Technology Corridor Stay Online With Partial Discharge by drg59916


									                                                                                                       activity along a medium-voltage power
   Technology Corridor                                                                                 cable while the cable remains online.
                                                                                                           The PD Detection Online With
                                                                                                       Localization (PD-OL) system provides
Stay Online With Partial                                                                               significant benefits over the current
                                                                                                       offline test systems. While the offline

Discharge Testing                                                                                      systems require disconnecting a cable
                                                                                                       from the grid and energizing it with a
                                                                                                       separate power supply to generate PDs
How to maintain continuous power                                                                       and are performed on an occasional
supply while measuring for weak spots.                                                                 basis, PD-OL enables continuous on-
BY PETER VAN DER WIELEN AND BARRY WARD                                                                 line cable monitoring, allowing utilities
                                                                                                       the benefits of:
    ailures in medium-voltage power          inates from weak spots along a cable.                         s Maintaining continuous power

F   cables and their components cause a
    large proportion of annual power
service interruptions, especially in high-
                                                 In its current form, PD testing
                                             occurs offline and involves some draw-
                                             backs in terms of cost, risk of damage
                                                                                                              supply during measurement;
                                                                                                           s Testing the cable system under
                                                                                                              exact operating conditions,
density urban areas. Locating and            to the cable, safety, and operational                            including over voltages and
repairing weak areas in cables at an early   impact. However, a new measuring                                 load variations;
stage can improve the reliability of the     technique has been developed that can                         s Cost-effective operation with
energy supply considerably.                  diagnose and locate PD activity on a                             minimal personnel effort
   Analytical methods for asset manage-      medium-voltage power cable while the                             required after installation;
ment that depend on cable-specific fail-     cable remains in service. The patented                        s Continuous registering of data
ure models work best when the condi-         system, which was created by KEMA                                that captures time-related infor-
tion of the cable is known. Partial dis-     in partnership with the Eindhoven                                mation, such as variation of PD
charge testing is a proven condition         University of Technology in the Nether-                          activity in time, the effects of
assessment/test methodology. A partial       lands, uses inductive sensors to detect,                         sudden over-voltages, and cable
discharge (PD) is a short pulse that orig-   diagnose, and locate partial discharge                           temperature variations; and
                                                                                                           s Detecting PDs occurring shortly
                                                                                                              before failure.
                                                                                        Source: KEMA

     Rendering of KEMA PD-OL Computer
     (left) and Sensor (right).
                                                                                                       Improving Asset Management
                                                                                                       and Reliability
                                                                                                       Many of North America’s critical cus-
                                                                                                       tomer areas are served through under-
                                                                                                       ground distribution systems, at voltages
                                                                                                       between 5 kV and 35 kV, using paper
                                                                                                       insulated, lead covered or solid dielec-
                                                                                                       tric (polymeric) cables. Although these
                                                                                                       systems have given good service in the
                                                                                                       past, wear and tear at several utilities are
                                                                                                       driving up failure rates, operating prob-
                                                                                                       lems, and service interruptions as the
                                                                                                       systems age. Repair and replacement
                                                                                                       costs, as well as service quality problems
                                                                                                       due to circuit and equipment outages,
                                                                                                       are expected to escalate in the future
                                                                                                       unless preventative action is taken.
                                                                                                           An asset-management strategy that                                                                                           OCTOBER 2006 PUBLIC UTILITIES FORTNIGHTLY 73
                                                                                                                                                       Source: KEMA

                                                 INSERT PD-OL:                    • digitizer
              Transf.-r      RMU left            • PD sensor                      • pulse injector                Transf.-r    RMU right
                                                 • pulse injection coil           • modem

                                               PD-OL left          Cable under test      PD-OL right

includes some form of proactive cable                 PD-OL Technology                                 ing from other adjacent equipment.
replacement, life extension, condition                The PD-OL methodology has been                       For online PD detection and local-
assessment, and continuous monitoring                 tested and developed by KEMA in                  ization, the only effective solution is
to manage cable failures is essential to              Europe and is entering the pilot stage in        placing sensors at both cable ends illus-
avoid these future problems. In some                  the United States. The system locates            trated in Fig. 1. While this solution
cases, load reduction, switching,                     the origins of PDs by using two induc-           may look simple, achieving accurate
dynamic ratings, and temporary over-                  tive sensors, each at one cable end. A           time synchronization of the digitizers at
loads also can be used as strategies to               patented solution is used for the time           both cable ends is problematic. In the
reduce failure rates and mitigate reliabil-           synchronization of the data intake at            case of a required PD localization accu-
ity impacts of aging cables. However,                 both cable ends and for the online cali-         racy of about 1 percent of the cable
an overall cable-management strategy                  bration. One computer at a remote                length, the related time synchronization
is essential for long-term improvement                location collects data from various PD-          of the digitizers should be in the order
in cable-system operation and electric-               OL measuring systems for final inter-            of 100 ns for a 2 km long. For this syn-
system reliability to the customers.                  pretation and presentation, which can            chronization, KEMA has a known and
For example, a KEMA study at a major                  be made visible on a secured Web site            patented solution with GPS, which is
U.S. investor-owned utility predicted                 for the network owner.                           used for offline PD measurements on
that the number of cable failures on its                  PD measurements can help to diag-            long (up to 10 km) and branched cable
system dramatically would increase in                 nose medium-voltage cables. The offline          networks. However, this is an expensive
the coming years unless certain preven-               test systems applied today can measure           and impractical solution for long-time
tative measures are taken. Currently,                 and locate PDs by placing one sensor at          monitoring. A possible alternative, as
this utility experiences about 600 cable              one cable end and using PD pulse reflec-         suggested by many, is the use of atom
failures each year on an installed base of            tion. This implies that the cable has to         clocks, but these either are not stable
approximately 25,000 circuit miles of                 be disconnected from the grid and ener-          enough or even more expensive.
several different types and vintages of               gized with a separate power supply to                The PD-OL measuring system uses
cables. Based on a statistical analysis of            generate the PDs. The primary reason             pulse injection for synchronization.
the data, this number could increase by               PDs currently are not measured and               Pulses are injected at one cable end on
a factor of 10 over the next 30 years. An             located online is that under online con-         a regular basis and measured at both
average customer served by this                       ditions, PD pulses barely will reflect at        cable ends. These pulses are injected
utility currently experiences about                   the cable ends, making localization with         with an additional coil along the meas-
167 interruption minutes per year,                    one sensor practically impossible. The           uring sensors at the cable termination.
with 13 interruption minutes being                    reflections become too small to detect           This method of synchronization, a cru-
caused by cable outages. The number                   because of the non-infinite impedance            cial and patented element in PD-OL,
of interruption minutes is likely to                  of the equipment connected to the cable          is both inexpensive and very effective.
increase to more than 280 minutes if                  under test at the transformer house or               PD-OL also uses inductive sensors,
only a reactive cable repair policy is                substation. Furthermore, measuring               which have the advantage of being cost-
employed, thereby significantly reduc-                with one sensor at one cable end, it is          effective and easy to install. Such induc-
ing the reliability of electric service               impossible to distinguish pulses from            tive sensors have certain disadvantages.
to its customer base.                                 the cable under test and pulses originat-        In general, inductive sensors placed in

74 PUBLIC UTILITIES FORTNIGHTLY OCTOBER 2006                                                                           
a ground lead will pick up more             tion or GPRS to a server. Here, the               CAREER OPPORTUNITIES
noise/interference than do capacitive       final PD map is made, based on PDs
sensors. In addition, inductive sensors     and their time of arrival.
                                                                                                  Operations Manager,
only can be applied on terminations            The costs of the PD-OL equipment
                                                                                                   Member Services
where the metal screen at the cable end     together with the interpretation of the
has a separate ground lead (a path for      results are only a fraction of the costs of          Operations Manager, Member
the PD pulse) to the general grounding      replacing a cable connection. The costs            Services, E Source, Boulder, CO.
                                                                                                Responsible for consulting and
system. In a termination where the          that will be reduced are the costs related
                                                                                               research services on E-Business
metal enclosure of, for instance, the       to outages. These costs usually are only          strategies for the energy industry.
switchgear is fully bonded with the         partly direct costs for the utility itself.           Requires master’s degree in
metal screen of the power cable, such       Furthermore, if the knowledge of the                   Business Administration,
an ideal location for an inductive sensor   condition of the cable network                      Management or related fields.
cannot be found. Since PD-OL                improves (which happens when using                       Apply by email to
currently is based on inductive sensors,    PD-OL) it will help the utility with        
this measuring system is optimized          replacement strategies. If utilities can
for terminations without full metal         postpone cable replacement invest-
enclosures.                                 ments, benefits are huge compared
    Before a PD can be measured, it         with the costs of PD-OL.                          PUBLIC UTILITIES
is important to calibrate the system.
In PD-OL a smart solution is applied.       Current Status
At each cable end, steep pulses are         PD-OL has been demonstrated success-                 Utility Rates and Studies Office,
injected, and measured separately at        fully on several sections of differing          Naval Facilities Engineering Com-
both cable ends. Magnitude and shape        types and lengths of cable in the               mand needs to fill a position with a
of resulting signals measured with the      Netherlands, and is now at the first            highly qualified person to perform
inductive sensors depend on the imped-      production stage, with approximately            utilities cost of service studies.
ances of the power cable under test and     70 systems on order for several utilities
                                                                                                 For details and how to apply visit
the connected equipment at the trans-       in Europe. The activities foreseen in
former house or substation. In this way,    the second half of 2006 are directed at
                                                                                            Announcement No. NW6-1130-14-
the transfer impedance for a pulse com-     bringing PD-OL to the market, com-
                                                                                            K1421969-DE. This is a permanent
ing from the power cable and entering       bined with extensive testing in the field.
a sensor is calculated. This information    In addition, further work on the PD-            full time position located in the
is used for the actual calibration of PD-   OL methodology focuses on the appli-            Washington Navy Yard. For info call
OL. Again, the pulse injection method       cation of PD-OL in difficult networks           202-685-0130. Navy is an E.O.E.
is crucial here.                            (branched, cross-bonded, many RMUs,
    PD data acquisition is carried out      etc.) and to the further enhancement
at both cable ends. There the first step    of knowledge rules.                           also so-called “short term” defects can
of the data analysis is done to extract         The advantages make PD-OL                 be traced prior to breakdown. F
possible PD signals from the noisy data,    attractive in a much wider environment
remove interfering signals and thereby      of cable diagnostics. Especially because      Peter van der Wielen is a consultant and
reduce the size of the data to be com-      (1) a control room can monitor                Barry Ward is a senior consultant at KEMA.
municated. The resulting data is sent by    all cables connected to PD-OL; and            Contact van der Wielen at
LAN, modem, and telephone connec-           (2) not only long-term defects, but 

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