Final Project Report of Cloudgrid

The CloudGrid project is done in cooperation between NTNU, IPE, ZHAW, Chalmers and STRI/ABB. The work is split in six work packages where ABB/STRI, ZHAW, Chalmers and NTNU were package leaders. Besides the authors of this report, the following persons contributed to the project: I. Zikmanis (IPE), M. Calder (ABB), M. Javdani (ABB), A. Babak (Chalmers), A. Bahmani (Chalmers), P. Chen (Chalmers), C. Agathokleopus (Chalmers), O. Lennerhag (STRI/ABB), G. Pinares (STRI/ABB), A. Perez (STRI/ABB) and S. Sanchez (NTNU). The cooperation between the parties is established on many levels. Laboratory facilities and field measurements were shared to a certain extent and data sharing through a cloud was accomplished. This has a high significance for future smart grids where data exchange is essential for stable operation as well as short term and long-term planning. The scope of work covers of the work packages covers a broad range of topics, including stability of current and future power grids, ancillary services, energy management as well as multiterminal HVDC links and high power converter design and control

Analysis of High-Frequency Electrical Transients in Offshore Wind Parks

In this thesis, a study of high frequency electromagnetic transient phenomena is performed. Models of various components needed for simulations of high frequency transients, such as transformers, cables and breakers are developed. Also, measurements of high frequency transients are performed in the cable laboratory in ABB Corporate Research in V\ue4ster\ue5s, Sweden for the purpose of parameter estimation of models and for verification of simulations. Some critical cases where the voltage surges of the magnitude and/or rise time above basic lightning impulse voltage level appear, are identified. Also, some transient protection schemes are analyzed and the performance of different transient mitigation devices is studied. Furthermore, the energizing transient of the Utgrunden wind park is analyzed and the simulation model is verified using the measurements.The energizing transient simulation predicted accurately the magnitude and the frequency of the transient voltages and currents. Simulations of the high frequency transients are in a very good agreement with the measurements obtained in the cable lab. Simulations predicted accurately critical surges with the highest magnitude and matched with good accuracy surge waveform recorded during the measurements. During the testing and simulations, surges which exceed the basic lightning impulse voltage level of dry-type transformers specified by IEEE standards, both in magnitude and rise time are observed even when surge arresters are used. It is confirmed both in simulations and measurements that use of additional transient protections devices such as surge capacitors and RC protection, decreased the magnitude of surges below the critical level

Analysis of High-Frequency Electrical Transients in Offshore Wind Parks

In this thesis, a study of high frequency electromagnetic transient phenomena is performed. Models of various components needed for simulations of high frequency transients, such as for transformers, cables and breakers are developed. Measurements were performed in Chalmers research laboratory as well as at ABB Corporate Research in V\ue4ster\ue5s, Sweden for the purpose of parameter estimation of models and for verification of simulations. Critical cases where the voltage surges of the magnitude and/or rise time above basic lightning impulse voltage level appear, are identified. Also, some transient protection schemes are analyzed and the performance of different transient mitigation devices is studied. Furthermore, the voltage distribution along the winding during very fast transients is studied in order to estimate turn-to-turn voltages and the critical voltage envelope. In the work it was found that simulations of the high frequency transients are in a very good agreement with the measurements obtained in the laboratory. Simulations predicted accurately critical surges with the highest magnitude and matched with good accuracy surge waveforms recorded during the measurements. The accuracy of the rise times is within 10%, while the magnitudes during the critical cases are within a 5% margin. During the testing and simulations, surges which exceed the basic lightning impulse voltage level of dry-type transformers specified by IEEE standards, both in magnitude and rise time are observed at the transformers even when surge arresters are used to protect the transformers. Furthermore, obtained voltage surges exceeded the proposed critical voltage envelope. It is shown that the most critical voltage strikes are obtained with dry-type transformers, where the rise time is five to ten times shorter compared to oil-insulated transformer with the same rating. It was confirmed both in simulations and measurements that the use of additional transient protections devices such as surge capacitors and RC protections, decreased the magnitude of surges to be below the critical level. The analysis of the voltage during very fast transients showed that the rise time of the transients directly influences the magnitude of the turn-to-turn voltages. Furthermore, during the breaker closing transient, turn-to-turn voltages, measured in delta connected dry-type transformers, were 2.5 times higher than the voltages obtained during the same transient with a wye connected dry-type transformer, or during the stress with a 4.4 p.u. lightning impulse voltage

Analysis of High-Frequency Electrical Transients in Offshore Wind Parks

In this thesis, a study of high frequency electromagnetic transient phenomena is performed. Models of various components needed for simulations of high frequency transients, such as transformers, cables and breakers are developed. Also, measurements of high frequency transients are performed in the cable laboratory in ABB Corporate Research in V\ue4ster\ue5s, Sweden for the purpose of parameter estimation of models and for verification of simulations. Some critical cases where the voltage surges of the magnitude and/or rise time above basic lightning impulse voltage level appear, are identified. Also, some transient protection schemes are analyzed and the performance of different transient mitigation devices is studied. Furthermore, the energizing transient of the Utgrunden wind park is analyzed and the simulation model is verified using the measurements.The energizing transient simulation predicted accurately the magnitude and the frequency of the transient voltages and currents. Simulations of the high frequency transients are in a very good agreement with the measurements obtained in the cable lab. Simulations predicted accurately critical surges with the highest magnitude and matched with good accuracy surge waveform recorded during the measurements. During the testing and simulations, surges which exceed the basic lightning impulse voltage level of dry-type transformers specified by IEEE standards, both in magnitude and rise time are observed even when surge arresters are used. It is confirmed both in simulations and measurements that use of additional transient protections devices such as surge capacitors and RC protection, decreased the magnitude of surges below the critical level

Analysis of High-Frequency Electrical Transients in Offshore Wind Parks

In this thesis, a study of high frequency electromagnetic transient phenomena is performed. Models of various components needed for simulations of high frequency transients, such as for transformers, cables and breakers are developed. Measurements were performed in Chalmers research laboratory as well as at ABB Corporate Research in V\ue4ster\ue5s, Sweden for the purpose of parameter estimation of models and for verification of simulations. Critical cases where the voltage surges of the magnitude and/or rise time above basic lightning impulse voltage level appear, are identified. Also, some transient protection schemes are analyzed and the performance of different transient mitigation devices is studied. Furthermore, the voltage distribution along the winding during very fast transients is studied in order to estimate turn-to-turn voltages and the critical voltage envelope. In the work it was found that simulations of the high frequency transients are in a very good agreement with the measurements obtained in the laboratory. Simulations predicted accurately critical surges with the highest magnitude and matched with good accuracy surge waveforms recorded during the measurements. The accuracy of the rise times is within 10%, while the magnitudes during the critical cases are within a 5% margin. During the testing and simulations, surges which exceed the basic lightning impulse voltage level of dry-type transformers specified by IEEE standards, both in magnitude and rise time are observed at the transformers even when surge arresters are used to protect the transformers. Furthermore, obtained voltage surges exceeded the proposed critical voltage envelope. It is shown that the most critical voltage strikes are obtained with dry-type transformers, where the rise time is five to ten times shorter compared to oil-insulated transformer with the same rating. It was confirmed both in simulations and measurements that the use of additional transient protections devices such as surge capacitors and RC protections, decreased the magnitude of surges to be below the critical level. The analysis of the voltage during very fast transients showed that the rise time of the transients directly influences the magnitude of the turn-to-turn voltages. Furthermore, during the breaker closing transient, turn-to-turn voltages, measured in delta connected dry-type transformers, were 2.5 times higher than the voltages obtained during the same transient with a wye connected dry-type transformer, or during the stress with a 4.4 p.u. lightning impulse voltage

Improvement of admittance measurements for modeling of an ungrounded reactor/transformer winding

In this paper, a model of an ungrounded reactor winding is developed for transient voltage studies. The developed model is a black box model where the state-space model is developed using the directly measured admittance matrix of the reactor winding. Furthermore, an improved model is developed, where the accuracy of the admittance matrix measurement set in the low frequency band is obtained indirectly using an additional set of voltage ratio measurements. When the measured admittances are low, the accuracy of the diagonal admittance vectors in the low frequency band is low as well. However, the accuracy at low frequencies obtained using the indirect measurements is improved at nodes with the lowest admittances. In this paper, different approaches for indirect measurements are studied and the most accurate method is identified considering the physical structure of windings

Modeling of the energizing of a wind park radial

This paper presents measurement and modeling ofthe energizing of a wind park radial. The measurements in this paper are taken from the Utgrunden wind park that consists of seven wind turbines each of 1.5 MW rated power. This park is connected to the 6land 55 kV grid at the Degerhamn station. The wind turbines in the Utgrunden wind park are of the pitchregulated doubly-fed induction generator (DFIG) type. The transient voltage and the energizing current graphs are presented and both active and reactive powers are analyzed as the radials to the wind park is connected. Furthermore, the model of the Utgrunden wind park radial is built using PSCAD/EMTDC software. For the cable model, the frequency-dependent (phase) model built in model is used. A built-in model with the saturation modeling is simulated for the purpose of the transients analysis and the results are compared with measurements. The results show that the measured resonance frequency agrees well with the measurements while the damping is underestimated in the simulations

Voltage Stress in a Transformer Winding During Very Fast Transients Caused by Breaker Closing Event

Transformers connected at the medium-voltage level in cable grids, such as wind park collection grids and industrial grids, are exposed to the stress of very fast transients. These electric transients are mainly generated during breaker switching operations and the rise time of the transient voltage in such systems is much shorter compared to the rise times of transients generated in transmission systems at a high-voltage level. In this paper, the internal voltage stress is studied during very fast transients generated during transformer energization. Instead of using a breaker, the energizing tests are performed using a low-impedance pulse generator that can generate lightning impulse-shaped waveforms and voltage steps with rise times varying between 35 and 500 ns. Experiments show that during very fast transients with a 35-ns rise time and 1-p.u. magnitude, the interturn voltage exceeds the level obtained with a lightning impulse-shaped voltage waveform of 4.4 pu. Furthermore, during a specific switching scenario with delta-connected transformers, where the winding is excited from both ends, the same 1-p.u./35-ns voltage step generates an interturn voltage that exceeds the 1-p.u. level, which is more than 2.5 times higher voltage stress than during a lightning impulse test

Transformers internal voltage stress during current interruption for different wind turbine layouts

The voltage transients generated during the breaker operations in cable systems, for instance, a wind park collection grid, can reach very low rise times. The rise times of these transients can be almost 50 times shorter than the rise time of a lightning pulse. Such transients can generate a very high voltage stress on the internal transformers insulation. In this paper, a test case is studied using verified models of different types of transformers and wind turbine layouts in order to account for typical wind turbine layouts found in modern wind farms. A critical switching scenario is chosen in order to provoke the highest possible voltage transients during a current interruption. Furthermore, internal overvoltages are estimated using model of a similarly sized winding. Simulations show that the magnitude of the voltage transients is higher than the basic lightning impulse insulation level (BIL) defined by present standards. Moreover, it is found that the rise time of the voltage surges is much shorter than the rise time of the lightning pulse. The shortest rise time of 40ns is obtained in a wind turbine layout where the wind turbine breaker is placed near the transformer. Due to very short rise times of the transients, very high internal overvoltages are estimated in dry-type transformer windings. These internal overvoltages are much higher than overvoltages recorded at the basic lightning impulse level. For a wind turbine layout where a breaker is placed in the bottom of a tower and a dry-type transformer in a nacelle, the highest turn-to-turn voltage of about 1.5pu is estimated. This is almost 4 times higher turn-to-turn voltage then the voltage obtained during the BIL test. In a wind turbine layout where a breaker is placed close to the transformer, the amplitude of the turn-to-turn voltages reached 1.8pu due to lower stray capacitances and thus a shorter rise time of voltage strikes

Comparison of switching surges and basic lightning impulse surges at transformer in MV cable grids

The electric transients generated during the switching of a breaker placed near the transformer in a system with a substantial amount of cables can have a very short rise time and can reach very high magnitudes. In practice, the magnitude of the voltage surges is usually limited using the surge arresters while the rise time of the surge to which the apparatus is exposed, remains unchanged.In this paper, the characteristic of the switching transients generated in the MV cable grids are analyzed. For this purpose, cases with both dry-type and oil insulated transformers are studied. These cases are typical for a MV industrial grid and a wind park (WP) collection grid. Furthermore, the case with and without surge arrester protection is analyzed in order to observe the impact of the surge arresters on the voltage surges.Simulations show that the magnitude of the voltage surges exceeds the basic lightning impulse insulation level (BIL) defined by present standards. Furthermore, the rise time of the voltage surges is much shorter compared to the rise time of the lightning impulse especially in the case of the dry-type transformers and the WP collection grid. When surge arresters are used, the magnitude of the surges is limited to the BIL but the rise times of the surges can be twenty five times shorter compared to the BIL. Only when surge arresters are used with other fast transient mitigation equipment such as RC protection or surge capacitor protection, the magnitude of very fast voltage surges are brought down