CCVTs are devices capable of dual function. One function they can perform is to provide highly accurate voltage conversion for measuring devices, protection relays, and automatic control systems, while the other is to couple high-frequency power-line carrier (PLC) signals onto the transmission system for communication and control purposes. There are other effects due to filtering of the higher frequency components of the pulse-wave as a result of attenuation by the steel and the coils own inductance. The speed of propagation of the pulse wave is, however, the key aspect that allows the surge test to detect bad turns. Validation of insulation, ratio and burden at rated voltage requires large and heavy test equipment used in conjunction with expensive instrumentation. For this reason, these tests are commonly used during the manufacture of CCVTs but are impractical in the field. Nevertheless, field measurements of the CCVT burden, as an aid to ensuring that the burden rating of the device is not exceeded, are possible.

On-Line testing – that is, testing while the motor is running – can also be carried out on an as-needed basis with a portable on-line monitor like the Baker EXP4000. In addition to this, the Megger TPT420 LCD/LED Two-Pole Voltage Tester features a Continuity function which will alert the user to the presence of continuity between 0 and 500kΩ via a visual and audible alert. The Megger TPT420 LCD/LED Two-Pole Voltage Tester will also issue an audible alert upon detection of voltage and will continue to warn the user of dangerous test voltages even when its batteries have expired. This voltage tester has a safety rating of CAT IV 1000V. Basic insulation testing, PF tests at 1 Hz, and tip-up testing can be performed with the Megger Delta4000 test set and, with the addition of a DMM, a basic ratio test can be carried out. Ratio validation to an accuracy of ±0.1 % can be carried out with the Delta4000 test set and an accessory. The Megger MRCT test set can also perform basic insulation testing and ratio validation to an accuracy of ±0.1 %, with the added benefit of being able measure the overall burden of the CCVT, thus ensuring that the burden rating has not been exceeded. CCVT construction varies between manufacturers, models and year of fabrication. Knowledge of the construction is critical when deciding what and how to test. As mentioned previously, some CCVTs are equipped with a potential grounding switch located below C1, at the intermediate voltage terminal (IVT), and a carrier grounding switch below C2 at the low voltage terminal (LVT), as shown in Figure 3. In some CCVTs, however - mainly modern types - the low voltage terminal may not accessible. Understanding the construction characteristics and location of the IVT and LVT is therefore important when determining appropriate connections for testing and deciding whether the ground switches need to be open or closed for the required measurements to be made. Capacitance and line frequency power factor (PF) measurements should be made routinely on CCVTs. Insulation power factor tests are most informative when the amount of insulation included in the test is minimised. For this reason, tests are performed on each individual component of the CVD (e.g., C1-1, C1-2, …, and C2). Typical overall PF values range from 0.2 % to 0.5 %, but power factor values under 0.05 % are normal depending on the insulating materials used for construction.In fact, it will typically take the pulse 100 ns to travel from one coil-turn to the next, which is equal to the rise time of the pulse. The result is that the pulse produces a significant voltage difference between adjacent turns of the coil, something that it is impossible to achieve with any other test technique. The surge test will, therefore, reveal turn-to-turn insulation weaknesses. The Megger TPT420 LCD/LED Two-Pole Voltage Tester is an easy-to-use and ergonomic voltage meter optimised for safety and reliability. It is designed for testing electrical installations making it ideal for electricians and electrical engineers. However, it can also be used to test automotive, fire alarm, and solar/ PV systems. Megger TPT420 LCD/LED Two-Pole Voltage Tester Key Features When the probes of a voltage tester are connected to two points in an electrical circuit (commonly one end to a suspected live conductor and the other to a ground source), a closed circuit or loop is formed. This allows electrical current to flow from the higher potential to the lower potential.

Instrument transformers perform the important function of providing windows on the power grid’s electrical behaviour. Protection, control, and measuring devices require these ‘windows’ yet they also need electrical isolation from the grid as they function at much lower voltages and currents. Instrument transformers provide the solution; they are go-betweens that provide isolation by magnetically coupling secondary monitoring and measuring devices to the grid. There are several types of instrument transformers, but one of the most common on higher voltage transmission systems is the coupling capacitor voltage transformer (CCVT). FIGURE 4: Surge test voltage vs the dielectric strength of a 460 V motor and typical voltage spikes it sees in service On older style CCVTs, the potential terminal is typically accessible. However, for modern CCVTs such as those supplied by Trench, the potential terminal is inaccessible. Even in these cases, however, C2 can still be tested. CCVTs have a potential ground switch that provides the means to ground the potential terminal. With the potential ground switch closed, the carrier terminal can be energised, a low voltage lead connected to the line terminal (top of C1), and a C2 test performed in the GST-guard mode. Note that the carrier terminal must be disconnected and isolated from ground potential and the drain coil (also, if applicable, from any accessory leads) for the C2 test. In addition, the test voltage (typically 500 V) used to energise the carrier terminal must not exceed the voltage rating of the terminal. In summary: Go/no-go tests are an invaluable part of every maintenance regimen. Options include insulation resistance, leakage current, polarisation index and step voltage testing, and there are few who would argue against the benefits of these core electrical tests. However, these tests will not reveal one of the most common early initiators omotor faults: inter-turn insulation breakdown. To detect such a breakdown, it is necessary to use a surge test. Unfortunately, this is the area where some confusion has been generated in relation to the ‘high’ voltages involved.At its core, a voltage tester is a simple device designed to indicate the presence of electrical voltage in a system or a component. The underlying principle lies in Ohm's Law. This states that the current passing through a conductor between two points is directly proportional to the voltage across the two points.

The Megger TPT420 voltage tester is designed to provideelectricians and electrical engineers with an easy-to-use voltage indicating instrument. The meter will continue to warn the operator of dangerous test voltages even when the batteries are exhausted. With both LCD and LED displays, the TPT420provides both AC and DC voltage measurement from 12 to 1000 V AC and up to 1500 V DC. In addition, a continuity function within the range 0 to 500 kΩ, is included and for added safety, continuity, and voltage measurements are also accompanied by an acoustic sounder. The MET1000 All-in-one true RMS electrical tester is a multifunctional voltage andcurrent tester that gives electricians and electrical engineers a versatile and robust handheldmeterwith detachable test leads. Featuring both LED and LCD displays, it gives youa 200 A AC and1000 V AC/DC tester designed for everyday use. CATIV 600 V/CATIII 1000 V rated, with an IP65 housing for added protection, it reduces the number of testers required in the professional’s tool bag. And it’s the same with a surge test generated by a Baker DX tester. It applies a voltage and rise-time to enable you to see the inter-coil response, but with a signal controlled in voltage, time and energy, so that the impact on the motor is similar to the spikes that the motor receives as a result of typical power-system variations during everyday operation. In some circumstances, the CVD capacitive reactance can resonate with the magnetizing reactance of the inductive voltage transformer and the compensating reactor cores. This unwanted effect is called ferro-resonance and can give rise to large and damaging voltages across the inductive and capacitive elements. To avoid this, a ferroresonance damping circuit is installed in parallel with one of the secondary windings. The Megger TPT420 Two-Pole Voltage Tester is designed to provide electricians & electrical engineers with a simple, intuitive voltage indicator compatible with both AC and DC voltages. This device can measure AC voltage between 12 to 1000 V and DC voltage between 12 to 1500 V. The TPT420 will even continue to alert the user of a present voltage once the batteries have been completely exhausted. Including a continuity function within 0 to 500 kΩ and a frequency range of 40 to 400Hz, this unit can be applied to numerous situations such as automotive, fire alarm & solar/PV systems.A bad turn will short and this will be shown by a jump in the ringing frequency of the coil (it becomes in effect a ‘different’ coil at this point in the test). Large frequency jumps can be clearly seen on the tester display, but the instrument’s software also uses mathematical analysis to reveal anomalies that are less easy to spot by eye. When testing motors, it’s important to keep in mind that they are complex electro-mechanical devices with a complex set of failure modes and related diagnostic options. We will look at the failures of motor electrical/insulation systems, how to manage the life of motors by adopting an appropriate test regime and the concerns that are sometimes voiced about the ‘high’ voltages used for surge testing. In addition to the unit's adaptable auto-ranging voltage and continuity tests, the Megger MET1000 has a built-in current clamp for AC current measurement to 200 A. With true RMS AC measurements, a phase rotation test, and single pole voltage test built in, the MET1000 is a truly multi-purposeelectrical test instrument, ideal for use in commercial, industrial, and domestic environments. It even comes with GS38 shrouds, provided as standard, to ensure compliance with the latest standards. For a motor in normal operation, which has not endured significant electro-mechanical thinning of the insulation, the surge-test voltage is far below the dielectric strength of the insulation. The basic insulation tests just described can be usefully augmented by adding PF measurements at different frequencies and a tip-up test. A PF test at 1 Hz specifically, provides a better indicator of developing issues, facilitating early detection. 1 Hz PF is particularly sensitive to moisture contamination, a commonplace CCVT failure mode. Narrowband DFR tests include power factor tests at several discrete frequencies, including 1 Hz up to 505 Hz.