Patent Application
20200003811
The present invention generally concerns the monitoring of electrical distribution networks. More specifically, the invention is directed to a method for determining mutual voltage sensitivity coefficients between several measuring nodes in an electric power network without the knowledge of the network parameters. Once the values of the voltage sensitivity coefficients have been determined, the availability of this data can be made of use for operating the electric power network in a manner that is technically secure and economical. In particular, the availability of the voltage sensitivity coefficients can be made of use for voltage control.
The continuously increasing connection of highly intermittent distributed generation in electric power grids, essentially composed of renewable energy resources, leads to violations of operational constraints and calls for the development of dedicated monitoring and control mechanisms. In particular, voltage control is one of the typical controls expected to be deployed in distribution systems.
Schemes for controlling the voltage in grids (or microgrids) have already been proposed. Patent document U.S. Pat. No. 7,687,937 in particular discloses exemplary implementations of what is known as a droop-control method for voltage control. Droop-control methods involve local sensing of the voltage at the connection point between the grid and controllable resources, and the adjustment of the reactive and/or active power injection of the various resources according to a specific voltage droop characteristic. Such methods rely on locally available data and do not take into account grid topology and parameters. This is problematic as droop-control methods tend to lead to suboptimal solutions and, in some cases, to non-feasible operating conditions or even system collapse.
A known alternative approach for controlling the voltage in grids (or microgrids) is described for example in patent document WO2015/193199. This document teaches to apply offline power flow analysis in order to model relationships between, on the one hand, values of voltage, active power and reactive power at each generator in a power distribution network, and on the other hand, remote system conditions of the network. This modeling is done by first calculating sensitivity coefficients on the basis of prior knowledge of both the network's topology (e.g. total number of nodes) and the network parameters (i.e. series conductance and susceptance of the branches, shunt conductance and susceptance of the nodes). In a second step, a nonlinear regression technique is applied to the calculated sensitivity coefficients in order to derive the above-mentioned relationships. The modeled relationships can then be used for controlling the voltage of the distribution networks.
According to this second approach, controlling the voltage of a distribution network involves defining explicit set-points for active and reactive nodal power injections, in such a way as to directly control the grid. These power set points are typically computed as a solution to an online optimization problem in order to guarantee optimal operation of the grid. Such an approach has the advantage of guarantying some level of optimality compared to droop-control methods. However, accuracy of the method depends on having access to an accurate and up-to-date model of the network. Such an up-to-date model is not always available. In particular, in the case of low voltage networks, the networks' topology tends to be adapted quite frequently. Furthermore, feeder parameters, and the status of breakers for instance, can possibly change without this information reaching the distribution network operator (DNO) or distribution system operator (DSO) in due time.
It is therefore an object of the present invention to alleviate the above-mentioned problems in the prior art by providing a method for the calculation of the voltage sensitivity coefficients of a power distribution network, according to which knowledge of the electrical parameters of the network is not required. The invention achieves this object and others by providing a method for determining mutual voltage sensitivity coefficients between several measuring nodes in an electric power network according to the annexed claim 1.
The expression “downstream” should be understood as meaning further away from the connection to the main grid.
If the amount of power injected or consumed at a particular place in an electric power network changes, it causes the voltages at every location in the network to change as well. However, the change in power affects some nodes in the network more than others. The method of the invention allows for computing a matrix of the mutual voltage sensitivity coefficients between a plurality of measuring nodes in an electric power network, without requiring the knowledge of the network parameters (for example: series conductance and susceptance of the branches, shunt conductance and susceptance of the nodes, etc.). Knowledge of these voltage sensitivity coefficients allows in turn predicting the voltage change, at any particular node, when the amount of power injected or consumed at the same node or at any other particular node changes. This knowledge can then be used, for instance, for determining explicit power set-points or voltage-droop characteristics of controllable resources connected to the network.
Implementation of the invention requires the availability of a basic monitoring infrastructure comprising metering units, a metering unit being arranged at each one of the measuring nodes. The metering units are each arranged for measuring, locally, a voltage, a current and a phase difference between the voltage and the current. The monitoring infrastructure further comprises a communication network, to which the metering units are connected so as to allow for the transmission of data acquired by the metering units to a processing unit. One should note however that, in spite of the need for a monitoring infrastructure, implementation of the invention does not require highly synchronized measurements. This means, in particular, that the metering units do not need to be expensive Phasor Measurement Units (PMUs) having a permanent link to a common time reference, but can be conventional metering devices.
Other features and advantages of the present invention will appear upon reading the following description, given solely by way of non-limiting example, and made with reference to the annexed drawings, in which:
The subject matter of the present invention is a method for determining mutual voltage sensitivity coefficients between several measuring nodes in an electric power network. As the field, to which the invention applies, is that of electric power networks, an exemplary network will first be described. Actual ways in which the method can operate will be explained afterward.
The substation transformer is connected to network 1 through a circuit breaker 9 and a first bus N1. In the network of the illustrated example, several feeder lines branch out from the bus N1. One of these feeder lines (referenced L1) is arranged to link a subset of five residential blocks and one agricultural building to the low-voltage network. It should be understood that the remaining 52 residential blocks and 8 agricultural buildings can be linked to the bus N1 by other feeder lines that are not explicitly shown in
The feeder line L1 connects the bus N1 to a second bus (referenced N2). As can be seen in
Still referring to
One can observe that, according to the present example, the photovoltaic power plants G1 and G2 provide a maximum power of 226 kVA.
Besides an electric power network, the physical environment within which the method of the invention is implemented must also comprise a monitoring infrastructure. According to the invention, the monitoring infrastructure comprises metering units provided at a selection of nodes of the network (in the following text, nodes of the network that are equipped with at least one metering unit are called “measuring nodes”). As previously mentioned, the exemplary low voltage electric power network 1 illustrated in
According to the invention, the monitoring infrastructure further comprises a communication network, to which the metering units are connected so as to allow for the transmission of data to and from a processing unit 7. In the very schematic illustration of
According to the presently described implementation of the invention, the different metering units in the network are synchronized by means of the Network Time Protocol (NTP) via the GSM network that serves as the communication network for the monitoring infrastructure. Advantages of NTP are that it is easy to implement and readily available almost everywhere. A known disadvantage of NTP is that it is not extremely precise. However, contrarily to what might be expected, experience shows that the synchronization provided by NTP is good enough for the method of the invention to produce satisfactory results. It should be understood however that NTP is not the only synchronization method usable with the method of the invention. In particular, according to a considerably costlier implementation, the metering units could be PMUs having a permanent link to a common time reference or a GPS synchronization.
As previously mentioned, the active power and of the reactive power at each measuring node are computed from local values of the measured voltage and current, and from the phase difference between the voltage and the current. This computation can be based on the following relations:
P=V·I·cos φ and Q=V·I·sin φ
According to the presently described first exemplary implementation, computation of the values of the active and reactive powers is implemented locally by each one of the metering units. According to a first variant of the first implementation, a buffer is provided in each metering unit, and the successive time-stamped measurements are saved in a buffer at least until the end of a time window τ during which the voltage, the current and the phase difference between the voltage and the current are measured. Computation of the active and reactive powers is then done on the entire batch of saved measurements once a day for example. As the values obtained by the metering units for the voltage, the current, and the phase difference between the voltage and the current are time-stamped, the subsequently computed values for the active and the reactive powers can “inherit” the timestamps associated with the data from which these values were computed. According to an alternative variant of the first embodiment, computing of the active and the reactive power takes place online in real time, and the obtained values for the voltage and for the active and reactive powers are time-stamped and then saved temporarily in the previously mentioned buffer provided in each metering unit.
It should further be understood that, according to other implementations of the invention, computation of all the values of the active and reactive powers takes place in the processing unit. According to a particular one of these other implementations, the monitoring infrastructure comprises a high speed communication network, and the measurements from all the measuring nodes are transmitted to the processing unit as soon as they are obtained. In this way, the processing unit can compute the values of the active and reactive powers in real time.
According to the invention, measurements of the voltage, the current and the phase difference between the voltage and the current, that take place at different measuring nodes are synchronized to the extent discussed above. According to the present example, the metering units measure the voltage, the current and the phase difference repeatedly, preferably at regular intervals, within a given time window. The number of successive measurements is preferably comprised between 200 and 5000 measurements, preferably between 1000 and 3000 measurements, for instance 2000 measurements. It should be understood however that the optimal number of measurements tends to increase as a function of the number of measuring nodes. On the other hand, the optimal number of measurements tends to decrease with improving accuracy of the measurements provided by the metering units, as well as with improving accuracy of the synchronization between the metering units.
As the values measured by the metering units are not instantaneous values, but average values measured over at least half period of the AC power, the minimal time interval between successive measurements should be equal to several periods of the AC power. Actually, according to the first exemplary implementation, the length for the time intervals separating successive measurements is preferably between 60 ms and 3 seconds, and most favorably between 60 ms and 1 second.
The second box (referenced 02) in the flow chart of
According to the first exemplary implementation of the invention, in order to accomplish the above computation, the processing unit first accesses the communication network and downloads the timestamped values for the voltage {tilde over (V)}(t), the active power {tilde over (P)}(t), and the reactive power {tilde over (Q)}(t) from the buffers of the different metering units. The processing unit then computes variations of the measured voltage and of the active and the reactive powers by subtracting from each downloaded value of the voltage, of the active power and of the reactive power respectively, the value of the same variable carrying the immediately preceding timestamp. One should keep in mind in particular that the times t ϵ {t1, . . . , tm} refer to timestamps provided by different metering units. As, for example, P1(t1) and PN(t1) were computed from measurements out of different metering units, and that according to the first exemplary implementation their respective clocks were synchronized using NTP, measurements at time t should therefore be understood as meaning measurements at time t±a standard NTP synchronization error.
The processing unit then associates each variation of the voltage at one particular measuring node Δ{tilde over (V)}i(t) with the variations of the active power Δ{tilde over (P)}j(t) and of the reactive power Δ{tilde over (Q)}j(t) at all measuring nodes (where j ϵ {1, . . . , N}, specifies the j-th measuring node) at the same measuring time (where t ϵ {t1, . . . , tm}, stands for a particular measuring time or timestamp). As exemplified by table V (next page), the result can be represented as a set of N tables each containing the variations of the voltage at one particular measuring node i in relation to concomitant variations of the active and reactive powers at all measuring nodes 1 to N. The timestamps {t1, . . . , tm} correspond to the successive measurement times. These measurement times cover a given time window τ=[t1, tm]. According to the invention, m>2N, and preferably m»N.
The third box (referenced 03) in the flow chart of
To sum up, multiple parametric regression analysis allows predicting the values of the voltage variation ΔVi(t), for t ϵ {t1, . . . , tm} and i ϵ {1, . . . , N}, as a function of the concomitant variations of the active power {Δ{tilde over (P)}1(t), . . . , Δ{tilde over (P)}N(t)} and of the reactive power {Δ{tilde over (Q)}1(t), . . . , Δ{tilde over (Q)}N(t)} at all measuring nodes.
Due to the statistical nature of the method, individual measured values tend to deviate to some extent from their predicted value. Accordingly, each measured voltage variation equals the corresponding predicted voltage variation plus/minus an error term. That is:
Δ{tilde over (V)}l(t)=ΔVi(t)+ωi(t), where ωi(t) is the error term.
According to the invention, the multiple parametric regression analysis takes negative first-order autocorrelation into account. This means that the multiple parametric regression analysis assumes that a substantial negative correlation exists between the errors ωi(t) and ωi(t+Δt), where t and t+Δt are two consecutive time-steps. In the present description, the expression a “substantial correlation” is intended to mean a correlation, the magnitude of which is at least 0.3, is preferably at least 0.4, and is approximately equal 0.5 in the most favored case.
According to preferred implementations of the invention, the multiple parametric regression analysis further assumes that no substantial correlation exists between the errors from two non-consecutive time-steps. The expression “no substantial correlation” is intended to mean a correlation, the magnitude of which is less than 0.3, preferably less than 0.2, and approximately equal to 0.0 in the most favored case. Accordingly, the correlation between the errors in two non-consecutive time steps is contained in the interval between −0.3 and 0.3, preferably in the interval between −0.2 and 0.2, and it is approximately equal to 0.0 in the most favored case. As the number of successive measurements is m, there are m−1 error terms ωi(t) for each measuring node, and therefore (m−1)×(m−1) error correlation terms.
K
PQi=(Δ({tilde over (P)}, {tilde over (Q)})TΣi−1Δ({tilde over (P)}, {tilde over (Q)}))−1Δ({tilde over (P)}, {tilde over (Q)})TΣi−1Δ{tilde over (V)}i
where Σi is the correlation matrix of the errors.
The error correlation matrices Σi are preferably not preloaded into the processing unit, but created only once the tables of voltage vs. power variations have been created (box 02). Indeed, the size of the (m−1) by (m−1) error correlation matrices is determined by the length m−1 of the voltage vs. power variations tables. Accordingly, the variant of
In the present example, as is the case with any correlation matrix, the entries in the main diagonal of each one of the N (m−1) by (m−1) correlation matrices are all chosen equal to 1. According to the invention, the entries in both the first diagonal below, and the first diagonal above this, are all comprised between −0.7 and −0.3, and finally all other entries are comprised between −0.3 and 0.3. In the present particular example, the correlation coefficients of the errors between two non-consecutive time-steps are equal to zero, and the correlation coefficients of the errors between two consecutive time-steps are assumed to be −0.5. In this case the error correlation matrices correspond to the tridiagonal matrix shown next page:
In the field of electric power networks, the condition in which a portion of the utility grid (in the illustrated example, network 1) becomes temporarily isolated from the main grid but remains energized by its own distributed generation resources (in the illustrated example, G1, G2, G3 and 15) is known as “islanding operation”. Islanding may occur accidentally or deliberately. Intentional islanding operation may be desired in cases where the central grid is prone to reliability problems. In this case, the interconnection is designed to permit the particular portion of the grid to continue operating autonomously and provide uninterrupted service to local customers during outages on the main grid. Usually, protective devices must be reconfigured automatically when transitioning between islanded and grid-connected modes.
Referring again to
In the following discussion, the level of the voltage that the substation transformer would output if it was an ideal transformer, having zero impedance, is referred to as the “slack voltage” of the transformer. It should be understood that the slack voltage of the transformer is “pegged” to the voltage supplied to the substation transformer by the medium-voltage network 3, or in other words that, in the case of an ideal transformer, the ratio of the output voltage over the input voltage is constant. Again referring to
Vslack(t)=|
Comparing the flowchart of
In a similar manner to what was previously described in relation to
According to the illustrated example, a set of measurements is considered suitable for the statistical estimation of the voltage sensitivity coefficients if the amount of variation of the active power {tilde over (P)}i(t) and of the reactive power {tilde over (Q)}i(t) at each measuring node i (for i ϵ {1, . . . , N}) is large enough. This condition ensures that the voltage sensitivity coefficients can be properly estimated using the measurements. According to the described implementation, the amounts of variation of the active and of the reactive power are assessed by computing the standard deviations of the timestamped active power values {tilde over (P)}i(t) and reactive power values {tilde over (Q)}i(t) for timestamps t ϵ {t1, . . . , tm} originating from a predefined time window. For instance, the measurements can be considered suitable if the standard deviations of {tilde over (P)}i(t) and {tilde over (Q)}i(t) are larger than a predefined threshold value. The predefined threshold value can be selected, for example, to be equal to 10% of the absolute value of the corresponding mean, and preferably to be equal to 20% of the absolute value of the corresponding mean.
Comparing the flowchart of
Although the method of the invention has been illustrated and described in greater detail by means of exemplary implementations, the invention is not restricted by the disclosed examples and various alterations and/or improvements could be derived therefrom by a person skilled in the art without departing from the scope of the present invention defined by the annexed claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 16166721.7 | Apr 2016 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2017/052116 | 4/12/2017 | WO | 00 |
