Field testing kWh meters is easy, field testing CTs when live is much more difficult. Our admittance tester can test CTs live with 50Hz or 60Hz current in the metering circuit. This technique does not confirm the accuracy of a CT but it will detect faults that give metering errors, such as shorted turns, metering circuits shorted to ground and tampering of CTs. CT faults are not very common, but they can cause large revenue losses. Admittance testing only adds 10 minutes to a routine polyphase meter test, giving cheap insurance.

The Model 505B is a battery powered, field portable instrument for checking metering CTs and secondary metering circuits. The 505B is a 2 terminal device that is inserted into the CT secondary loop and uses 1.6kHz to inject a small signal superimposed on the 50Hz current already flowing. From this test the 1.6kHz admittance is measured.

It can also measure 50Hz voltage and current and then calculate the CT ( or VT ) burden. A 50Hz and 60Hz version is available, and both 5A and 1A CTs can be tested. The USB interface on the front panel is provided for downloading data to a PC.

The 505B can perform the following measurements:

1.6kHz admittance test on a CT secondary.
1.6kHz admittance test on a CT secondary plus complete metering loop.
50Hz (or 60Hz) CT secondary current.
VA burden of the metering loop downstream from the test block, or at the CT secondary terminals.
VA burden of the metering loop from VT secondary terminals or at its test block.

Utilities have always found a major problem in field testing CTs. The test equipment available for testing is generally very bulky, heavy, expensive and difficult to use. This new product from Red Phase employs a novel technique to test CTs. The operator keys in data for the CT under test. This data includes primary current, 1A or 5A secondary, VA rating, burden PF, accuracy class and serial number. The CT primary current loop must be open circuit, but with LV CTs the primary side voltage busbar or cable can still be live for the test.

From this keyed data, the software in the 590G calculates the correct voltage which must be injected into the CT winding to measure turns ratio and secondary winding admittance. The secondary winding resistance is also measured.

From all the keyed data and measured parameters, the 590G can predict the performance of the CT under various burdens from 10% to 100%, various secondary currents from 1% to 200% rated current and any PF from 0 to 1.0. At the end of the test the 590G will give CT error at typically 10 test points. A complete test takes about 6 minutes.

The accuracy of measuring the CT performance is better than 0.1% for CTs which are in the higher ratio bracket, usually 400/5 and upwards. With lower ratio CTs the accuracy is still 0.1% for high quality types, but many lower ratio CTs use parallel windings and sometimes split cores. Parallel winding types are still measured to 0.1% where the inherent errors of the CT are low. Split core CTs have high inherent errors and are heavily compensated to meet the nameplate accuracy. The accuracy of measuring these CTs is 0.2%. The 590G tests an inductive PT by simply measuring the turns ratio. The Model 590D-1 accessory, which is used in conjunction with the 590G, enables full load tests to be simulated for PTs up to 500VA rating. The accuracy for load tests is 0.1%.

The 590G also measures 1.6kHz admittance of a CT, and the burden of both CT and PT metering and protection circuits.

There are customers who cannot be disconnected when you want to test their metering CTs. The Model 590F is a retrofit kit available for our popular Model 590C and 590G CT testers. The first part of the kit is the 590F-1, which is a clipon unit mounted on the end of a hot stick. This unit has a battery powered electronic compensation circuit for the clipon which greatly improves accuracy over a wide operating range, typically 25A to 1450A. A microprocessor is also used to control the clipon compensation, drive relays for internal range changing and provide communications to a fibre optic interface.

The fibre optic cable runs between the 590F-1 and a small interface unit at the base of the hotstick. From there the signal is transmitted through a 100M drum of RS485 cable to the 590G. The combination of hot stick and cable has been tested to 100kV, line to ground. The breakdown voltage of the hotstick and fibre optic cable is much greater than 100kV, but for operator safety we suggest not using this test set above 100kV. However, with a longer hotstick and suitable precautions the 590F-1 unit can be used at higher voltages.

The CT primary current measured by the clipon is sent via the cable to the 590G, which can be up to 100M distant. Extra drums can be daisy chained up to 400M total.The 590G measures the CT secondary current, either 1A or 5A nominal, and then calculates the CT ratio and phase error to 0.1% accuracy.

For testing live LV CTs two optional electronically compensated clipons are available. One is for primary side cables to 1,000A and the other is a large jaw clipon for busbars up to 2,500A.

This is an accessory for the Model 590C and 590G to perform the full load testing of inductive PTs. It has a high power switch mode ( typically 1.2kW ) source for full load performance testing of PTs under simulated load conditions to 0.1% accuracy. It uses the existing measuring circuitry and microprocessor of the 590G to the greatest extent possible but also has its own microprocessor to monitor and control its operation in conjunction with the 590G. Communication between the two units is via a serial link.

The unit connects to the 590G and inductive PT under test and measures:-

Primary & Secondary winding resistance.
Secondary winding admittance with primary short circuited.
Primary winding admittance calculated from turns ratio.
Turns ratio and no load voltage ratio.
Secondary admittance with primary open circuit.

Finally the microprocessor calculates the PT errors at 6 test points from the measurements. The test points are typically 25% and 100% of VA burden, and 80%, 100%, 120% of rated voltage. The actual test points are programmed into the 590G to suit the customer's choice.

We have recently started manufacturing a CVT tester which can test a de-energised CVT . The maximum voltage applied during testing is 2kV. The 590K makes a number of automatic measurements to characterise the CVT, then calculates the performance under load. A powerful PC104 mini computer is built into the instrument, running a Windows operating system. The large colour display on the front panel presents a large amount of information and is easy to read.

There are many CVTs in service throughout the world, but it is not possible for us to test more than a very small sample of them. So far we can reach our target accuracy of 0.1% on most of those we have tested. The 590K can test CVTs up to 500kV rating with up to 3 secondary windings. The test results are stored for later downloading into a PC.

The Model 594 is a low cost, comparative tester for the rapid testing of batches of new CTs against a known sample. The test set gives a digital indication of the turns error and polarity, current error (inherent losses) and phase error (inherent losses). The unit can be preset to 12 different types of CT’s by trimpot adjustment so that the readout is the same as the sample CT. The test method applies only very low voltages and currents to the CT secondary winding. Total test cycle time is about 30 seconds.

Protection CTs are tested by injecting a high voltage into the secondary and measuring the current produced as well as the voltage. From these measurements the error of the CT can be calculated at different test points.

Our Model 661 is a portable high power voltage source designed for testing protection CTs. It has all the measurement circuits needed for determining the overload characteristic of a protection CT up to 20 times overload.

To test the CT the DC winding resistance is measured first, then the voltage is ramped up by the microprocessor control and the current measured at a number of points. From these measurements the error of the CT is calculated at various overload levels.

So that the complete protection system is checked, the 661 can also measure the ratio of the CT and the burden of the circuit the CT is connected to.

This small battery powered tester is ideal for carrying out a quick check on the burden in CT metering and protection circuits.

The clipon CT is connected to the CT secondary wiring and the crocodile clips attached to the CT terminals. The instrument calculates the burden as the ratio of voltage over current.

The burden measurement accuracy is typically 1% at 5A and 5V, and decreases to 5% at 0.01A and 0.01V.

For conventional primary injection testing of CTs using a reference CT and burden box, the actual errors are measured by a comparator, or test bridge, such as the Model 820. This instrument has digital readouts for ratio and current error, plus it measures the current in the CT secondary, the burden voltage across the terminals and the burden power factor.

The overall accuracy of error measurement depends on the errors in the reference CT plus the measurement error of the comparator. The 820A is a 19" rack mounting instrument
usually built into a laboratory or workshop test bench.

For checking mini CTs as used in electronic kWh meters we also make the Model 820A. This model has an internal reference CT with multiple taps, rated up to 100A primary current. It can test any mini CT with ratios from 100:1 up to 3000:1.

To cater for a comparator used in field testing we have the Model 820B which is essentially the same as the 820A, but
this time housed in a portable carry case.

We make reference CTs to order. Multiple primary turns are fitted to increase accuracy for testing low ratio CTs. The error of a reference CT is very dependent on the ratio selected for testing. These are not electronically compensated, and have errors that are better than 0.01% for high ratios. The CT is contained in a wooden box with terminals on the top.

Two small fans are fitted to cool the primary turns when continuous operation is required.