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
