Delta MM300 Instruction Manual download pdf (Page 34)

CHAPTER 2: INSTALLATION ELECTRICAL INSTALLATION

MM300 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL 2–15

2.2.3.2 Two CT configuration

Each of the two CTs acts as a current source. The current that comes out of the CT on

phase A flows into the interposing CT on the relay marked CT1. From there, the current

sums with the current that is flowing from the CT on phase C which has just passed

through the interposing CT on the relay marked CT3. This summed current flows through

the interposing CT marked CT2 and from there, the current splits up to return to its

respective source (CT).

Polarity is very important since the value of phase B must be the negative equivalent of

A + C in order for the sum of all the vectors to equate to zero.

Only one ground connection should be made as shown. If two ground connections are

made, a parallel path for current has been created.

In the two CT configuration, the currents will sum vectorially at the common point of the

two CTs. The diagram illustrates the two possible configurations. If one phase is reading

high by a factor of 1.73 on a system that is known to be balanced, simply reverse the

polarity of the leads at one of the two phase CTs (taking care that the CTs are still tied to

ground at some point). Polarity is important.

NOTE

NOTE:

Change CT wiring only if the system is de-energized!

Figure 17: Two CT connection vector diagram

To illustrate the point further, the following diagram shows how the current in phases A

and C sum up to create phase "B".

Figure 18: Two CT connection currents

Once again, if the polarity of one of the phases is out by 180°, the magnitude of the

resulting vector on a balanced system will be out by a factor of 1.73.

On a three-wire supply, this configuration will always work and unbalance will be detected

properly. In the event of a single phase, there will always be a large unbalance present at

the interposing CTs of the relay. If for example phase A was lost, phase A would read zero

while phase B and C would both read the magnitude of phase C. If on the other hand,

phase B was lost, at the supply, phase A would be 180° out-of-phase with phase C and the

vector addition would equal zero at phase B.

853715A1.CDR

1.73

60°

60° 60°

808701A1.CDR

1.73

Two-phase CT currents

Two-phase CT currents,

180° out-of-phase

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Delta MM300 Instruction Manual Page 34