Power quality

• What are interharmonics?

  Harmonic frequencies are multiples of the fundamental frequency. Interharmonics are harmonic frequencies that are between the harmonic orders. Typically, these values would be very low. However, if the inter-harmonic frequency happens to be at resonance value of a piece of equipment, it can be amplified significantly. This can create catastrophic failures in equipment. When equipment such as capacitor banks are out on a system, a harmonic and interharmonic study should be performed to ensure there are no potential resonance issues.

• What is mains signalling?

  As the smart grid grows, we will see more and more communications between equipment. Some of that communication can be through the use of ripple signals sent over the power lines. Utilities can use these signals to turn equipment on and off. This allows them to turn off large loads during peak times and turn them back on at a later time. When these signals are sent, they can cause some issues. There have been reports of customer appliances resonating when the signal is sent. There have been reports of street lights turning on during the day and off at night. The MPQ analyser will not only detect when a mains signaling event has occurred, but will capture the waveform as well. This means you can view in your trended data when a signal was sent and view the actual signal.

• What is RVC?

  In today’s modern grid power can be obtained from many renewable sources. These can include both solar power and wind power. These particular forms of power do not supply constant power, meaning their output can fluctuate. This leads to Rapid Voltage Changes (RVCs) on the power lines. RVCs are typically small changes less than a dip/sag or swell and can happen quickly. RVCs can lead to problems such as lights flickering and equipment tripping off line. RVCs can cause flickering in any type of lighting system, not just incandescent bulbs. Many analysers record the flicker parameter which uses a weighted curve that is based on the incandescent lamp. Additionally, RVCs can affect any type of lamp.

• Why analyse waveforms through 128th order?

  AC is converted to DC using rectifiers that create harmonics. In addition we now see DC being converted to AC using inverters. Modern inverters are using higher switching frequencies, which create higher order harmonics. This is especially true in solar and wind applications where higher order harmonics can be seen. Today, these high frequency harmonic orders between the 39th and 49th harmonic order can be seen. However, as switching frequencies increase, these orders increase also. The MPQ analyser gives us the ability to measure and analyse these high frequency harmonics.

• Why Auto CT ID?

  It is not uncommon to find operators connecting the wrong value CT to the analyser. This means the CT range selected in the analyser’s configuration file does not match the range of the actual CT connected to the analyser. A recording would be started with the wrong value CT connected. The analyser could be left in the field for a week or more. They would retrieve the unit only to find that the data was no good. They would then have to correct the analyser’s configuration and repeat the test. The auto CT identification will now let them know there is a mismatch and ask them if they would like to adjust / change the CT or would like the analyser to automatically alter the configuration to match the CT range that is connected. This eliminates the possibility of mismatched current clamps.

• Why capture transients down to 1 microsecond?

  High speed transients have limited energy because it is a function of time and a high speed transient can be as small as a microsecond. These transients are typically dampened out by just a few meters of cable. This is why high speed transients will typically not be an issue on distribution lines. However, these can be an issue in areas where there are limited cable runs. Traditionally, these areas would include offshore oil platforms as well as mines.

  But, now we see residential and small commercial buildings getting solar panels. In a wind turbine when the wind changes, the output of the turbine does not change instantly. However, in a solar panel when the solar radiation changes, the output of that panel changes immediately. This can lead to repeating high speed transients. Repeated high speed transients can damage sensitive electronics. There have already been reports of homeowners having to replace their microwave ovens every 15 months. The MPQ analyser can capture high speed transients down to 1 microsecond.

• Why Class A?

  Different instruments can aggregate their data differently. This means that if you have two different instruments you can get different readings on each. Which is correct? A Class A instrument will aggregate the data per the IEC61000-4-30 standard. (Now adopted by IEEE1159.) This means any two instruments that are Class A will get the same measurements. The majority of revenue meters used today are Class A, as are most monitors in substations. Therefore, they calculate their RMS voltage per the IEC61000-4-30 standard. A Class A instrument is needed in order to get the same measurements as other Class A meters.

• Why configuration verification?

  One of the most common problems we see is operators connecting the analyser incorrectly. They would start a recording and leave the analyser for a week or longer. When the analyser was retrieved, the operator would find the data was no good. Therefore, they would have to repeat the test, wasting a great deal of time. Configuration verification solves this problem. The analyser will examine the phase angle of all the channels and verify they are correct for the configuration selected. The analyser will let the operator know if channels are connected incorrectly or if a current clamp is backwards, ensuring proper connection before the recording is started.

• Why event and timed waveforms?

  Most power quality analysers today will record a waveform when an out-of-limit event occurs, such as a dip / sag or a swell. However they may not capture periodic timed waveforms. If you have a recording that has no out-of-limit events then you do not record any waveforms. Waveforms tell us a great deal about the system. These are needed to truly understand a system. The MPQ analyser will capture both event-triggered waveforms and periodic timed waveforms. This ensures you always have the information needed to determine the health of your system.

• Why IEC and ANSI unbalance?

  Today, the recommended method to measure unbalance is to use the IEC unbalance method which follows the IEC61000-4-27 standard (now adopted by IEEE1159). This method allows you to see negative sequence or zero sequence unbalance. Sequence components provide a better way to analyse the data and determine what issues it may be causing. The majority of source-side faults are asymmetrical which means they will not only cause changes in phase magnitude but also in phase sequence. The ability to view the changes in phase sequence allow for quicker analysis.

  ANSI unbalance is another method of measuring unbalance. This method averages the phases together, then compares each individual phase to the average. Many utilities may have legacy ANSI unbalance data.

• Why phase angle shift detection?

  In today’s modern grid, we can generate power from a variety of renewable sources. These can include wind energy as well as biomass. Some of these sources can produce a great deal of reactive power. For example, wind turbines use induction generators. These draw reactive power from the grid to create their magnetic field. When the wind is blowing, and they are producing power, we see good power factors at approximately 0.98. However, when the wind is not blowing they are not creating power, but are drawing reactive power. This can lead to very poor power factors, down to approximately 0.40. This can lead to low voltages in the transmission lines.

  In some cases, these renewable systems need to be islanded from the grid. When they come back on the grid, the phase needs to sync up. This can be seen as a momentary change in phasing. The MPQ analyser can record an event when it sees a phase shift which can allow the operator to see when islanded systems are coming back on line.

• Why THD and TDD?

  When analysing voltage harmonics, the Total Harmonic Distortion (THD) measurement is a good method to use. THD is the sum of all the harmonics referenced to the fundamental value. The voltage fundamental value is typically always present. However, the same cannot be said of current. When loads turn off, current drops and can drop to near zero. When this occurs, misleading THD values can be seen on the current channels. For example, if there is 1A of harmonics and 100A of fundamental, then the THD will be 1%. However, if there is 1A of harmonic noise and the current drops to 0.5A, the THD can be 200%, which can be misleading. IEEE519 recommends using Total Demand Distortion (TDD) when measuring current.

  TDD will reference the total current harmonics to the maximum average current recorded during the test interval, which indicates the reference value will always be valid. Some utilities have their own reference value they use throughout the grid. The Megger PQ PC software allows them to enter this value to use as a reference when viewing TDD.