High-Efficiency Voltage Regulator for Rural Networks
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Transcript of High-Efficiency Voltage Regulator for Rural Networks
HIGH-EFFICIENCY VOLTAGE REGULATOR FOR
RURAL NETWORKS
ABSTRACT
This paper presents a high-efficiency voltage regulator, which combines robustness, low
costs and easy maintenance without power electronics components. Power quality is the
combination of voltage quality and current quality. Quality of supply is a combination of voltage
quality and the non-technical aspects of the interaction from the power network to its customers.
These characteristics make it suitable for rural networks, where investments and operational cost
in power quality improvement are limited. The regulator consists of a multi winding reduced-
power transformer, and provides serial voltage compensation.
This paper presents a new voltage regulator that fulfills the rural networks needs: high efficiency,
robustness, easy maintenance and low cost. Section II presents the design of the voltage
regulator, describing its power circuit and control system. Some practical considerations
regarding the design of the voltage regulator are presented in Section III. And finally, Section IV
presents the operation experience data of voltage regulators installed in the distribution network.
Different voltage compensation steps are obtained by modifying the connection and the
polarity between the primary and secondary windings. The transformer design has been
optimized to obtain a high-efficiency and low-cost regulator. An automatic controller monitors
the output voltage and sets the optimal compensation step. At present more than 400 units of the
voltage regulator are in operation.
Experimental records for the operation of installed voltage regulators have shown their
reliability, high efficiency, and their capacity to improve power quality in rural networks.
I. INTRODUCTION
LONG-duration voltage variation (undervoltage and overvoltage) is a central issue in
distribution network power quality. Supply voltage and power quality are regulated by certain
standards, such as the European EN 50160 [1] or the American ANSI C84-1 [2]. These standards
are complemented in each country or state by specific codes and rules [3]. The European EN
50160 stipulates that the maximum voltage amplitude variation accepted is 10%, while the
American ANSI C84-1 defines a normal operating range of 120 V 5%. National rules usually
define more restrictive voltage ranges; for instance, the Spanish rule for voltage quality [4] sets
the maximum variation of the voltage at the load connection point at 230 V 7%. The value of
voltage amplitude is an important quality issue, because loads are designed to work correctly
within a specific voltage range. Several problems in domestic and industrial equipment are
associated with long duration undervoltages, such as malfunctioning in relays and contactors,
incandescent lighting dim, switch-off of discharge lighting, failure of nonlinear loads (e.g.,
computer power supplies), and torque reduction in induction machines. On the other hand, long
duration overvoltages usually result in the overheating of loads (motors and transformers), and
hence a reduction in their expected durability. Low voltage rural distribution networks compared
with urban networks are more susceptible to long-term voltage variations, due to the dispersed
configuration of customers. Voltage variations in rural areas are usually associated with long
distances between the loads and the distribution transformer. Nowadays, the integration of non-
controllable dispersed generation in these networks is a new potential source of voltage variation
problems. To minimize long-term voltage variations in rural networks, distribution companies
have traditionally performed different actions: 1) tap change control in the main distribution
transformer; 2) installation of compensation equipment, such as capacitor banks, voltage
regulators, boosters, or auto-boosters; and 3) as a last resort, because it is the most expensive
alternative, the distribution company upgrades the low voltage network (increasing the line
capability, or changing the network rated voltage) [5]. In rural areas, the ratio of contracted
power per connection point is much smaller than for urban areas; therefore, investments to solve
specific voltage problems are limited. In this situation, the use of compensation equipment such
as voltage regulators becomes an interesting alternative. Moreover, the distribution company also
takes into account the operation and maintenance costs and the energy losses resulting from the
different options for solving voltage problems. Consequently the cost-efficiency of voltage
regulators is also a key issue. Currently, there are different technologies for voltage regulators
both in commercial devices and in the literature: tap-switching, ferroresonant, and electronic [6].
The most advanced commercial voltage regulators are based on power electronics and provide
accurate voltage output. Nowadays there are few approaches to voltage regulators in rural
networks [7]–[10]; moreover these approaches still do not completely cover the needs of rural
distribution networks. This paper presents a new voltage regulator that fulfills the rural networks
needs: high efficiency, robustness, easy maintenance and low cost. Section II presents the design
of the voltage regulator, describing its power circuit and control system. Some practical
considerations regarding the design of the voltage regulator are presented in Section III. And
finally, Section IV presents the operation experience data of voltage regulators installed in the
distribution network.
Power distribution control
Distribution SystemElectrical power is transmitted by high voltage transmission lines from
sending end substation to receiving end substation. At the receiving end
substation, the voltage is stepped down to a lower value (say 66kV or 33kV
or 11kV). The secondary transmission system transfers power from this
receiving end substation to secondary sub-station. A secondary substation
consists of two or more power transformers together with voltage regulating
equipments, buses and switchgear. At the secondary substation voltage is
stepped down to 11kV. The portion of the power network between a
secondary substation and consumers is known as distribution system. The
distribution
system can be classified into primary and secondary system. Some large
consumers are given high voltage supply from the receiving end substations
or secondary substation.
The area served by a secondary substation can be subdivided into a number
of sub- areas. Each sub area has its primary and secondary distribution
system. The primary distribution system consists of main feeders and
laterals. The main feeder runs from the low voltage bus of the secondary
substation and acts as the main source of supply to sub- feeders, laterals or
direct connected distribution transformers. The lateral is supplied by the
main feeder and extends through the load area with connection to
distribution transformers. The distribution transformers are located at
convenient places in the load area. They may be located in specially
constructed enclosures or may be pole mounted. The distribution
transformers for a large multi storied building may be located within the
building itself. At the distribution transformer, the voltage is stepped down to
400V and power is fed into the secondary distribution systems. The
secondary 14 distribution system consists of distributors which are laid along
the road sides. The service connections to consumers are tapped off from the
distributors. The main feeders, laterals and distributors may consist of
overhead lines or cables or both. The distributors are 3- phase, 4 wire
circuits, the neutral wire being necessary to supply the single phase loads.
Most of the residential
and commercial consumers are given single phase supply. Some large
residential and commercial consumer uses 3-phase power supply. The
service connections of consumer are known as service mains.
The consumer receives power from the distribution system. The main part of
distribution system
includes:-
1. Receiving substation.
2. Sub- transmission lines.
3. Distribution substation located nearer to the load centre.
4. Secondary circuits on the LV side of the distribution transformer.
5. Service mains.
Power Flow
For distribution system the power flow analysis is a very important and
fundamental tool. Its results play the major role during the operational
stages of any system for its control and economic schedule, as well as during
expansion and design stages. The purpose of any load flow analysis is to
compute precise steady-state voltages and voltage angles of all buses in the
network, the real and reactive power flows into every line and transformer,
under the assumption
of known generation and load.
During the second half of the twentieth century, and after the large
technological developments in the fields of digital computers and high-level
programming languages, many methods for solving the load flow problem
have been developed, such as Gauss-Siedel (bus impedance matrix),
Newton-Raphson’s (NR) and its decoupled versions. Nowadays, many
improvements have been added to all these methods involving assumptions
and approximations of the transmission lines and bus data, based on real
systems conditions.
The Fast Decoupled Power Flow Method (FDPFM) is one of these improved
methods, which was based on a simplification of the Newton-Raphson’s
method and reported by Stott and Alsac in 1974. This method due to its
calculations simplifications, fast convergence and reliable results became the
most widely used method in load flow analysis. However, FDPFM for some
cases, where high R/X ratios or heavy loading (Low Voltage) at some buses
are present, does not converge well. For these cases, many efforts and
developments have been made to overcome these convergence obstacles.
Some of them targeted the convergence of systems with high R/X ratios,
others those with low voltage buses. Though many efforts and elaborations
have been achieved in order to improve the FDPFM, this method can still
attract many researchers, especially when computers and simulations are
becoming more developed and are now able to handle and analyze large size
system.
Objectives of Radial Distribution System:-1. Planning, modernization and automation.
2. To provide service connection to various urban, rural and industrial
consumer in the allocated area.
3. Maximum security of supply and minimum duration of interruption.
4. Safety of consumers, utility personnel.
5. To provide electricity of accepted quality in terms of :-
(a) Balanced three phase supply.
(b) Good power factor.
(c) Voltage flicker within permissible limits.
(d) Less voltage dips.
(e) Minimum interruption in power supply.
Advantages of Radial Distribution System:-
(a) Radial distribution system is easiest and cheapest to build.
(b) The maintenance is easy.
(c) It is widely used in sparsely populated areas.
Drawback of Radial Distribution System:-
(a) The end of the distributor nearest to the feeding point will be heavily
loaded.
(b) The consumers are dependent on a single feeder and single distributor.
Therefore, any fault on the feeder or distributor cuts off supply to the
consumers who are on the side of the fault away from the sub-station.
(c) The consumers at the distant end of the distributor would be subjected to
serious voltage fluctuations when the load on the distributor
The single line diagram of a typical low tension distribution system.History of Distribution System
In the early days of electricity distribution, direct current DC generators were
connected to loads
at the same voltage. The generation, transmission and loads had to be of the
same voltage because there was no way of changing DC voltage levels,
other than inefficient motor-generator sets. Low DC voltages were used (on
the order of 100 volts) since that was a practical voltage for
incandescent lamps, which were then the primary electrical load. The low
voltage also required less insulation to be safely distributed within buildings.
The losses in a cable are proportional to the square of the current, the length
of the cable, and the
resistivity of the material, and are inversely proportional to cross-sectional
area. Early transmission networks were already using copper, which is one of
the best economically feasible
conductors for this application. To reduce the current and copper required
for a given quantity of
power transmitted would require a higher transmission voltage, but no
convenient efficient method existed to change the voltage level of DC power
circuits. To keep losses to an economically practical level the Edison DC
system needed thick cables and local generators.
Modern Distribution System
The modern distribution system begins as the primary circuit leaves the sub-
station and ends as the secondary service enters the customer's meter
socket. A variety of methods, materials, and equipment are used among the
various utility companies, but the end result is similar. First, the energy
leaves the sub-station in a primary circuit, usually with all three phases. The
most common type of primary is known as a Wye configuration (so named
because of the shape of a "Y".) The Wye configuration includes 3 phases
(represented by the three outer parts of the "Y") and a neutral (represented
by the centre of the "Y".) The neutral is grounded both at the substation and
at every power pole. The other type of primary configuration is known as
delta. This method is older and less common. Delta is so named because of
the shape of the Greek letter delta, a triangle. Delta has only 3 phases and
no neutral. In delta there is only a single voltage, between two phases
(phase to phase), while in Wye there are two voltages, between two phases
and between a phase and 27 neutral (phase to neutral). Wye primary is safer
because if one phase becomes grounded, that is, makes connection to the
ground through a person, tree, or other object, it should trip out the circuit
breaker tripping similar to a household fused cut-out system. In delta, if a
phase makes connection to ground it will continue to function normally. It
takes two or three phases to make connection to ground before the fused
cut-outs will open the circuit. The voltage for this configuration is usually
4800 volts.
Requirement of Distribution system
A considerable amount of effort is necessary to maintain an electric power
supply within the
requirements of various types of consumers. Some of the requirements of a
good distribution
system are: proper voltage, availability of power on demand, and reliabilit
Proper Voltage:
One important requirement of a distribution system is that voltage
variations at consumers’ terminals should be as low as possible. The changes
in voltage are generally caused due to the variation of load on the system.
Low voltage causes loss of revenue, inefficient lighting and possible burning
out of motors. High voltage causes lamps to burn out permanently and may
cause failure of other appliances. Therefore, a good distribution system
should ensure that the voltage variations at consumers’ terminals are within
permissible limits. The statutory limit of voltage variations is +10% of the
rated value at the consumers’ terminals. Thus, if the declared voltage is 230
V, then the highest voltage of the consumer should not exceed 244 V while
the lowest voltage of the consumer should not be less than 216 V.
Availability of Power Demand: Power must be available to the consumers in any amount that they may
require from time to time. For example, motors may be started or shut down,
lights may be turned on or off, without advance warning to the electric
supply company. As electrical energy cannot be stored, therefore, the
distribution system must be capable of supplying load demands of the
consumers. This necessitates that operating staff must continuously study
load patterns to predict in advance those major load changes that follow the
known schedules.
Reliability:
Modern industry is almost dependent on electric power for its operation. Homes and office
buildings are lighted, heated, cooled and ventilated by electric power. This calls for reliable
service. Unfortunately electric power, like everything else that is man-made, can never be
absolutely reliable. However, the reliability can be improved to a considerable extent by (a)
inter-connected system, (b) reliable automatic control system and (c) providing additional
reserve facilities.
Classification of Distribution System
A distribution system may be classified according to:
(i) Nature of current:
According to nature of current, distribution system may be classified as (a) d.c. distribution
system and (b) a.c. distribution system. Now-a-days a.c. system is universally adopted for
distribution of electric power as it is simpler and more economical than direct current method.
(ii) Type of construction:
According to type of construction, distribution system may be classified as (a) overhead system
and (b) underground system. The overhead system is generally employed for distribution as it is
5 to 10 times cheaper than the equivalent underground system. In general, the underground
system is used at places where overhead construction is impracticable or prohibited by the local
laws.
(iii) Scheme of connection:
According to scheme of connection, the distribution system may be classified as (a) radial
system, (b) ring main system and (c) inter-connected system. Each scheme has its own
advantages and disadvantages.
Radial Distribution System
A radial system has only one power source for a group of customers. A power
failure, shortcircuit, or a downed power line would interrupt power in the
entire line which must be fixed before power can be restored. The figure of
Radial Distribution System is shown as :-
Radial Distribution System
In this system, separate feeders radiate from a single sub-station and feed
the distributors at one end only. Figure (a) shows a single line diagram of a
radial system for d.c. Distribution where a feeder OC supplies a distributor
AB at point A. Obviously, the distributors are fed at one point only i.e. point A
in this case. Figure (b) shows a single line diagram of radial system for a.c.
distribution. The radial system is employed only when power is generated at
low voltage and the sub-station is located at the centre of load. This is the
simplest distribution circuit and has the lowest initial cost.
Single Line Diagram of Radial Distribution System
Node Radial Distribution Network:-
Objectives of Radial Distribution System:-1. Planning, modernization and automation.
2. To provide service connection to various urban, rural and industrial
consumer in the allocated area.
3. Maximum security of supply and minimum duration of interruption.
4. Safety of consumers, utility personnel.
5. To provide electricity of accepted quality in terms of :-
(a) Balanced three phase supply.
(b) Good power factor.
(c) Voltage flicker within permissible limits.
(d) Less voltage dips.
(e) Minimum interruption in power supply.
Advantages of Radial Distribution System:-
(a) Radial distribution system is easiest and cheapest to build.
(b) The maintenance is easy.
(c) It is widely used in sparsely populated areas.
Drawback of Radial Distribution System:-
(a) The end of the distributor nearest to the feeding point will be heavily
loaded.
(b) The consumers are dependent on a single feeder and single distributor.
Therefore, any fault on the feeder or distributor cuts off supply to the
consumers who are on the side of the fault away from the sub-station.
(c) The consumers at the distant end of the distributor would be subjected to
serious voltage fluctuations when the load on the distributor
MODELINOGF DISTRIBUTIOSYNS TEMCOMPONENTS
The individual components of a distribution system are modeled by their mathematical
equivalents. The three-phase modeling of distribution system components is given . The series
impedance matrix of a three-phase line section is given by equation
This equation is obtained after Kron's reduction. It takes care of the effects of the neutral or
ground. At each bus i, the complex power S, is given by,
where P:pe' and Q;," are the specified real and reactive powers respectively of bus i. The
equivalent current injection at bus i for the kfh iteration is given as,
THREE-PHASE DISTRIBUTION LO AD FLOW ANALY.S.I S
Most of the distribution'systems are. radial in nature with a single voltage source. This special
property of the distribution system is used to derive various formulations. Different iterative
methods similar to Gauss-Seidel method are discussed in this paper. In this section, the
algorithms for these methods are given.
A. Implicit Z-bus Method
The implicit Z-bus method is the most commonly used method 151. The method works on the
principle of superposition as applied to the system bus voltages. According to the principle of
superposition, only one type of source is considered at a time for the calculation of bus voltages.
An iterative procedure is used in this method. Initially, all the bus voltages are assumed to be
equal to the swing bus voltage (only swing bus is considered as the source in the system with all
the current injections at load buses taken as zero). In the next step, since the current injections
and bus voltages are .dependent on each other, these quantities are required to be determined
iteratively. The swing bus is short-circuited while calculating the component of bus voltages due
to the current injections.
The following steps are involved in this algorithm:
1. The bus voltages are assumed to have some initia1,value. The Y-bus (Y,) is formed.'
2. The current injections are computed by using equation 3 for which the recent values of bus
voltages are taken.
3. The voltage deviations (VD) due to current injections are computed by the factorization of Y-
bus,
f = [YB] [ VD]' (4)
4. The voltage deviations calculated in step 3 are superimposed on the no load bus voltage
(VNL)H. ence, the bus voltages are updated'as. 3" = VNL + [ VD]' (5)
5. The convergence is checked. If the method has not converged. then steps from 2 to 4 are
repeated.
B. ModiJied Gauss-Se2.d Method
The implicit Z-bus method described earlier requires the factorization of the full Y-bus matrix,
adversely affecting the performance in terms of speed. .Hence, a new method has been suggested
in [6] by blending the implicit Z-bus method and the Gauss-Seidel method to improve the
computational efficiency. For a distribution system with n buses, where P:pe'and Qtpec are the
specified powemat bus i, the bus voltage for k'" iteration can be calculated by using the Gauss-
Seidel method
as.
The values of voltages used in the modified Gauss-Seidel method are the most recently
computed values, whereas thevalues of voltages used in the implicit Z-bus method are the
Two matrices are developed, viz. the bus injection to branch current (BIBC) matrix and branch
current to bus voltage (BCBV) matrix. By . . using simple matrix multiplication of these two
matrices, the Two developed matrices, BIBC and BCBV are used to obtain the load flow
solution. The development of these two matrices is explained with reference to Fig. . The figure
shows a simple distribution system. It has sub-station at its bus number I, and bus numbers 2 to 6
are the load buses loadflow‘solution is obtained
D. Forward-Backward Substitution
In all the previous methods, the voltages at all the buses in the system are calculated in one step,
by using the matrices. In forward-backward substitution, the KCL and KVL are applied at each.
node and branch respectively. By solving these equations iteratively, the solution is obtained [SI.
The following steps involved in this method:
Optimal ordering of nodes: Nodes are renumbered according to source node - load node relationship to facilitate the
forward and backward substitution. Thus, a forward path is created from the source node to the
load node and a backward path is traced from the load node to the source node. The branch node
nearer to the source is called as the parent node and the other node is called as the child node.
Initially, the flat voltage start is assumed.
Backward substitution:
This is used to calculate the current in each branch. The current in the last branch is equal to the
current injection at the corresponding end node. The voltage values are kept constant. The
network is traced in the backward direction. The currents in all the other branches can be found
out by using KCL as given by the equation.
where I,, ("1). Ib (U,) and I, (ut) are the branch currents of line section m, and ib,, iu and
iLc are the equivalent current injections at the child node (i) of branch m. M is the set of line
sections connected to mrh branch at its child node (p is the number of a line section which is an
element of M).
respectively. These values of the voltages are used for calculating the currents by backward
substitution in the next iteration.
Check for convergence:
The forward and backward substitutions are performed in each iteration of the load flow. The
voltage magnitudes at each bus in an iteration are compared with their values in the previous
iteration. If the error is within the tolerance limit, the procedure is stopped. Otherwise, the steps
of backward substitution, forward substitution and check for convergence are repeated
E. Ladder Network Theory
The ladder network theory given in [9] is very much similar to the forward-backward
substitution method. Though the basic principle of both the methods is same, there are
differences in the steps of implementation. In the ladder network theory, the optimal ordering of
nodes is done first. In the backward substitution, the node voltages are assumed to be equal to
some initial value in the first iteration. The currents in each branch are computed by KCL using
equation 22. In addition to the branch currents, the node voltages are also computed by using
equation. Thus, the value of the swing bus voltage is also determined. This calculated value of
the swing bus voltage is compared with its specified value. If the error is within the limit, then
the load flow converges; otherwise the forward substitution is performed as explained in the case
of forward-backward substitution method. Thus, in the ladder network theory, the bus voltages
are calculated twice in the same iteration as compared to only once for the forward-backward
substitution method. .The convergence is checked in the ladder network theory by comparison
between the specified and calculated voltage values of the swing bus, whereas the difference
between the values of bus voltages at the present and previous iterations is considered for
convergence in the forward-backward substitution method.
Forward sirbstitufion
This is used to calculate the voltage at each node (starting from the child node of the first branch) by using KVL. The swing bus voltage is set to its specified value. The current in each branch is held constant at the value obtained in the backward substitution. Thus, using the branch currents
calculated in the backward substitution, the values of voltages are calculated by using the equation,
Fast Decoupled Power Flow for Radial Distribution System
In Radial Distribution System, the large R/X ratio causes problems in convergence of conventional load flow algorithm. Therefore for the better convergence some modified load flowmethods are used. For the purposes of power flow studies, we model a radial distribution system as a network of buses connected by distribution lines, switches, or transformers to a voltage specified source bus. Each bus may also have a corresponding load, shunt capacitor, and/or co-generator connected to it. The model can be represented by a radial interconnection of copies of the basic building block shown in Figure 4.2 the dotted lines from the co-generator, shunt capacitor, and load to ground are to indicate that these elements may be connected in an ungrounded delta-configuration. Since a given branch may be single-phase, two-phase, or three-phase. The basic building block of radial distribution systemis shown on the next page as:-
Figure
The Basic Building Block of Radial Distribution System.
One of the key concepts behind our formulation is that the voltage and current at one bus can be
expressed as a function of the voltage and current at the next bus. If we let
the equations [1] as
The branch update function [1] is given below as:
Where Wk is a vector containing the real and imaginary parts of the voltages and currents at busk. The function gk is determined by the sub-laterals attached at bus k as well as the models fordistribution lines, switches, transformers, loads, shunt capacitors, and co-generators. From Vk wecan compute the currents injected by the loads, shunt capacitors, and co-generators. Given Ik + 1and the currents Ij injected into sub-laterals branching off from bus k, we apply KCL at bus k tocaculate current [1] given as:-
Where Ak is the set of buses adjacent to bus k on sub-laterals.From the following equation (28), we can solve for the voltage and current at the primary giventhe voltage and current at the secondary [1] as:-
Therefore by solving equation (27) , we get
So that by using this method we get the converged value easily and fast than the other ordinarymethods.
In mathematics, an incidence matrix is a matrix that shows the relationship between two classes of objects. If the first class is X and the second is Y, the matrix has one row for each element of X and one column for each element of Y. The entry in row x and column y is 1 if x and y are related (called incident in this context) and 0 if they are not. There are variations; see below.
Graph theory
Undirected and directed graphs
An undirected graph
In graph theory an undirected graph G has two kinds of incidence matrix: unoriented and oriented. The incidence matrix (or unoriented incidence matrix) of G is a p × q matrix (bij), where p and q are the numbers of vertices and edges respectively, such that bij = 1 if the vertex vi and edge xj are incident and 0 otherwise.
For example the incidence matrix of the undirected graph shown on the right is a matrix consisting of 4 rows (corresponding to the four vertices) and 4 columns (corresponding to the four edges):
The incidence matrix of a directed graph D is a p × q matrix [bij] where p and q are the number of vertices and edges respectively, such that bij = − 1 if the edge xj leaves vertex vi, 1 if it enters vertex vi and 0 otherwise. (Note that many authors use the opposite sign convention.)
An oriented incidence matrix of an undirected graph G is the incidence matrix, in the sense of directed graphs, of any orientation of G. That is, in the column of edge e, there is one +1 in the row corresponding to one vertex of e and one −1 in the row corresponding to the other vertex of e, and all other rows have 0. All oriented incidence matrices of G differ only by negating some set of columns. In many uses, this is an insignificant difference, so one can speak of the oriented incidence matrix, even though that is technically incorrect.
The oriented or unoriented incidence matrix of a graph G is related to the adjacency matrix of its line graph L(G) by the following theorem:
where A(L(G)) is the adjacency matrix of the line graph of G, B(G) is the incidence matrix, and Iq
is the identity matrix of dimension q.
The Kirchhoff matrix is obtained from the oriented incidence matrix M(G) by the formula
M(G)M(G)T.
The integral cycle space of a graph is equal to the null space of its oriented incidence matrix, viewed as a matrix over the integers or real or complex numbers. The binary cycle space is the null space of its oriented or unoriented incidence matrix, viewed as a matrix over the two-element field.
Signed and bidirected graphs
The incidence matrix of a signed graph is a generalization of the oriented incidence matrix. It is the incidence matrix of any bidirected graph that orients the given signed graph. The column of a positive edge has a +1 in the row corresponding to one endpoint and a −1 in the row corresponding to the other endpoint, just like an edge in an ordinary (unsigned) graph. The column of a negative edge has either a +1 or a −1 in both rows. The line graph and Kirchhoff matrix properties generalize to signed graphs.
Multigraphs
The definitions of incidence matrix apply to graphs with loops and multiple edges. The column of an oriented incidence matrix that corresponds to a loop is all zero, unless the graph is signed and the loop is negative; then the column is all zero except for ±2 in the row of its incident vertex.
Hypergraphs
Because the edges of ordinary graphs can only have two vertices (one at each end), the row of an incidence matrix for graphs can only have two non-zero entries. By contrast, a hypergraph can have multiple vertices assigned to one edge; thus, the general case describes a hypergraph.
Incidence structures
The incidence matrix of an incidence structure C is a p × q matrix [bij], where p and q are the number of points and lines respectively, such that bij = 1 if the point pi and line Lj are incident and 0 otherwise. In this case the incidence matrix is also a biadjacency matrix of the Levi graph of the structure. As there is a hypergraph for every Levi graph, and vice-versa, the incidence matrix of an incidence structure describes a hypergraph.
Finite geometries
An important example is a finite geometry. For instance, in a finite plane, X is the set of points and Y is the set of lines. In a finite geometry of higher dimension, X could be the set of points and Y could be the set of subspaces of dimension one less than the dimension of Y; or X could be the set of all subspaces of one dimension d and Y the set of all subspaces of another dimension e.
Block designs
Another example is a block design. Here X is a finite set of "points" and Y is a class of subsets of X, called "blocks", subject to rules that depend on the type of design. The incidence matrix is an important tool in the theory of block designs. For instance, it is used to prove the fundamental theorem of symmetric 2-designs, that the number of blocks equals the number of points.
Create graphs from an incidence matrix
Description
graph.incidence creates a bipartite igraph graph from an incidence matrix.
Usage
graph.incidence(incidence, directed = FALSE, mode = c("all", "out",
"in", "total"), multiple = FALSE, weighted = NULL, add.names = NULL)
Arguments
incidence The input incidence matrix. It can also be a sparse matrix from the Matrix package.
directed Logical scalar, whether to create a directed graph.
mode A character constant, defines the direction of the edges in directed graphs, ignored for undirected graphs. If ‘out’, then edges go from vertices of the first kind (corresponding to rows in the incidence matrix) to vertices of the second kind (columns in the incidence matrix). If ‘in’, then the opposite direction is used. If ‘all’ or ‘total’, then mutual edges are created.
multiple Logical scalar, specifies how to interpret the matrix elements. See details below.
weighted This argument specifies whether to create a weighted graph from the incidence matrix. If it is NULL then an unweighted graph is created and the multiple argument is used to determine the edges of the graph. If it is a character constant then for every non-zero matrix entry an edge is created and the value of the entry is added as an edge attribute named by the weighted argument. If it is TRUE then a weighted graph is created and the name of the edge attribute will be ‘weight’.
add.names A character constant, NA or NULL. graph.incidence can add the row and column names of the incidence matrix as vertex attributes. If this argument is NULL (the default) and the incidence matrix has both row and column names, then these are added as the ‘name’ vertex attribute. If you want a different vertex attribute for this, then give the name of the attributes as a character string. If this argument is NA, then no vertex attributes (other than type) will be added.
Incidence Matrices
by Dr Richard Klitzing
(reproduced with permission)
Taken purely abstract, polytopes are described by their surtopial elements plus the relative incidences. The most basic way to give those incidences are 0-1-matrices with
1 meaning "incident" and 0 "not". But already the easiest polytopes would ask for huge matrices. This is the entrance for symmetry, the symmetry of the polytope itself. Alike surtopial elements now can be classed together via symmetrical equivalence, and the incidence relation will be given for the classes instead. This reduces the size of the matrix considerably. The diagonal elements of these reduced matrices will give the total count of elements of each of the respective equivalence classes. The non-diagonal elements I_(n,m) will provide the numbers of incident surtopes of class m with any of the elements of class n. The subdiagonal parts of the rows thus still describe the surtopial element classes. The superdiagonal parts of the rows describe their environmental aspacts, i.e. vertex figures, edge figures etc.
Regular polytopes are bound to provide a single class of surtopes per dimension, but in general there will be more symmetry-inequivalent elements of the same dimensionality. Therefore it is conveniant to display the dimensional borders as well as a superimposed guiding grid. Vertex-transitivity for instance can be read off from an incidence matrix directly, as those polytopes show up only a single vertex class.
Here as an example the incidence matrix of the truncated cube is given. The Dynkin diagrams of the relative classes are provided in addition in front of the rows.
o3x4x
. . . || 24 | 2 1 | 1 2------++----+-------+----. x . || 2 | 24 * | 1 1. . x || 2 | * 12 | 0 2------++----+-------+----o3x . || 3 | 3 0 | 8 *. x4x || 8 | 4 4 | * 6
This matrix shows that there are 24 vertices, all having the same symmetry (upper-left element). The lowest two rows show that the 2-dimensional elements have 3 or 8 vertices (lower-left block) and therefore are triangles or octagons. The rightmost entries of the first row show further that at each vertex 1 such triangle and 2 octagons are incident. Further there are 2 types of edges, the upper one is incident to 1 triangle and 1 octagon, the other one is incident to 2 octagons only. The middle block of the bottom two rows shows that the triangle will have all edges of the first type clearly, but the octagons do use edges of both types (alternatingly). Altogether there are 8
triangles and 6 octagons (lower-right block).
Two relations on these numbers are generally valid. The one is the equation I_(n,n)*I_(n,m)=I_(m,n)*I_(m,m). This is true whether incident representants of those classes of subpolytopes do exist or not, as in the latter case the corresponding non-diagonal elements are both zero. The other observation is, and this derives right from the diagrammatic representation of the 0-1-matrix, the so called Hasse diagram, that this diagram read top-down instead of bottom-up would describe the dual abstract polytope. The same is even true for the reduced matrices, where the matrix of the dual polytope can be read off by just rotating the matrix half way around an axis orthogonal to the writing plane, thereby interchanging counts of vertices and facets, or dualising the numbers of the vertex figures into those of facets and vice versa. Further-on to each of the subdiagonal parts of the rows, the superdiagonal parts of the rows, and the diagonal itself, the Euler formula might be applied; but appropriate extensions like genus, density etc. would have to be considered.
Note that the same polytope might be a fix-element under different symmetry groups. Thus there could be different (reduced) incidence matrices, all describing the same polytope. Especially the identity map, taken as reducing symmetry, would reproduce the 0-1-matrix. On the other hand incidence matrices just like Hasse diagrams only depend on the structure of the abstract polytope. That is, different isomorph realisations of it would have the same incidence matrix. For instance a convex polygon {n} abstractly can not be distinguished from the polygram {n/d} as long there are no incidences of different types.
POWER QUALITY
The contemporary container crane industry, like many other industry segments, is often
enamored by the bells and whistles, colorful diagnostic displays, high speed performance, and
levels of automation that can be achieved. Although these features and their indirectly related
computer based enhancements are key issues to an efficient terminal operation, we must not
forget the foundation upon which we are building. Power quality is the mortar which bonds the
foundation blocks. Power quality also affects terminal operating economics, crane reliability, our
environment, and initial investment in power distribution systems to support new crane
installations. To quote the utility company newsletter which accompanied the last monthly issue
of my home utility billing: ‘Using electricity wisely is a good environmental and business
practice which saves you money, reduces emissions from generating plants, and conserves our
natural resources.’ As we are all aware, container crane performance requirements continue to
increase at an astounding rate. Next generation container cranes, already in the bidding process,
will require average power demands of 1500 to 2000 kW – almost double the total average
demand three years ago. The rapid increase in power demand levels, an increase in container
crane population, SCR converter crane drive retrofits and the large AC and DC drives needed to
power and control these cranes will increase awareness of the power quality issue in the very
near future.
POWER QUALITY PROBLEMS
For the purpose of this article, we shall define power quality problems as:
‘Any power problem that results in failure or mis operation of customer equipment, manifests
itself as an economic burden to the user, or produces negative impacts on the environment.’
When applied to the container crane industry, the power issues which degrade power quality
include:
• Power Factor
• Harmonic Distortion
• Voltage Transients
• Voltage Sags or Dips
• Voltage Swells
The AC and DC variable speed drives utilized on board container cranes are significant
contributors to total harmonic current and voltage distortion. Whereas SCR phase control creates
the desirable average power factor, DC SCR drives operate at less than this. In addition, line
notching occurs when SCR’s commutate, creating transient peak recovery voltages that can be 3
to 4 times the nominal line voltage depending upon the system impedance and the size of the
drives. The frequency and severity of these power system disturbances varies with the speed of
the drive. Harmonic current injection by AC and DC drives will be highest when the drives are
operating at slow speeds. Power factor will be lowest when DC drives are operating at slow
speeds or during initial acceleration and deceleration periods, increasing to its maximum value
when the SCR’s are phased on to produce rated or base speed. Above base speed, the power
factor essentially remains constant. Unfortunately, container cranes can spend considerable time
at low speeds as the operator attempts to spot and land containers. Poor power factor places a
greater kVA demand burden on the utility or engine-alternator power source. Low power factor
loads can also affect the voltage stability which can ultimately result in detrimental effects on the
life of sensitive electronic equipment or even intermittent malfunction. Voltage transients created
by DC drive SCR line notching, AC drive voltage chopping, and high frequency harmonic
voltages and currents are all significant sources of noise and disturbance to sensitive electronic
equipment
It has been our experience that end users often do not associate power quality problems
with Container cranes, either because they are totally unaware of such issues or there was no
economic Consequence if power quality was not addressed. Before the advent of solid-state
power supplies, Power factor was reasonable, and harmonic current injection was minimal. Not
until the crane Population multiplied, power demands per crane increased, and static power
conversion became the way of life, did power quality issues begin to emerge. Even as harmonic
distortion and power Factor issues surfaced, no one was really prepared. Even today, crane
builders and electrical drive System vendors avoid the issue during competitive bidding for new
cranes. Rather than focus on Awareness and understanding of the potential issues, the power
quality issue is intentionally or unintentionally ignored. Power quality problem solutions are
available. Although the solutions are not free, in most cases, they do represent a good return on
investment. However, if power quality is not specified, it most likely will not be delivered.
Power quality can be improved through:
• Power factor correction,
• Harmonic filtering,
• Special line notch filtering,
• Transient voltage surge suppression,
• Proper earthing systems.
In most cases, the person specifying and/or buying a container crane may not be fully
aware of the potential power quality issues. If this article accomplishes nothing else, we would
hope to provide that awareness.
In many cases, those involved with specification and procurement of container cranes
may not be cognizant of such issues, do not pay the utility billings, or consider it someone else’s
concern. As a result, container crane specifications may not include definitive power quality
criteria such as power factor correction and/or harmonic filtering. Also, many of those
specifications which do require power quality equipment do not properly define the criteria.
Early in the process of preparing the crane specification:
• Consult with the utility company to determine regulatory or contract requirements that must be
satisfied, if any.
• Consult with the electrical drive suppliers and determine the power quality profiles that can be
expected based on the drive sizes and technologies proposed for the specific project.
• Evaluate the economics of power quality correction not only on the present situation, but
consider the impact of future utility deregulation and the future development plans for the
terminal.
THE BENEFITS OF POWER QUALITY
Power quality in the container terminal environment impacts the economics of the terminal
operation, affects reliability of the terminal equipment, and affects other consumers served by the
same utility service. Each of these concerns is explored in the following paragraphs.
1. Economic Impact
The economic impact of power quality is the foremost incentive to container terminal operators.
Economic impact can be significant and manifest itself in several ways:
a. Power Factor Penalties
Many utility companies invoke penalties for low power factor on monthly billings. There
is no industry standard followed by utility companies. Methods of metering and calculating
power factor penalties vary from one utility company to the next. Some utility companies
actually meter kVAR usage and establish a fixed rate times the number of kVAR-hours
consumed. Other utility companies monitor kVAR demands and calculate power factor. If the
power factor falls below a fixed limit value over a demand period, a penalty is billed in the form
of an adjustment to the peak demand charges. A number of utility companies servicing container
terminal equipment do not yet invoke power factor penalties. However, their service contract
with the Port may still require that a minimum power factor over a defined demand period be
met. The utility company may not continuously monitor power factor or kVAR usage and reflect
them in the monthly utility billings; however, they do reserve the right to monitor the Port
service at any time. If the power factor criteria set forth in the service contract are not met, the
user may be penalized, or required to take corrective actions at the user’s expense. One utility
company, which supplies power service to several east coast container terminals in the USA,
does not reflect power factor penalties in their monthly billings, however, their service contract
with the terminal reads as follows:
‘The average power factor under operating conditions of customer’s load at the point
where service is metered shall be not less than 85%. If below 85%, the customer may be required
to furnish, install and maintain at its expense corrective apparatus which will increase the Power
factor of the entire installation to not less than 85%. The customer shall ensure that no excessive
harmonics or transients are introduced on to the [utility] system. This may require special power
conditioning equipment or filters. The IEEE Std. 519-1992 is used as a guide in Determining
appropriate design requirements.’
The Port or terminal operations personnel, who are responsible for maintaining container
cranes, or specifying new container crane equipment, should be aware of these requirements.
Utility deregulation will most likely force utilities to enforce requirements such as the example
above. Terminal operators who do not deal with penalty issues today may be faced with some
rather severe penalties in the future. A sound, future terminal growth plan should include
contingencies for addressing the possible economic impact of utility deregulation.
b. System Losses
Harmonic currents and low power factor created by nonlinear loads, not only result in
possible power factor penalties, but also increase the power losses in the distribution system.
These losses are not visible as a separate item on your monthly utility billing, but you pay for
them each month. Container cranes are significant contributors to harmonic currents and low
power factor. Based on the typical demands of today’s high speed container cranes, correction of
power factor
alone on a typical state of the art quay crane can result in a reduction of system losses that
converts to a 6 to 10% reduction in the monthly utility billing. For most of the larger terminals,
this is a significant annual saving in the cost of operation.
c. Power Service Initial Capital Investments
The power distribution system design and installation for new terminals, as well as
modification of systems for terminal capacity upgrades, involves high cost, specialized, high and
medium voltage equipment. Transformers, switchgear, feeder cables, cable reel trailing cables,
collector bars, etc. must be sized based on the kVA demand. Thus cost of the equipment is
directly related to the total kVA demand. As the relationship above indicates, kVA demand is
inversely proportional to the overall power factor, i.e. a lower power factor demands higher kVA
for the same kW load. Container cranes are one of the most significant users of power in the
terminal. Since container cranes with DC, 6 pulse, SCR drives operate at relatively low power
factor, the total kVA demand is significantly larger than would be the case if power factor
correction equipment were supplied on board each crane or at some common bus location in the
terminal. In the absence of power quality corrective equipment, transformers are larger,
switchgear current ratings must be higher, feeder cable copper sizes are larger, collector system
and cable reel cables must be larger, etc. Consequently, the cost of the initial power distribution
system equipment for a system which does not address power quality will most likely be higher
than the same system which includes power quality equipment.
2. Equipment Reliability
Poor power quality can affect machine or equipment reliability and reduce the life of
components. Harmonics, voltage transients, and voltage system sags and swells are all power
quality problems and are all interdependent. Harmonics affect power factor, voltage transients
can induce harmonics, the same phenomena which create harmonic current injection in DC SCR
variable speed drives are responsible for poor power factor, and dynamically varying power
factor of the same drives can create voltage sags and swells. The effects of harmonic distortion,
harmonic currents, and line notch ringing can be mitigated using specially designed filters.
3. Power System Adequacy
When considering the installation of additional cranes to an existing power distribution system, a
power system analysis should be completed to determine the adequacy of the system to support
additional crane loads. Power quality corrective actions may be dictated due to inadequacy of
existing power distribution systems to which new or relocated cranes are to be connected. In
other words, addition of power quality equipment may render a workable scenario on an existing
power distribution system, which would otherwise be inadequate to support additional cranes
without high risk of problems.
4. Environment
No issue might be as important as the effect of power quality on our environment.
Reduction in system losses and lower demands equate to a reduction in the consumption of our
natural nm resources and reduction in power plant emissions. It is our responsibility as occupants
of this planet to encourage conservation of our natural resources and support measures which
improve our air quality
Rural areas
Rural areas are large and isolated areas of an open country with low population density. The
terms "countryside" and "rural areas" are not synonyms: a "countryside" refers to rural areas that
are open. Forest, wetlands, and other areas with a low population density are not a countryside.
About 91 percent of the rural population now earn salaried incomes, often in urban areas. The 10
percent who still produce resources generate 20 percent of the world’s coal, copper, and oil; 10
percent of its wheat, 20 percent of its meat, and 50 percent of its corn. The efficiency of these
farms is due in large part to the commercialization of the farming industry, and not single family
operations
Voltage control
In those pre-digital MIDI days, synths used a different system to control themselves and each
other. Instead of digital bits and bytes, information was passed between modules through wires
that carried a voltage.
A voltage is just a measure of how much electrical 'push' a circuit has. Plug a voltage source -
like a battery, or synth module - into a circuit and it will push the electricity around so it starts
flowing. The amount of this push - you can think of it as electrical pressure - is measured in units
called Volts.
In an analogue synth, voltages are used to control how much each module does what it´s
designed to do. Turn up the voltages to an amplifier, for example, and the sound gets louder. Do
the same to an oscillator and its pitch pitch goes up. Try it with a filter and the filter opens.
Modulation sources - low-frequency oscillators, ADSRs, and so on - are cunningly designed
boxes that ramp voltages up and down automatically in predicted ways. Without them, You and
Your friends would have to turn knobs and dials by hand every time You hit a note. Most synths
use a standard voltage-control system. This defines a common one-volt-per-octave scale - in
other words, every time the control voltage goes up by one volt, the frequency of an oscillator
doubles. Turn up the voltage by 1/12th of a volt, and the pitch goes up by a semitone.
Consequently, a 4V signal would cause an oscillator to produce a pitch one octave higher than a
3V signal, and that´s the theory of one-volt-per-octave.
One fo the clever -and strange- things about analogue synthesis is that you can interchange
control voltage and audio lines, because there´s no practical difference between the two. An
audio signal is just a voltage that´s wobbling up and down particularly fast, but it´s still basically
a voltage, just like you´ll get from any module.
This means you can use the output of an oscillator to change the pitch of another, or of a filter, or
the gain of a amplifier (vca). This gives you acces to strange and unusual effects that you can´t
create any other way.
Gate and Trigger
The gate signal told the synth that you had pressed a key. This voltage, usually in the 5V-10V
range, would remain constant as long as the key was held. As soon as the key was released, the
gate signal would stop (drop to OV), and the synth's envelope would immediately jump to its
release stage.
The trigger signal also told the synth you were playing a note, but unlike a gate, it was a
momentary (about 5ms) rather than continuous signal, and could not tell the synth to produce a
sound; it worked in conjunction with the gate. The trigger signal's purpose was to start the synth's
envelope generators, thus articulating the attack of the note. Whenever the synth received a new
trigger, the envelope generators would be restarted, and the attack of that new note would be
articulated. Without a trigger signal, a new note would sound using the current state of the
envelope generator - much the same as what hoppens in the 'legato' mode found on modern
synths.
Triggers came in two varieties. The first, used by ARP instruments, was a momentary spike
where the voltage jumped from OV to 10V then back down to OV. The other type, called an S-
trigger (or switch trigger), was used on Moog synthesizers. It consisted of a continuous gate-type
voltage that dropped to OV when a key was pressed. This voltage was used to control a switch
that produced a trigger when it closed in response to the voltage drop.
VOLTAGE REGULATOR PRINCIPLE OF OPERATION
The proposed voltage regulator provides discrete serial voltage compensation. Voltage is
compensated using tapswitching, which combines two regulation principles: voltage
ratio regulation and polarity selection. The voltage regulator has a transformer with two
independent primary windings, which is fed from the network, and a secondary
compensationwinding, which is serial connected. Different compensation values can be obtained
by changing the connections between the primary and secondary windings, using three power
contactors, with four poles each. An automatic controller measures the output voltage and selects
the optimal voltage compensation connection. The characteristics of the proposed design make it
suitable for the needs of rural distribution networks:
—Step voltage regulation: voltage is adjusted within the required quality range, and for
industrial or commercial applications in rural networks there is usually no need for an accurate
voltage regulation based on small steps or continuous regulation.
—Robustness: the voltage regulator will be usually placed outdoors, in dispersed locations, some
of them with difficult access. For greater reliability and easy maintenance, electromechanical
contactors are preferred to power electronics.
—Low cost: using serial voltage compensation instead of a full power converter reduces the
device size and cost, increasing the efficiency notably. Moreover, the higher reliability
reduces the maintenance costs. Section III in this paper presents a cost analysis of the voltage
regulator. The regulator is intended for the use in distribution networks, both indoors or
outdoors; for instance, pole mounted in overhead lines (Fig. 1). The design proposed in this
paper can be used for one-phase and three-phase voltage regulators. This paper is focused on the
one-phase voltage regulator, including a description of its power circuit and the control system.
A. Power Circuit
The power circuit of a one-phase voltage regulator consists of a multiwinding transformer (see
Fig. 2), with two primary windings and a secondary serial compensation winding. In addition,
three power contactors are used to connect the windings and the network. With this design, five
different voltage ratios can be achieved in the transformer by changing the connection between
the windings (see Table I). The selection of the adequate compensation step is performed by a
microprocessor-based control unit. The primary windings of the transformer P1 and P2 have the
same number of turns , and they are connected to the input voltage with contactor C1. The
primary windings can be parallel or series connected using contactor C2, so the effective number
of turns can be or 2 respectively. Each connection determines a certain voltage ratio, as indicated
in Table I. The secondary winding of the transformer S is the serial compensation winding, and
can be series connected with the distribution network or bypassed using contactorC1. Finally, the
polarity of the magnetic coupling is set by contactor C3. The output voltage is then increased or
decreased depending on contactor C3. The output voltage can be formulated as the input voltage
plus the compensation voltage set by the regulator (1). The compensation voltage is given by the
ratio of the secondary winding turns and the primary winding turns , times
the voltage at the input of the device
where:
— is the number of turns of windings P1 or P2;
— is the number of turns of winding S;
— is the connection constant, which can be 1 or 2 for parallel
and series connection respectively of P1 and P2. In addition this value will be positive for direct
winding coupling, and negative for inverse coupling of windings. The proposed design of the
voltage regulator has five different compensation steps can be achieved by changing the position
of the three contactors, as is shown in Table I. The standby mode of the voltage regulator is set
with the three contactors opened, and guarantees that the device is disconnected from the
distribution network. Hence, the secondary winding is short-circuited and input and output
voltages are the same, as . This stand-by mode protects loads connected to the voltage regulator
from any failure of the device. Given the design of the voltage regulator, the rated power of the
transformer is lower than the power that the voltage regulator can supply (2). The power
difference depends on the ratio between the compensation and the rated voltages. For instance,
the transformer of a voltage regulator with 40-V compensation and 230-V rated voltage, will
have a rated power of 17% the maximum load that the voltage regulator can supply
B. Control System
The objective of the voltage regulator is to improve the line voltage whenever it can be achieved,
and to guarantee system security in the event of a failure of the voltage regulator or severe
contingency in the distribution network. In this situation, the device will be automatically
disconnected from the network. For this purpose, the voltage regulator includes a control system
that consists of three modules (Fig. 3).
—The Measure Module registers voltage and current at the voltage regulator output. Every 32
cycles, average RMS voltage values are calculated, and these are used to decide if voltage
compensation is needed. For the sake of security, if the voltage exceeds the security range ( , )
the voltage regulator will be disconnected from the network.
—The Comparison Module compares the average value of a set of output voltage values with the
reference voltage range ( , ), and decides if a new compensation step is required.
—The Compensation Module receives a step-change order from the Comparison Module and
opens or closes the three contactors to achieve a new compensation step in the voltage regulator.
A voltage compensation example is presented in Fig. 4, using a voltage regulator with a 40-V
secondary, and two 230-V primary windings. The reference voltage values, and , are in dash
lines, and the solid lines represent the theoretical relationship between the input and output
voltages for different compensation steps. In the example the initial voltage at the output is 220
V at point A, which is within the reference voltage range. Voltage suddenly decreases to 190 V
(point B). Then, the
voltage regulator corrects the voltage with two maneuvers, using steps 4 and 5 (path B-C-D).
II. DESIGN AND CONSTRUCTION: PRACTICAL CONSIDERATIONS
This section discusses various aspects of the construction of the voltage regulator, given the need to guarantee its correct and safe operation.
A. Transformer Design
A shell-type transformer has been selected for its hardness and its lower operation temperature,
which makes it more appropriate for achieving greater efficiency in the functioning of the
voltage regulator. A software tool has been developed to analyze the costs of different
transformer column designs. This tool analyzes the active material cost and the energy losses
cost of the transformer [11]. The cost analysis for a 230/40-V transformer is shown in Fig. 5,
where different designs for the column length of the magnetic core are presented. The cost
calculation assumes the following prices: the steel lamination cost is Euros 3/kg, the copper cost
is Euros 8.5/kg, and the energy losses for 60 000 hours at Euros 0.1/kWh. Given the results in
Fig. 5, the optimal transformer design has a column length of 93 mm. For this design, total
transformer costs are Euros 588.52, which can be split into Euros 492.62 for energy losses and
Euros 95.9 for active material cost. A detailed electro-magnetic analysis for the selected length
of the 230/40-V transformer has been performed with ANSOFT’s
Maxwell Software [12]. This analysis corresponds to the operation at point D of the example
presented in Fig. 4, and voltages in the windings are shown in Fig. 6. The maximum flux density
distribution in the transformer for the voltage regulator is shown in Fig. 7. We observe that the
transformer has been designed with a low flux density in order to reduce the energy losses in the
steel laminations.
B. Voltage Reference Range Definition
The reference voltage range will be defined usually in accordance with the quality
standards in each country. For instance, if voltage requirements at the load are 230 V
7%, the proposed settings for the voltage regulator will be
, and . However, for certain voltage-sensitive applications, a reduced reference voltage range
may be required. In this case, some adaptations in the design are required to avoid oscillations in
the compensation maneuvers. If the control system step-ups because output voltage is below
the reference , the new compensated voltage
should always be lower than the reference voltage , to avoid oscillations. This constraint can be
formulated for each step c as follows:
The proposed design of the voltage regulator can be then adapted to a small-voltage reference
range. The values can be achieved by selecting the adequate winding turns of the primary
and secondary windings.
B. Compensation Maneuvers Sequence
As defined in the previous section, a compensation maneuver changes the position of the
contactors of the device. The compensation maneuver starts with the disconnection of the
contactor C1. Then, contactors C2 and C3 are opened or closed depending on the compensation
step needed. This operation is performed off-load, which minimizes possible transients and
extends the service life of contactors C2 and C3. And finally, C1 is closed, and a new
compensation step is obtained. Experimental results for the compensation maneuver are shown
in Fig. 8, which shows voltage in the coil of contactor C1, and voltage in the primary winding,.
C. Switching Control
The reliability and expected life of the voltage regulator is mainly determined by the power
contactor C1, which operates on-load. Unfavorable switching conditions will shorten the life
of the contactor, and some malfunctioning can occur when the contactor fails to open (because
contactor contacts are welded) or fails to close (because contacts have lost their conducting
surface). Three improvements have been implemented in the design of the voltage regulator to
enhance the switching maneuver of contactor C1.
—Two poles of contactor C1 are parallel connected to open the load current (see Fig. 1). The
other two poles are series connected to open the rated voltage.
—A capacitor is connected in the primary winding P1.
—The Compensation Module guarantees the switching of contactors to zero current crossing
[13].
D. Efficiency Test
The efficiency of a voltage regulator, whose technical data are enclosed in the Table II, has been
tested. The tests have been carried out with a variable voltage source, in order to emulate the real
operation in a non-constant voltage network. Adjustable loads at power factor 1 and power factor
0.8 have been used to
charge the voltage regulator. The results of the efficiency tests are shown in Table III and Table
IV. Although the voltage regulator has 4.3% total energy losses, its efficiency at any load is
remarkable. Similarly noteworthy is the fact that, due to its 3 000 VA transformer, this voltage
regulator can manage apparent power at the output of up to 16 500 VA. In short, the voltage
regulator design presented in this paper offers low cost and high efficiency.
REFERENCES
[1] “Voltage characteristics of electricity supplied by public distribution networks,” in Proc.
CENELEC, Brussels, Belgium, 2001.
[2] ANSI C84.1 American National Standard for Electric Power Systems and Equipment—
Voltage Ratings (60 Hz) American National Standards Institute, 2006.
[3] R. Malaman, J. Afonso, L. Lo Schiavo, A. Romero, C. Sepulveda, R. Vrolijk, and B.
Wharmby, Quality of Electricity Supply: Initial Benchmarking on Actual Levels, Standards and
Regulatory Strategies Council of European Energy Regulators (CEER), 2001.
[4] RD 1955/2000 de 1 de diciembre por el que se regulan las actividades de transporte,
distribución, comercialización, suministro y procedimientos de autorización de instalaciones de
energía eléctrica B.O.E., Dec. 27, 2000, Spain [Online]. Available: http://www.boe.es
[5] G. Salis and A. Safiagianni, “Economically optimization of a radial power network based on
voltage criterion,” Adv. Eng. Softw., vol. 22, no. 1, pp. 1–20, 1995.
[6] W. E. Kazibwe and M. H. Sendaula, Electric Power Quality Control Techniques. New York:
Van Nostrand Reinhold, 1993.
[7] D. H. Jang and G. H. Choe, “Step-up/down AC voltage regulator using transformer with tap
changer andPWMAC chopper,” IEEE Trans. Ind. Electron., vol. 45, no. 6, pp. 905–911, Dec.
1998.
[8] S. G. Peschel, A. Molden, and O. Tonello, “In-Line Buck/BoostVoltage Regulation Systems
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