Megger TPT420 LCD/LED Two-Pole Voltage Tester

Product Information

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. 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. 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. 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.

FIGURE 4: Surge test voltage vs the dielectric strength of a 460 V motor and typical voltage spikes it sees in service When testing between phase and earth on any circuit protected by an RCD, RCBO, and Safety Breaker, the TPT420 is designed to work below the tripping threshold of these devices to help avoid unintentional disconnection. The phase rotation indication test has been simplified, which now avoids the crossing of the test probes. The TPT420 can also perform a single-pole voltage indication test. 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. Ratio validation to confirm that the performance of the CCVT matches its nameplate values for ratio and phase accuracy requires the use of specialised test equipment. For this reason, it is not commonly carried out in the field. If validation testing is performed, however, the results should fall within the appropriate accuracy parallelograms given in standards, such as IEEE C57. 13- 2016. One such example is provided in Figure 2, wherein a CCVT is used for metering purposes.The EMU, in addition to an inductive voltage transformer, contains a tuning circuit and protection against ferroresonance (Figure 1). The tuning circuit is a reactor that compensates for magnitude errors and phase shift caused by the CVD, making it possible to have the CCVT with a characteristic on the secondary side that is similar, in terms of error and phase deviation, to that of a purely inductive voltage transformer. Power frequency (50 / 60 Hz) power factor / tan delta testing has been the standard for evaluating the condition of solid and liquid insulation in transformers for over 100 years.With the introduction of 1 Hz power factor / tan delta testing, Megger is revolutionizing insulation diagnostics. When testing at 1 Hz, the dielectric losses due to moisture, contamination and increase liquid conductivity are better observed. Power factor or tan delta test at 1 Hz provides superior detection sensitivity to early and advanced insulation deterioration. Combined with power frequency measurement, 1 Hz delivers immediate insulation assessment without the need for trending or proprietary databases. But what overall voltage is necessary to show up these turn-to-turn faults, and will this voltage be harmful if applied to a fault-free motor? How do the test voltages relate to the dielectric strength of the windings?

To answer these questions, let’s look at an example. When a 415 V motor is assembled, the insulation applied to the wire used to wind the stator has a dielectric strength of approximately 8000 V. During its lifetime, this insulation will degrade primarily due to heat in the motor, but also as a result of environmental conditions and the coil movements which arise from starting, stopping and load changes. 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. 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. 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.

Megger TPT420 LCD/LED Two-Pole Voltage Tester Technical Specifications 

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. 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:

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.

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. 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.