information model for the computation of the power supply continuity indicators for a power...

INFORMATION MODEL FOR THE COMPUTATION OF THE POWER SUPPLY CONTINUITY INDICATORS FOR A POWER

DISTRIBUTION BRANCH

RĂŞCANU VALENTIN GRIGORE 1, ALBERT HERMINA 2, GOLOVANOV NICOLAE 3, PĂUN DAN STELIAN 1, PĂUN CLAUDIU MIRCEA 1,

Keywords: Information model, Power continuity indicators, Spanning tree, Connectivity status function, Data production and aggregation areas

This paper presents an information model conceived to support the computation of the power continuity indicators. The model consists of three layers: the physical layer, the data layer and the application layer. A tree-like structure that spans the graph structure of the power grid and the hierarchical structure of the power distribution branch is used to describe the relationships between this branch and its consumers. The model allows the computation of the quality indicators at various granularity levels by defining a number of monitored areas that span sub-trees of the spanning tree of different sizes. For the description of the connectivity state of the consumer to the power grid during the monitored period of time a status function is also defined.

1. INTRODUCTION

The quality of the power supply service, especially for the industrial consumers, has an important influence on the economic performance of a company.

Although all power quality indicators have their own importance for the evaluation of the performance level of a power network operator, among them the information about the short- and long-term power interruptions is of outmost importance due to the fact that these events have the greatest impact on the consumer.

1 ) Felix IT SA, str. Ing. George Constantinescu nr. 2, sector 2, cod 020339, Bucureşti, tel: 021-242.01.92, fax: 021-242.70.41, e-mail: [email protected] ) ISPE SA Bucureşti, Bd. Lacul Tei nr. 1-3, cod 020371, sector 2, tel: 021-206.13.07, fax: 021-206.13.07, e-mail: [email protected] ) Facultatea de Energetică, U.P. Bucureşti, Splaiul Independenţei nr. 131, cod 060042, sector 6, tel.: 021-316.96.43, fax: 021-316.96.45, e-mail: [email protected]

Rev. Roum. Sci. Techn.– Électrotechn. et Énerg., 54, 1, p. , Bucarest, 2009

mailto:[email protected]



12 Răşcanu Valentin Grigore et al. 2

In this respect, the Performance standard for the electrical energy distribution service issued by ANRE (The National Energy Regulating Agency) defines a series of quality indicators regarding the frequency and the duration of the power supply interruptions. These indicators describe:

- the power supply continuity of a consumer;- the power supply continuity of a residential area (e.g. : transformer

points);- the power supply continuity of a busbar (node) of the electrical grid;- the power supply continuity of the consumers of an electrical energy

supplier (at the LV, MV and HV levels).The computation of the power quality indicators requires the continuous

acquisition and processing of the data from the monitored nodes of the power grid.At present, only particular solutions for data acquisition and computation of

the power supply continuity indicators are available on the Romanian market. These solutions depend on the type of the equipment being used for the data acquisition and on the particular structure of the distribution branch and are difficult to modify, reuse and even reconfigure.

This paper presents an information model that solves the problem presented above by using a general purpose tree-like structure that describes the relationship between the customer and the distribution branch for the computation of the power supply continuity indicators.

2. THE POWER GRID STRUCTURE

The interconnected power system propagates the perturbations caused by different events to all the grid supply points. The impact of the perturbation depends on the short-circuit current in the analyzed node and on the impedance between this node and the location where the event took place.

Basically, the structure of an electrical distribution network, depicted in figure 1, includes [1, 2]:

- the 110kV network (usually containing loops) which supplies electrical power to the 110/20kV substations (level I);

- the 110 kV/20kV substation which supply electrical power to the consumers that are directly connected (usually in a radial network) to the 110kV busbar or to the consumers connected to the 20kV busbars by means of the 110kV/20kV transformers (level II);

- the 20kV network (usually having a radial or a looped topology) which supplies electrical power either directly to the consumers connected to it or to the consumers connected by means of the 20/0.4 kV transformers.

3 Information model for the computation of the power supply continuity indicators 11

Starting form the 20kV busbars, the energy may be fed to the feed points too (level III);

- the LV network (mainly a radial network), which supplies electrical power to the small consumers from the residential or rural areas (level IV).

Fig. 1 The structure of the power distribution grid

The data required for the computation of the power supply continuity indicators, at each of these voltage levels, are the number of power interruptions, the duration of these interruptions, the number of the affected consumers and the interrupted power. It is also important to know if the interruption was planned or it happened by accident, knowing that in case of the planned interruptions the costs can be avoided.

In a radial network, these data can be directly acquired either from the consumers (level IV), from the transformer points (level III), from the 20kV busbars of the distribution substation (level II) or from the 110kV bus bars of the substation (level I).

We have to take into account, when we analyze the acquired data, the use on the 20kV busbars, for power reservation purposes, of the automatic throw over

20 kV

S

110 kV110 kV 110 kV

S

PT

PA

PA PA

PT

PT


switches (ATS) and of the isolating switches used to configure a loop in a radial network.

3. THE POWER SUPPLY CONTINUITY INDICATORS

According to the present legislation, every distribution branch must compute the yearly values of the continuity indicators SAIDI, SAIFI and AIT and of the ENS quantity.

The SAIFI indicator (The System Average Interruption Frequency Index) is commonly used as a reliability indicator by electric power utilities. SAIFI is the average number of interruptions with duration longer than 3 minutes that a customer would experience. It is usually measured over the course of a year, usually taking values that are smaller than 2, and it is calculated as:

SAIFI = (∑Nk=1 The number of interrupted customers during the k

interruption) / Total number of customers served (1)

where N is the number of power supply interruptions during a year.SAIFI is measured in units of interruptions per customer.. The calculation of

this continuity indicator requires information about the number of customers connected to every feeder leaving a substation or a feed point and who experience interruption as the respective feeders are disconnected from the power system.

The SAIDI indicator (The System Average Interruption Duration Index) is another indicator commonly used as a reliability indicator by electric power utilities. SAIDI expresses the average outage duration for each customer served, and is calculated as:

SAIDI = (∑Nk=1 The duration of the k interruption x The number of

interrupted customers during the k interruption) / Total number of customers served

(2)

where N is the number of power supply interruptions during a year.SAIDI is measured in units of time, often minutes or hours. It is usually

measured over the course of a year and it requires information about the time the circuit breakers stayed tripped-off during every interruption. The SAIDI indicator could take values that are less than 200 minutes pro year.

The ENS (Energy Not Supplied) quantity may be determined using the following formula:

ENS = (∑Nk=1 The power interrupted during the k interruption x The

duration of the k interruption) (3)

where N is the number of power supply interruptions during a year.


The power that is interrupted may be determined using the information delivered by the power meters, if they exist or else a constant value, like the power to be delivered by contract, may be used.

The AIT indicator (Average Interruption Time) may be determined using the formula:

AIT = 8760 * 60 * ENS / AD (4)

where AD (Annual Demand) it’s the total energy demanded by the consumers over an entire year (MWh/year, no losses), while the ENS (Energy Not Supplied) unit of measurement is the MWh.

The calculation of the AIT indicator it’s based on the calculated ENS value, while the AD value it is computed using the value of the power level that was contracted by the consumers.

For the computation of the above-mentioned power supply continuity indicators we need an information model that will allow us to systematically tackle the complex structure of the electrical grid.

The proposed information model covers the HV and MV substations belonging to a distribution branch. Although it has been conceived to support the computation of the continuity indicators at the branch or even at company level, the model is highly scalable and allows to computation these indices at the desired granularity level (distribution branch, substation, busbar or feeder).

4. THE PRESENTATION OF THE INFORMATION MODEL

The information model (figure 2) consists of three layers: the physical layer, the data layer and the application layer. Besides these three layers there is a fourth one, the user layer, which doesn’t actually belong to the model. It is nonetheless useful for the description of the way in which the continuity indicators are used by the users in the problem domain.

A layer provides for data services to the layer above it and uses the data services of the layer below it.

The physical layer is implemented at the electrical grid level by means of which the electrical power is supplied to the consumers. This grid, which is often referred as a complex system, can be described from a topological point of view by a graph. The data types used by the model at this layer are the primary data (e.g.: r.m.s. values of the voltage magnitude).

The data layer is dedicated to the production of the events that describe the continuity of the power supply to the consumer. These events are produced the moment when the r.m.s. value of the supply voltage magnitude goes under the separation threshold Vsep for more than 3 minutes (where Vsep= 0.05 * UN).


User layer

Application layerData layer

Physical layer

Fig. 2 – The information model used for the computation of the continuity

The application layer is the place where the acquired events are processed and aggregated and the continuity indices are computed.

The user layer is the place where the data coming from the application layer are being used in decision systems in order to facilitate the solving of the problems caused by the poor electrical energy quality or to generate reports.

The primary data from the physical level are obtained by continuously (every half-period) measuring the r.m.s. value of the supply voltage magnitude. In order to determine the events related to the power supply interruptions for the set of consumers of a distribution branch connected at a certain voltage level C={c1, c2, … ,cn}, we attach each of these consumers a connectivity status function S=f(ui).

The function S: R -> {0,1} is defined as following:S(u) = 0, u < 0.05 * UnS(u) = 1, u >= 0.05 * Un

where: u=f(t), u:R->R is a function of time that gives the r.m.s. valueof the supply voltage magnitude in the feed node;Un= the r.m.s. value of the contracted voltage magnitude.

The S function, defined on the R set and taking values in the {0,1} set, indicates if at any particular moment of time the consumer is connected to the power grid (taking the “1” or “true” value) or not (taking the “0” or “false” value).

We can say that the set of the values these functions take at the time moment t=t1 {S (u1 (t=t1)), S (u2 (t=t1))… S (un (t=t1))} completely defines the connectivity status of the members of the C set at that particular moment. We can conclude that, if we are going to record the all the values that the S i functions took for a particular period of time, we can determine the power supply interruptions by selecting those situations when the function S took the 0 value for more than 3 minutes.

Function value Moment of change [dd mmm yyyy hh:mm:ss.ms] Duration [ms]

0 10 May 2008 22:43:59.250 200.000… … …1 12 May 2008 12:23:45.120 123


Due to the fact that the S functions can take only to values (0 or 1), we need to record only the moments when the function change its output value together with the new value of the function and the change duration.

In the table above, the rows that record the moments when the function S takes the value 0 for more than 3 minutes are of particular interest, because they represent the events of the interruption of the power supply to the customer.

In order to compute the continuity indicators at the application layer, we need to define a topological structure that will match both the physical structure of the electrical grid (it has to include the nodes that produce the events which are used in the computation of the continuity indicators) and the organizational structure of the power distribution system (it has to describe the relationships between the entities of this system and the consumer). The most adequate solution for describing the relationships between the distribution branch and the consumer is the use a tree structure named the spanning tree (figure 3). This name stems from the fact that this tree spans the entire above mentioned structures.

Fig. 3 The structure of a spanning tree of a distribution branch

A spanning tree is a hierarchical structure that places elements in nodes along branches that originate from a unique node called the root. The nodes in a tree are subdivided into levels, where the topmost level holds the root node. Any node in a tree can have multiple successors at the next level, excepting the nodes belonging to the last level (the leaf nodes). Each node in a tree is the root of a subtree, which consists of the respective node and all of its descendants.


Using analogies from a family tree, we associate the terms parent and children to describe the relationship between a node and its successor nodes. The fact that each nonroot node has a single parent ensures that there is a unique path from any node to itself or one of its descendants. Practically, in the case of the tree structure, starting from the root node we can visit all the nodes exactly one time.

The spanning tree is made of nodes that represent either physical objects (the busbars and the circuit breakers), which are data producers, or abstract objects (the distribution company, the distribution branch, the substation) which are data aggregators.

The root node is allocated to a data aggregator (the distribution branch, depending where we start the hierarchy), the intermediate nodes hold data aggregators (the distribution branch, the substation, and the busbars) and the leaf nodes hold data producers (the circuit breakers which connect the consumer to the network grid). The spanning tree is described from the point of view of the relationships between nodes (“has a” or subordination relationships) by a number of configuration files.

This storage of the events takes place in the leaf nodes of the spanning tree which partially overrides the graph structure of the electrical grid, including only those vertices that directly participate in the event production (the busbars and the circuit breakers). The line that connects the leaf nodes of the spanning tree represents practically the border between the consumers and the distribution network operator (figure 4).

Fig. 4 The structure of a distribution branch viewed from the perspective of its relationship with the customers


The physical nodes are monitored by means of specialized equipments that continuously acquire real-time data regarding the continuity of the supply service, data which is transformed in events that are eventually stored in historical databases. The structure of the historical database records that store these events represents an interface that separates a variable domain (the data format) from the rest of the model. The model allows the import of data that are acquired in other formats.

Two assumptions have been made regarding the decision of wether a vertex of the electrical grid graph is included in the spanning tree or not.

First, taking into account that the absence of the supply voltage at any point on the path between the busbar and the consumer automatically leads to the absence of supply voltage at the consumer connection point, no intermediate vertex (usually implemented by a circuit breaker) belonging to this path shall be included in the tree.

Second, the path between the busbar and the circuit breaker that connects the consumer to the electrical grid represents an abstract relationship (although it is based on a physical reality). So, from the model standpoint, it is immaterial whether the power is feeded from the respective busbar or from another power source (as in the case when we reconfigure the network by building a loop).

One important attribute of the leaf nodes is their multiplicity. The node multiplicity represents the number of the consumers directly connected to that node at the same voltage level. Keeping this thing in mind, a MV substation represents only one consumer for a HV substation, no matter how many MV consumers are connected to the first one. This fact holds for a MV substation with respect to the LV consumers connected to a feed/transformer point too (they count as a single consumer at the MV level to the substation).

The abstract nodes contained by the spanning tree belong to the organizational structure of the power distribution system and they include the electrical substation, the distribution branch or the distribution company. These nodes do not directly produce events and they are included in the spanning tree because, at the application layer, we need them to compute the continuity indicators.

From the point of view of their position to the information production and consumption processes, we can divide the nodes of the spanning tree between servers (data producers) and clients (data consumers). In this respect, the leaf nodes are servers, the intermediate nodes are both servers (the busbars) and clients (the substations and the distribution branches) and the root node is a client (the distribution branch or the distribution company).

The data collection which a server presents to its clients defines the address space of that server. These data can be either attributes, which are primitive types of data (e.g.: r.m.s. value of the voltage magnitude or mains frequency values), or


complex data types consisting of primitive data types and which have a certain structure (e.g.: the events which record the power interruptions).

Taking into account this point of view, the circuit breakers from the substations or from the feed points form together a so called data production zone (DPZ), while the busbars, the substations and the distribution branches which use these data in order to compute the continuity indicators form around them a data aggregation zone (DAZ). Both the DPZ and the DAZ contain subtrees of the spanning tree. The root node in case of the DPZ is a busbar and in case of the DAZ is a substation, a distribution branch or a distribution company, depending on the area covered by this zone.

Fig. 5 The relationship between the structure of the spanning tree and the way data are produced and consumed.

The correspondence between the structure of the spanning tree (figure 3) and the way the data are produced and consumed during the power supply continuity monitoring and computation processes are presented in figure 5.

The relationships between the spanning tree nodes and the way they belong to the data production and aggregation zones are depicted by a number of configuration files.


The configuration files store static information about the subtrees of the spanning tree. The model uses two type of configuration files: the data production zone (DPZ) configuration file which stores information about the nodes belonging to such a zone (e.g.: the circuit breakers) and the data aggregation zone (DAZ) configuration file which stores information about the data production zones that are included in a data aggregation zone (e.g.: the substations).

The DPZ configuration file describes a certain subtree of the spanning tree which contains physical nodes and which has two layers: the root node (busbar) layer and the leaf nodes layer (the circuit breakers which the customers are directly connected to).

The nodes of the subtree depicted by a DPZ configuration file are described by a number of entries containing an unique identifier (e.g.: the serial number of the monitoring equipment and the identifier of its input channel connected to the node), the number of the consumers connected to the node, the power that has to be supplied to these users by contract, the area type to which the customer belongs (urban or rural area), the voltage level supplied to the customer (HV or MV), the path to the historical database where the dynamic information belonging to the node is stored, the name of the data production node and the name of the respective node.

The DAZ configuration file includes information about the DPZs that belong to the respective data aggregation zone, this information being described by the entries included the file. The file contains one entry for each DPZ, each entry including the path to the database that stores the dynamic information acquired in a DPZ, the path to the DPZ configuration file, the name of the DPZ and the name of the DAZ.

The configuration files represent another interface that separates another zone of variability (the specific structure of a distribution branch) from the rest of the model.

In order to compute the power supply continuity indicators at the application layer we need to use, besides the static information contained in the configuration files, the dynamic information from the historical databases that store the data acquired form the monitored grid nodes by the specialized equipments.

The power supply continuity indicators can be computed at different granularity levels by reading the static information contained in the configuration files and by corroborating this information with the one contained in the historical databases.

5. CONCLUSIONS


The information model presented in this paper uses a general purpose tree-like structure that describes the relationship between the customer and the distribution branch, for the computation of the power supply continuity indicators.

Compared with the solutions that are developed or used at present in the distribution sector in Romania, this solution has the following advantages:

- proposes a general purpose information model for the computation of the power supply continuity indicators;

- separates the variable part of the model (the input data format and the distribution branch structure) being a number of clearly defined interfaces;

- allows the description of the relationship between the distribution branch and the consumer by means of an easily to be transversed tree structure;

- offers a flexible mechanism for the description, by means of this tree structure, of the particular topologies of the distribution branches;

- the proposed model is scalable, allowing the computation of the power supply continuity indicators at the granularity level chosen by the user (distribution branch, substation, busbar, feeder).

The information model was experimentally implemented in an integrated power quality monitoring system deployed in an Electrica distribution branch in Romania

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REFERENCES

1. Hermina Albert, I. Florea, Alimentarea cu energie electrică a întreprinderilor industriale, Editura Tehnică, Bucureşti, 1987.

2. N. Golovanov, ş.a., Instalaţii electroenergetice şi elemente de audit industrial, Editura N'ERGO, Bucureşti, 2008.

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