IES60396B2 - A.C. mains monitoring circuit - Google Patents

Description

A.C. Mains Monitoring circuit This invention relates to a circuit for monitoring a three-phase A.C. mains.

According to the present invention there is provided a mains monitoring circuit for a three-phase A.C. mains, including a current transformer having at least one primary winding and a secondary winding, the primary winding(s) being connected to the three phases of the mains such that sero current is induced into the secondary winding in the absence of a mains fault whereas a non-zero current is induced into the secondary when there is a mains fault, the circuit further including means to detect a current induced into the secondary winding.

In a preferred embodiment of the invention a single primary winding is connected between mains neutral and a common connection to the three phases. However, it is possible to provide a respective primary winding connected between mains neutral and each of the three phases.

Embodiments of the invention will now be . described, by way of example, with reference to the accompanying drawings wherein Figure 1 is a circuit diagram of a first embodiment and Figure 2 is a circuit diagram of a second embodiment.

Referring to Figure 1, the circuit comprises a current transformer 10 having a core 12 and two windings T1 and T2. T2 is a primary winding and T1 is a secondary winding. One end of T2 is connected to mains neutral N. The other end of T2 is connected to each ox three mains phases Pl, P2 and P3 via three equal-valued resistors Rl, R2 and R3 respectively. winding T1 is connected to a monitoring means 14 responsive to a current Ir flowing in Tl.

Each of the three phases of the mains Pl, P2 and P3 will normally be at the same potential, and since Rl = R2 = R3, resultant currents II, 12 and 13 of equal magnitude will flow through the respective resistors Rl, R2 and R3.

The voltages on each of the phases, normally of equal amplitude, are phase shifted by 120 degrees with respect to each other, with the result that the vector sum of the three equal amplitude currents II, 12 and 13 will be zero at the common connection point of Rl, R2 and R3, such that no net current will flow to the neutral point through the winding T2.

! If one or two of the three mains phases Pl to P3 are disconnected, their respective currents will cease to flow to the common junction of Rl, R2 and R3, such that the vector sum of the three currents II to 13 will now be greater than zero, and a net current IN will flow through T2 to neutral- Current IN will induce a current Ir into winding Tl, which will be detected by the monitoring means 14.

Thus the mains monitoring circuit can detect the absence of one or two of the three phases in a three phase installation.

If the three mains phases Pl to P3 are not of equal voltage amplitude, their respective currents will not be of equal amplitude, and. therefore the vector sum of the three currents at th® common junction of Rl, R2 and R3 will be greater than zero, which will cause a net current IN to flow to neutral- Current IN will induce a current Ir into winding Tl, which will be detected by the monitoring means 14Thus the mains monitoring circuit can detect the difference in amplitudes between three phases of a three phase installation.

If the three phases are phase shifted by an angle other than 120 degrees the vector sum of their respective currents II, 12 and 13 will be greater than zero at the common junction of Rl, R2 and R3, such that a net current, IN will flow to neutral. Current IN will induce a current Ir into winding Tl, which will be detected by the monitoring means 14.

Thus the mains monitoring circuit can detect a > difference in the phase angle between the three phases if it is not 120 degrees. r Normally closed switches, SI, S2 and S3 are connected in series with each phase Pl to P3 as shown. Because these switches are normally closed, they will not affect the detection of missing phase, phase voltage differences, or phase angle differences.. By opening one or two of the switches, the integrity of the other phases and their associated components and windings T1 and T2 and the neutral connection N and the monitoring means 14 can be checked to confirm correct operation.

Thus the mains monitoring circuit incorporates a test circuit to check the integrity of the circuit and its associated components.

The monitoring means 14 detects the presence of 20 current Ir induced into winding Tl, and is responsive to this currentCurrent Ir can be monitored and measured by the monitoring means 14, which can produce an output signal equal or proportional to the magnitude of Ir, thereby providing a means of determining the magnitude of the non-zero vector sum current which caused Ir to be induced. Thus the mains monitoring circuit can be used to measure the magnitude of an out of balance current which causes Ir to be generated, such out of balance , current being due to any of the aforementioned reasons, or due to an additional current conductor being passed ) through the transformer core 12 for monitoring purposes.

Th® induced current Ir monitored end measured by the monitoring means 14 can be compered to an internal reference, and if above a predetermined level, an output signal can be generated to activate an alarm and/or a tripping mechanism to cause disconnection of a circuit connected to the mains.

Turning now to Figure 2, components which are the same as in Figure 1 have the same references. The difference in Figure 2 is that rather than having a single primary winding T2 connected in common to the three mains phases Pl to P3, as in Figure 1, each phase is connected to mains neutral N through a respective primary winding T21, T22 or T23, each primary winding having the same number of turns.

However, the operation of the circuit is basically not changed. In Figure 1, the vector sum of the three currents II to 13 is formed prior to the transformer 10 at the junction of the three resistors R1 to R3, and then applied to the single primary T2. In Figure 2 the vector sum of II to 13 is formed at th® transformer 10 itself. In either case the secondary winding T1 responds to the vector sum of II to 13.

The Figure 2 embodiment may be modified to make the values of the resistors R1 to R3 unequal, provided that the number of turns in each winding T21 to T23 is correspondingly adjusted so that the number of ampere turns in series with each phase Pl to P3 remains equal. In this case, in the absence of a mains fault, the currents II to 13 will not all be equal, and hence their vector sum will not be zero. However, the combined affect of II to 13 at the secondary winding T1 will be , zero, i.e. the vector sum of the currents induced into the secondary winding T1 by these primary currents II to 1 13 will be zero.

F.R. KELLY 5 CO., Agents for the Applicants

Claims (4)

CLAIMS:

1. A mains monitoring circuit for a three-phase mains, including a current transformer having at least one 5 primary winding and & secondary winding, the primary winding(s) being connected to the three phases of the mains such that zero current is induced into the secondary winding in the absence of a mains fault whereas a non-zero current is induced into the secondary 10 when there is a mains fault, the circuit further including means to detect a current induced into the secondary winding»

2. A circuit as claimed in Claim 1 wherein a single 15 primary winding is connected between mains neutral and a common connection to the three phases.

3. A circuit as claimed in Claim 1 wherein a respective primary winding is connected between mains neutral and 20 each of the three phases..

4. A circuit substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings..