During Severe System Disturbances in the Swiss Unterengadin Region, excessively high harmonic voltages have resulted in damage to some medium-voltage (MV) and low-voltage (LV) equipment. The MV subnetworks of the region are fed from substations supplied by the 380-kV transmission lines that tie Switzerland to Austria and Italy.

Measurements have shown that during certain network configurations, the 5^th harmonic voltage content is close to the International Electrotechnical Commission's (IEC) harmonic limit level and the 7^th harmonic voltage level exceeds the IEC's limit. Swiss utilities launched extensive studies to determine the exact cause of these problems, analyze the influencing factors and evaluate possible solutions.

THE PROBLEM

The Unterengadin region is situated in southeast Switzerland in the middle of the Alps. The regional distribution networks are fed from the 380-kV Pradella Substation, which is connected by a 380-kV overhead line to Sils, Switzerland, and the Swiss and European extra-high-voltage (EHV) transmission systems. In 1991, the EHV transmission system was extended with a double-circuit overhead line from Pradella to Westtirol in Austria. Figure 1 shows the EHV transmission and regional distribution systems.

Since this circuit was commissioned, harmonics in excess of IEC 1000-3-6 standard limits were occurring in the distribution networks of the Swiss Unterengadin during certain network configurations. In particular, high levels of 5^th and 7^th harmonic distortion (the latter being the highest) were recorded when the 380-kV Filisur-Sils line was disconnected; therefore, this was considered a critical circuit configuration. However, the voltage distortion was reduced when the hydropower plants in Pradella, Ova Spin and Martina were in service, and the harmonic current flow on the Westtirol intertie was unaffected.

Harmonic distortion was first recorded in the 1990s, when the 16-kV Samnaun network experienced occasional severe disturbances, resulting in damage to LV equipment including customer load control receivers and couplings. The problem has increased in recent years, prompting Swiss utilities to study this regional voltage pollution problem.

HARMONIC MEASUREMENTS

Continuous harmonic measurements on behalf of the regional utilities in various substations on the MV networks and in one substation on the 380-kV system have been made since 1995. All the measurement equipment has been connected to the LV side of the high-voltage (HV) or MV transformers. Hence, the measurement results are limited to the frequency range of 50 Hz to 500 Hz.

Figure 2 shows examples of the 7^th harmonic voltage recorded at different locations between Aug. 3 to Aug. 31, 2004. Trace 3 in Fig. 2 shows the 7^th harmonic course at the central 380-kV node of Pradella. With a value varying from near 0% to 4.4% maximum, the trace shows different levels — with the levels being superimposed by regular daily load variations — attributable to various system configurations. Under the normal network configuration, when all the 380-kV lines were energized, the specified 7^th harmonic level of 2% was exceeded most of the time. Disconnecting one of the two parallel lines between Pradella and Westtirol did not significantly alter this level, but when both lines were de-energized, there was almost no 7^th harmonic.

The maximum value of this harmonic occurred when the Schyn-N line was disconnected and all the other lines were energized. The harmonic voltage courses in the MV systems, the 10-kV Unterengadin (Trace 4, Gurlaina) and the 50-kV Davos (Trace 2) showed a very similar shape, whereas in the subordinate 25-kV system of Samnaun (Trace 1), which was equipped with a LV active filter for the 7^th and 5^th harmonics, the overall level was lower. An interesting fact is that the 7^th harmonic level in the Samnaun network decreased with system voltage. However, before installing the active filter, values of 14% were recorded on the Samnaun 0.4-kV network.

Based on the harmonic measurement studies, the following conclusions were drawn:

• The 2% acceptable 7^th harmonic standard was exceeded during some circuit configurations on the 380-kV and 50/60-kV systems.

• On the radially connected MV systems, 7^th harmonic voltages of 6% to 9% were recorded, coincident in time with the maxima on the EHV system (unless damped by local filters).

• ANALYSIS OF SYSTEM STUDIES

  The harmonic distortions occurred predominantly in the evening and at low load periods.

• With the EHV lines to Westtirol disconnected, no 7^th (and 5^th) harmonic voltages appeared in the remaining EHV system and subordinate MV systems. Therefore, the harmonic sources (currents) stemmed mainly from the Westtirolean section of the system. These were attributable to wind-farm generators in Germany.

To verify the results taken during the measurement period and to study possible countermeasures, a three-phase electromagnetic transient model of the monitored EHV, HV and MV systems was developed. Transformers and overhead lines were modeled by means of frequency-dependent elements, and loads were modeled by series-parallel R-L-C equivalents. The complete model network comprising voltage levels from 380 kV to 6 kV was tested in sections and as a whole for load flow and short-circuits conditions.

NETWORK TOPOLOGY

Frequency scan calculations were performed, whereby a current of 1 A was injected into the central 380-kV node of Pradella, and the voltage response was determined for frequencies over a 10- to 1000-Hz range. For a normal system configuration with all the 380-kV lines connected, the first resonance (peak) was detected at about 325 Hz. However, when the current was injected near Westtirol, the first resonance shifted closely toward 350 Hz (7^th harmonic). Figure 3 shows the frequency scan results for the 380-kV nodes Pradella and Filisur, a 50-kV node of the Davos network, a 60-kV node of the Unterengadin (Gurlaina) network and the 6-kV network of Samnaun when the Schyn-N line was disconnected. Injection of a 7^th harmonic current of 10 A near Westtirol results in harmonic voltages of 7.7% of line voltage (13.3% of phase voltage) in Samnaun, matching the worst-case scenario.

Measurements in the Swiss Unterengadin region confirmed that during low load periods under certain network configurations, the 7^th harmonic voltage level exceeded the IEC harmonic limit level, whereas the 5^th harmonic voltage level remained close to the limit. For example, the 7^th harmonic levels were recorded in the LV networks of Samnaun (14%), Gurlaina (8.8%) and Davos (6.5%), and in the EHV Pradella Substation (6.7%).

PASSIVE SHUNT FILTERS

The frequency scan analysis showed that the transfer impedances between the 380-kV Westtirol or Pradella nodes and the 16-kV Samnaun and 50-kV Gurlaina, Zernez and Davos nodes had a common parallel resonance peak at around 350 Hz. That meant a relatively small 7^th harmonic current flowing on the 380-kV transmission line from Westtirol to Pradella could produce very high 7^th harmonic voltages on the LV subnetworks connected to these substations.

The grid owners investigated four countermeasure techniques: changing the network topology, employing passive shunt filters, blocking series filters and installing active filters at strategic locations in the network.