Wang Zanji

Performance of DSSS against Repeater Jamming

Direct sequence spread spectrum (DSSS) technique has been widely used in both military communications and commercial communications. The normal jamming types, including broadband noise, partial-band noise and so on, are ineffective at the current jamming power level when the processing gain is large enough. This paper proposes a design of the repeater jamming based on radio frequency memory (RFM). The interference effect of repeater jamming on victim receivers code acquisition is discussed, and the bit error probability of the DSSS system interfered by repeater jamming is obtained. Moreover, geometric limitation on repeater jamming is mentioned. The result of simulation shows the deduction is right. The repeater jamming proposed in this paper is a kind of correlative jamming types which are more effective than normal jamming types, and it can be used in the field of DSSS communication countermeasures.

The improvement of symbol rate estimation by the wavelet transform

The symbol rate estimator by the wavelet transform proposed in Ho et al., and Chan et al. (1997) suffered from difficulties of choosing wavelet scales and the weak ability of anti-noise. In this paper we proposed an improved version of such an estimator. The main modification of our algorithm is that we move the carrier frequency of the received waveform to zero before the wavelet transform, i.e. we extract the symbol rate from the WT magnitudes of the baseband modulated signal instead of those of the MF signals. We find that with this modification there will be no invalid wavelet scales as occurred in Ho and choosing large scale not only reduces the additive noise, but also enhances the strength of the desired spike series, which remarkably improve the anti-noise ability of the estimator. Simulation results verified the effectiveness of our algorithm.

Transmission characteristics of low-voltage distribution networks in China and its model

Communication over the low-voltage distribution networks is of great importance to the electrical engineering and networks engineering. The signal transmission characteristics over the distribution networks are the key factors to be considered in designing the communication systems. In this paper, the signal transmission characteristics in frequency band of 100 kHz /spl sim/400 kHz over 220-V distribution networks in China is investigated and its mathematical models are derived from the measured data. After the procedure of transmission measurement based on HP4395A is discussed briefly, the measurement results including in-phase and cross-phase transmission are described and analyzed. The impulse response of the channel is estimated from the frequency domain measurement based on the linear minimum mean square error in which the measurement noise is reduced during the parameters estimation processes, the estimation shows that the channel could be well described by the 20-taps finite-impulse response model. On the basis of a large amount measurements, we believe the transmission channel is the stationary stochastic process and a stochastic channel model of in-phase transmission is discussed and a six-order autoregressive of the channel model was given out.

Study on the Control of Shunt Active DC Filter for HVDC Systems

A shunt active dc filter (ADF) has been applied in high-voltage dc transmission systems for harmonic suppression. Some key problems in the design of ADF control systems have not been investigated thoroughly due to high harmonic frequency and the complexity of system modeling. In this paper, a high-gain feedback control strategy is proposed, which is independent of circuit parameters. The authors draw a comparison between the harmonic suppression effects of ADF and that of a passive filter and prove that the former can increase the series impedance of the hybrid filter, while the latter can decrease the equivalent voltage. Based on circuit analyses, solutions, such as selection of the dc-side voltage, regulation of the modulation index, system stability conditions, filter design, and switching noise are presented. The consistency between theory analyses and experimental results verifies the feasibility of the proposed control strategy.

A Measuring Method of the Single-Phase AC Frequency, Phase, and Reactive Power Based on the Hilbert Filtering

This paper presents a novel measuring method based on the Hilbert filtering for the digital monitoring of the single-phase ac frequency, phase, and reactive power. According to this method, the sampled voltage signal passes a pair of the Hilbert filters. Then, the ac frequency and phase can be obtained by the phase detection and the phase-locked loop algorithm. Additionally, the sampled current signal also makes its phase shift by the other Hilbert filter. The current output is multiplied by the one of the phase-shifted voltage signals, and the reactive power is acquired from the dc component of the product. Because a type of infinite impulse response (IIR) Hilbert filters with low-order and small phase-shift error is adopted, this measuring method cannot only obtain fast and accurate digital monitoring results but also be easily realized with the DSP or other microprocessor programming codes. This method has been successfully applied in the design of a digital multifunctional power instrument

Harmonic detection method for HVDC DC-side active power filter

One of the most critical issues associated with APF control is to find an efficient algorithm that can obtain an accurate harmonic reference signal for control purpose. This paper presented two novel methods of the harmonic detection in an advanced active power filter (APF) for the compensation of instantaneous harmonic current components in high voltage direct current (HVDC) transmission system. One is based on the ameliorated adaptive noise canceling theory and the other is grounded on mean value filtering method. The two methods are implemented on TMS320C32 DSP system. Both of them can efficiently determine the instantaneous harmonic reference signal in real time. The validity of two algorithms is substantiated via simulations and experiments.

A Circuit Model in a Wide Frequency Range for Power Transformer and Analysis of its Characteristics

In order to predict VFTO (very fast transient overvoltage) distributions in the windings during transformer design stage, it is important to acquire the frequency range of the surge wave that could enter the transformer winding, and then model the windings in that frequency range. In GIS (gas insulated station), transformer is connected to bus through bushing and lead wire. The bushing capacitance to ground will bypass the high frequency components, so there should be a upper limit of the frequency components of the surge wave at the entrance of the transformer. In this paper, the transformer windings are modeled using MTL (multitransmission-line) model firstly, then the lumped equivalent circuit is formulated with the -type circuits, and finally the driving-point impedance and voltage transfer functions of the equivalent circuit are calculated using nodal analysis. The driving-point impedance of a model winding is calculated and compared with the measured result, and the validation of the equivalent circuit of transformer winding in the frequency range below 20 MHz is verified. Then the driving-point impedances of two real transformer windings are calculated with the equivalent circuit. Taking the lead impedance into consideration, the impedance of the winding is compared with the one of the bushing capacitance. The results show that most of the energy of the frequency components above 10 MHz in the surge wave will pass through the bushing capacitance and will not cause damage to transformer insulations.

The active DC filter in HVDC resolved by the multiple PWM inverters

Because the environmental demand is becoming stricter and stricter, more and more attention should be paid to the harmonic problem in high-voltage direct current (HVDC) transmission systems. This paper details the principle of active DC filter applied in the HVDC system. It is clearly stated that the contradiction between the switching frequency of power switch and the high frequencies of harmonics in the DC side of HVDC is the main bottle-neck for the application of ADF. Multiple pulse width modulation (PWM) can increase the equivalent switching frequency, decrease the stress on switch and improve the performance of ADF in high frequency. Both results of simulations and experiments validate the proposed multiple PWM.

Modeling and simulation of magnetizing inrush current of large power transformers

This paper presents a model for simulating magnetizing inrush current in large power transformer. The voltage and current distribution in the transformer winding under energisation conditions is formulated on the basis that the magnetic field can be considered as the composition of main flux in the iron core and leakage flux in the coil space. The calculation of the circuit parameters and the methods for solving the equations are also discussed. The simulation results for a single-phase, three-winding autotransformer and a three-phase, five-legged, three-winding transformer show the effect of energisation angle, the magnitude and the direction of the original residual magnetism on the inrush current. The difference of the B-H curves obtained from the measurements of the material of the iron core and the laminated iron core is also indicated. The proposed model is suitable to all kinds of structures of large power transformers.

A Digital Controller of Hybrid Filter Based on Pseudo-Inverse Filtering Technique

A new digital controller of hybrid passive-active power filter which employs pseudo-inverse filtering control strategy is discussed in this paper. The proposed controller is composed of a bank of digital selective frequency band-pass filters, phase-shifters and amplifiers that are implemented in TMS320C32 DSP. Digital band-pass filters employs IIR algorithm to extract the corresponding harmonic components to be canceled, and then the signals are conditioned according to the magnitude and phase distortion caused by the passive filter to reconstruct the harmonic signals to be eliminated. The results of both simulations based on EMTDC/PSCAD and experiments on a 30 kVA library HVDC system validate that the digital controller can ensure the good performance of hybrid filter stably