and logic to determine if tripping is necessary, which all sounds very similar to a relay.  Op-amps and transistors perform analog signal processing and a microcontroller performs logic. The end goal of an AFCI is definitive detection of a hazardous arc condition of two types, parallel and series, resulting in breaker tripping.  In the parallel type, an arc will travel from line to line, line to neutral or line to ground and the amount of current available is dependent upon the power source.  In a series type, the arc occurs within the conductor itself and the amount of current available is limited to the load on the circuit.  An example would be a conductor that has pulled apart or a loose connection at a receptacle.  Parallel is the more serious of the two arc types.

     The key to detection of these two arc types is the ability to tell the difference between a normal and a dangerous arc condition.  A normal arc condition would be that of a motor in an electric drill. Arcing that takes place in a drill is established and extinguished at a rate relevant to the revolutions per minute of the drill.  The internal arcing does not have a direct correlation to the AC source, since the arc breaks at each gap in the stator. The electronics of the AFCI detects this normal condition when it compares the periodic function of the current waveform to the voltage waveform.  In a dangerous arc condition, severe broadband noise is generated that can range between tens of kilohertz to 1 gigahertz and exists only during the conduction of current.  The AFCI looks for certain waveform characteristics, such as DC offset and zero crossing behavior, that typically exist whenever noise is being generated to the atmosphere.

     AFCI's resemble Ground Fault Circuit Interrupters (GFCI) in that they both have a test button, but they differ in functionality.  GFCI's protect against electric shock, whereas AFCI's protect against the threat of structure fires, caused by electric hazard. A GFCI detects leakage current, whereas an AFCI detects an abnormal current waveform, by looking for certain characteristics that are indicative of an arc hazard.  AFCI's are of similar shape and construction to a normal circuit breaker and can easily be installed in the home circuit breaker panel, but will have a yellow "AFCI" label and a test button next to the switch.

     Currently, AFCI protection of branch circuit wiring in dwelling unit bedrooms is required on new installation per NEC Code 210.12.  The NEC Code panel wants the industry to gain experience with these devices in bedroom circuits so that in the future their usage might be expanded to other rooms and facilities that could benefit by the added protection they provide.

     Article by Dan Scrobe III

What is Commissioning?

    "Apollo 13 Flight Controllers.  Listen up!  Give me a go / no-go for launch..."  Gene Kranz, Apollo Flight Director, Space Center Houston.  When watching Ed Harris play this role in Ron Howard's 1995 movie, you get a clear sense that commissioning substation equipment into service for the first time is strikingly similar to the many verification steps that Kranz had to make on all systems and to the green light he gave to launch control at the Kennedy Space Center on April 11, 1970.  Dr. Allen Morinec, who teaches electrical engineering at Cleveland State, used this analogy to rally a group of engineers at First Energy's MonPower center in Fairmont WV, to appreciate the importance of the commissioning process, which uses methodical and calculated steps to catch and correct any mistakes before releasing for normal service all new substation equipment on the electric grid, such as circuit breakers, capacitors and transformers.

     "Go!" Booster Systems Engineer.  This person was responsible for all propulsion matters during prelaunch and ascent.  The power to make something move could be compared to the DC power in substations, that is required for conversion to the immediate, on-demand, mechanical energy that is necessary to move the large contacts of circuit breakers to an open or close position.  All substations have rows of wet cell batteries that provide the high DC currents required for these breaker operations.

     "Go!" Network Officer.  This person was responsible for supervising the network of ground stations that relayed telemetry and communications from the spacecraft to Space Center Houston's flight officers.  This can easily be compared to a substation's RTU (Remote Terminal Unit), which is an onsite computer that sends information back to transmission control centers in Ohio and West Virginia in order for operators there to analyze what the grid voltage is and the power that is flowing thru there.  This is necessary to help maintain system reliability.  Just as Houston had to communicate to flight commanders, James Lovell, John Swigert and Fred Haise, transmission operators sometimes have to communicate to substation equipment and command a breaker to open or close, for the purpose of redirecting power flow, isolating faulted equipment or protecting human life/equipment from harm/damage.

     "We're go, Flight!" Electrical, Environmental and Consumables Manager.  This person was responsible for support systems in the spacecraft such as cabin cooling, vehicle lighting and cryogenic monitoring for the fuel cells.  These support systems are easily compared to a substation's station service, which provides low voltage power to heating and cooling systems, relaying and control mechanisms.

     "3, 2, 1...ignition!" Kennedy Space Center.  You will notice in the movie that at any time before ignition, Kranz had the ability to abort the launch.  It was his job that all checks were verified leading up to that beginning moment of the mission and to stop the mission if there was anything that was wrong.

     "We have liftoff!" Kennedy Space Center.  As Apollo 13 ascends in the movie clip we are shown, Dr. Morinec so enthusiastically boasts, "I commissioning engineer release this equipment for normal service!"  The analogy is successfully made to demonstrate all that goes into the steps leading up to the beginning of a mission.  It is also a time of documentation to provide a snapshot of history to show to future technicians, operators and engineers how a piece of equipment was put into service with perfection.  For Apollo 13, its mission was very short indeed, but for substation equipment, its mission will hopefully last 20-30 years of providing electrical service to customers.

     Article by Dan Scrobe III

FR Clothing

     In the electrical industry today, not enough can be said about safety.  Every day, whether on conference calls in the office or on tailgate discussions in the field, the inherent dangers of the electrical system are considered, mishap prevention is discussed and all incidents are evaluated to promote a safer working environment.  One particular, but very serious, danger when working near live conductors is the risk of arc flash.

     An arc flash, also called a flashover, is a rapid discharge of electrical energy across an insulating medium.  As the integrity of the medium begins to fail, a conductive plasma can form and reach a temperature of 36,000 degrees Fahrenheit, roughly the temperature of a particular surface layer of the sun.  This plasma is the fourth state of matter, ionized gas, and produces extreme heat and light, that are backed by the electrical energy of the system.  The propagation of heat from this plasma, in the form of plasma spray and infrared radiation, is what can cause substantial equipment damage and injury.  The available energy to feed this spectacle varies according to system fault duty, construction of electrical apparatus and fault clearing time.  Incident energy is what is received by the worker and it depends on the arc energy, the propagation directionality of the heat energy and the distance between the arc and the worker.  To protect the worker from the threat of arc flash, workers in the field are required to wear flame-resistant (FR) clothing, in compliance with OSHA regulations 1910.269.  The development of this type of clothing can be accredited to the work of Alice Stoll.

     In the late 1950's and early 60's, the United States Navy performed research on the effects of heat energy transfer to the human body's surface as part of its aerospace program.  Sailors would line up to volunteer to subject themselves to first degree burns in exchange for weekend passes at the Naval Air Development Center in Warminster, PA, near the Naval Air Station Joint Reserve Base Willow Grove. Headed by research physiologist Alice Stoll, this elaborate testing program lead to construction of a mathematical model, which related heat energy transfer to the body's surface as a function of three factors:  exposure time, level of heat and increase in temperature.  The term "second degree" burn was coined from these experiments as it was the point in burn severity, in which the epidermis would separate from the dermis.  This separation is what caused blisters to form.  Second degree was also the point at which human tissue could no longer dissipate heat by itself and was found to happen at an exposure to an incident energy level of 1.2 calories per centimeter squared, a level of heat energy that can be experienced if you were to hold the surface of your hand at the tip of a cigarette lighter flame for one second.  The mathematical model for human tissue was soon made relevant to other kinds of surfaces such as protective fabrics.  For the first time, any kind of fabric could be graded numerically on its effectiveness to withstand combustion.  Stoll clearly demonstrated that fabric constructed of inherently flame-resistant fibers was far superior to flame-retardant treated fabric.

     Today, regulation of FR clothing is a major portion of companies' electrical safety programs, mandated to have per NFPA 70E-2012, Standard for Electrical Safety in the Workplace.  FR protects the worker in that it will withstand combustion up to its rated heat energy value, will extinguish itself when the energy source is removed and will not melt.  These characteristics comply with OSHA's stipulation that clothing should not add to a worker's injury.  FR clothing will have the "FR" logo stitched onto it and is composed of natural fibers such as cotton.  Use of clothing that contain synthetic fibers, such as nylon and polyester, is forbidden as these fabrics have shown to continue to burn after the removal of incident energy.

     In the picture shown, a 100% cotton garment from Carhartt has a rated heat energy value of 8.8, also called an Arc Thermal Protective Value (ATPV).  This number means that the garment will withstand combustion when exposed up to and including 8.8 calories/centimeter squared of heat energy.  This doesn't mean that the garment will shield all the heat energy from underlying surfaces.  There is a 50% chance that the worker could receive a first or second degree burn.  Although this particular garment is cotton, only fibers that have been tested in the laboratory can be assigned an ATPV rating, as thickness, type of weave and color can play a role in flame resistant qualities.  The ATPV level required for use in certain applications is specified in a company's electrical safety program.  Met-Ed uses Hudson Workwear and Tyndale for its FR clothing.  An excellent blog on latest developments in arc flash safety is Electrical Arc Safety. 

     Article by Dan Scrobe III

Transformer Connections

     Sometimes, transformers at substations need to be switched out for maintenance.  However, if there is only one transformer at a substation, a mobile transformer needs to be installed in parallel before the transformer is switched out.  The key to connecting a mobile transformer to the system lies in an understanding of the transformer nameplate.  

     The nameplate is typically drawn in an aerial view fashion in order to determine the physical location of the primary (H1, H2, H3) and secondary (X0, X1, X2, X3) bushing terminals, and it is typically located on the side of the transformer, as that of the X0 bushing, which is used for a neutral connection.  Since high voltage bushings will generally be larger than low voltage bushings, it is easy to identify the X0 bushing and nearby nameplate.  The vector diagram from the nameplate shows how the windings in the transformer are connected to each other and symbolizes the vector relationship of the three phases of electric connected to them.

     In Met-Ed, 69kV Delta transmission is used to source a distribution transformer, that provides power to a 13.2kV Wye distribution system.  Before switching out this transformer, it is important to visually trace out its connected conductors to phase markings outside of the area being disturbed, such as on a pole outside of the substation, in order to determine proper connections to the mobile transformer.  Met-Ed uses phase markings to indicate A-B-C phase rotation.  

     The picture shows primary leads entering the high side of the transformer called "Bank 1" in a B-C-A configuration to H1, H2 and H3 terminals respectively.  The vector diagram from its nameplate is drawn with its matched phase markings.  Since three phase power rotates counter-clockwise on the electric grid, visualize the vector diagram rotating counter-clockwise and you can confirm Met-Ed's A-B-C rotation.  If you visualize rotating the vector model on the nameplate of the mobile by 180 degrees, you will see that it matches up to that of Bank 1.  This check confirms that you have a compatable transformer.

     To connect the primary leads to the mobile, there are actually three ways to do it and still maintain A-B-C rotation.  In this example, for simplicity, it was chosen to connect the primary leads to the mobile transformer called "Mobile 7" in a B-C-A configuration to H1, H2 and H3 terminals respectively, to keep the same physical run of the conductors.

     To ensure that the secondary voltages of Mobile 7, once energized on the primary side, are in phase with the 13.2kV distribution system, secondary phase connections are chosen to ensure the in-phase relationship between primary and secondary vectors.  For example, it was chosen to connect secondary leads to Mobile 7 in B-C-A configuration to X1, X2 and X3 terminals respectively.  It can be seen from looking at Bank 1 that primary voltage vector A-B is in phase with secondary voltage vector N-A.  This can be proven also by looking at the mobile connections.  As long as both primary and secondary vectors are in the same geometric direction among both transformers, phasing will be successful.  Same visual check can be performed for the other phases.

     Article by Dan Scrobe III