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