Tech & Ser­vices

Power Watch India - - CONTENTS - By Da­mon Mount

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

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.

LO­CAL mode

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.

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