Tech & Services
Phase verification in power networks by Damon Mount, Power Sales Manager, Megger UK.
Incorrect phasing in power networks can have devastating and even deadly consequences. Yet phasing has, until now, been surprisingly hard to verify. This has changed with the introduction of an innovative phase verification system that makes use of the ultra-precise time-base provided by the signals from GPS satellites.
Phasing is often far from readily apparent. Sometimes there is no phase identification available, phase identification can be incorrect due to line transposition, crossover in joints or there could just be incorrect labelling of conductors or terminations. This can open the door to misunderstandings amongst staff, and dangerous phasing errors when adding new equipment, connecting or re-connecting a network.
The consequences of incorrect phasing can be catastrophic. If a link is inadvertently made between two wrongly phased supplies in a power network, the instantaneous currents that flow are likely to be enormous. In a properly designed system the protection will operate very quickly, but in the very short time before this happens or if the protection fails, the energy produced by the fault current could damage or destroy equipment, and possibly even cause an explosion.
It’s worth noting that phasing mistakes can lie dormant for months or years. Often open ring distribution networks incorporate switches. One of these switches can remain open in normal circumstances, but could be closed to reconfigure the network if there a need for planned maintenance or correcting a fault. Closure of such a switch will temporarily connect two supply sources that normally operate as two radial feeds into a closed ring. If the phasing is incorrect – possibly because of changes that have been made to the network since it was originally commissioned – the results can be dire.
The solution to all of these problems is to use a dependable method of phase verification. Before discussing this, however, it’s important to emphasise one key point: phase verification is not the same as checking phase rotation. It’s perfectly possible to have two supplies that are phase displaced with respect to each other, yet have the same phase rotation. Phase rotation testing is no substitute for phase verification; for safe network operation, it is essential to be able to determine the absolute phasing at every point in the network.
Absolute phasing can only be determined when measurements are made with respect to a known reference phase. Unless the system is very small,
this means that a two-part instrument is needed to make the measurement – one part to act as the base station that provides the reference information, and the other to perform the measurement at the point in the network where it is required to measure the absolute phase.
This is the concept used in a novel phase verification system that is now available. It comprises two identical devices. One is configured as the base station and is connected to a low-voltage reference phase while the other is configured as the phasemeasuring device, which, for convenience, can be referred to as the mobile device, as it will normally be used at a location remote from the base station. An ultra-precise timebase for synchronisation of the devices is established using signals from the GPS satellites that are more usually associated with satellite navigation systems. Synchronisation data is passed between the devices via the mobile phone network using a GSM module that can operate in standard data mode (circuit switched data, or ‘CSD’) as well as in GPRS mode.
If voltage for the mobile device is up to 400 V, you can connect directly to the phase that you need to identify. You can use the same connections when capacitive measuring taps on switchgear are available.For higher voltages, you could use a high-voltage sensor with wireless data transmission to the measuring device with an insulated pole approved for use at the appropriate voltage.
If the base station is not connected to L1 as the reference phase, the corresponding correction angle of +120º or -120º must be entered into the mobile device. Also, depending on the application, there may be transformers with the same or different vector groups between the base station and the measuring device. These vector group shifts, which will lead to a particular phase shift, can then be entered into the mobile device in order to obtain the correct absolute phase indication.
To cater to the widest possible range of applications scenarios, this innovative phase verification system offers four operating modes.
NET mode is applicable when a low-voltage supply, such as a mains socket, is available at the location where the phase measurement is to be performed. In this case, the mobile device is connected to the low-voltage supply and a one-time synchronisation process is carried out with the base station. The mobile device determines the phasing of the low-voltage supply and uses this the local reference phase for all measurements carried out at this location.
The low-voltage source must remain connected to the mobile device throughout the entire measure- ment process.
The major advantage of this operating mode is that GPS and GSM reception are only needed for a short period while the one-time synchronisation process is carried out. After that, all measurements are carried out with respect to the local reference phase, which means that the measurements can be performed very quickly.
NO NET mode
Where no local low-voltage supply is available – for example, when carrying out measurements on overhead lines – the mobile device operates from its internal rechargeable battery. If immediate phase display during measurement is needed in such cases, the mobile device must be continuously synchronised with the base unit via GSM, and GPS reception must be available.
NO NET/NO GSM mode
There are a few locations where no local low-voltage supply is available and there is also no GSM reception. To make measurements in these locations, NO NET/NO GSM mode is used. In this mode, when the measurement is being made, the mobile device stores only the GPS time signals and the voltage zero crossings. When the mobile device is subsequently moved to a location where GSM reception is available, post-synchronisation is performed – the absolute phase identifiers are determined and stored in a measurement file for later analysis. Post-synchronisation can be performed at any time up to 10 days after the measurement data is recorded.
In this mode, only the mobile device is used. It is connected to a local supply with a known reference phase, and all measurements are made with respect to local reference phase. No synchronisation with the base station is needed.
The novel phase verification system described in this article allows safe, fast, and accurate phase identification at all voltage levels. The range of operating modes available makes the system convenient and easy to operate in virtually every application. The use of this system helps to eliminate the serious safety hazards associated with incorrect phasing in a network, as well as ensuring that documentation and any future additions will be correct. The ultimate benefits are, therefore, significant overall improvements in network reliability, efficiency and safety.