Zhanqing Yu

Design and application of an integrated electro-optic sensor for intensive electric field measurement

Impulse electric field measurement should satisfy some special requirements such as intensive signal amplitude, appropriate insulation, broadband response, and small size. Three types of integrated electro-optic sensors, designed and fabricated for measuring intensive impulse electric field, was implemented in this paper. The theoretical concerns and specific characteristics of these integrated electro-optic sensors were illustrated in terms of their structures and key parameters. The time domain input/output characteristic and frequency response of the sensors were examined too. Comparison of the time domain characteristics of these sensors revealed that the mono-shield sensor was more efficient than those with antennas in measurable amplitudes, which is critical for high electric field measurement in gas breakdown research. In addition, the application of the sensor in the lightning impulse experiment upon air gap not only verify the existence of streamer charge but also ensured that this sensor is expected to be widely used in transient measurement investigations such as the measurement of lightning electric field, air breakdown procedure, and maybe surface discharge.

Vibration and Audible Noise Characteristics of AC Transformer Caused by HVDC System Under Monopole Operation

When the HVDC system works in monopole ground return mode, dc current flows through the neutral of ac power transformer with the neutral grounded, which will enhance its vibration and audible noise. In this paper, dc bias experiments on ac power transformers carried out on 500-kV power transformers both in the transformer factory and the substation are described; the experimental results show how the vibration and audible noise of the transformer change under the dc bias current or windings voltage. The frequency characteristics of the vibration and audible noise of the dc-biased transformer are discussed under the conditions with and without loads. The direct connections between the vibration peak value and the dc bias current, and between the sound pressure and the dc bias current are revealed. Long-term online monitoring shows that the vibration of the transformer changes seriously even if there is not a dc system operating in monopole earth return mode, so when we analyze the influence of dc bias on the vibration and audible noise of the transformer, other influential factors should be expelled.

Development and Application of Integrated Optical Sensors for Intense E-Field Measurement

The measurement of intense E-fields is a fundamental need in various research areas. Integrated optical E-field sensors (IOESs) have important advantages and are potentially suitable for intense E-field detection. This paper comprehensively reviews the development and applications of several types of IOESs over the last 30 years, including the Mach-Zehnder interferometer (MZI), coupler interferometer (CI) and common path interferometer (CPI). The features of the different types of IOESs are compared, showing that the MZI has higher sensitivity, the CI has a controllable optical bias, and the CPI has better temperature stability. More specifically, the improvement work of applying IOESs to intense E-field measurement is illustrated. Finally, typical uses of IOESs in the measurement of intense E-fields are demonstrated, including application areas such as E-fields with different frequency ranges in high-voltage engineering, simulated nuclear electromagnetic pulse in high-power electromagnetic pulses, and ion-accelerating field in high-energy physics.

Effective Protection Distances of SPDs for Household Electrical Appliances

The installation of surge protective devices (SPDs) is important for the protection of todays household appliances against in-rushing surge. The effective protection distance should be taken into account for the installation to avoid the influence of oscillation phenomenon. For household appliances, their port impedance characteristics are more complex and frequency dependent, and for some appliances, the impedance characteristics are different in operating state and nonoperating state. With the method proposed in this paper, operating state impedance characteristics of 16 different kinds of appliances are measured and the equivalent circuits are obtained by vector fitting and circuit synthesis. The protection effect of SPDs is simulated by electromagnetic transient analysis software using a practical distribution system model, and the effective protection distances of SPDs with different protection level for these appliances are obtained. Several analyses are made about relationships between the protection distance and factors such as the type of appliance, operating condition, and protection level. A case of protection for multiappliance is also analyzed.

Simulation Analysis on Conducted EMD Caused by Valves in

Wideband frequency electromagnetic (EM) noise generated during the turn-on and turn-off processes of thyristor valves in converter stations is conducted along the primary circuitry, and may interfere with secondary systems. The EM interference (EMI) problem in a plusmn800 kV ultra-HVDC converter station is an important issue that should be studied carefully in the design stage. A method to analyze the conducted EM disturbance (EMD) based on time-domain wideband circuit simulation is proposed here, which is verified through the field test results of a plusmn500 kV converter station. Models of major equipment are described. The EMD levels of a plusmn800 kV converter station, which is currently being planned and built in China, are calculated, and compared with those of a plusmn500 kV converter station.

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HVDC transmission system operating in ground return mode often affects the neighboring ac power system seriously. This paper presented some field experimental results, including audible noise, voltage harmonics, and oscillation of the transformer while dc current passes through the neutral point. The relationship between the dc neutral current and those effects were discovered clearly by analyzing these data. Based on the analysis of the characteristics of the dc current distributed under ground and in the ac network, some useful measures to decrease or block the dc current through the neutral point of the transformer, such as the injection of reverse dc current, blocking capacitor, and series resistor are compared. And the series resistor method is chosen after carefully study of its feasibility in the ac power system. The parameters as well as the protection method of the resistor were selected by implementing several large current experiments. Finally, field experiments were carried out to observe the effect of reducing the neutral current with a series resistor device. Experimental results show that series resistor method is a simple structure, low cost, and of high reliability, which can restrain the neutral current effectively.

800 kV UHVDC Converter Station

In order to analyze the lightning transient of a thin-wire system, a time-domain method of reducing the thin-wire conductor system into an equivalent active multiport network based on the partial-element electric-circuit model is presented in this paper. A modified-mesh-current (MMC) approach is developed, uses node charge and mesh current as its variables, and takes into account both capacitive coupling and inductive coupling among conductors. Equations given by MMC are expressed in a state-space form. These equations are reduced to an active multiport network model using the trapezoidal scheme. The active multiport network is described by a constant-input-conductance matrix and a vector of currents that are updated using passed values. The multiport network can easily interface with linear and nonlinear components in an electromagnetic transient program. Since the proposed method requires only one-time calculation of the inverse of the system matrix, it is more efficient in comparison to the conventional full-wave method in the frequency domain. For a large system, well-established model-order-reduction techniques can be applied to the MMC formulation to reduce it to a small scale one. The proposed method is validated by comparing its results with NEC-calculated ones and published field measurements. Finally, the method is used to simulate two lightning-strike scenarios.

Study on Restraining DC Neutral Current of Transformer During HVDC Monopolar Operation

Electric field (E-field) in air gap streamer discharges under positive lightning impulse was measured by specifically developed integrated electro-optic sensors. An E-field step phenomenon was observed. The E-field firstly agreed with the Laplace field, then suddenly increased with a rise time of μs. The occurrence probability of this phenomenon increased as the applied voltage increased. The discharge current waveforms and photos taken by a fast camera prove the E-field step was caused by the space net charge. From the E-step rise time and the corona area range, the average electron drift speed under the experiment situation was estimated about 0.2 × 106 – 0.6× 106 m/s.

A Time-Domain Multiport Model of Thin-Wire System for Lightning Transient Simulation

Lightning is always a major cause of outages in power systems during these years, the description of insulator string breakdown process is a key part in lightning back flashover analysis. This paper builds a model to accurately depict the insulator breakdown process, which is applied to typical porcelain insulator or typical composite insulator under positive or negative impulse. The breakdown experiments of insulators of different string lengths for 500-, 220-, and 110-kV transmission lines are carried out under short-tail impulse. For comparison, the insulator breakdown experiments under standard lightning impulse are done as well. Then, utilizing the records of voltage, current data, and photos taken by high-speed cameras, the leader progression model is developed. Finally, the models are validated by being embedded in PSCAD software.

Electric field step in air gap streamer discharges

The evaluation of the lightning shielding failure of a transmission line is very important in power transmission system design, and the operation voltage has rarely been considered before. However, when the operation voltage reaches 800 kV and the tower height is near 100 m, the influence of operation voltage may not be ignored. In this paper, an experimental system consisting of an air gap between a plate/rod and a conductor was established, where the dc voltage could be applied to the conductor and was accompanied by a negative impulse voltage on the plate/rod electrode. An upward leader up to 1.54 m in length was observed, and the leader velocity and charge density were obtained in a 11-m-long air gap, whereas the voltage on the conductor was 800-kV dc. The influence of dc operation voltages on the upward leader was studied. The experimental results showed that the upward leader will emerge much earlier and may progress longer with dc operation voltages. That is, the lightning attachment to the conductor may occur more easily when the voltage of the conductor is opposite in polarity to that of the downward lightning than when the conductor has no operation voltage.