US 6,154,709 AGranted

Method for determining the load capacity of an AC network

AssigneeDaimlerChrysler AG

InventorsKai Mossig, Peter Terwiesch

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Abstract

In low-power AC networks, it is desirable when a load is connected, for example an arc furnace or a vehicle, to know the load capacity of the AC network in order to avoid undesirable drops in the network voltage U.sub.N. To this end, load voltage signals which are proportional to the amplitude of a load voltage U.sub.L are detected at the point of the load L at a 1st measurement time which can be predetermined and at a 2nd measurement time which can be predetermined and is later. At the same time, load current signals are detected which are proportional to the amplitude of a load current I.sub.L. Furthermore, a phase difference .DELTA..phi. is determined, which is obtained with reference to the phase angle of the network voltage U.sub.N at the 1st measurement time. These measurements are used as the bases to calculate a magnitude of the network impedance of the AC network and the network phase angle .phi..sub.N between the network voltage U.sub.N and the network current. A maximum possible power P.sub.L of the load L can be calculated in accordance with P.sub.L =U.sub.L.sup.2 .multidot.cos .phi..sub.N /Z.sub.N, where Z.sub.N =network impedance.

A method for determining the load capacity of an AC network at the point of an electrical load (L),

a) which is fed from the AC network,
b) wherein at a 1st measurement time t.sub.A which can be predetermined and
c) wherein at a 2nd measurement time t.sub.B which can be predetermined and is later
d) wherein load voltage signals U.sub.LA and U.sub.LB which are proportional to the amplitude of a load voltage (U.sub.L) are detected,
e) wherein, at the same time and furthermore, load current signals I.sub.LA and I.sub.LB which are proportional to the amplitude of a load current (I.sub.L) are detected, the indices A and B relating to detection at the 1st measurement time t.sub.A and at the 2nd measurement time t.sub.B, respectively,
f) wherein, at the 2nd measurement time t.sub.B, a phase difference .DELTA..phi. is detected from a signal which is proportional to the network voltage (U.sub.N), which phase difference .DELTA..phi. is obtained with reference to the phase angle of the network voltage (U.sub.N) at the 1st measurement time t.sub.A,
g) wherein a magnitude of the network impedance Z.sub.N of the AC network is formed at least approximately in accordance with:
.vertline.Z.sub.N .vertline.={[(U.sub.LA -U.sub.LB .multidot.cos .DELTA..phi.).sup.2 +(U.sub.LB .multidot.sin .DELTA..phi.).sup.2 ]/[(I.sub.LA -I.sub.LB .multidot.cos .DELTA..phi.).sup.2 +(I.sub.LB .multidot.sin .DELTA..phi.).sup.2 ]}.sup.0.5
or
h)
.vertline.Z.sub.N .vertline.=.vertline.-[(x.sub.A -x.sub.B)+j.multidot.(y.sub.A -y.sub.B)/(p.sub.A -p.sub.B +j.multidot.(q.sub.A -q.sub.B)].vertline.
and
i) wherein a network phase angle .phi..sub.N between the network voltage (U.sub.N) and the network current (I.sub.N) is formed at least approximately in accordance with:
##EQU2## or j)
.phi..sub.N =arctan (w/v)
where
v=-[(p.sub.A -p.sub.B).multidot.(x.sub.A -x.sub.B)+(q.sub.A -q.sub.B).multidot.(y.sub.A -y.sub.B)]/[(i p.sub.A -p.sub.B).sup.2 +(q.sub.A -q.sub.B)],
w=[(q.sub.A -q.sub.B).multidot.(x.sub.A -x.sub.B)-(p.sub.A -p.sub.B).multidot.(y.sub.A -y.sub.B)]/[(p.sub.A -p.sub.B).sup.2 +(q.sub.A -q.sub.B).sup.2 ],
U.sub.LA =x.sub.A +j.multidot.y.sub.A,
U.sub.LB =x.sub.B +j.multidot.y.sub.B, I.sub.LA =p.sub.A +j.multidot.q.sub.A
and
I.sub.LB =p.sub.B +j.multidot.q.sub.B.

19 more elements

Abstract

Claims4