Why your protection isn’t to blame
Electrical power supplies are such an important feature in nearly every industry that they are often taken for granted. Loss of power to an office or production facility has such an impact that operators will often strive to restore power at the exclusion of all else. When this happens the tendency is to want to blame the item which caused power to be switched off; the protection. This is like blaming the safety barrier for damaging your car when you come off the road. The following are some of the reasons why protection systems are blamed for outages and how to make sure it doesn’t happen to your business.

If you want tips on driving your car, you’re on your own.
“My protection didn’t operate”
Do you mean that it didn’t operate at all, or that it didn’t operate the tenth time you tried to switch it back on? “Weeeelll…”  

The switch-it-back-on-and-see-what-happens philosophy might work for finding a blown lightbulb at home, but should never be confused with proper fault-finding. If you keep ramming that crash barrier with your car, eventually it will fail. A good protection scheme will keep records of any trips that have occurred; more advanced schemes will even hold data centrally in a SCADA system. CEE’s NP900 relays are perfect for this task since they have built-in fault recorders for recording current and voltage graphs before and during a fault. They also have different communication options for transmitting data to a SCADA system or control panel. If you are really concerned about unauthorised re-setting of trips, why not use the NP900’s three-level password access system?  
When was the last time you reviewed your protection settings?
“The protection was designed and set by a reputable company; it has worked fine since then”. Until now.  

Any change to your power distribution system could have an impact on the running currents, fault currents and therefore on the protection settings themselves. If the crash barrier stops your car successfully, it might not be so effective if you drove a lorry instead. Some changes may be implemented by your utility company (DNO in Britain) and are therefore beyond your control: 
  • Higher fault levels lead to higher fault currents; switchgear may not operate or may be destroyed by the fault current. 
  • Lower fault levels can mean that protective devices operate more slowly than they were designed for, or high-impedance faults might not be detected at all.  
CEE can perform protection studies to ensure that your settings are appropriate and that your equipment is rated to handle the predicted fault current. Also consider inter-tripping schemes to clear a fault using an up-stream circuit breaker with a higher rating. Use the NP900’s GOOSE messaging system (see our January Newsletter) to send fast, reliable status messages between protection relays.  
“My protection keeps tripping but I can’t find a fault”
Nuisance trips are part of the reason that protection systems get bad press. If your operators get used to re-setting trips, eventually one of them will be genuine. There are several reasons why a trip might occur that are hard to track down:
You are looking for the fault in the wrong part of the system.
Poor discrimination between protective devices can lead to an up-stream device tripping for a fault that has occurred much further down in the system; much like a barrier that stops you from driving off the side of a bridge by preventing you from getting to the bridge in the first place. CEE can perform discrimination studies on your behalf, or you can verify your own system using the Power*Tools for Windows (PTW) software.

Your equipment is drawing extra current on start-up.
No matter how straight the road is, if you swerve briefly to one side you are more likely to crash. Even if you have accounted for circumstances like motor starting currents in your protection, if the impedance of your cables is too high or if your network is too weak, there will be a voltage dip that will cause other equipment to draw even more current to maintain the same power. This is often a problem when systems are powered by a back-up generator that cannot supply as much inrush current as a utility could. CEE’s NP900 relays have up to 8 configurable setting groups to allow you to adapt your system to multiple different operating scenarios. This might include having different overcurrent settings when on generator power or a load-shedding scheme.
You have harmonic current problems. Harmonic currents can cause spurious trips in two ways:
  • Triplet-order (3rd, 6th, 9th etc.) harmonics in three-phase systems add together at the neutral of a star-connected system, causing unexpected current flow. Some protection schemes may interpret this as a fault. 
  • Additionally, harmonic currents cause conductors to heat up; for simple thermal protection trip units this can actually lead to a spurious trip. Like a crash barrier that melted in the sun? I don’t know – use your imagination! 
CEE can perform harmonic studies on your system to predict potential problems. If you need a corrective solution, the NP900 relays are also available with harmonic overcurrent protection and measurement of voltage harmonics (up to 31st harmonic).  

CEE recommends reviewing your protection settings whenever there are major changes to your network. If you have protection problems, CEE can also perform tests on your protection relays.

Drive safely!