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The power-line infrastructure has been identified as an efficient system suitable for broadband power-line communication (BPLC) to connect and control various end users. However, the network is affected by stochastic attenuations due to the number of interconnected branches, their line lengths, associated terminal loads, etc. There is yet another parameter that could influence the above stated attenuations or distortions depending on the way the signals are allowed to return to the transmitting end. In this paper, we investigate whether a finitely conducting ground return could be used for BPLC and to investigate its performance over the conventional methods where one of the adjacent power-line conductors is used as signal return. This study could be helpful to those who are proposing the use of ground as a return conductor in BPLC systems. It will be shown that the use of ground return for the BPLC system is effective or better only when the ground conductivity is high (>50 mS/m). When ground conditions are poorer, attenuations increase with frequency, making them unsuitable for BPLC. There are situations where poor ground conditions can still be used but only the transmission-fine lengths are shorter. The analysis presented here is based on transmission-line solutions both under lossless (without ground return) and lossy (with ground return) conditions and are applied to typical low-voltage and medium-voltage channels. Comparisons are also made based on the power spectral densities and channel capacities. Index Terms--Branched network, broadband power line, channel capacity, frequency response, ground impedance, low-voltage (LV) channel, medium-voltage (MV) channel, multi-path, power-line channel, transfer function.

This paper presents a novel converter configuration, based solely on conventional thyristors and aimed for the use with high voltage dc (HVDC) transmission. The converter utilizes resonant turn off and forced commutation with auxiliary thyristors to aid commutation in the converter switches. The PSCAD/EMTDC simulation confirms that the HVDC inverter is capable of operating with a wide range of firing angles, including operation with reactive power export. Further simulation proves that the system is immune to commutation failure even in the case of most severe close single-phase faults. The harmonic generation is comparable to the conventional converter and also the thyristor voltage stress is not significantly increased. The auxiliary thyristors do increase costs, but this is offset by the elimination of reactive power support and the improvements in performance. Index Terms--Commutated circuits, HVDC converters, HVDC transmission, reactive power.

This paper concerns the determination of the electric current requirements for an anti-icing technique based on the Joule effect. The minimum current intensity needed for preventing ice accretion depends on several conductor parameters, including external diameter, electrical resistance, as well as surface geometry (number and diameter of external strands). It depends also on meteorological conditions, such as air temperature, wind velocity, and liquid water content. The study comprises the elaboration of a mathematical model and the laboratory experiments for validation. This research work is mainly concerned with power-line conductor and atmospheric parameters. Therefore, four different types of single A1/S1 power-line conductors are investigated. The analytical model was validated with the experiments performed in the wind tunnel of CIGELE Icing Research Pavilion at the University of Quebec, Chicoutimi. In order to complete the mathematical model, it is necessary to assess the overall heat transfer coefficient (HTC) for stranded conductors. The HTC measurements are presented for conductors with different surface geometries. Index Terms--Atmospheric parameters, heat-transfer coefficient, ice prevention, Joule effect, stranded conductors, wind tunnel.

This paper is intended to present a comprehensive and objective approach to the behavior of grounding electrodes when subjected to lightning currents. Index Terms--Grounding, lightning currents, transient grounding behavior.

This paper presents an improved design method for a novel transformer inrush current reduction scheme. The scheme energizes each phase of a transformer in sequence and uses a neutral resistor to limit the inrush current. Although experimental and simulation results have demonstrated the effectiveness of the scheme, the problem of how to select the neutral resistor for optimal performance has not been fully solved. In this paper, an analytical method that is based on nonlinear circuit transient analysis is developed to solve this problem. The method models transformer nonlinearity using two linear circuits and derives a set of analytical equations for the waveform of the inrush current. In addition to establishing a set of formulas for optimal resistor determination, the results also reveal useful information regarding the inrush behavior of a transformer and the characteristics of the sequential energization scheme. This paper also proposed a method, the use of surge arrester, to solve the main limitation of the sequential phase energization scheme--the rise of neutral voltage. Performance of the improved scheme is presented. Index Terms--Inrush current, power quality, transformer.

This paper analyzes four basic ground methods for personal protection during maintained distribution transformers considering reverse energy from end users. The test in this paper considers that no loads are connected at both the primary and secondary of the distribution transformer. The results are summarized to determine the effective grounding method of protecting distribution line workers on de-energized lines during accidental-line energization from the end users. The secondary voltage at the transformer was recorded with various configurations of temporary grounds, and the primary voltage is then computed by using the transformer turn ratio. As a result, the case of installing no temporary ground is considered lethal. Moreover, the cases of installing temporary ground with a short circuit at either the primary or secondary of the transformer are considered to be the effective means of protecting distribution line workers. Index Terms--Electric shocks, overhead distribution lines, temporary grounding conductor.

In order to measure the grounding impedance of a grounding system more precisely and easily, two key problems of the measurement system: the location of the voltage electrode and the elimination of the noise and interference must be conquered. The new theoretical analysis and measurement system of the grounding impedance based on short-current lead wire and a swept frequency ac source is proposed. The compensating position of the voltage electrode to obtain the real grounding impedance is evaluated by analyzing the soil structure, the grounding system under test, and the position of the current electrode. This new system measures the ground impedance at several frequencies other than power frequency, then interpolates them to acquire the ground impedance at the power frequency. The mutual inductance between the potential and current wires is analyzed and eliminated. The industrial computer-based grounding impedance measurement system is developed and the analysis software is programmed. A field test was performed and compared with the traditional method in the substation and the novel system worked well. Index Terms--Grounding impedance, quasi-synchronous sampling algorithm, substation, swept frequency ac source.

A nonuniform transmission line approach is adopted in this paper for modeling the transient behavior of different types of grounding systems under lightning strikes in time domain by solving Telegrapher's equations based on finite-difference time-domain (FDTD) technique. Electromagnetic couplings between different parts of the grounding wires are included using effective per-unit length parameters (l, c, and g), which are space and time dependent. The present model can predict both the effective length and the transient voltage of grounding electrodes accurately, while, an uniform transmission line approach with electrode length dependent per-unit length parameters [19]-[22] fails to predict the same. Unlike the circuit theory approach [1]-[4], the present model is capable of predicting accurately the surge propagation delay in the large grounding system. The simulation results for buried horizontal wires and grounding grids based on the present model are in good agreement with that of the circuit and electromagnetic field approaches [3], [9]. From an engineering point of view, the model presented in this paper is sufficiently accurate, time efficient, and easy to apply. Index Terms--Grounding, lightning, transient analysis, transmission line and finite-difference time-domain (FDTD) methods.

This paper presents a new approach for the dynamic control of FACTS apparatus, such as the STATCOM and UPFC. The proposed approach allows for fast and independent control of real and reactive power while decoupling unbalanced disturbances from one bus to another. The control concept is based on a linearization of the dq inverter model. Traditional feedforward techniques are used to decouple the d and q-axis control. To ensure only balanced line currents flow, an asymptotic tracking control strategy is used which suppresses second harmonic in the dq-quantities. Experimental validation of the approach is carried out on a laboratory UPFC setup. The results demonstrate the fast and independent power flow control as well as the balanced line current control ability. They also show that the effects of AC system parameter variations are neglectable over a wide range and have practically no impact on the dynamic behaviour or the stability of the system. Keywords: Power Transmission, Power Electronics, FACTS Equipment, Unified Power Flow Controller (UPFC), Unbalanced Systems, Power Stability Enhancement, Vector Control

This paper presents laboratory and field tests of the filters constructed to reduce harmonic currents generated by three phase systems of fluorescent lamps. The main idea is to develop a filter with high impedance for harmonic currents to reduce harmonic distortion and a with low impedance for flow of the fundamental in asymmetrical regimes. This filter is included in the neutral conductor with the primary role of reducing the zero sequence harmonics. The filter design procedure is based on 'levels of aspirations' to reduce harmonic currents. With the use of a lamp equivalent circuit, it provides the required filter impedances for a given harmonic frequency. Rosenbrock's optimization procedure is employed to search for the optimal filter parameters, minimizing the objective function formulated. Laboratory and field tests show that a tuned filter in the neutral conductor can an effective method to reduce harmonic current distortion. It can be applied to any set of discharge lamps arranged as a three phase circuit. The method is especially suitable for high rise buildings and industrial installations where a large amount of electrical energy is consumed by lighting. Keywords: Harmonics, fluorescent lamps, filters.

This paper presents a new approach to the detection of high impedance fault (HIF). The proposed approach (or detector which implements this approach) has four distinct features. First, the detector input signal is the three-phase unbalanced current (or feeder 3[I.sub.0]), rather than the conventional three individual phase-currents. Secondly, the detector is designed by monitoring the energy variance for the second, fourth and sixth harmonics of 3[I.sub.0] individually, and by requiring the increment of randomness of all these harmonics to indicate an HIF. Thirdly, a self-tuning scheme based on the statistical confidence is applied to the automatic setting of threshold level for the energy variances monitored. Fourthly, effective counters are designed to detect both persistent and intermittent arcings. The detector has been tested on 51 staged faults and 8 unfaulted feeders. The test results show that the detector can have 100% fault detection rate so long as the arcing current at fault location is not less than 5A. As to the dependability tests, no false alarm (or tripping) has happened for all of the unfaulted feeders under test. These feeders either supply special (including arc furnace) load, or have leakage current (referring to the intermittent zero-sequence load due to the feeder location along coast), or are under the normal capacitor or load switchings. The test results and the investigation for the high sensitivity and dependability and for the limitation of proposed fault detection method are presented. Keywords: Distribution feeder, High impedance fault, Digital relaying, Arcing fault, Staged fault test, Dependability test, Three-phase unbalanced current.

Recently the return voltage measurements have been proposed for the diagnostics on the high voltage equipment characterising the dielectric properties of insulation systems. The application of return voltage method at the diagnostics of the HV current transformers are presented in this paper. The return voltage characteristics such as maximum of return voltage, the initial slope of return voltage and the time to peak value dependencies on charging time are discussed. These characteristics vary depending on the ageing changes of the insulation. That indicate for a possible applicability of the technique for assessing the state of transformer insulation. The investigations of temperature influence on the rerum voltage characteristics were aimed to determine the state of the insulation under operating conditions. The theoretical analysis of the results allows to calculate the temperature of insulation that corresponds to the thermal breakdown of insulation. Keywords: current transformer - oil-paper insulation - ageing - diagnostics - return voltage

The performance of the Clapham SVC after a control retrofit is described. The replacement of an analogue with digital control was completed in 1995. The retrofit was accomplished well within the ten days of allotted shut down time. With the original control system ac system voltage unbalance caused by untransposed lines had to be balanced by manual adjustment. The primary improvement provided with the new control is automatic and continuous voltage balancing. Key Words: control retrofit, untransposed lines, negative sequence motor currents.

The present work investigates the electromagnetic field and the eddy currents of a system consisting of a faulted power transmission line above parallel conductors buried in nonhomogeneous earth. The electromagnetic field diffusion equation has been numerically solved, using finite element method (FEM). Using FEM results, magnetic vector potential distribution in the cross-section of the parallel exposure, as well as eddy currents induced in all conductive parts, are calculated for various nonhomogeneous earth models. An analysis concerning the most important operational parameters in determining power transmission line influence is presented. Such parameters are the depth of the first earth layer, the resistivities of the different earth layers and the number of the mitigation wires. Keywords: power transmission lines, eddy currents, finite element methods, nonhomogeneous earth.

This paper presents a comparative analysis of the effects of expulsion and current-limiting fuse (CLF) operations in distribution systems on power quality and system protection. This analysis included field tests of both types of fuses on a distribution system in Florida. To further analyze fuse effects on the voltage dip duration and spread of the overvoltages created during the fuse operations throughout the system, computer simulations were also performed. Unique digital fuse models were developed for use with the EMTP/ATP program. This paper shows that CLFs improve power quality by supporting system voltage during faults, reducing the voltage dip duration, and do not affect load operation. Additionally, CLFs reduce the fault let-through [I.sup.2]t. Key words: Fuse, Current-Limiting Fuse, Modeling, EMTP, ATP, Distribution Systems, Power Quality

The equations given in IEC 287 and in the Neher-McGrath paper for the internal thermal resistance of 3-core cables were developed for paper insulated cables. For such cables, it was assumed that the insulation and filler materials have the same thermal resistivities. In reality, in 3-core cables a variety of materials are used as fillers. The majority of these will have a higher thermal resistivity than the insulation. In this paper, a new formula is developed to compute the value of this thermal resistance taking into account the thermal resistivity of the filler. The effect of filler resistivity on cable ampacity is also discussed.

A new synthetic test circuit is presented to test the 765kV - 63kA - 50Hz circuit-breakers. The new synthetic test circuit consists of a combination of a current injection circuit and a voltage injection circuit. This configuration is especially appropriate for the interrupting test of Ultra-High-Voltage circuit breakers. After the description of the new circuit and its operation, the test circuit parameter combination and the calculated results are studied in this paper. The new synthetic test circuit can be used for voltage ratings up to 1100kV. Keywords: Ultra-high-voltage, circuit breaker, synthetic test circuit, switchgear

This paper presents a survey of voltage and current harmonic distortion levels at 40 power transformers and 10 capacitor units in Northern Taiwan. Through one year measurement, about 12,000 sets of harmonic voltage and current data are recorded and analyzed. The types and levels of harmonics are showed in each power transformer which supplies different residential, commercial, and industrial customers with various loads. At the same time, a suggestion is given to the representative value of background level of harmonic voltages with 95% cumulative probability which is adopted by Taipower. At last, effects of harmonics on capacitors and power transformers are discussed. Keywords: harmonics, distribution substation, survey, electric power quality.

This paper describes a new technique for sampling synchronisation in a numerical differential feeder protection relay. The protection uses a Global Positioning System satellite receiver for sampling synchronization at each end of the protected feeder. The EMTP/ATP power system simulator was used to simulate a typical distribution feeder with faults at various locations. The current signals generated at both ends of feeder were used to evaluate the performance of the protection. These tests were carried out using a protection simulator. The results illustrate the effect of current transformer saturation on the sensitivity and stability of the protection. Finally it is shown that a precisely synchronized differential protection is fast, selective and accurate. On an internal fault it provides a high operating sensitivity, whilst it remains stable on all external faults. Keywords: - Synchronized sampling, Current differential feeder protection, Global Positioning System.

Harmonic resonance has been observed, measured and modeled on parallel 500-kV lines that are about one wavelength at 2100 Hz, the 35th harmonic. A seemingly small harmonic injection at one location on the system causes significant problems some distance away such as telephone interference.

The high requirement of rated power of an active power filter leads to high costs and limits the range of operation especially for high voltage applications. To overcome these problems a combined system of a passive filter, using only one LC filter tuned to the lowest typical line current harmonic, and a shunt active power filter, sharing the filter capacitor to be decoupled from the fundamental line voltage, is proposed. These features strongly decrease the rated power of the active filter which is controlled only to suppress the harmonics of higher order and to eliminate resonance effects between the line impedance and the passive filter.

Current transformer (CT) saturation may cause relays to malfunction. The conventional method to deal with the problem is overdimensioning of the core so that CTs can carry up to 20 times the rated current without exceeding 10 % ratio correction. However, this not only reduces the sensitivity of relays, but also increases the CT core size. This paper presents a technique of estimating the secondary current corresponding to the CT ratio under CT saturation. The proposed algorithm can improve the sensitivity of relays to low level internal faults, minimize the instability of relays for external faults, and might ultimately assist in reducing the dimension of the required CT core cross section.

The purpose of this paper is to provide the relay engineer with a basic understanding of the physical and electrical characteristics of high voltage ac cables, 115 kV and above, and how this applies to protective relaying. It will also discuss the application of various protective schemes with the advantages and disadvantages associated with those schemes.

The performances of XLPE and EPR as materials candidate for insulation of high voltage power cables are investigated. Data obtained from laboratory tests performed on cable models aged under single thermal and electrical stresses, as well as under the combination of the two stresses, are considered. The drawing of electrical, thermal and multi-stress life lines provides a synthetic and exhaustive description of the behavior of the tested materials, and allows the determination of endurance indices, useful for electrical, thermal and multi-stress characterization.

In this paper, we propose an approach to quantifying the distortion caused by a single customer, when there are many customers in the network. We have described a method for isolating the contributions to waveform distortion from a customer side and the supply side. The voltage and current measurements at a single point are sufficient. This approach will be useful in arriving at equitable ways of settling customer complaints, sharing the cost of waveform distortion through rate structures, penalties, etc. The principle is illustrated using six example loads. The method proposed can be implemented in any power quality measurement device, which monitors the voltage as well as the current harmonics simultaneously. Key words: Harmonics, origin, direction, Load, power quality, measurement, high voltage.

The paper deals with the power properties of three-phase circuits with unbalanced loads supplied from a three-wire line. It discusses various definitions of the apparent power in such circuits and shows that they cannot be interpreted in terms of any physical phenomena. The paper shows that the unbalanced power does not occur as an effect of the power oscillations and explains why this power does occur. The paper presents a decomposition of the supply current into the active, reactive, and unbalanced currents and shows that this decomposition can be considered as a fundamental for the comprehension of the power phenomena in such circuits and for the power factor evaluation. Key words: unbalanced systems, apparent power, unbalanced current.

An experimental investigation was performed on a UHV tower model for the EMTP multiconductor calculation of lightning overvoltage at substations associated with back-flashover at an adjacent transmission tower. The various lightning surge response characteristics were measured on an actual UHV tower, and parameters of a multistory transmission tower model that can reproduce voltages across the insulator strings, voltages of the crossarms, and voltages of the power lines were determined. A value of 120 [ohms] was determined as the surge impedance at each section of the multistory tower model, which closely agreed with the tower surge impedance measured for the UHV tower alone. Keywords: Lightning surge, EMTP, UHV, Multistory transmission tower model, Tower surge impedance, Back-flashover

To examine the sensitivity to geomagnetically induced currents (GIC) of the Hydro-Quebec 735 kV power system, model calculations were made of the GIC produced by uniform electric fields directed northwards (azimuth of 0 [degree]) and with azimuths increasing in 15 [degree] increments. The results show that the maximum GIC was produced in the system for an electric field beating 15 [degrees]. In addition the model results allow the variation of GIC amplitude with electric field direction to be examined for individual substations. Maximum GIC occurred at substations at the ends of a long transmission line when the electric field was aligned with the transmission line.

When an HVdc scheme is isolated from its receiving ac system, the inventor may continue to operate, generating its own ac bus voltages; this is defined here as islanding. If islanding is allowed to continue unrestricted, then main circuit components may in some conditions be damaged and it is therefore necessary to provide a suitable protection system. This paper outlines the protection scheme developed for the McNeill Back-to-Back HVdc link in Alberta, Canada, to prevent damage due to islanding while still permitting the link to automatically restart on reclosure of the isolating breaker. Oscillograms showing the protection in operation on both the GEC ALSTHOM HVdc simulator and during tests carried out as part of the commissioning of the McNeill HVdc link are included.

Bipolar dc corona is known to be a strong function of conductor voltage but little has been known about the influence of load current on bipolar corona. To evaluate the influence of load current, a bipolar cylindrical corona cage was constructed and operated with simulated dc load current flowing in the conductors. Measurements of load current, corona current, ion-current density, and conductor temperature showed that corona conductance was a function of load current. The dependence on load current was via temperature rather than magnetic field.

An HVdc fault location scheme is described which relies on very precise detection of the time of arrival of fault created surges at both ends of the line. Such detection is achieved by a very accurate data acquisition and processing system combined with the time reference signals provided by a global positioning system receiver. Extensive digital simulation is carried out to determine the voltage and current waveforms, to identify the main sources of error and suggest possible compensation techniques.

Negative-sequence overcurrent elements do not respond to balanced load and can thus be set to operate faster and more sensitively than phase overcurrent elements for phase-to-phase faults. This paper demonstrates that on a radial distribution system, negative-sequence overcurrent elements need only be coordinated with downstream phase overcurrent devices for phase-to-phase faults. Coordination for other fault types is then achieved with no further analysis required. The effects of negative-sequence load current and open phase conductors on negative-sequence overcurrent elements are also discussed.

The design of High-Voltage, Direct-Current transmission cables insulated with oil-impregnated cellulose paper is constrained by the maximum temperature and the maximum steady-state and transient stresses (voltage gradients) that the cable insulation can reliably withstand over the desired operating lifetime. Because the electrical stresses in the insulation are related in a very complex fashion to temperature, applied voltage and cable geometry, iterative techniques are required to compute the values of these stresses. Digital computers are ideally suited for these iterative techniques. This paper presents the principal equations and logic needed to develop suitable computer programs.

Audible noise (AN) produced by corona discharges from high voltage transmission lines is one of the more important considerations in line design. Therefore, line designers must predetermine the AN using prediction formulas. This paper presents the results of applying evolutionary computation techniques using AN data from lines throughout the world to develop new, highly accurate formulas for predicting the A-weighted AN during heavy rain and stable rain from overhead ac lines. Calculated AN's using these new formulas and existing formulas are compared with measured data. Index Terms--AN database, audible noise prediction, corona, evolutionary computations, genetic algorithm, genetic programming, transmission line design.

A CCC has superior commutation ability when compared to a line commutated converter used in a conventional HVdc system. This paper models the commutating and post commutation period of the converter circuit in theoretical style, and formularize the commutation and extinction equation with mathematical treatment. The equations express the relations among firing angle, overlap angle, effective margin angle, state variables and converter parameters, and compose nonlinear simultaneous equations. The fine CCC commutation characteristics are shown by using the equations to several commutation capacitor configurations. The authors propose the predictive firing angle calculation method for the constant margin angle control (A[Gamma]'R) operation of a CCC-HVdc system as the practical application of the obtained equations. The obtained equations cannot be solved algebraically for their complexity, then convergence calculations of numerical analysis are assessed, but it is hard to secure the derivation to the various operating conditions. The authors propose to apply an approximate polynomial equation from the off line calculated characteristics to achieve the firing angle predicting equation. The design index for approximation polynomial of A[Gamma]'R is presented to make the polynomials to low order. The suitability of the proposed predictive A[Gamma]'R is studied with simulations by applying it to the converter controller and it can make the most of the CCC availability. Index Terms--CC, CCC, effective margin angle, firing angle predictions, HVdc.

Recently back-to-back HVdc systems have greatly increased. They interconnect large power systems offering numerous technical and economic benefits. The design of AC system filters, however, is not an easy task. Also, filters do not remove all the harmonics for which they are tuned, are costly and have a complicated transient behavior. In this paper a 36-pulse filterless configuration for back-to-back HVdc systems is proposed, whereby harmonics are eliminated within the converter, thus opening the possibility of advantages in aspects of design, reliability and capital savings. Index Terms--HVdc Systems, Filters, Harmonics.

The subject of this paper is the analysis of the behavior of an ac/HV (400 kV) cable line, compensated with shunted inductors, in the event of a fault. Comparisons are made between, and the consequences assessed for a three-phase, single and double phase-to-ground short circuit, starting from various operating conditions. The paper presents criteria applied for assessing the worst fault and the influence of compensating parameters under fault conditions, and for locating the most severe type of fault. Index Terms--Cable Transmission Lines, Short Circuits.

Zinc oxide surge arresters using zinc oxide (ZnO) elements have been widely used for insulation coordination in the world power systems. These ZnO elements have basically reference voltage of about 200 V/mm. Recently, new ZnO elements having about 1.5, 2 times high voltage gradient zinc oxide element have been developed. This paper describes applications of high voltage gradient 300 V/mm ZnO elements to high performance porcelain type surge arresters and oil immersed surge arresters, and 300 V/mm or 400 V/mm ZnO elements to gas insulated tank type surge arresters for power system. Adequate selection of voltage gradient of ZnO elements makes more compact design of the surge arresters. Index Terms--Surge Arrester, Zinc Oxide Elements.

In recent years, online condition assessment of high voltage equipment is increasingly asked for by power utilities. However, most systems on the market today are merely monitoring solutions with hardly any diagnostic capabilities. This report reviews the basic principles for condition diagnosis from the field of artificial intelligence and introduces a novel strategy, named Model-Aided Diagnosis (MAiD). It was initially developed for condition assessment of high voltage circuit breakers but is also suitable for other technical devices. Theoretical considerations as well as practical results show that MAiD is a useful strategy for condition assessment. In addition, with its low hardware requirements it allows online condition diagnosis of high voltage circuit breakers locally by a low-cost monitoring device. Index Terms--Fault diagnosis, circuit breakers, modeling.

The light-triggered thyristor (LTT), which can be directly fired by light, does not need gate electronics on the high-voltage side and is thus a near-ideal semiconductor device for a HVDC valve. In providing an overvoltage protection for LTT without losing its advantage of simple circuits associated with the thyristor, the overvoltage protection system adopted to the conventional electrically triggered thyristor is not necessarily appropriate. We have shown that high-speed protective gate pulses generated from the valve base electronics together with the valve arrester are sufficient for the LTT valve overvoltage protection. The adequacy of this protection strategy is demonstrated by factory type tests and also excel lent service experiences. Index Terms--HVDC, Thyristor valve, LTT, Overvoltage protection, Forward recovery protection.