|Title of Invention||
A METHOD AND A DEVICE FOR SELECTING AND DIMENSIONING MEASURES IN CASE OF INSTABILITY IN AN ELECTRICAL POWER SYSTEM
|Abstract||The invention relates to a method for determining/dimensioning measures for restoring an electrical power system, which experiences or is heading for a voltage instability, to a steady-state condition, wherein there is no immediate risk of instability by determining the actual voltage/phase angle in the electrical power system, determining the power unbalance within at least one sub-area in the electrical power system, determining suitable power-balancing measures, dimensioning the extent of the respective measure, and - carrying out the power-balancing measures. The invention also relates to a device for carrying out the method.|
|Full Text||Technical field
The present invention relates to a method for protecting, an electrical power system which is in an unstable condition and which is heading for voltage collapse, as well as for selecting and dimensioning those measures that need to be raken for r.he electrical power system -o attain voltage recovery and return to a stable condition.
The measures that may need to be taken according to the method are controlled disconnection of certain load from the electrical power system or connection of additional power to the electrical power system, or a combination of these measures.
Ihe invention also relates to a device for carrying out the method according to the invention, wherein the device comprises electronic measuring and control equipment intended to be used in an electrical power system.
Background of the invention and prior art
It is already known to plan, dimension and design an electrical pov?er system for normal as well as disturbed operation by using dimensioning criteria, set in advance, for various system quantities, such as operating voltage and mains frequency.
The vcltags levels of the electrical power system at different network points :r nodes constitute a measure of the conditicn cf the electrical power system, that is, the ability of the system to continue to supply the loads cf connected areas with the desired power. During normal opera-zLzzi, rhe cperatinr voltage at a network point or a node is zo lie wLzsiLr. a pre-alloved interval, normally within a few
percent of the stated nominal voltage. Examples of normally occurring nominal voltages are 400 kV, 130 kV and 50 kV. The construction, function, operation, automatic and protective equipment of an electrical power system are described, as far as Swedish conditions are concerned, in our Swedish patent SE 0101061-0 entitled "an electric power plant with means for damping power oscillations".
In case of disturbances in the electrical power system, preferably when the transmission capacity is weakened or decreasing, but also in the event of loss of electricity production plants or loss of other components with a voltage-control ling function, and combinations of these, the voltage distribution in the electrical power system is changed in such a way that the voltage level across the loads drops, which leads to a drop also of the voltage level in the transmission network. The corresponding phenomena may also arise in case of too fast a load increase.
Common in both cases is that the electrical power system is not capable, in the steady state, to supply the load connected to the network. Unless measures are taken, the different component-protective functions of the electrical power system, which are adapted to react on changes in voltages and/or currents, will successively disconnect components in the electrical power system. Normally, disconnection of transmission lines is the first thing that happens. Discon-ii.ecr.ion of components may occur over time, from a few seconds up to several hours,
A typical scenario is that the voltages et the measuring points cf the iinpedance-ifteasuring line protection device decrease and the currents increase, whereby the most sensitive nrsasuring zone cf the protection device disconnects the line in question. The reiaaining lines, in the network t-hus weakened, are then further loaded, whereby more transmission lines are disconnected. This proceeds until the remaining electrical power network is able ro maintain a con-
stant electricity operation and achieve a balance between production, transmission capacity and load.
Examples of events such as those mentioned above are the major power failures in the USA/Canada, Sweden and Italy in 2003. Scenarios such as those described above are, of course, desirable to avoid and there are a plurality of methods to detect incipient voltage instability in an electrical power system. In such systems, voltages and reactive power flows are measured, and devices for limiting the current on generators for detecting voltage instability have also been used, see, for example, Ingelsson, Karlsson, Lindstrom, Runvik, Sj5din: "Special Protection Scheme against Voltage Collapse in the South Part of the Swedish Grid" , CIGRE Conference r Paris, 1995 .
Still more advanced detectors, with or without possibilities of communication, have been presented and tested. One such example is the so-called VIP algorithm (Voltage Instability Predictor) described in Begovic, Milosevic, Novosel: WA Novel Method for Voltage Instability Protection", Proceedings of the 35th Hawaii International Conference on System Sciences, 2 002. This system compares the impedance at a certain network point, in the direction of the load, with the impedance in a direction towards the production source. The relation between the load impedance, thus measured, and uhe source impedance may then be used as a measure for defecting incipient voltage instability,
i One common method, which is efficient in this context, for
mitigating or preventing voltage instability, after such instability has been detected, is to disconnect parts of the load in the network. 3y disconnecting a load corresponding to some ten per cent of the total load in the inflicted : area, the remaining load may often be adequately supplied. By intentionally and in a controlled manner disconnecting some load objects in a stressed, opera-ing situation., at least the operation of the major part cf the inflicted area is saved, v;hile at the saute time rhe transmission network is
3) How much load should be disconnected?
'he voltage level is normally a sufficiently good criterion :or determining where che load disconnection should take place.
Disconnection may take place " without unnecessary delay", that is, as quickly as possible after a margin has been allowed for reserve -discsraiection of a shunt fault, -hat is, short circuits and ground contacts. The speed should also be adapted to the speed of the on-load tap changer control of the power transformers connected in the vicinity thereof. The time delay should normally be a few seconds.
However, the extent of the load disconnection is more diffi-cult to determine. One method is to proceed by trial and error by disconnecting one or more loads, load areas, at a time. This is a relatively slow method since the system response to each disconnection must be awaited.
Another method is to use a comfortable margin and disconnect a larger number of loads. The disadvantage of such a method is that an unnecessarily large part of the load is disconnected and also that this involves a risk of obtaining high voltages in the electrical povrer system.
At present, there is no known method of dimensioning or determining the magnitude of a necessary load disconnection, when the need of such a measure has been detected. The fulfilled criteria normally result directly in load disconnection via a preselected circuit-breaker function with a suitable time delay, typically a few seconds.
US 6,219,551, wVclt.age instability predictor (VI?} - method and system f~r performing adaptive control to improve voltage stability Lr. power sys terras", shows a method for detecting ~haz zis rower system, or parts thereof, is/are heading fcr instability. However, the method only mentions the need of load disconnection in general, as a suitable measure fcr
how this threateningInstability is to be cancelled or mitigated.
The invention according to US 6,242,719; 'Applications and methods for voltage instability predictor (VIP)", is a further development of the invention according to US 6,219,591, which, among other things, relates to masked transmission networks. However, none of these publications suggests any measure of magnitude of that quantity cf lead disconnection that is required for stabilizing the system.
Several patent specifications deal with the VI? algorithm, which suggest stability measures of various kinds and predict an imrrdnent voltage collapse, but do not suggest the extent of or magnitude of the load disconnection that is necessary to prevent a power failure or a collapse in the electrical power network.
Objects and most important characteristics of the invention
The object of the present invention is to solve the above-mentioned problems and suggest a method for determining the extent of the load that needs to be disconnected from the electrical power network or the power that needs to be supplied to the electrical power network for the purpose of achieving voltage recovery in the electrical power network and to cause the system to resume a stable state.
The above object and further objects are achieved according to the invention by a method according to claim 1, by a device according to claim 22, and by a computer program according to claim 25.
Brief description of the accompanying drawings
The invention will be described in greater detail below with reference to the accompanying figures.
Figure 1 shows, in principle, an electrical power syster..
Figure 2 schematically shows an electrical power system comprising a transformer station, loads, switching means and a unit according to the invention.
Figure 3 is a simple flow diagram of the method according to the invention.
Figure 4 shows a priority table of various loads included in the electrical power system vrriich may be disconnected, and their mutual order.
Figure 5 shows a concrete arithmetical example of a load disconnection.
Description of preferred embodiments of the invention
In the description, the same designations are used for the quantities that occur in lines, protective devices and loads, as for the measured values and signals/calculated values which correspond to these quantities and which are supplied to and treated in the protective device.
Figure 1 shows a single-line diagram of an electrical power system 1 coitprising an equivalent generator G which delivers the voltage Us to the electrical power system and which has a source impedance Zs, as viewed from a busbar 3. The busbar 3 functions as a nodal point in the electrical power network and collects and distributes the electric power flow.
The load Z is here divided into one part which is to remain ■undisturbed, z1 and one part which is to be disconnected, 2LS, in case of voltage instability. Both that part of the load Z15 which is to be disconnected and that part cf the load Z-, which is to continue to be connected are normally constituted by a plurality of line bays with associated circuit breakers C3a, b.
A calculation of the volcage U
equations (I) and (2) below, where I is the current into the busbar 3 under consideration from source G, and U is the voltage prevailing on the busbar 3 under consideration:
If UT designates the desired voltage, after load disconnection, an eruaticn (3 5 for voltage division is ob-ained as follows:
Equation (3) gives zL. Thereafter, zLS may be calculated with the aid of equation (2), and thereafter the corresponding values of active and reactive power, Pls and QLs, respectively.
As soon as the required amount of load disconnection has heen determined, a trigger impulse is sent to the switching member (s) CBa,b which best correspond(s) to the calculated required load disconnection. In this context, different disconnection costs or mutual priorities for different types of loads may also be taken into consideration. Information about the power consumption of different loads may be estimated with differing degrees of accuracy, from fixed values, which, for exainple, are updated in view of the season, to real-time measurements. Then, loads are chosen from a priority table established in advance; see Figure 4, which, shows the priority of the loads and the power levels occurring.
^ne value of nay alternatively be regarded as known, or be calculated regularly, fcr example every 5 minute, whereupon the calculation of the change of the source impedance Zs during a vol-age instability scenario becomes trivial.
The source impedance 2S is calculated based on changes in voltage and current during, for example, tap-changer control or connection with shunt components. The source impedance Zs may also be estimated by measuring current and voltage at different load levels. If the above-mentioned continuous updating of the magnitude of the source impedance for some reason should be considered too complex, the source impedance Zs may also be assigned a definite value. In transformer stations from transmission level (or subtransmission level) to medium-voltage level, knowledge of the size and short-circuit impedance of the transformers is often insufficient, to estimate the source ixapedance with an accuracy of ±25 %, since the short-circuit power of uhe transformer in such cases is predominant and the short-circuit power of the rest of the network has a minor influence.
Another way of determining the source impedance Zs is to utilize negative-sequence quantities for current and voltage as well as assuming that the impedance of the source in relation to positive- and negative-sequence currents, respectively, is the same. In connection with the development of the VIP algorithm, methods have been obtained for determining the source impedance Zs.
The method according to the invention may also be used step by step, for exarple if the voltage did not turn out to be the expected/calculated one at the nodal point in question after the first disconnection phase or if the impedance in supply networks increases and Tine voltage thus decreases at the monitored point.
According tc the invention, this is achieved by detent:-ing the quantity cf the load, or that part of the load, which is to be disconnected on the basis of measured actual voltage and desired vclzage, EZ zhe node after the load disconnection.
With knsvleige cf measured and desired voltage, or voltage recovery, as well as actual load and source impedance Z£ - or
a corresponding quantity such as, for example, source admittance - the required load disconnection, for achieving the desired voltage in the node again, may be obtained by a normal circuit calculation.
Voltage recovery may, for example,1 be the difference between voltage before load disconnection, that is, the "measured* voltage, and voltage after load disconnection, that is, the * desired* voltage, but it: rnay also be defined as a percentage of the measured voltage.
The load disconnection takes place by circuit-breaker tripping or by means of other switching members. It is also feasible to use a centralised telecontrol system which, via a communication channel, orders disconnection of certain individual load objects, for exairrple hot-water heaters, air-conditioning plants etc. Such ordering may also take place via a "price change", wherein certain loads, via the communication system, react automatically on a price change.
In a preferred embodiment of the invention a table is used, comprising all switching members which are included in the load-disconnection system in question, which may comprise a larger geographical area or a single transformer station. The switching members are listed in the table in the order in which they are to be disconnected from the electrical power network in a position where instability has been detected. The order of priority is based, for exaircple, on the cost of disconnecting the respective load.
The table also contains information about the prevailing load power for che respective switching member, that is, how much power is disconnected during a switching-off operation vith the respective svizching member. In this way, the automatic control system runs through the list/the table to identify and operate these switching members, taken by order cf priority, which restore trie iesired voltage in the node and the electrical power system with a minimum. cf costs/drawbacks.
The table is regularly updated, for example where necessary, manually with regard to the order of priority, and automatically and in real time with regard to the load per switching member.
The lines in the priority table in Figure 4 are sorted by order of priority according to column 2. From the dimensioning algorithm, the rnagnitude of the load that needs to be disconnected is obtained. In the table, one or more switching members are selected in the order of priority, until the necessary amount of disconnected load has been achieved. Within the last selected group, the switching members may be selected so as to correspond to the necessary load in the best way.
Figure 5 shows a concrete numerical example. The necessary
load disconnection is here calculated to be 6 MW at cos In another preferred embodiment of the invention, the extent of the necessary power addition is dimensioned in the same way as for load disconnection, for example via a dc connection, in order co obtain, based on a certain actual measured voltage, a certain desired voltage in the node in question after the power addition, with knowledge of the source impedance in the node.
The method according to whe invention may, at least partly, be carried out with the aid cf program codes that are run in a processor or in a cxcacer, and these program codes may be stored on a computer-readable medium such as a hard disk, ~ diskette, a C3-ROX, or a=y other volatile memory.
Although the invention sieve has been described by means of & few different erbodinenrs, the invention is not limited
thereto but other embodiments and variants thereof are, of course, feasible within the scope of protection of the -claims. Thus, it is feasible for a device according to the invention to trigger an alarm signal in case of instability in the system, based on a suitable criterion, and for the load disconnection thereafter to be executed manually-
1. A method for determining/dimensioning measures for resto
ring an electrical power system, which experiences or is
heading for a voltage collapse, to a normal condition,
- determining the actual voltage/phase angle in the
electrical power system,
- determining the power rebalance within at least one
sub-area in the electrical power system,
- determining SUITAable power-balancing measures,
- dimensioning the extent of the respective measure, and
- carrying out the power-balancing measures.
2. A method according to claim 1, characterized in that
the determination of the actual voltage/phase angle is per
formed by measuring in at least one node in the sub-area.
3. A method according to claim 1 or 2, characterized in that
the determination cf the actual voltage/phase angle in the
electrical power system is performed by measuring in at least
one node and by calculation..
4. A method according to claim 1, 2 or 3, characterised in
that the power unbalance is determined based on the actual
voltage/phase angle and the desired voltage/phase angle.
5. A method according to one or more cf the preceding claims,
characterized in that the power unbalance is determined
starting from a circuit calculation based on the actual and
the desired voltage/phase angle.
5. A method according to claim 4, characterized in that. the power unbalance is aetenrdiied starting from a comparison of the acrual voltage, the voltage drop across a magnitude related to t±_e source impedance, and the equivalent voltage of the source.
7. A method acccriing to claim 6, characterized in that
the magnitude related to the source irapedance is source impedance, source admittance, short-circuit power or short-circuit current.
8. A method according to one or more of the preceding claims,
characterized by disconnection of a load corresponding to the
determined power unbalance, such that the voltage/phase angle
returns to the desired/predetermined level.
9. A method according to one or more of the preceding claims,
characterized in that power, corresponding to the determined
power unbalance, is supplied to the electrical power system
such that the voltage/phase angle returns to the desired/pre
10. A method according to one or more of the preceding
claims, characterized in that power, corresponding to the
determined power unbalance, is redistributed within the
electrical power system by controlling reactive power
resources such that the voltage/phase angle returns to the
11. A method according to one or more of the preceding
claims, characterized in that power, corresponding to the
determined power -unbalance, is redistributed within the
electrical power system by controlling dc connections such
that the voltage/phase angle returns to the desired level.
12. A method according to one or more of the preceding
claims, characterized in that the power unbalance is
determined based on a simultaneous comparison of the actual
phase angle and the desired phase angle arid cf the actual
voltage and the desired vcltage.
13. A method according to one or raore of the preceding
claims, characterized in that determination/dimensioning of
measures is based on the isarnitude cf the defected power
■unbalance and that possible power-balancing means in the area.
14. A method according to one or more of the preceding
claims, characterized in that addition of power to the
electrical power system and disconnection of loads from the
electrical power system are combined such that the power-
balancing measures together correspond to the determined
15. A method according to one or mere of the preceding
claims, characterized in that -disconnection of loads is
performed in a predetermined order of priority.
15. A method according to one or more of the preceding claims, characterized in that the order of priority is stated in a table.
17. A method according to claim 16, characterized in that
the table contains information about which switching members are available within the area.
IS. A method according to claims 16 and 17, characterised in that the table contains information about what power change is caused by activation of the respective switching members.
19 A method according to one or more of the preceding claims, characterized in that, based on the information in, the table, a required number of switching members is selected so that -he necessary power change is achieved.
20. A method according to one or more of the preceding
claims, characterized in that the table is regularly updated.
21. A method according to one cr mere of the preceding
claims, characterised in that the load disconnection is
22. A method according to one or iriere of the preceding
claims, characterized in -hat the load disconnection is
23. A device for determining /dimensioning measures for resto
ring an electrical power system, which experiences or is
heading for a voltage collapse, to a normal condition,
characterized in that
- means are arranged for determining the actual
voltage/phase angle in the electrical power system,
- means are arranged for determining the power unbalance
within at least one sub-area in the electrical power
- means are arranged for determining suitable power-
- means are arranged for dimensioning the extent of the
respective measure, and that
- means are arranged such that the selected measures can
enable the electrical power system to be restored to a
24. A device according to claim 23, characterized in that
means are arranged to determine the actual power unbalance
starting from a circuit calculation based on the actual
voltage/phase angle and the desired voltage/phase angle.
25. A computer program for carrying out the method steps
according to one or more of claims 1-22.
26. A computer-readable medium containing at least parts of a computer program according to claim 25.
27. A computer program according to claim 25 which is at least partly transferred via a network such as, for example, the Internet:.
|Indian Patent Application Number||2378/CHENP/2006|
|PG Journal Number||26/2010|
|Date of Filing||29-Jun-2006|
|Name of Patentee||ABB AB|
|Applicant Address||Kopparbergsvagen 2, S-721 83 Vasteras|
|PCT International Classification Number||H02J3/12|
|PCT International Application Number||PCT/SE2004/02002|
|PCT International Filing date||2004-12-23|