7 practical tips for setting up a good measurement system
1. Start from the need: what do I want to measure?
A single electrical parameter or all electrical parameters
Measurement system: There are several product families on the market: instruments that measure a single electrical parameter (voltage, current, frequency and phase angle cosϕ), typically used in single-phase systems, as on-machine instrumentation, and instruments that measure and display all electrical parameters. for both the single-phase system and the three-phase system.
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This type of multifunction instrument is ideal in panels where space is limited, in substation panels and in industrial main panels.
If not only the electrical parameters, but also the energy consumption are to be monitored, measuring instruments should be chosen that also have active and reactive energy measurement.
2. Selection of the measurement system
Single parameter, multifunction, analog or digital instrument
The instrument should be selected according to the type of distribution system. In a single-phase system, analog and digital instruments are selected to measure voltage, current, frequency and power factor.
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In a three-phase system, instruments can be installed that measure the single electrical parameter, one per phase, or a voltmeter and a current can be installed with the voltage and current switches, allowing the readings to be displayed in sequence. , phase by phase.
Choosing an analog instrument ensures good reading stability, thanks to the mechanical inertia of the stylus and the fact that the reader immediately knows if the instrument is operating normally or if the measurement is out of range.
The analog instrument indicates the point of the measurement scale at which it is located, indicating the upper and lower limits.
Moving coil counters (photo credit: directindustry.com)
This indication is not the case with digital instruments. The only possible reference is to read the displayed value, for example the current. Some gauges have bar indicators that show the current level as a percentage of the set full scale.
Choosing a digital instrument guarantees better readability, even in low light conditions, especially for instruments with LED displays, and an immediate response to measurement variations.
3. System size, choice of CT
The sizing of the measuring system starts with knowing the main parameters of the plant; in particular, the characteristics of the protection switch, the type of distribution system, the rated current, the rated voltage and the busbar type can be known.
Current transformers used in metering equipment for 400A three-phase power supply (photo credit: Wikipedia)
After determining the type of instrument that best suits the needs, the accessories of the measurement system, such as current and voltage transformers, should be carefully selected if the measurement is performed by indirect insertion.
In most cases, if 800 A current is to be measured, the instrument cannot be connected directly to the line. A current transformer suitable for the application must therefore be selected. The parameters chosen for a current transformer are not only the rated current, secondary current and power, but also the type of mounting. Flexible and rigid cables or rods for power transmission can be installed in a power panel.
Transformers can be of different types depending on the mounting system: a continuous cable or a cable with a primary coil, transformers for mounting on horizontal or vertical bars.
4. Wiring and Wiring Diagrams
Connecting analog instruments is very easy; you only need to connect the phase and neutral cables to the instrument. For digital instruments, two auxiliary power cables must always be connected.
Multifunctional instruments can be used in different distribution systems.
Three current transformers are required in three-phase distributed neutral systems. In three-phase systems without distributed neutral in which the loads are balanced and symmetrical, an insertion can be made, i.e. two current transformers can be used instead of three; the instrument calculates by difference the third phase that is not measured directly, and considers it identical to the other two.
With multifunction instruments, not only the cables connected to the measurement, but also the serial RS485 port and the analog and digital inputs and outputs must be wired.
5. Instrument and Ground Protection
To ensure that the instrument is properly protected, fuses should always be installed on the power cables of the digital instruments and on the measurement inputs of the voltmeter.
Grounding of CT secondarys provides grounding if the transformer fails and does not affect the measurement. If there is a large potential difference between neutral and ground, this can negatively affect the measurement, in the case of instruments with non-galvanically isolated measurement inputs.
6. Customization of digital instruments
Before digital instruments start to work, they must be set up with measurement system parameters and communication parameters. The main measurement parameters are the transformation ratios of CTs and VTs, which are defined as the mathematical relationship between the nominal value and the secondary value;
For example, if you set the transform ratio of a CT3/100 scanner with a secondary to 5 A, that means you have kCT = 100 : 5 = 20
7. Troubleshooting during the last test
The main problems encountered during the testing phase may be due to incorrect installation of instruments and accessories. Always check that the wiring corresponds to the user manual.
Errors during installation
The following mistakes are the most common mistakes when installing a measuring instrument:
Reverse CT secondary’s
Reverse the phases of the current and voltage measurement inputs
No elimination of the secondary CT . short circuit
Setting an incorrect transformer ratio.