US 3704380 A
An arrangement which provides for automatically shedding the separable loads of a power distribution area in accordance with predetermined priorities established, not on a basis of system frequency or rate of system frequency change as is present day conventional practice, but on a basis of actual area capability as compared to area load. The total prevailing capability of the power sources of the area is compared with the prevailing area load to produce a signal indicative of the prevailing area reserve power. That signal is compared with the preferred reserve and the difference is then the excess or deficiency with regard to the preferred reserve, defined as the prevailing area margin. That margin in turn is compared with the margin settings for each separable load so that when the margin decreases to a value equal to or below the margin setting for each separable load, that load is automatically sequentially disconnected.
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Description (OCR text may contain errors)
United States Patent Cohn [ 1 Nov. 28, 1972 I LOAD SHEDDING APPARATUS  Inventor: Nathan Cohn, Jenkintown, Pa.
 Assignee: Leeds & Northrup Company,
221 Filed: May 6,1971
21 Appl.No.: 140,774
 US. Cl ..307/52, 307/57 [5i Int. Cl ..l-l02j 3/38  Field of Search ..307/52. 57, 29
[ 56] References Cited UNITED STATES PATENTS 3.27 l ,5 80 9/1966 Pope ..307/29 3,510,637 5/1970 Ross ..307l57 X Primary Examiner.lames D. Trammell AltorneyWilliam G. Miller, Jr. and Raymond F. MacKay ABSTRACT An arrangement which provides for automatically shedding the separable loads of a power distribution area in accordance with predetermined priorities established, not on a basis of system frequency or rate of system frequency change as is present day conventional practice, but on a basis of actual area capability as compared to area load. The total prevailing capability of the power sources of the area is compared with the prevailing area load to produce a signal indicative of the prevailing area reserve power. That signal is compared with the preferred reserve and the difference is then the excess or deficiency with regard to the preferred reserve, defined as the prevailing area margin. That margin in turn is compared with the margin settings for each separable load so that when the margin decreases to a value equal to or below the margin setting for each separable load, that load is automatically sequentially disconnected.
LOAD SHEDDING APPARATUS BACKGROUND OF THE INVENTION This invention relates to apparatus for selectively shedding blocks of load in an area of an interconnected power system in accordance with predetermined area priorities so that the power system can continue in operation when the generating and tie line capabilities of the area are inadequate to meet the normal total connected load of the area.
In power distribution systems, including those which have interconnected areas maintaining generation to match their own load changes and interchanging power with other areas, the procedures which have been used for load shedding have been based upon the system frequency or the rate of change of system frequency. More specifically, load shedding practices have involved the progressive shedding of load in steps as the frequency drops in order to bring the load into balance with generation with the load shedding being initiated at a particular frequency, such as 59 hz. in a 60 hz. system, and with all of the load shedding preferably being accomplished before the frequency reaches 57 hz.
The practice of shedding load on the basis of frequency drop has an inherent limitation in that it does not serve to identify the area in which the reason for the dropping frequency exists and thus all areas shed load, whereas it may only be necessary for one area to shed load. In addition, it will be evident that the shedding of load on the basis of a drop in frequency results in the shedding occurring only after the problem or emergency has already arisen.
Under certain circumstances, as for example when the cause for the frequency drop is in one particular area of an interconnected group, if other areas assist by shedding load, the result can be a higher than permissible tie line flow toward the area in need, causing its ties to trip and depriving the area of all assistance from the remainder of the interconnection.
It is an object of this invention to provide apparatus for shedding of load wherein an individual area of an interconnected power system can identify any imbalance between its load and its capability before the imbalance actually occurs and can therefore shed load in good time to prevent loss of area operability and possible catastrophic conditions for the area and the system.
SUMMARY OF THE INVENTION In carrying out this invention there is provided apparatus for automatically shedding load in a power dis tribution area in which a plurality of sources are connected to supply power to a plurality of loads. This apparatus includes a means operable to produce a signal which is representative of the magnitude of the prevailing reserve power of the area and other means which are operable to disconnect at least one of the loads of the area when the prevailing area reserve power drops to a predetermined value.
DEFINITION OF PREVAILING AREA CAPABILITY When an area is part of an interconnected system, its ability to satisfy its own area load may be defined in various ways, depending on agreed upon relationships with its neighbors. The area may consider that it can count on the full incoming power capacity of its tie lines to augment its own on-line generating capacity. It may count on only a portion of its incoming power capacity as a firm commitment from its neighbors. On the other hand, it may have a firm commitment to supply power to its neighbors when they require it either in the amount of its own tie line capacity or a firm portion thereof, the amount of which in either case it would subtract from its own on-line generating capacity to establish its area capability. Finally, it may choose to define its capability only in terms of its own generating capacity.
Thus the following relation may be written for prevailing area capability, C:
where 26,, is the capacity of the areas n on-line generators, ET, may be defined by the area as either:
a. total incoming capacity of the areas m ties with its neighbors in which case it has a minus sign;
b. something less than incoming capacity of the areas m ties with its neighbors, representing a firm commitment from neighbors, and having a minus sign;
c. total outgoing capacity of its m ties with its neighbors, inwhich case it has a plus sign;
d. a firm power flow commitment outward to its neighbors but less than outward capacity, in which case it also would have a plus sign; or
e. zero, reflecting no firm dependence on, or commitment to tie line flow in calculating area capability, the area capability simply being its own area on-line generating capacity.
The load shedding apparatus described in this invention can readily be accommodated to any one of the foregoing definitions of area capability.
OTHER TERMINOLOGY AND DEFINITIONS Prevailing area load is designated L, and is defined where 26,, is prevailing area generation representing sum of the generation of the n generators in the area,
ET, is prevailing area tie line flow for the m ties to neighboring areas and is plus for outgoing power and minus for incoming power.
Note that in some instances an area may wish to include only incoming tie line power in the calculation of L by the foregoing relationship.
Prevailing area reserve power is designated R, and is defined as:
Preferred area reserve is designated P, and is the reserve level above which the area would prefer that the prevailing area reserve power be.
Prevailing area margin is designated M, and is defined as:
Margin settings, designated M with suitable sequential subscripts, are set to margin levels at which selected loads are to be shed.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing the circuit of the invention in one form.
FIG. 2 shows another modification of a portion of FIG. 1 for taking into account tie line interchange of only one direction in computing total reserve.
Description of the Preferred Embodiments In FIG. 1 there is shown a plurality of generators (3,, G and G which are connected by lines ll, 12 and 13, respectively, to the area bus 16 upon the closing of the respective breakers B B, and B The area which includes the generators 0,, G, and G is shown as being connected to other areas by interconnecting tie lines T, and T which connect to the bus 16 through breakers B" and B12.
The area bus 16 is shown as having individual interconnecting lines 21, 22, 23 and 24 which connect the loads L L L and L respectively, to the area bus 16. The connections between the bus 16 and loads L,, L, and L,, are shown as being made through breakers having contacts 5,, S and S respectively, while L, is shown as a load which is not subject to shedding. The 8,, S and S breakers will normally be located at a point distant from breakers B,B
If all of the generators G,-g;, in the area are being utilized as power sources and power is being imported or exported over both tie lines T and T breakers B,-B and B and 8,, will be closed as well as switches A and D. In determining when load is to be shed, an account will be taken both of the capability of the generators 0, and the capability of the tie lines T, and T, to supply power.
Each of the breakers 8,, 8,, B B, and B is shown as incorporating two contacts. One completes the circuit carrying the power to or from the bus 16 and the other completes a circuit from each of the respective capability setters G G T, and T, to a summing circuit consisting of operational amplifiers 30 and 32.
The operational amplifier 30 is operative to sum the potential established by the capability setters T, and T, while the operational amplifier 32 is effective to sum the output of amplifier 30, connected through closed switch A to one of the input lines of amplifier 32, and the outputs of the other capability setters G,- G,,'. As a result of this summation, there is produced as an output from amplifier 32 on line 34 a signal C representative of the prevailing capability of the power sources of the area shown. Thus,
where T, and T are plus for firm commitments of power out of the area, minus for firm power commitments into the area, and each is zero if tie line capacity is not to enter into the capability computation.
With switch D closed as well as switch A there is provided by another summing circuit a signal L on line 36 representing the total actual power at bus 16, which is the prevailing area load. This signal is provided by summing the output of each of the generators 6,, G, and G and power flow measured on lines T and T,. The tie line power flow measurements are made by utilizing transducers 41 and 42 which are designed to produce on the respective lines 43 and 44 DC. potentials of magnitude and polarity indicative of the power flow in the respective tie lines T. and T,. The lines 43 and 44 are input lines to operational amplifier 48 which suns the inputs so as to produce on line 50 a signal representative of the total tie line power flow into or out of the area. With switch D closed, the potential on line 50 provides an input to operational amplifier 54 which also receives inputs from transducers 61, 62 and 63. These transducers may be any one of a number of available watt transducers capable of providing a DC. output potential proportional to the power flow on the connected power line. Thus,
where T, and T are plus for outgoing power and minus for incoming power.
The operational amplifier 66 receives its inputs from lines 34 and 36 with these lines being connected so as to provide a comparison between the signals C and L on those lines or, in other words, a subtraction of those signals to produce on the output line 68 of amplifier 66 a signal representative of the available reserve power R of the area. Thus:
The signal R would then represent the total prevailing reserve of the area. This computed total prevailing reserve may be compared to a preferred reserve P by utilizing amplifier 70 to compare the signal on line 68 with the signal on line 72. The signal on line 72 is derived from a manual setter, identified as block 74, which produces on its output line a signal representative of the preferred reserve P of the system. That signal will be provided as an input on line 72 when switch 76 is closed. Thus, amplifier 70 provides a comparison of the signal on line 72 with that on line 68 so as to provide a subtraction to produce on line 78 a signal M which represents the margin for the system in terms of power. Thus:
The signal M can be utilized as a basis for determining when the various loads should be shed in order to maintain the desired relationship between area capability and area load. The value of P may in some cases remain fixed. in such a case the value of M can be said to be representative of the prevailing reserve as is the signal R.
The criteria which establish the basis for the shedding of the several loads are the magnitudes of the signals produced by the margin setters M,', M, and M, which, respectively, set the available margin below which the respective loads L L, and L should be shed, and the arrangement of the sequence contacts S, and S, which determine the sequence in which the load shedding taka place. The signals generated by the margin setters M,', M, and M, may be referred to as margin setter signals.
Thus, for example, the output of setter M, on line 81 is a margin setter signal of magnitude M, which is compared with the magnitude of signal M in the comparing circuit represented by block 82, and if M is equal to or less than M,', there is then produced a potential on line 84 which is effective to energize actuator 85 to open breaker contact S, and thereby disconnect or shed load L,. Simultaneously with the opening of S, the contact S, is closed so that the load which is next in the desired sequence for shedding can be shed under appropriate conditions. The contacts S, and S, will remain in those positions due to the action of the mechanical latch 86 shown associated with the actuator 85. The other actuators 90 and 94 are similarly equipped to be latched. As a result of the shedding of load L,, there will be an increase in the magnitude of the signal M by an amount equal to that of load L The output of setter M, on line 88 is compared by the comparing circuit of block 87 with the magnitude of signal M and when M is equal to or less than M,', the output on line 89 through closed contact 8 causes an energization of actuator 90 to disconnect and latch breaker contact S and connect contact S and thereby shed load L and set up load L as the next load to be dropped in the established sequence. After load L is shed the magnitude of M will rise by an amount equal to L The signal on line 91 from setter M is compared with the magnitude of signal M in the comparison circuit 92. When M is equal to or less than M actuator 94 is energized by the output of block 92 which is connected through contact S Breaker contact 8,, is disconnected and latched upon energization of actuator 92 and load L is shed. M will then increase by the magnitude L The signals M M and M produced on lines 81, 88 and 91, respectively, may be established so that the loads L L and L are shed at different values of M. Alternatively, M,', M and M;,' can all be set at the same value since S, and S, are effective to establish the sequence for the shedding. Alternatively, M and M could be omitted and actuator 85 could be coupled to operate a sequential switch which could disconnect the loads sequentially based in each case on the comparison of M, and M.
The comparing circuits, such as 82, may incorporate a time delay so that the associated load, such as L will not be shed until the tie lines to the area have had a chance to respond. Such an arrangement would be used when the area is depending on the ties to provide help in meeting the area load requirement.
Under some circumstances it may be desirable to open switches A and D and thereby omit from the operation of the load shedding system shown in FIG. 1 any consideration of the power flow on the tie lines T, and T As an example of one illustration of the manner in which the circuit of FIG. 1 operates, consider the sequence of events detailed in the following paragraphs.
Assume the signal on line 36 represents a prevailing area load of 1,050 MW, or in other words, a total supply of power from generators 0,, G and G, and tie lines T, and T to the bus 16 of 1,050 MW for the supply of loads L,L Also, assume that the signal on line 34 represents a prevailing area capability of 1,200 MW representing the maximum power which can be supplied by the generators and ties to the bus 16 for the loads L,L Under those conditions the prevailing area reserve signal on line 68 would represent 150 MW, and if we assume that the preferred reserve represented by the signal on line 72 with switch 76 closed represents 50 MW, then the signal on line 78 would be a positive signal representing 100 MW which is the prevailing area margin or the increase in the loads L -L. that can occur before the margin is reduced to zero. Thus, the prevailing area load can be increased l00 MW to equal the prevailing area capability minus the established preferred reserve before the margin becomes zero.
Assuming that the loads L,, L,, L and L. are increasing in magnitude and assuming that the settings M,, M, and M, are respectively 0 MW, l0 MW and l 5 MW, then it will be evident that when the signal on line 36 has reached a value representative of a load of l, l 50 MW, the signal on line 68 will represent a prevailing reserve of 50 MW while the prevailing margin represented by the signal on line 78 will be zero. Load L will then be disconnected. If we assume that the load L at the time of disconnection represented 30 MW, there will, as a result of the normal load-frequency control in the area, be a compensating change in the generation of the generators G -G and the signal on line 36 will then represent 1,120 MW so that the signal on line 68 represents MW and the signal on line 78 represents a 30 MW margin.
If after the disconnection of load L, the loads L L and L increase so that the signal on line 36 represents a prevailing area 1 equal to 1,160 MW, then the load, L, will be disconnected. Assuming that in disconnecting load L, the only loads left are loads L and L and that that prevailing load is of the magnitude of 1,100 MW, then as a result of the load-frequency control on the generators G G and G the signal on line 36 will represent 1,100 MW and the signal on line 68 will then represent l00 MW while the signal on line 78 will represent +50 MW. After the disconnections of loads L,, L, there must be an increase in the loads L and L of 65 MW to a value of 1,165 MW total before the limit M is reached and the load L is disconnected since at that value for the sum of L and L, the signal on line 78 will have reached a value representative of l 5 MW.
The above example of the manner in which the circuit of FIG. 1 operates may be further illustrated by the In still other circumstances it might be desirable to omit the inclusion of the signal on line 72, representative of the preferred reserve, in which case the switch 76 would be opened and the signal on line 78 would then be equivalent to the signal on line 68 and the loads would be shed in accordance with the established priorities based upon the prevailing area reserve established by the signal on line 68. The computation of that reserve would, of course, be independent of tie line capability and tie line power flow when the switches A and D are open. It would be independent of tie line capability but would include tie line flow in the computation of area load when switch A is open and switch Dis closed.
In the circuit of FIG. 1 there is shown a means for automatically disconnecting or shedding load in a particular area, it being assumed that the reconnection of the load may be advantageously accomplished by manual means as the generating capability of the area increases or the load decreases to the point where additional load can be added in the area.
Under certain circumstances it is desirable to take into account the power flow over the tie lines T, and T, only when it is in a direction providing power flow to the bus 16. FIG. 2 shows a circuit diagram in block form for accomplishing that mode of operation. The circuit of HO. 2 differs from that shown in FIG. 1 only in that it incorporates diodes 110 and 112, each of which is shown with its cathode connected to ground and its anode connected to the output of the associated transducers 41 and 42, so as to prevent input signals from appearing on lines 43 and 44 whenever the outputs of the transducers 41 and 42, respectively, are positive in potential, as when power is flowing out of the area over tie lines T and T The diodes 110 and l 12 still allow the negative potential outputs on lines 43 and 44 to be summed by the amplifier 48. Thus, an area utilizing the arrangement of FIG. 2 would not take into consideration any power flow from the area over tie lines T, and T in calculating the magnitude of prevailing area load, L.
What is claimed is:
1. Apparatus for automatically shedding load in a power distribution area in which a plurality of sources are connected to supply power to a plurality of loads comprising a first means operable to produce a signal representative of the magnitude of the prevailing reserve power of the area, and
a second means operable to disconnect at least one of the loads of said area when said prevailing reserve power signal drops to a predetermined value.
2. Apparatus as set forth in claim 1 in which said first means includes means responsive to the power flow to or from the sources of the area for producing a first signal representative of the prevailing area load,
means for producing a second signal representative of the total prevailing capability of said sources,
means for comparing said first and second signals to produce said signal representative of the prevailing reserve power.
3. Apparatus for automatically shedding load in a power distribution area in which a plurality of sources are connected to supply power to a plurality of loads, comprising:
means operable to produce a signal representative of the magnitude of the prevailing reserve power of the area. including;
l. means responsive to the power flow to or from the sources of the area for producing a signal representative of the prevailing area load, including;
a means for producing first signals respectively representative of the power flow from each of the generated sources connected to supply power to said area,
b. means for producing second signals respectively representative of power flow on each of the tie lines connected to supply power to or from said area, and
c. means for summing said first and second signals to produce said prevailing area load signal;
2. means for producing a signal representative of the total prevailing capability of said sources, including;
a. means for producing third signals respectively representative of the capability of each of said generating sources,
b. means for producing fourth signals respectively representative of the capacity of each of said tie lines, and
c. means for summing said third and fourth signals to produce said total prevailing capa bility signal; and
3. means for comparing said prevailing area load signal and said total prevailing capability signal to produce said signal representative of the prevailing reserve power; and
means operable to disconnect at least one of the loads of said area when said prevailing reserve power signal drops to a predetermined value.
4. Apparatus as set forth in claim 1 in which said second means includes means for producing for each portion of the load to be shed a margin setter signal representing the magnitude of prevailing reserve power at which the associated load should be shed,
means for individually comparing each of said margin setter signals and said signal representing the prevailing reserve power, and
means responsive to said last named means for disconnecting each of said loads when the signal representing the prevailing reserve power equals the associated margin setter signal.
5. Apparatus as set forth in claim 1 in which said first means includes means responsive to the power flow from said sources for producing a first signal representative of the total power flow to the loads of said area, means for producing a second signal representative of the total capability of said sources,
means for producing a third signal representative of the preferred reserve power, and
means for comparing the difference between the values of said first and second signals and the value of the third signal to produce a resultant signal representative of the prevailingarea margin.
6. In a power distribution area having a plurality of sources providing power to a network to which a plurality of loads are connected, apparatus for selectively shedding said loads in accordance with pre-established priorities comprising transducer means associated with each of said sources for producing signals representative of the power being provided to said network from each of said sources,
capability setter means each associated with one of said sources for producing capability signals each representative of the power producing capability of the associated source,
means responsive to said power signals and said capability signals for producing a signal representative of the prevailing reserve power of said sources,
means for producing a signal representative of the preferred reserve capability of said area,
means for comparing the prevailing reserve power signal and the preferred reserve capability signal to produce a signal representative of the prevailing margin of the area,
means for producing for each of the loads a margin setter signal representing the magnitude of the prevailing margin of the area at which the associated load is shed,
comparison means for comparing each of said margin setter signals to the prevailing margin signal and operative when said last named signal decreases to the value of a margin setter signal to disconnect the associated load from the network when that load is next in priority to thereby avoid loading said area beyond the prevailing area margin while maintaining priorities in the shedding of said loads.
7. Apparatus as set forth in claim 6 which includes means operable to prevent the transducer means associated with those sources which are tie lines from producing signals which are of sense representative of power flow from said area over said tie lines.
* III l