GB2531697A - Electrical measurement apparatus and method of measurement - Google Patents

Abstract

An apparatus and method for electrical measurement comprising a meter, and at least one current transformer or transducer (CT). The meter is arranged in use to apply a selected linearizing curve 170 for the CT in accordance with a selected primary current rating 160 for the CT. The apparatus may provide multiple linearizing curves for the CT. The CT may be recognised by the meter by means of a sensing component which may be a sense resistor. The meter may be arranged to provide a user with a range of optional ratings for a recognised CT. The meter may also determine a current 150 in the CT and select a linearizing curve 160, or curves according to the detected current.

Description

Electrical Measurement Apparatus and Method of Measurement The present invention relates to electrical measurement apparatus, and is concerned particularly with current 5 transformers and current transducers, used with a meter, and with methods of using the same.

In commercial premises particularly, the electricity usage of several devices or appliances, hereinafter referred to generally as "loads", is often monitored using separate meters for each load. In such cases, in order to derive valuable data about the energy usage of each load it is necessary to collate metered values manually, and subsequently enter the data manually on a computer for processing.

A previously considered example of electricity meter brings together a fixed number of metering units and combines them in a unitary housing, together with a common visual display 20 and processing means to manipulate and present the data collected by the individual metering units. Signal wires are used to carry the measurement signals from current detectors such as current transducers or transformers (referred to as CTs) located locally at each load. The 25 combined multi-meter load is particularly suited to modern premises in which the electrical supply enters the building at a single location, and is controlled from a single control panel.

Modern electronic electricity meters are designed to measure a variety of load types and sizes.

Current inputs to the meters are standardised to accept a specific signal type and value which represents a larger measured value of current at the load. A range of external current transformers or transducers are used to convert the detected "primary" current into a representative "secondary" signal that may be measured by the metering circuit.

For example a meter may accept a 0.333 Vac signal which represents any nominal primary current determined by the selection of an appropriate external current transducer. Typical external transducers may be of a split, or toroidal type, such as 100 Amp/0.333V or 500 Amp/0.333V.

When a metering system is installed the user must select the most appropriate transformers/transducers for the measured load dependent on the maximum current that the load would draw in normal operation. These devices may be physically located some distance away from the meters themselves. For example the transformers/transducers may be located in a separate switch enclosure or in a different room. Many meters may be installed together and may be connected to different ranges of transformers/transducers.

During commissioning of the metering system the installing engineer must program the individual meters to provide readings that are scaled in proportion to the specific transformers or transducers to which they are respectively connected.

If mistakes are made during installation Or, commissioning these may remain undetected for long periods, and indeed may never be picked up. However, such mistakes can be costly. For example if a 200 Amp transducer is connected to a meter which is programmed to scale for a 150 Amp transducer, when 200 Amps is detected by the transducer the secondary signal will provide 0.333 V to the meter. The meter is scaled to assume that 0.333 V is equivalent to 150 Amps so will display readings which are in error by the ratio 150/200 (i.e. a 25% error). This discrepancy may not be obvious to the meter reader, and the power/energy readings accepted may lead to errors in billing and possibly the taking of inappropriate management decisions based on the erroneous data. Larger errors in scaling may be less likely to escape detection.

Normally a current transformer or current transducer is designed to provide a nominal output signal in proportion Lo a nominal input signal. Typical examples include Current Transformers: 100A: 1A; 200A:5A and Current Transducers: 150A: 0.33V; 400A: 1V. For current transformers this is achieved by applying a fixed number of primary/secondary turns of wire on a core to give the transformation ratio (e.g. 100A:5A may be achieved with one Primary wire and twenty turns of secondary wire).

For transducers the same primary/secondary wire approach is 25 used but a resistor ("burden") is added to convert the secondary current into a secondary voltage.

Figure 1 shows a previously considered specific type of three-phase CT, generally at 10, having three apertures 1030 1, 10-2 and 10-3, each for receiving a cable to be monitored.

Figure 2 shows the schematic circuit of the three-phase CT 10, with three independent transformers CT1, CT2 and CT3, each with a primary winding coil Tl-L1, T2-L2 and T3-L3 and each primary coil being associated with a secondary winding coil (S1-S2).

The particular CT also has an optional sensing resistor RS which enables a metering device to identify the specific CT type.

This particular CT also uses an RJ12 socket S to terminate the secondary coils in a manner convenient for cabling by the user. Other connectors may be used such as screw terminals.

Each CT may also have secondary burdens fitted (transducers). Units may contain a single or a multiple of Current transformers and/or current transducers.

The transformation ratio of a fixed current transformer is linear over a relatively narrow range of input currents. For example a 250A:5A CT will typically be linear over a range of input currents from 300A to 30A. CTs are specified in this way (for example Class 1.0 means accurate to within 1% over a range of 120% to 10% of nominal input current. The customer is therefore required to match the CT to each load he wishes to measure. For a 250A circuit breaker he would choose a 250A CT, for a 50A circuit breaker a 50A CT etc. It is difficult for the installer to pre-empt how many of each CT may be needed for all installations and therefore CTs are specified individually for each job and ordered at the last minute.

Before meters/CTs can be purchased a thorough site-survey is required to determine the range of currents in each load over a period of time. This is expensive, time consuming and may not always be undertaken with the required degree of accuracy.

Embodiments of the present invention aim to provide a current transformer and a method of using the same in which the above problems are addressed.

The present invention is defined in the attached 15 independent claims, to which reference should now be made. Further, preferred features may be found in the sub-claims appended thereto.

According to one aspect of the present invention, there is provided electrical measurement apparatus comprising a meter and at least one current transformer or transducer (CT), wherein the meter is arranged in use to apply a selected linearizing curve for the CT in accordance with a selected primary current rating for the CT.

Preferably the meter is arranged in use to provide multiple linearizing curves for the CT.

The meter may be arranged in use to recognise the CT by 30 means of a sensing component associated with the CT, which may comprise a sense resistor in the CT.

The meter may be arranged in use to provide to a user a range of optional ratings for the recognised CT. The meter may be arranged in use to determine a current in the CT and to select a linearizing curve for the CT according to the detected current.

The apparatus may comprise a plurality of CTs.

In another aspect, the invention provides a method of measuring an electrical parameter, using a meter and at least one current transformer or transducer (CT), wherein the meter applies a selected linearizing curve for the CT in accordance with a selected primary current rating for the CT.

Preferably the method comprises providing multiple linearizing curves for the CT.

The method may comprise recognising the CT by means of a 20 sensing component associated with the CT, which may comprise a sense resistor in the CT.

The method may comprise providing to a user a range of optional ratings for the recognised CT. Preferably the method comprises determining a current in the CT and selecting a linearizing curve for the CT according to the detected current.

The invention may include any combination of the features 30 or limitations referred to herein, except such a combination of features as are mutually exclusive, or mutually inconsistent.

A preferred embodiment of the present invention will now be described. By way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a previously considered three-phase CT; Figure 2 shows the CT of Figure 1 in schematic circuit form; and Figure 3 is a schematic diagram illustrating an auto-ranging process, in accordance with an embodiment of the present invention.

Embodiments of the present invention propose a CT/Meter combination where the CT non-linear characteristics and phase angle are known to the meter beforehand. These are a characteristic of the CT physical design.

The meter provides linearization for the CT characteristics. In accordance with this embodiment of the invention, the meter provides a set of multiple linearizing curves, to be applied in software, dependent on the customer selection of an optimum CT primary current.

In this embodiment, the meter is able to recognise the CT by means of the sensing resistor RS. However, where this feature was not available, the user would need to indicate to the meter which type or range of CT was being used.

The meter firmware then provides the user (via user interface or other communications link -not shown) with a range of optional nominal primary ratings for the particular current transformer or transducer.

The same or separate rating(s) may be applied independently 5 to each CT. The user selection determines the linearizing curve applied in the firmware.

Example:

CT physical design is fixed 250A -0.333V.

The customer is provided, in the meter, with the option to select a preferred nominal CT primary current rating: * Customer selects 250A -Meter linearizes gain and phase to provide optimum accuracy over the range 250A - 25A * Customer selects 200A -Meter linearizes gain and phase to provide optimum accuracy over the range 200A - 20A * Customer selects 150A -Meter linearizes gain and phase to provide optimum accuracy over the range 150A -15A * Customer selects 100A -Meter linearizes gain and phase to provide optimum accuracy over the range 100A - 10A * Customer selects 50A -Meter linearizes gain and phase Lo provide optimum accuracy over the range 50A -5A In this embodiment there is also provided CT primary recognition. In this case the rangeable CT 250/200/150/100/50 is recognised by the identifying component (RS) and the customer is presented with the 30 option to select his preferred CT primary on each installation but without changing the physical CT used in the installation.

There are numerous advantages in using the inventive apparatus and method. For example, in normal use a piece of switchgear is made with everything set to maximum ratings and a circuit breaker fitted to ensure the maximum, future load can be supplied.

The metering equipment, including CTS, is then normally specified to suit this maximum load requirement.

For example if switchgear is supplied for a production line, with an expected load of 60A, the installing engineer would rate the switchgear at 100A to cover future expansion. He would then buy and fit 100A CTs and 100A circuit breakers to cover this requirement.

When the line is actually run it is determined that the actual maximum current is lower. For example, this could be due to loads with different power factors, or overestimating from rating plates on machines. For the purposes of example, let us assume that the actual current is 50A peak.

Using a prior system CTs, there is now a metering system, designed to measure accurately over a range of 10A-100A, but actual currents are only 50A maximum. This reduces the accuracy of the system as the CT has an accurate range of 10% to 100% load.

However, with the auto-ranging apparatus and method of the present invention, the user can modify the CT primary, over the life of the switchgear, to suit the peak primary current actually measured in the system. So, as loads are added/removed, Lhe CT can be "upgraded" from the meter user interface or remotely to suit the actual load.

Figure 3 shows schematically a process for setting a CT range, in accordance with an embodiment of the present 10 invention.

At a first Step 100, a CT is connected to the meter. If it is judged at a Step 120 that the CT is identifiable automatically, then the meter sets the CT type at a step 130. Alternatively, if at Stop 120 it is determined that the CT is not automatically identifiable, then at Step 140 a user sets the CT type in the meter via an interface or else using a communications link.

At Step 150, the user determines the maximum current from stored load survey data. A best fit for the nominal primary is selected from a list of linearising curves and input to Lhe meter via an interface or communications link at Stop 160.

Next, at step 170, the meter applies linearising calibration constants to match the current range. At Step 180 the optimum current range is selected.

In a further embodiment (not shown), since meters measure peak current, the CT primary can optionally be automatically set to track this, thus maintaining optimum accuracy without user intervention.

Whilst endeavouring in the foregoing specification to draw 5 attention to those features of the invention believed to be of particular importance, it should be understood that the applicant claims protection in respect of any patentable feature or combination of features referred to herein, and/or shown in the drawings, whether or not particular 10 emphasis has been placed thereon.

Claims (11)

1. CLAIMS1. According to one aspect of the present invention, there is provided electrical measurement apparatus comprising a meter and at least one current transformer or transducer (CT), wherein the meter is arranged in use to apply a selected linearizing curve for the CT in accordance with a selected primary current rating for the CT.
2. 2. Apparatus according to Claim 1, wherein the meter is arranged in use to provide multiple linearizing curves for the CT.
3. 3. Apparatus according to Claim for Claim 2, wherein the meter is arranged in use to recognise the CT by means of a sensing component associated with the CT.CD--
4. 4. Apparatus according to Claim 3, wherein the sensing component comprises a sense resistor in the CT.
5. 5. Apparatus according to any of the preceding claims, wherein the meter is arranged in use to provide to a user a range of optional ratings for the recognised CT.
6. 6. Apparatus according to any of the preceding claims, wherein the meter is arranged in use to determine a current in the CT and to select a linearizing curve for the CT according to the detected current.
7. 7. Apparatus according to any of the preceding claims, wherein the apparatus comprises a plurality of CTs. 12
8. 8. A method of measuring an electrical parameter, using a meter and at least one current transformer or transducer (CT), wherein the meter applies a selected linearizing curve for the CT in accordance with a selected primary current rating for the CT.
9. 9. A method according to Claim 8, wherein the method comprises providing multiple linearizing curves for the CT.
10. 10. A method according to Claim 8 or 9, wherein the method comprises recognising the CT by means of a sensing component associated with the CT, such as a sense LC) 15 resistor in the CT.
11. 11. A method according to Claim 8 or 9, wherein the method comprises providing to a user a range of optional ratings for the recognised CT. Preferably the method--comprises determining a current in the CT and selecting a linearizing curve for the CT according to the detected current.