Earth fault current compensation

In Finnish networks the high voltage side of the primary transformer is earthed only in some particular points defined by Fingrid Oyj, but basically the transmission network is unearthed. As a result the earth fault in transmission network does not effect on the cur-rents or voltages in the medium voltage network. Same applies in case of earth fault in medium voltage network: it does not effect on the primary side network of primary trans-former. The primary transformers are mainly Yd coupled, which means that there is no neutral point in secondary side available. Therefore in order to connect an earthing device to neutral point of medium voltage network a neutral point needs to be created. This is usually done with earthing transformer, which usually is Znyn-coupled. The compensa-tion coil is connected to neutral point of earthing transformer via disconnector that ena-bles the disconnection of the coil during for example maintenance. The centralized Pe-tersen coil is automatically adjustable. In Finland compensation degree is kept slightly under compensated, but for example in Sweden it is kept slightly overcompensated. Be-hind this practice is the idea that it is more likely for some network part to be disconnected from network, which changes the network closer to resonance point. [11]

2.5.2 Distributed compensation

Instead of one large controlled coil in the HV-MV substation it is possible to install small compensation devices around the system. Each of these devices comprises a star point transformer and Petersen coil without automatic control or transformer that consist of YN connected Petersen coil for earth fault current compensation and distribution transformer [10]. Distribution transformers with only earth fault current compensation capability are Zn(d)yn or Znzn0-coupled. These couplings give the magnetic balance to the transformer in case of an earth fault at LV-side of distribution transformer, which disables the earth fault current to be seen in MV-voltage side. [11] If these coils are properly located in individual feeders around the network, no additional automatically adjusted arc-suppres-sion-coil is required. The disconnection of the compensation equipment when the associ-ated feeder is isolassoci-ated from the network keeps the compensation level at sustainable state regardless of the switching arrangements on the network. [1]

2.5.2.1 Effect of distributed compensation on earth fault current transportation

In system where series reactance can be neglected, equivalent circuit of earth fault in system with distributed compensation is equal to one in case of system with centralized compensation except that zero sequence network consists of multiple parallel inductances

and possible resistances. Earth fault current and neutral point displacement voltage can be calculated with equations (6) and (7) presented in Section 2.1.2.

The distributed compensation is one solution to limit the resistive losses caused in trans-portation of reactive current and non-ideal neutral point reactors. In Figure 13 the se-quence networks in case of earth fault in busbar is presented. In system presented in left side of the figure only centralized compensation coil is used. In the system presented in the right side of the figure combination of centralized and distributed compensation is used.

Figure 13. Earth fault at the busbar in single long cable system with centralized (left) and distributed compensation (right) [3]

If the cable is long and only centralized compensation is used the zero sequence series impedance is not negligible. If also distributed coils are used and those are dimensioned to compensate for the capacitive current generated in the system, and the distance between the coils is limited, the total shunt impedance is very large and the series impedance can therefore be neglected. [3]

2.5.2.2 Coil rating

The effect of arc suppression coils is based on the inductive earth fault current generated in the coil, which compensates the capacitive earth fault current. Distributed compensa-tion coils are manually adjustable and there are coils of different sizes. ABB produces coils with compensation capacity of 3,5 - 5 A, 5 - 15 A and 15 - 25 A. [10] The rating of the coil can also made by the shunt impedances of the system. For the influence of series impedance of the system to be as small as possible, the equivalent zero sequence shunt impedance should be as large as possible. The shunt reactance of system can be calculated with equation (18).

𝑋_{𝑠ℎ𝑢𝑛𝑡} = 𝜔3𝐿_{𝑙𝑜𝑐𝑎𝑙} ∙ 1 𝜔𝐶_{0𝑥𝑘𝑚}

𝜔𝐶_{0𝑥𝑘𝑚}1 − 𝜔3𝐿_{𝑙𝑜𝑐𝑎𝑙} (18)

Where Xshunt is the shunt reactance of compensated line section, 𝐿_{𝑙𝑜𝑐𝑎𝑙} is the inductance of distributed coil and 𝐶_{0𝑥𝑘𝑚} is the equivalent capacitance of the cable section.

While the reactance of coil approaches the reactance of cable section the total shunt reac-tance approaches infinity. In reality the capacireac-tance to earth is distributed and the shunt admittance is therefore finite. As the distance between coils increases, shunt admittance of the system decreases and resistive losses in zero sequence network increases. [4] In addition to resistive losses of in lines resistive losses are also generated in compensation coils. Resistive losses of compensation coils are approximately 2,5 % and therefore losses increases while the coil size increases. [10] The current provided by compensation coil can be calculated with equation (19).

𝐼_𝐿 = 𝑈₀

√3∙ 1

𝑅_𝐿+ 𝑗𝜔𝐿 (19)

Where 𝑈₀ is the zero sequence voltage affecting on distributed coil, 𝑅_𝐿 is the series re-sistance of the coil and L is the inductance of the coil.

2.5.2.3 Coil location

The location of distributed arc suppression coils effects on the resistive losses during earth fault. It is recommended in Licentiate thesis of Anna Guldbrand that 15 to 20 km of each cable feeders are compensated by centralized compensation coil and the rest of the feeders are compensated with local compensation coils.

In reference [13] J. Jaakkola and K. Kauhaniemi have been investigated the effect of density of distributed coil. In that paper the smallest variation in fault currents in different fault locations was reached in distributed compensated networks where distributed coils were placed in 5 km intervals. However, there was no big difference compared to situation where coils were placed in 10 km or 20 km intervals. Therefore it is probably cost efficient to install coils with 10 or 20 km intervals. In case of centralized and distributed coils it turned out to be good solution to compensate first 10 km of feeders with centralized com-pensation coil and the rest of the feeders with distributed coils with interval of 10 km.

[13]

During a pilot project in Savon Voima Verkko Oy Dyn11+YN coupled distributed coils were installed in cabled rural distribution network. Esa Virtanen proposes that on the basis of experience in that project the optimum location of coils is about 10 km from each other’s, and that in mixed network 50 % of earth fault current should be compensated centrally and 50 % with distributed coils. [10]

2.6 Analysis method for compensated system equipped with