0626GIC occurrences and GIC test for 400 kV system transformer

GIC Occurrences and GIC Test for 400kV System Transformer

Matti Lahtinen,Member,IEEE,and Jarmo Elovaara

Abstract—The geomagnetic induced currents in transmission network may saturate transformers and cause additional losses,a high level of harmonics,and a lack of reactive power in the grid. Also,malfunctioning of protection circuits is reported in this paper. The field tests made for two463MV A system transformers show that tested transformers may tolerate very high dc currents for times typical to geomagnetic storms.The effects of the voltage con-trol and voltage quality in network are also considered,and results of earlier Finnish GIC risk studies are reported.

Index Terms—DC-current,geomagnetic,immunity,trans-former.

I.I NTRODUCTION

F INLAND is situated in a zone which is prone to the con-

sequences of the changing conditions of near-earth space. The changes are mostly induced by sunspots and solar flare ac-tivity.In the electric power transmission system,one of these consequences is the geomagnetically induced current(GIC). The magnitudes of the GICs have been measured almost contin-uously in the Finnish400kV grid since1977,first in a400kV neutral point of one400MV A system transformer.At present, a continuous GIC-monitoring system is operating at three dif-ferent400kV substations as a part of a system,which monitors and records the power quality.With the present microprocessor-based measuring system,the GIC-recordings can be combined with the simultaneous measurements of reactive power flow and harmonic current and voltage.

The grid owner has financially supported the Finnish Me-teorological Institute(FMI)which has developed theoretical models and has estimated the occurrence probabilities of high GIC-events.The models are used to estimate the risks that GICs may cause to the Finnish power system.

The earlier Finnish GIC-recordings and Nordic experiences are reported,e.g.,in[1].Finland has never experienced an outage or an equipment failure or malfunctioning caused by the GIC although very high neutral-dc-currents have been measured during geomagnetic storms(e.g.,165A/10s on January4,1979,175A/10s,and200A/1min on March24, 1991).However,clear evidence is obtained that GICs have caused saturation of transformers because the harmonic content of line currents increases during the high GIC.

Because power transformers might be subjected to a high risk under large dc-magnetization conditions,a dc magnetiza-tion test was already done in1980for a three-phase,five-legged,

Manuscript received April10,2000;revised October11,2001.

The authors are with Fingrid Oyj,Helsinki,Finland(e-mail:htinen@ fingrid.fi;jarmo.elovaara@fingrid.fi).

Publisher Item Identifier S0885-8977(02)02750-4.full-wound,core-type YNynd11-connected400/400/125MV A transformer to measure the thermal behavior of the transformer and its different structural parts.Nothing alarming was noticed in this test[2].

In20years time,the internal design of the power transformers used in Finland has developed considerably.Therefore,it was decided to make new dc-magnetization test on site and to also record some of the network effects.This paper describes the main results of the tests but also gives statistical background for the decision that no mitigation techniques be used in Finland against the possible adverse effects of the GICs.

II.P ROBABILITY OF S EVERE G EOMAGNETIC S TORM

O CCURRENCE IN F INLAND

FMI is continuously collecting geomagnetic data and has de-veloped software packages and performed studies for evaluating the GICs in an arbitrary network,[3]–[5],[11].Here,we refer to the study made in1991through1992[3],[4],and to a newer study[11].In the first mentioned study,the basic input data was the ionosphere current which could be modeled as an indefi-nitely long sheet or line current.The resistivity of the earth could be modeled with large homogeneous geoelectric blocks or as a perfect conductor which was situated at a depth of300km.To be able to combine the geomagnetic data with the help of dif-ferent models to the GIC-data,a special measurement campaign was carried out in June1991through May1992.The location of the selected measurement stations is shown in Fig.1.The de-veloped theoretical models were fitted to the observed GIC data by determining the relative significance of each model at several occurrence probability levels.After this process,the statistics of the external geoelectric field was known and the GIC statistics in the whole network could be determined.

In the study of[3]and[4],the geomagnetic activity of a spe-cific observation station or network of stations was described in the

index

-,-com-ponents

of)can be derived from the statistics of the in-

dices