GIC observations and studies in the Hydro-Quebec power system

With the fast-paced changing technologies in the power industry, new references addressing new technologies are coming to the market. Based on this fact, there is an urgent need to keep track of international experiences and activities taking place in the field of modern transformer design. The complexity of transformer design demands reliable and rigorous solution methods. A survey of current research reveals the continued interest in application of advanced techniques for transformer design optimization. This paper conducts a literature survey and reveals general backgrounds of research and developments in the field of transformer design and optimization for the past 35 years, based on more than 420 published articles, 50 transformer books, and 65 standards.

Transformer Design and Optimization: A Literature Survey

In the early 1990s, Hydro-Que/spl acute/bec undertook a joint project with GE to develop and commission a dc current-blocking series capacitor for the transmission lines forming part of its main power system in the area surrounding the grounding electrode of the Radisson HVdc substation. The aim of this project was to minimize the harmful effects of dc current flowing through the main network during the ground-return operating mode of the Radisson-Sandy Pond multiterminal HVdc link. With a view to having available technology applicable anywhere in the network, TransE/spl acute/nergie and IREQ, two divisions of Hydro-Que/spl acute/bec, later took part in the development of a dc current-blocking device that could be installed in transformer neutrals. This work was undertaken not only with immediate needs in mind but also to solve specific problems that could appear sporadically at different network locations. Tests on a prototype of the new blocking device were performed in the High Power Laboratory at IREQ, followed by tests on the preproduction unit, in accordance with the specified requirements. The neutral dc current-blocking device (NCBD) was then installed at a regional substation for a one-year operating period to verify its immunity under normal network events and switching conditions. During this period, no disruption to the smooth operation of the substation or protection of the ac system occurred. The NCBD operated correctly and suffered no fault or unsolicited operation.

Development of a DC current-blocking device for transformer neutrals

There is some misconception in the electric power industry today that geo - magnetically induced currents (GIC) have caused, and would cause, significant damage to the majority of large and medium power transformers installed on power grids in North America and around the world. The purpose of this paper is to present, both qualitatively and quantitatively, the true effect of GIC on power transformers, based on thorough knowledge of power transformer design and performance. The paper also provides concise explanation of the cases of transformer failure / overheating reported in the published literature to have been caused by GIC currents. The paper demonstrates that, because of the nature of the GIC currents, the great majority of power transformers would not experience much overheating or damage due to even high levels of GIC. Only some specific transformers could suffer some winding damage due to high winding circulating currents if exposed to high levels of GIC. The paper also presents the real issue with GIC; namely, the narrow pulse of the magnetizing current which results from part - cycle, semi - saturation of transformer cores when subjected to high levels of GIC currents. This one current pulse / cycle could cause relays and capacitive components in power systems, such as SVC (s), to trip, which may contribute to grid instability. Also, the current pulse is associated with 2nd and higher order harmonics. As a result, resonance may occur, differential relays may operate, and stability of the grid may be compromised.

Effects of GIC on power transformers and power systems

Saturation time of transformers under dc excitation

The temperature rise in power transformers subjected to a DC source of excitation was measured on core-form single-phase 735 kV autotransformers rated 370 MVA and 550 MVA. The measurements were compared to the temperature rise values obtained on the same transformers operating in overexcitation at 1.95 T. The results show that the tie plates of this particular type of transformer are the components most susceptible to rapid temperature rise. Smaller-scale tests on 100 kVA transformers were performed to take a specific look at these tie plates under the effect of a temperature increase. Finite-element simulations combined with analytical studies of temperature rise were performed in an attempt to determine a tolerable DC current limit based on permissible temperature standards.

Study of the acceptable DC current limit in core-form power transformers

The temperature rise in power transformers subjected to a dc source of excitation was measured on core-form single-phase 735-kV autotransformers rated 370-MVA and 550-MVA. The measurements were compared to the temperature rise values obtained on the same transformers operating in overexcitation at 1.95 T. The results show that the tie plates of this particular type of transformer are the components most susceptible to rapid temperature rise. Smaller-scale tests on 100-kVA transformers were performed to take a specific look at these tie plates under the effect of a temperature increase. Finite-element simulations combined with analytical studies of temperature rise were performed in an attempt to determine a tolerable dc current limit based on permissible temperature standards.

V. Q. Pham

Papers

Study of the acceptable DC current limit in core-form power transformers

Study of the acceptable dc current limit in core-form power transformers. Discussion