Analysis of Power Transformer under DC/GIC Bias
Abstract
Power transformers are key components in the interconnected bulk power transmission grid. Moreover, to ensure the reliable and stable operation of the power grid, the interaction of the transformers and the power grid during normal and abnormal operation conditions were studied. To study abnormal operation conditions of power transformers it may be necessary to include the non-linear hysteresis characteristics of the transformer cores in electromagnetic transient studies. The modelling of the hysteresis characteristics of the transformer cores requires detailed information about the transformer core design and material. If this information is not available, it is challenging to establish an adequate electromagnetic transformer model. Especially during deep saturation conditions, typically near two Tesla for gain-oriented steels, an accurate modelling of the hysteresis characteristics can be essential for the calculated phase currents. Such saturation conditions could be caused by geomagnetically induced currents (GIC's) or direct current (DC) bias caused e. g. by power electronic devices. This work is a follow-up investigation, motivated by increased transformer sound, which could be traced back to GIC s in the high and extra high-voltage transmission grid. This work presents a measurement based modelling approach to establish electromagnetic topology models of power transformers, including the transformer’s core hysteresis characteristics. First the AC saturation test was developed with the idea to saturate the outer two legs of a three-phase transformer core by two elevated 180° phase-shifted single-phase voltages. The AC saturation test was successfully used to parametrise the hysteresis model of two transformer topology models, using the inductance-reluctance and the capacitance-permeance analogy. Because the AC saturation test requires a sufficiently large power source, it was further developed to the DC hysteresis test. Instead of using a 50/60 Hz sinusoidal voltage, a DC with reversal polarity was used. The DC hysteresis test was also successfully used to parametrise the transformer hysteresis models. The implementation of the DC hysteresis test in a portable transformer test allows to conduct this test in the laboratory and in the field. Together with the principle of variable core gap inductance the transformer topology models of a 50 kVA reveal a high accuracy of the calculated and measured current waveforms during the AC saturation and the standard no-load test, as well as the corresponding power demand. For the measurement of transformer neutral point currents, including geomagnetically induced currents (GICs), an existing measurement system was further developed to minimise the constraints of the monitoring system on grid operations. The utilisation of a split-core current transducer around the earthing switch, together with a software-supported correction of the offset drift, reveals a low long-term offset drift of the measured transformer neutral point current. In addition to the measurement of the transformer neutral point current, the measurement system was extended to monitor a direct current compensation (DCC) system, installed in several transformers in the transmission grid. The analysis of the DCC measurements, which allows a calculation of the DC per phase, reveals an equal distribution of the DC between the high-voltage phases and the capability of the system to minimise the effects of GICs in transformers.
... The excitation current is measured with 100 A/60 mV dc shunts to record the dc voltages. The accuracy in phase angle and amplitude of the dc shunts was validated in [27]. Before each measurement the transformer is demagnetized by over-excitation and continuously decreased 50 Hz sinusoidal voltage to prevent remanence in the transformer core. ...
Power transformers are key components in the
electrical energy transmission and distribution. Therefore, special
attention is paid to the condition of the transformers, using
electrical, chemical, acoustical, and mechanical techniques to
assess the transformer health status. However, many diagnostic
methods require the transformer to be taken out of service, which
can be difficult due to operational restrictions. This paper deals
with the detection of distorted excitation voltage of transformers
utilizing vibration measurements on the transformer tank. We
investigate the applicability of vibrations measurements on the
transformer tank to assess changes in the electrical excitation
of the transformer. We are using standard vibration measure-
ment equipment for on-load tap changer diagnostic to detect
voltage harmonics and dc bias via vibration measurements on
the transformer tank. We found that small dc bias in the
supply voltage and harmonics with amplitudes below the power
quality limits can be detected. These results may be useful to
investigate increased transformer sound on-site and to detect
other transformer faults with minimum effort on-site.
We need a typical method of directly measuring geomagnetically induced current (GIC) to compare data for estimating a potential risk of power grids caused by GIC. Here, we overview GIC measurement systems that have appeared in published papers, note necessary requirements, report on our equipment, and show several examples of our measurements in substations around Tokyo, Japan. Although they are located at middle latitudes, GICs associated with various geomagnetic disturbances are observed, such as storm sudden commencements (SSCs) or sudden impulses (SIs) caused by interplanetary shocks, geomagnetic storms including a storm caused by abrupt southward turning of strong interplanetary magnetic field (IMF) associated with a magnetic cloud, bay disturbances caused by high-latitude aurora activities, and geomagnetic variation caused by a solar flare called the solar flare effect (SFE). All these results suggest that GIC at middle latitudes is sensitive to the magnetospheric current (the magnetopause current, the ring current, and the field-aligned current) and also the ionospheric current.
A short overview of the developments in the overall transmission and distribution industry in South East Europe is presented in this contribution. Furthermore, as the present journal focuses on breaking and switching in HV and MV switchgears, several technology innovations in T&D switchgear have been identified, at global level. In almost all cases, the ongoing energy transition is the main driver of these innovations. In this contribution, based on test-experience and overview of the industry, innovations are grouped by drivers, with each of them having its impact on testing technology.
These drivers are the following:
1. Increase of ratings: the ongoing trend of co-existence of micro-, local- and super-grids. The consequences for testing of switchgear for UHV and very large current are highlighted:
2. Offshore transmission: compactness and reliability are requirements stretched to the limit for this application. Very long cables for AC and HVDC power transmission, even in a multi-terminal topology will have impact on switchgear.
3. Health safety and environment: here, the major switchgear related discussion is on SF 6 replacement and reduction of electrical losses. Various issues related to SF 6 replacement will be highlighted. In addition, a recent project intended to quantify the hazard of aluminium bus bars in power stations affected by long duration fault arcs is described.
4. HVDC and power electronics: circuit breakers are going to be installed in multi-terminal HVDC grids. Testing of HVDC breakers, regarding the complete interruption current process is described.
5. Digitalization: switchgear is going to be communicating with the IEC 61850 communication protocol. This will impact testing for which laboratories need to prepare. The introduction of smaller micro-electronics (sensors, IED) closer to primary HV components and its high-frequency transients need careful consideration regarding operation, endurance, and lifetime.
6. Resilience and fault mitigation: Resilience against severe weather conditions, vandalism and cyber-attacks calls for resistant equipment and robust mobile equ ipment/substations that can 6be deployed very rapidly still having an extreme degree of readiness and availability.
7. Quality assurance of products in the face of a pandemic and a lockdown requires adequate methods of a remote digital access to test, and factory sites for type testing and inspection.
This paper is devoted to improvements of the low-frequency topological model of a three-legged stacked-core transformer. It is shown that B − H hysteresis loops employed in the transformer model and −i curves measured at transformer terminals are quite different in shape. A distinction is made between transformer modeling at moderate and deep saturations, separated conditionally by the level of technical saturation (near 2 Tesla) typical for grain-oriented steels. In the former case, in addition to the magnetic coupling of different phase windings through the core, an important part is played by air gaps at core joints. It is found that the effective gap length depends on the instantaneous magnetic flux and this dependence is proposed to implement by a variable inductance. It is shown that regardless of the core saturation depth, an important role in the transformer modeling belongs to the distribution of the zero-sequence path between all three phases. In addition to accurate replications of the special purpose saturation test, the modeled results are in a close agreement with positive-and zero sequence data measured on a 50 kVA transformer, as well as with the measured inrush currents.
Geomagnetic disturbance (GMD) arises during space weather and solar activity can result in geomagnetically induced current (GIC) flow in the grounded power transformers in the power network. This GIC may cause half-cycle saturation of transformers and lead to severe damage or blackout. To block the GIC flow into the power transformers, the neutral blocking devices (NBDs) based on capacitor banks are often installed in the neutral ground paths of transformers to mitigate the GIC. However, the high voltage (HV) can build up across these capacitors during ground faults and may cause a ferroresonance phenomenon in the power network. This phenomenon generates high voltages/currents in the transformer windings and results in transformer failure. This work investigates the effect of connected NBDs to the power transformers on the potential power network ferroresonance in Peninsular Malaysia. The complete analysis was carried out using the Power System Computer-Aided Design for Electromagnetic Transients including Direct Current (PSCAD/EMTDC) software. These transformers were selected in the power network due to the sensitivity of their locations to GMD events. The NBD systems were tested under different working conditions. The simulation results found that the metal oxide varistors (MOVs) arresters in NBDs fault protection mode effectively clamped ferroresonance overvoltages below the protection level under faulty conditions. Also, the results showed that GIC protection modes with 1
$\Omega $
and 3180
$\mu \text{F}$
in the mitigation systems had the lowest ferroresonance overvoltages in the neutrals of the transformers under faulty conditions. Based on the results, the recommendations were provided to the local power utility which will help to improve the reliability of the power supply to the consumers.
Unusually high transformer sound levels triggered the GIC research in Austria. The measurements revealed DC currents highly correlated with geomagnetic field variations. There are currently 9 self developed measurement systems running in different transformer neutral points in Austria. In order to investigate the transformer behaviour under GIC bias, transformer laboratory tests and simulation models were set up. The developed GIC-grid-simulation tool calculates the currents in the power grid from the measured magnetic field data. The calculation is based on an earth subsurface conductivity model combined with a model of the electrical transmission grid. The comparison between continuous measurements and simulations results in constantly improved calculation methods. We wish to provide forecasts of future GICs using the incoming solar wind measured at the Earth-Sun Lagrange point 1 (L1) as a basis. We aim to predict the two horizontal geoelectric field components, Ex and Ey, with the ground truth being the geoelectric field modelled from geomagnetic variations. The forecasting is carried out using a recurrent neural network (LSTM), which takes solar wind speed, density and magnetic field, as well as recent geomagnetic variations, as input and then outputs the maximum expected geoelectric field in the next 30-40 minutes, assumed to be homogenous across Austria. The fit of simulation to measurement will be shown with recent solar events of 2021.
Rosenqvist and Hall (2019), https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018SW002084 developed a proof‐of‐concept modeling capability that incorporates a detailed 3D structure of Earth's electrical conductivity in a geomagnetically induced current estimation procedure (GIC‐SMAP). The model was verified based on GIC measurements in northern Sweden. The study showed that southern Sweden is exposed to stronger electric fields due to a combined effect of low crustal conductivity and the influence of the surrounding coast. This study aims at further verifying the model in this region. GIC measurements on a power line at the west coast of southern Sweden are utilized. The location of the transmission line was selected to include coast effects at the ocean‐land interface to investigate the importance of using 3D induction modeling methods. The model is used to quantify the hazard of severe GICs in this particular transmission line by using historic recordings of strong geomagnetic disturbances. To quantify a worst‐case scenario GICs are calculated from modeled magnetic disturbances by the Space Weather Modeling Framework based on estimates for an idealized extreme interplanetary coronal mass ejection. The observed and estimated GIC based on the 3D GIC‐SMAP procedure in the transmission line in southern Sweden are in good agreement. In contrast, 1D methods underestimate GICs by about 50%. The estimated GICs in the studied transmission line exceed 100 A for one of 14 historical geomagnetic storm intervals. The peak GIC during the sudden impulse phase of a “perfect” storm exceeds 300 A but depends on the locality of the station as the interplanetary magnetic cloud hits Earth.
During solar storms, the Sun expels large amounts of energetic particles (SEP) that can react with the Earth’s atmospheric constituents and produce cosmogenic radionuclides such as ¹⁴C, ¹⁰Be and ³⁶Cl. Here we present ¹⁰Be and ³⁶Cl data measured in ice cores from Greenland and Antarctica. The data consistently show one of the largest ¹⁰Be and ³⁶Cl production peaks detected so far, most likely produced by an extreme SEP event that hit Earth 9125 years BP (before present, i.e., before 1950 CE), i.e., 7176 BCE. Using the ³⁶Cl/¹⁰Be ratio, we demonstrate that this event was characterized by a very hard energy spectrum and was possibly up to two orders of magnitude larger than any SEP event during the instrumental period. Furthermore, we provide ¹⁰Be-based evidence that, contrary to expectations, the SEP event occurred near a solar minimum.
Space weather alerting systems for power systems require real-time calculations of the electric fields that drive geomagnetically induced currents. In this paper, we present a new method for calculating the Earth impulse response that can be convolved with real-time magnetic field data to give the required electric fields. We start with the Earth transfer function which can be expressed in two ways: as a relationship between the electric field, E, and the geomagnetic field, B, (which has characteristics equivalent to that of a high pass filter), and between E and the time derivative of the magnetic field, dB/dt, (which has characteristics of a low pass filter). An inverse Fourier transform of these transfer functions should then give the corresponding Earth impulse responses in the time domain. This works well for a uniform conductivity model for which the inverse Fourier transform of the transfer function has an analytic solution. However, the inverse Fourier transform of the transfer function for a non-uniform conductivity model requires numerical calculations and produces an acausal impulse response with oscillations because of the Gibbs phenomena. To investigate the origin of the Gibbs oscillations, the real and imaginary parts of the transfer functions are transformed separately. This shows that the Gibbs oscillations arise from the inverse transform of the real and imaginary parts of the high-pass transfer function and the imaginary part of the low-pass transfer function. A new method is introduced that just transforms the real part of the low-pass transfer function and uses the requirement of causality to construct the full low pass impulse response. From this, the derivative theorem of convolution is used to obtain the high-pass impulse response. Electric fields can then be calculated by convolution of the low-pass impulse response with the rate of change of the magnetic field, dB/dt or by convolution of the high-pass impulse response with the magnetic field, B. Tests of the new method by comparison with analytic solutions for specified earth models and synthetic magnetic field data gave very high correlation coefficients, slopes near 1.0 and intercepts near zero, showing the accuracy of the new method. The method can be used with any Earth transfer function whether obtained from magnetotelluric measurements or from 1-D, 2-D, or 3-D conductivity models. Thus, it provides a versatile technique that avoids the Gibbs phenomenon and produces a causal impulse response suitable for time domain calculations of electric fields.
Power systems can and have been impacted by harmonics generated during geomagnetic disturbances (GMD) events. These harmonics are generated from the part-cycle saturation of power transformers caused by the flow of geomagnetically-induced currents through grid-connected transformers. This article describes a system level harmonic analysis tool needed by the industry to perform an adequate assessment of GMD-related distortion impacts. When the results of a properly performed harmonic assessment are combined with the fundamental frequency analyses, a much more accurate evaluation of grid security during GMD can be obtained. The tool is publicly available for engineers, students, and researchers. It provides multiple examples, including a validation against published measurements from an experimental setup consisting of two back-to-back connected transformers in the Fingrid power network. Final of this article briefly discusses the results of that validation.