Megger TPT420 LCD/LED Two-Pole Voltage Tester

Product Information

The instrument has abright, integrated LED torch to helpyou when working in poorly lit environments and, it is safetyrated at CAT IV 1000 V with an IP64 housing, and conforms to the latest IEC/EN 61243-3 and DIN VDE 0682-401 standards. For compliance, GS38 probe tip shrouds are provided as standard, along with a protective storage and carry pouch. 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. A basic ratio test can be carried out by exciting the primary side of the CCVT with a 10 kV source and measuring the secondary voltage with a digital multimeter. However, this supplies no measurement of phase deviation, which is required to validate the accuracy of the CCVT.

Transformers are an integral part of the power grid. Their reliability directly impacts the reliability of the grid. The failure of this critical asset can handicap the grid and increase its volatility. Because replacing a high voltage transformer requires planning for many reasons, including long manufacturing lead times that can exceed a full year, it is widely accepted that asset management, particularly of transformers, is a beneficial contribution to the operation of the grid. 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. 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.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.

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: 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. Figure 1 provides examples of surge traces for a good winding and a winding that has weak insulation or a turn-to-turn short. The three traces – corresponding to the three phases of the motor’s windings – should overlay as one on the graph but, as can be seen, the trace for a winding with a turn-to-turn short is different from the traces for the other two phases. In a surge test, a short-duration test current with a fast rise time – typically around 100 ns – is generated and applied to the motor coil. The test equipment captures the coil’s response, which takes the form of a decaying oscillation – or ‘ringing transient’. If the motor coil were in air, the pulse would be travelling at almost the speed of light and the voltage would be evenly distributed across the coils. But in a motor, the coil is not in air, it is wrapped around a steel core, and so the pulse travels much more slowly.

Megger VF5 Voltage Tester Key Features

Inside the voltage tester, this current passes through a known resistance. The voltage drop across this internal resistance is then measured and displayed on the tester's meter or screen. This provides the user with a reading of the voltage present. In some cases, for basic testers, this voltage may simply trigger a light or a sound to indicate the presence of voltage rather than providing a precise numerical value. 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. As mentioned in the previous article, there are many options for testing insulation, and Megger supplies an extensive range of testers for such applications, from 50 V to 15 kV insulation testers, through VLF and AC Tan Delta test sets to diagnostic Dielectric Frequency Response instruments and, under the Baker brand, specialist motor testing equipment up to 40 kV.