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Availability Assessment of the HVDC Converter Transformer System
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This paper presents a reliability assessment of the HVDC converter transformer system (CTS) comparing different component models and configurations. The CTS model is based on the Markov modelling approach, which is shown to be well suited for these relatively small systems. The failure rate data in the models is based on statistical surveys by OGRE. A number of scenarios are calculated in order to evaluate the impact of the availability of the CTS given different conditions. The result shows on the benefit in availability using a spare transformer, particularly at a close location of the HVDC station.
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... Conventionally, most of the researchers working on reliability evaluation of HVDC transmission systems have employed traditional methods, which can be categorized as analytical [11][12][13][14][15][16][17] and simulation methods [18][19][20][21]. Each method has advantages and disadvantages, so the appropriate method is determined by the type of evaluation desired as well as the nature of the problem. ...
... c P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P (1) 3(1) 1 (1) 2(1) 3(2) 1 (1) 2(2) 3(1) 1 (1) 2(2) 3(2) 1 (2) 2(1) 3(1) 1 (2) 2(1) 3(2) 1 (2) 2(2) 3(1) 1 (2) 2(2) 3(2) 1 (1) 2(1) 3(1) 1 (1) 2(1) 3(2) 1 (1) 2(2) 3(1) 1(2) 2(1) 3(1) 1(2) 2(2) 3(1) sys (16) where the final result is equal to the given in Eq. (11) after a rearrangement of the equation. ...
... The column vectors of the resulting C l matrix represent the event vectors for the component events, i.e. C 1 , C 2 , … C 16 . The vector p is obtained following the procedure previously described and its size is (2 16 × 1). ...
This paper presents a new reliability analysis method for High Voltage Direct Current (HVDC) transmission systems based on the Matrix-Based System Reliability method. The proposed method can compute the failure probability of HVDC transmission systems by use of efficient matrix-based procedures. Unlike conventional system reliability methods whose practicability strongly depends on the system size and the complexity of its multiple states, the proposed method can describe any general system event in a simple matrix form and therefore provides a straightforward way of handling the system events and estimating their probabilities. The main qualities of the proposed method are demonstrated by the reliability assessment of two multi-terminal HVDC (MTDC) transmission systems with multiple derated states. The performance of the proposed method and the results obtained were compared with two traditional assessment methods: Capacity Outage Probability Table (COPT) and Monte Carlo simulation. In this comparative analysis it is shown that the proposed method is a competitive alternative for reliability analysis of MTDC systems in terms of simplicity and efficiency.
... In order to assess the reliability of VSC-HVDC, the key components which significantly affect the reliability of the system need to be modeled. State space techniques based on Markov process are effective ways for system reliability modeling [5,6]. The model requires the collection of the corresponding reliability statistics for the key components. ...
... Based on the developed reliability model, the system reliability indices are normally evaluated by analytical [6,12,14,15] or Monte Carlo simulation methods [11,16,17]. Many of the state space concepts are effective for both methods as the main difference between them lies in the process of generating the system states. ...
... Both GIS and converter transformers' failure statistics can be categorized based on the voltage rating of the equipment. Reliability of HVDC converter transformers is of paramount importance, especially for LCC-HVDC systems [6]. Reliability surveys ( Table 2) point to transformers as the dominant contributors to failure statistics. ...
With the fast development of power electronics, voltage-source converter (VSC) HVDC technology presents cost-effective ways for bulk power transmission. An increasing number of VSC-HVDC projects has been installed worldwide. Their reliability affects the profitability of the system and therefore has a major impact on the potential investors. In this paper, an overview of the recent advances in the area of reliability evaluation for VSC-HVDC systems is provided. Taken into account the latest multi-level converter topology, the VSC-HVDC system is categorized into several sub-systems and the reliability data for the key components is discussed based on sources with academic and industrial backgrounds. The development of reliability evaluation methodologies is reviewed and the issues surrounding the different computation approaches are briefly analysed. A general VSC-HVDC reliability evaluation procedure is illustrated in this paper.
... The main components of the HVDC transmission systems are [10]; quadruple thyristor valve, converter transformer, smoothing reactor, specially designed DC filters, AC filter capacitor, AC filter reactor, circuit breaker, disconnected, current transformer, voltage transformer, surge arrester, earthing switch, and AC (power line conditioner) filter [92], [93]. In the following section, some of these components will be outlined: ...
... Nearly all HVDC classic stations in the world are designed with converter valves connected in a twelve-pulse configuration, in order to reduce the dominant current harmonics [92]. This configuration is in is turn built up with two six-pulse valve groups connected in series. ...
This paper is the 2nd part of the survey titled “High Voltage Direct Current Transmission - A Review, Part I”. The main converter technologies and HVDC systems' components will be discussed in this complementary paper.
... For the EHV system, each transformer costs around 80 million SEK to purchase and the residue value is estimated to be around 20 million (raw materials). The unexpected failure of one converter transformer causes the shutdown of the other five converter transformers in the link [6]. The repair time is estimated to be one week if there is local storage of spare units, which is often the practice in many regions. ...
... The alternating system is affected less in DC two ends back to back operation mode that it's not necessary for alternating system to provide source for large load and reactive-load compensation equipments in converter stations can be in balance with the ones in alternating system. [13] ...
This paper analyses reliability data of HVDC systems of Southern China Power Grid. The weak links of HVDC sys- tems' operational reliability are DC control and protection, valve hall and valve cooling system and transmission lines. Some improvement measures and HVDC system reliability technology are proposed in this paper. HVDC Transmission has obvious advantages in the re- spect of long distance transmission on a large scale and interconnection of power systems. It's significant for west-east power transmission and national interconnec- tion of power systems in China. The reliability in HVDC systems indicates the amount of power transmitted in specified time, conditions and environment. (1) Data shows that the availability rate of operating HVDC sys- tems is over 90 percent. It has been proposed in Inter- Mountain HVDC system that reliability level of bipolar HVDC should be higher than double-circuit AC lines. More requirements are raised for the reliability of HVDC systems with the technology of HVDC developing and more real projects go into operation. The improvement of reliability would bring great benefits to the safe, reliable and economic operation of the systems. Thus, reinforcing the work of analysis about reliability indices and man- agement of reliability is good for searching the weak links and influencing factors of reliability scientifically. Taking measures to improve HVDC systems reliability can guarantee the secure and available operation of HVDC systems.
... It is possible to compute the reliability parameters as the long term probability of the system being in a given state by solving a linear differential equations of the system. The graphical model constructed to display all possible transition paths between states is called state space diagram [ 6]. ...
The effect of tapping station on interconnected AC/DC transmission system is presented in this paper. Reliability values of AC and bipolar VSC-HVDC transmission systems are developed by using Markov modelling. The reliability indices are evaluated by DC line without tapping station and with tapping station. Reliability models of bipolar VSC-HVDC and AC systems are developed. A contingency enumeration technique with equivalent approach is used to determine the reliability indices. I INTRODUCTION Operation of DC links in parallel with AC lines has economical and technical advantages of supplying intermediate loads located between the sending and receiving ends through the AC facilities[1]. The first step in calculating the reliability of HVDC transmission systems by developing the appropriate models for all the components in the system[3,5] . The reliability model of DC line and AC lines are combined by a contingency is defined as a combination of generating unit and transmission line outages [ 2 ]. This paper examines the effect of tapping station on interconnected AC/DC line. A wide range of reliability indices are calculated at load level without and with tapping station on DC link. II TEST SYSTEM
The converter transformers (CTs) between the AC system and the converters are necessary for reliable operation of the ultra high voltage DC (UHVDC) system. Compared with the traditional HVDC system, 2×12 pulse UHVDC system has more complicated configuration and more capacity levels; the CTs with Y/Y and Y/Δ connections can't be replaced by each other, adding difficulty to the reliability evaluation, which has not be fully investigated yet. In this paper, reliability of the CTs for UHVDC system is quantified using the State Space (SS) and Frequency & Duration (F&D) methods. The state spaces of the single-phase CTs with three- and two-windings are proposed. Six operating modes are defined for the CTs. The spare for the three- and two- winding CTs is considered. The two-winding CTs with Y/Y and Y/Δ connections are differentiated and modelled separately to reduce number of the states. The numerical results are compared with the existing literatures to show accuracy and advantage of the proposed model.
Economics plays a major role in the application of reliability concepts and attainment of an acceptable level of reliability. Reliability is not free but poor reliability of electric power supply usually costs much more than good reliability. It is therefore important to determine the optimal reliability level at which the reliability investment achieves. The best results in reducing the customer damage costs or Lost of load value due to power supply interruptions. In this paper the effect of installation of VSC-tapping station cost beneficial or not is investigated and compared at the load point evaluating the value of lost of load. I. INTRODUCTION One of cost/ benefit analysis is to develop techniques which can appropriately evaluated the cost of value of loss of load or customer damage costs due to supply interruptions [5]. Generally the loss of any single generating unit or transmission line should not cause load interruption. This criterion does not explicitly consider the probability of component failures and the value of service to customers. It can therefore result in provide additional capacity. The criterion provides no economic input to the cost associated with a particular expansion to customers. The cost of HVDC system is depending upon the length of the transmission line, transmission voltage, conductor size, transmission losses [1]. Transmitting power from sending end to receiving end to meet the load demand. If any line of DC link under gone failure state or any deficiency in generation of power does not meet the load demand [ 2,3 ]. In such a case installing the tapping station at the middle of the transmission line. The effect of tapping station on reliability indices of bipole HVDC transmission system is discussed in [12]. Reliability models of HVDC transmission systems are presented in [2]. There are various methods to find the reliability of composite system. One of the best method is Markov modelling. This particular method is applicable to the system consists 'n' number states. In this method Markov modelling is used to find the reliability values of each component of HVDC transmission system, using these value lost of load of bipole VSC-HVDC transmission system is evaluated without and with installing the tapping station[ 6].
In the power system of the future, reliability will play an important role. This reliability is influenced by different developments like the increase in electricity consumption and the use of renewable and distributed generation. It is important to have a good insight into the reliability performance of the system. With probabilistic methods, the reliability of the power system can be analyzed and decisions can be made to maintain the high reliability level that we demand. This paper gives an overview of the different developments in power systems and describes how these developments influence the reliability of the future power system. The possibilities to implement these developments into the probabilistic reliability analysis in network planning and operational planning are discussed. By use of an example, the possibilities and challenges of probabilistic methods in operational planning are illustrated.
The transmission system is becoming ever more complex. Furthermore, demand for reliable supply of electricity is growing, increasing the need for a higher level of system reliability. A solution might be to incorporate controllable power components within the system. One such component is the HVDC link. However, in the domain of reliability models for HVDC and the component's impact on the overall transmission system reliability, there is still much work to be done. This is an important issue in future grid planning, especially with an increasing penetration of HVDC links within synchronous transmission systems acting as "firewalls". These links may prevent disturbances propagating in the system, which is a common cause of blackouts. This paper provides a broad introduction to the HVDC technology and a literature review of reliability assessments published within this area. It gives a background to and motivation for the technology. Published reliability assessments of the HVDC technology have been reviewed and categorized. One conclusion is that a large number of models and methods for the reliability evaluation of the HVDC system itself have been published, but very few on its impact on the composite system reliability. One future challenge to be solved is how the "firewall" properties of the HVDC can be quantified in a reliability assessment.
Quantitative reliability techniques are being increasingly used in system planning, design and operation. The fundamental parameters in building a model are the individual units/components and their failure and/or repair statistics. With complex unit configurations the system model complexity increases, hence high speed digital computers are used to solve them. This paper describes reliability modeling techniques in evaluating the reliability of the system.
Introduction Experimental Designs for ALT Parametric Models Used in ALT Nonparametric Models Used in ALT Problems
Methods proposed for software reliability prediction are reviewed. A case study is then presented of the analysis of failure data from a Space Shuttle software project to predict the number of failures likely during a mission, and the subsequent verification of these predictions.
High voltage auto-transformers represent an important component of bulk transmission systems and are used to transform voltage from one level to another. These auto-transformers are critical for regional load supply, inter-regional load transfers and for certain generator/load connections. Major or catastrophic failures to this equipment can have severe consequences to electric utilities in terms of increased operating costs and customer load losses. To minimize the impact of this type of failures, utilities may carry some spare units to guard against such events. These spare units are going to cost utilities money (utility cost) to purchase, to store and to maintain and utilities should try to strike the right balance between the utility cost and the risk cost (if spare units are not there). This paper describes a probabilistic approach for assessing the risk associated with catastrophic failures of auto-transformers used in bulk transmission systems with and without availability of spare units. The computed risk will be expressed in dollars and will be added to the cost of carrying any spare units. The required number of spare units (optimal) is obtained when the total cost, utility and risk costs, is minimal. Sensitivity studies are carried out to determine the system parameters that have dominant impacts on the optimal number of spares. An example is presented to illustrate the proposed assessment methodology.
In this paper, the problem of determining optimal burn-in time is considered under the general failure model. There are two types of failure in the general failure model. One is Type I failure (minor failure) which can be removed by a minimal repair, and the other is Type II failure (catastrophic failure) which can be removed only by a complete repair. In the researches on optimal burn-in, the assumption of a bathtub shaped failure rate function is commonly adopted. In this paper, upper bounds for optimal burn-in times are obtained under a more general assumption on the shape of the failure rate function, which includes the bathtub shaped failure rate function as a special case.