The large current flowing through transformer windings during external short-circuit event causes strong vibration and the tank emits multi-frequency sound. This paper investigates the transient acoustic signal theoretically and experimentally, and proposes to monitor the winding condition with the spectrum of the acoustic signal. A two-wire model is first built as a basic unit of winding structure. The dynamic equation of the model is constructed based on principle of least action. The solution shows that the spectrum of the winding vibration contains half power frequency and its multiples in some cases. If the winding is loosened and deformed, the motion space and vibration amplitude would be enlarged and the natural frequency of winding would decrease. As a result, the vibration spectrum would contain half frequency and its odd multiples. Then the acoustic signal of a 110 kV transformer is measured on a short-circuit test platform. The acoustic time-frequency spectrums are consistent with the theoretical analysis. The half frequency ratio (HFR) of the acoustic signal is proposed to characterize the winding condition. The results prove that HFR of acoustic signal is a more sensitive indicator than the short-circuit reactance, which can characterize the cumulative process of short-circuit on the winding.

Air-core reactors used in filters are significant equipment in HVDC converter stations. They are among the most serious noise sources under the action of multi-frequency magnetic forces. The vibration and noise characteristics of air-core reactors are investigated in this paper, which is the basis for noise control of HVDC projects. Firstly, based on the characteristics of magnetic forces, the generation process of vibration is analyzed theoretically. Then, Experimental Modal Analysis (EMA) is conducted and the results indicate that multiple natural frequencies exist and their mode shapes contain distinct peaks and troughs. Vibration shapes are also measured under different excitation frequencies with dense measuring points over the winding surface. The measured vibration shapes are consistent with the corresponding mode shapes. Furthermore, the noise distribution characteristics are analyzed by Boundary Element Method (BEM). The simulation results show that the noise radiation has obvious directional effect, which are mainly dominated by the corresponding vibrations and mode shapes. Based on the results of this paper, some suggestions for noise reduction are given in the last.

Filter capacitors are among the most serious noise sources in HVDC converter stations, and there are still no satisfactory noise reduction measures. This paper proposes the use of the mass tuning (MT) method to adjust the vibration frequency response characteristics and to realize vibration control and noise reduction. First, the theoretical model of the MT structure is developed based on continuum mechanics. Based on the theoretical model, it is revealed that inserting an MT component into the capacitor element package will cause a decrease in the resonant frequencies and antiresonant frequencies. Furthermore, the movement degree of the resonant frequencies increases as the position of the MT component becomes closer to the package end, while the movement degree of the antiresonant frequency is the largest when the MT component is located in the middle of the package model. The vibrations and the noise levels of an example MT capacitor are then compared with the results of an original capacitor. The comparison results show that the MT scheme designed in this paper can achieve a noise reduction effect of 12-15 dB(A). The proposed reduction method is proven to be both effective and efficient.

Power capacitors are widely used in power transmission systems. During their operation, an electric force acting on the electrodes of the power capacitors actuates mechanical vibrations and radiates an audible noise. Considering a power capacitor as a general system, the frequency response with the electric force as the input and mechanical vibration as the output have been measured by engineers in recent years and used to evaluate the acoustic and mechanical features of products. Accidentally, it was found that the frequency of the capacitor vibration was not consistent with its excitation due to electro-mechanical coupling. This electro-mechanical coupling had not been considered in previous vibration models of power capacitors. Therefore, a new vibration model of power capacitors was built up in this paper and a so-called multi-frequency vibration characteristic was revealed. A theoretical analysis showed that the electric force and mechanical vibration of the power capacitors were coupled, which resulted in the multi-frequency vibration. The vibration frequency response was measured and the result was consistent with the vibration model proposed in this paper. Once the frequency of the electric force was near half the natural frequency of the power capacitor, a predominant multi-frequency vibration was triggered and the power capacitor was in a superharmonic resonance.