IR/PI TEST

                                                STATOR SIDE TESTING

1.IR/PI TEST 

IR & PI TEST The insulation resistance & polarization index tests are useful indicators of contamination and moisture on the exposed insulation surfaces of a winding, especially when there are cracks or fissures in the insulation. The insulation resistance is the ratio of the dc voltage applied between the copper winding and ground to the resultant current.
Polarization index = Ratio of IR at 10min/IR at 1min

Polarization record test

Polarization record (PI) analyzers (in some cases known as dielectric breakdown analyzers) are utilized to evaluate protection wellbeing, distinguishing the collection of toxins just as actual changes inside the protection. The test includes emphatically charging the engine conductors and contrarily charging the casing for ~10 minutes. The test measures and diagrams the adjustment of current over these 10 minutes, where solid protection will "charge", or diminish current, while undesirable protection will stay steady. This test has become progressively hard to use as an independent acknowledgment test in view of fresher protection frameworks however is as yet valuable related to different tests to confirm results.

Insulation resistance (IR)

These electrical tests will find issues within the windings of a motor using resistance. It provides a test of the quality of insulation resistance (IR), which will start degrading as soon as the motor is in use due to

temperature effects. Motors in harsh environments (high humidity, dirt, particles) may need routine checks of the IR to avoid any failure, as small shorts from moisture or dust can lead to major issues if undetected. IR testing can highlight dead cables, shorts, loose connections, open circuits, or any other blatant winding issue that can alter the winding resistance. These resistance measurements must be compensated for temperature after each test; this way, the resistance measurements are standardized when compared over time. This test can also be used to balance 3 phase motors, as the phase-to-phase resistance of each winding can be compared to see how they differ from one another. Note that these tests are typically done on off-line motors – that is, motors that have been removed from their power source, for safety and ease of inspection.

A protection opposition analyzer (known as a "megger") is regularly used to rapidly evaluate the engine's protection obstruction. This gadget utilizes a DC voltage (100-5000+ V) to recognize protection breakdown within an engine. Similar as a multimeter, a megger has two leads where one interfaces with ground and the other to explicit engine leads. If the megger shows a low obstruction esteem when wired to the engine, it implies that the way to ground has been compromised and the engine should be fixed. Then again, if the megger shows a high obstruction esteem, it implies that it recognizes no significant spillage in the wire's coherence. It is a basic test that is non-intrusive and gives fundamental data about current spillage, winding shortcomings, and extreme pollution, yet because of its low voltage, a few flaws will go undetected.

Polarization Index Test (PI Value Test) alongside Insulation Resistance Test (IR Value Test) is led on HV electrical machine to decide administration state of the protection. IP test is led particularly to decide the dryness and tidiness of the protection.

 

In protection obstruction test, a high DC voltage is applied across the encasing. This applied voltage is then partitioned by flow through the electrical encasing to acquire resistive worth of the protector. Since, according to Ohm's law,

Without using separate source for direct voltage, voltmeter and ammeter for measuring corresponding voltage and current, we can use direct indicating potentiometer which is also locally called megger.

Megger gives required direct (DC) voltage across the insulator, and it also shows the resistive value of insulation directly in M – Ω and G – Ω range. We generally use 500 V, 2.5 KV and 5 KV megger depending upon the dielectric strength of the insulation. For example, we use 500V megger for measuring up to 1.1 KV rated insulation. For high voltage transformer, other HV equipment and machines, we use 2.5 or 5 KV megger depending up on the insulation level.

As all electrical insulators are dielectric in nature, they always have a capacitive property. Due to that, during application of the voltage across the electrical insulator, initially, there will be a charging current. But after few instants when the insulator becomes totally charged, the capacitive charging current becomes zero. Because of that it is recommended to measure insulation resistance at least after 1 minute (sometimes 15 seconds) from the instant of application of voltage across the insulator.

 Only measuring insulation resistance by megger may not always give the reliable result. As the resistive value of an electrical insulator may also vary with temperature.

This difficulty is partially solved by introducing polarity index test or in short PI value test. We will discuss the philosophy behind PI test, below.

When we apply a voltage across an insulator, there will be a corresponding current through it. Although this current is tiny and it is in milliampere or sometimes in microampere range, it has mainly four components.

 

Capacitive component.

Conductive component.

Surface leakage component.

Polarization component.

Each separator is hygroscopic in nature. Some impurity particles principally like dampness in the encasing are extremely polar. At the point when an electric field is applied across the encasing the polar atoms adjust themselves along the course of electric field. The energy needed for this arrangement of polar particles, comes from voltage source in type of electric flow. This current is called polarization current. It proceeds until every one of the polar particles aligned themselves along the bearing of electric field.

It takes around 10 minutes to adjust the polar particles along electric field, and that is the reason if we take megger result for 10 minutes, there would be no impact of polarizing in megger result.

Thus, when we take megger worth of an encasing for 1 moment, the outcomes mirrors, the IR esteem which is liberated from the impact of the capacitive part of current. Again when we take the megger worth of the encasing for 10 minutes, the megger result shows the IR esteem, liberated from impacts of both capacitive part and polarization part of the current.

Polarization file is the proportion of megger esteem taken for 10 minutes to the megger esteem taken for 1 moment.

The meaning of polarization file test.

Leave I alone the all out starting current during polarization record test or PI test.

IC is the capacitive current.

IR is resistive or conductive current.

Is surface spillage current.

IP is polarization current of the encasing.

From the above condition obviously, if the worth of (IR + IS) >> IP, the PI of protector ways to deal with 1. What's more, huge IR or IS or both show wretchedness of the protection.

The worth of PI turns out to be high if (IR + IS) is tiny contrasted with IP. This condition demonstrates that high polarization record of an encasing suggests strength of separator. For great protector resistive spillage current IR is exceptionally little.

It is constantly wanted to have polarization record of an electrical protector more than 2. It is dangerous to have polarization file under 1.5.

EQUIPMENT :

MEGGAR MIT S20/2 SKV INSULATION TESTER

PROCESS :

 

IR/PI TEST @ 2.5 kV  (BEFORE CLEANING) 

Temp: 28°C

Phase	IR 15sec (MW)	IR 60sec (MW)	IRsec 600(MW)	PI
U	401	1350	6150	4.56
V	408	1760	6800	4.68
W	310	700	1100	1.57
UVW	135	321	764	2.30

 

 

 IR/PI TEST @ 2.5 kV  (AFTER CLEANING) 

Temp: 42°C

 

Phase	IR 15sec (MW)	IR 60sec (MW)	IRsec 600(MW)	PI
U	361	807	1580	1.96
V	450	1110	2710	2.46
W	487	1290	3960	3.05
UVW	386	931	1920	2.06