This question comes always up together with stationary meter testing in laboratories. Isolation Current Transformers (ICT) are needed to test more than one direct connected energy meter on a test bench with multiple positions, assuming that the I-P links can not be opened. If you can’t find the links (red arrows) on the meter terminal block you need to use ICTs for testing. And why?
All electronic meters have a power supply, linked between phases and neutral. This power supplies have a consumption (see e.g. IEC62053-21, #7.7.1). According to the Kirchhoff’s Circuit Laws a fraction of the test current will be used by the power supply. This leads to a current-drop on the next test position and to an increasing error from position to position.
The smaller the test current, the higher is the impact on error measurement. And how does an ICT overcome this problem?
An ICT is principally a transformer with a 1:1 ratio. You have a primary side (where the source is injecting the current) and a secondary side with the connections to the meter. The test voltage is individually provided to each meter on the secondary side of the ICT. So all meters get the same test current. See for example our ICT CL2030 with advanced additional features like protection and remote access by PC-software. What about single phase meters?
Closed link single phase meters can be tested with ICTs. For single-phase test benches a Multi Secondary Voltage Transformer (MSVT) can be used. With a MSVT the test voltage is made galvanic free, while an ICT makes the currents galvanic free. Conclusion
To test single-phase meters with closed links you need to have a testbench with MSVTs.
To test three-phase meters with closed links you need to have a testbench with ICTs.
For testing of transformer operated meters (CT, CT/VT) we recommend a direct connection to the test bench. This meters have the current- and voltage circuits separated internally.
Modern reference standards and calibration devices have usually a graphical function to show the relationship between the voltages and the currents. Most equipment manufacturers call this function vector diagram or vectorial diagram.
In fact it is a phasor diagram. It represents a the phase relations of a sinusoidal rotating system at a certain time.
The system rotation (everything inside the circle) is anti-clockwise. The graph is shown either with
current phase L1 in zero degrees position
voltage phase L1 in 90° position
Actually the vector diagrams in test equipments are showing only the angles and not the amplitude of the phases. The reason behind is that you won’t see very small current vectors with the given resolution. Anyway, we can nicely read all amplitude values from the instrument. Common practice is to show the voltages with higher amplitude than the currents (voltages are on the outer circle).
Main use for vector diagrams is to check the proper connection of the instrument before you make error measurements.
If you see e.g. that the current of a phase is in opposite to the voltage, it is likely possible that the current clamp is connected in the wrong direction.
The simulation below is kept very simple. You can set the phase angles between I and U, the phase sequence and the reference for the system. The power values are calculated based on your settings.
With a right-click on desktop PCs you can save your drawing(s).
Together with tender documents and meter specifications you will find various subscripts related to current.
current I without subscript This is the actual current flowing trough the energy meter.
starting current Ist This is the lowest value of current at which the meter should register electrical energy at unity power factor and, for poly-phase meters, with balanced load.
minimum current Imin This is the lowest value of current at which the meter is specified to meet the accuracy requirements.
transitional current Itr At this value of current and above the meter must to lie within the smallest maximum permissible error corresponding to the accuracy class of the meter.
basic current Ib This term is used in IEC standards for direct connected meters. All accuracy values are related to Ib
nominal current In This is the same like Ib but for transformer operated meters
reference current Iref The term is only used in EN 50470-1. It is the reference current (for direct connected meters Iref = 10 x Itr = Ib according to EN 62052-11, 188.8.131.52; for CT-connected meters Iref = 20 x Itr = In
maximum current Imax This is the highest value of current at which the meter is specified to meet the accuracy requirements.
Now let’s look more in detail:
Starting current Ist according to IEC The starting current is a fraction of the basic current or nominal current. See the multiplication factors below:
IEC 62053-11 direct connected
class 1: 0.004 Ib class 2: 0.005 Ib
IEC 62053-11 transformer connected
class 0.5: 0.002 In class 1: 0.002 In class 2: 0.003 In
IEC 62053-21 direct connected
class 1: 0.004 Ib class 2: 0.005 Ib
IEC 62053-21 transformer connected
class 0.2S: 0.001 In class 0.5S: 0.001 In
IEC 62053-23 direct connected
class 2: 0.005 Ib class 3: 0.01 Ib
IEC 62053-23 transformer connected
class 2: 0.003 In class 3: 0.005 In
PASS/FAIL criteria: The meter has to start and continue recording energy. Means, we need to receive at least two pulses from the meter within a certain period of time.
How to calculate the time? We have the nominal voltage, the number of elements, the starting current and the meter constant. Now we can calculate:
This is the duration for one pulse for a meter with zero error. So, first we double the time because we need to receive two pulses. Then, the IEC does not specify any accuracy for starting test. So we need to consider the meter error. Best practice is to add 20 % to your calculated time for two pulses. This will be in most cases sufficient. If the meter fails with this time-out calculation you are allowed to extend it. The standard setting of CLOU test benches is 120 %. From the formula you can see that a higher meter constant is preferable because it saves testing time.
Minimum Required Time for Starting Test
IEC62053-11 DC class 1
IEC62053-11 DC class 2
IEC62053-11 CT/PT class 0.5
IEC62053-11 CT/PT class 1
IEC62053-21 DC class 1
IEC62053-21 DC class 2
IEC62053-22 CT/PT class 0.2S
IEC62053-22 CT/PT class 0.5S
IEC62053-23 DC class 2
IEC62053-23 DC class 3
IEC62053-23 CT/PT class 2
IEC62053-23 CT/PT class 3
Starting current Ist according to EN As usual we have to deal with more standards. For MID the relevant standards are EN 50470-1 and EN 50470-3 . Here is the starting current a fraction of Itr Itr is directly liked to Iref (see above). So, if we have a direct connected meter Itr is 10 % of Ib. For CT meters Itr is 5 % of In The PASS/FAIL criteria are exactly the same as described in #1: starting current Ist according to IEC
Starting current Ist according to OIML R46 OIML stands for ORGANISATION INTERNATIONALE DE MÉTROLOGIE LÉGALE, in English: International Organization of Legal Metrology. The OIML has published the recommendation R46 (please download the actual version from the OIML website). This description is based on the document r46-p-e12.pdf. PASS/FAIL criteria: The R46 requires an error measurement for starting current. This means, we set our test equipment to error measurement mode. Best practice is to make the error measurement for two pulses. During production and knowing the behavior of the meter you can also test with one pulse (leads to a higher error deviation). The maximum permissible error needs to be calculated.
Note: The 2010 edition had a more restrict calculation formula for the allowed error. Please make sure that you work with the actual document.
Note: This post is extremely simplified to give some brief information.
Generally a harmonic is a integer multiple of the fundamental frequency (nominal frequency fn). The fundamental frequency is either 50 Hz or 60 Hz.
So, when we have a nominal frequency fn of 50 Hz, the 3rd harmonics is traveling with a frequency of 50 Hz multiplied by 3 = 150 Hz.
We can have harmonics in the voltages and in the currents. Current harmonics
In a normal AC power system we have a sinusoidally current with the same frequency as the voltage. When a non-linear load is connected, the current waveform will become complex. But the wave can be split in harmonic sine waves with the help of the Fourier analysis. Don’t worry, the calculations are done by the CLOU reference standards and advanced AMI meters. What can cause current harmonics?
variable speed drives
PV- and wind power converters
Voltage harmonics are caused by the current harmonics, so they are typically smaller than the current harmonics.
Beside of a magnitude harmonics also have a phase angle, indicated relative to the fundamental wave.
What is the impact of harmonics?
Harmonics can cause
overheating of motors, transformers, cables
malfunction of relays and breakers
disturbance of PLC communication
damage of capacitors or capacitor banks for power factor compensation
What are the limits?
Most countries follow the IEEE 519 recommendations.
Voltages up to 1 kV are allowed to have a individual harmonics content of 5 %, the THD (total harmonic distortion) should not exceed 8 %. Odd current harmonics (between order no. 3 and 11) with a current between 50 A and 100 A are allowed to have 7 % for individual currents and a maximum THD of 8 %. Please note that utility regulations might be different.
With CLOU portable reference standards you can monitor the harmonics behavior on site during a calibration. Our AMI systems can provide long time statistics.
Harmonics with higher order than 1st are travelling faster than the fundamental frequency. The calculation of required degrees per cycle can be done by dividing 360° by the order no. of the harmonics. For example the 3rd harmonics finishes a cycle after 120°. So when you enter a phase shift of 180° it’s in fact the same like entering 60°.
We can not say whether a harmonic is leading or lagging.
phase shift °
The fundamental wave is blue, the total harmonic distortion is red.
If a meter has no possibility to open the internal link between voltage and current, the test must be performed with an Isolated Current Transformer (ICT) at each measurement position. The CL2030-D has a built-in electronic compensation. The ICTs can be remote controlled by PC. A safety protection by controller is built in. This assures that the secondary side is closed in case of high burden or open circuit. Each ICT will be delivered with a company calibration certificate. A certificate issued by an ILAC accredited laboratory is optional available.
Specifications ICT CL2030-D
100 A, max. load 120 %
1 : 1
±0.01 % (0.2 A … 120 A) ±0.03 % (0.05 A < 0.2 A) ±0.05 % (0.01 A < 0.05 A)
±0.3’ (0.2 A … 120 A) ± 3’ (0.05 A < 0.2 A) ± 8’ (0.01 A < 0.05 A)
The ICTs can operate stand-alone or together with a remote/control by PC. For remote control with 3rd party equipment a control box CL2030-3D-CB is needed. A interface-description is available on request.
This scanning head can be used for detection of metrological LED pulse outputs for static electronic meters together with CLOU portable meter test equipment. The TP-17 Wireless scanning head is based on the Nordic nRF24L01+, highly integrated, ultra low power (ULP) 2 Mbps RF transceiver for the 2.4 GHz ISM (Industrial, Scientific and Medical) band. It includes the Enhanced ShockBurst™ hardware protocol accelerator for a high-speed SPI interface. The no. of TP-17 Wireless scanning heads working in parallel is not limited.
Specifications receiver for scanning head TP-17 Wireless
3.3 V … 5 V DC
≤ 30 mA
output signal, high
≥ 4.5 V
output signal, low
≤ 0.3 V
The scanning system consists of
Scanning head TP-17 wireless 2.4 GHz transmitter, fixed at the energy meter
Fixing device TP-GS 2 – fastening to meter by two side-plates, can be moved up and down along the meter – can be used for rectangle- or round meters – adjustable width from 35 to 180 mm – provides magnetic adhesion to scanning probes – provides mechanical adhesion to scanning probes – probes can be moved left/right freely – made of insulation material, which ensures safety and portability
Yes, the scanning head TP17-wireless is designed and manufactured in conformity with health, safety, and environmental protection standards for products sold within the European Economic Area (EEA). The compliance was verified by MicroTest (external type test laboratory).
The term calibration is often misused.
Calibration means the comparison of a unit under test (in our case a energy meter or reference standard) with a test equipment with higher accuracy. It is a strict comparison to evaluate the accuracy error.
Adjustment involves a manipulation of the unit under test. Electromechanical meters have adjustment screws, electronic meters have special memory for correction values.
After production a energy meter is adjusted first. The target is to bring the meter as close to zero error as possible. Once the meter is adjusted, the performance must be verified.
This is done with a calibration.
accuracy error calculations [note]stand-alone with one pulse input socket, or together with error calculators in stationary test benches[/note]
Specifications Reference Standard CL3115-HW, Mains connection
85 V … 265 V AC
45 Hz … 65 Hz
< 30 VA
5 °C … 45 °C
Max. relative humidity
≤ 85 %, not condensate
Surge voltage protection
approx. 13 kg
19” 3 units,
H x W x D: 132,5 mm x 483 mm x 405 mm
Specifications Reference Standard CL3115-HW, measurement values
Test voltage, phase-neutral
30 V … 576 V
60 V, 120 V, 240 V, 480 V, auto range
1 mA … 120 A
10 mA, 20 mA, 50 mA, 0.1 A, 0.2 A, 0.5 A, 1 A, 2 A,
5 A, 10 A, 20 A, 50 A, 100 A, auto range
Frequency range, fundamental wave
40 Hz … 70 Hz
Voltage measurement accuracy
< 0.01 %
Voltage measurement drift
< 35 ppm / year
Current measurement accuracy < 25 mA
< 0.02 %
Current measurement accuracy ≥ 25 mA
< 0.01 %
Current measurement drift
< 65 ppm / year
Power measurement accuracy P, Q, S
< 0.02 % (current ≥ 25 mA and λ = 1)
Power measurement drift
< 100 ppm / year
< 0.01° (current ≥ 25 mA and voltage > 30 V)
Voltage temperature drift
< 2,5 ppm / K
Current temperature drift
< 5 ppm / K
Power temperature drift
< 7.5 ppm / K
Errors are independent of measurement mode and when using auto range. The reference standard provides a power proportional frequency output (160 kHz nominal).
The equipment can be operated manually with menu keys or via RS485 remote control. A interface description for integration into 3rd party meter test equipment is available on request. Below are some screen-shots
The reference standard CL3115 is by default delivered with a company calibration certificate. It can also be delivered with a calibration certificate issued by an ILAC accredited laboratory on additional cost.
A frequency proportional to the total power is generated by a signal processor, which applies to the pulse output fOUT of the CL3115. The energy output pulse constant can be set to automatic mode or manual mode. Setting range is: 1…2,000,000,000 i/kWh. The pulse output frequency is 160 kHz for End of range in voltage and current. The energy pulse is on TTL/CMOS level, burden capacity >20 mA. The range can be selected in Auto-mode or direct assigned. Calculation formula for the output frequency
Calculation formula for the meter constant
UR and IR are the ranges, the frequency is 160 kHz
There are two commonly used methods to calculate the apparent power (VA).
The traditional method is the vectorial calculation. Here we are adding the active power (W) from each phase and the reactive power (var) from each phase. As the reactive power vector has a 90° angle vers. the active power we can calculate the hypotenuse (VA). Old meter installations are using this method.
With electronic meters with microprocessor we are able to calculate the VA for each phase individually and then add the VA values. This method is called arithmetic calculation. It is more accurate.
CLOU meters are using the arithmetic calculation. Some meters can be set to vectorial calculation mode.