Showing posts with label Transformer. Show all posts

Friday, August 28, 2015

High voltage tension insulators

For high-voltage lines of anchor is the national (TEPCO other) popular type of stone.
It is also used for overhead ground wire outside the high-voltage lines.
There is no cover in one line side as TEPCO.

Friday, August 21, 2015

High pressure arm arm-

It is arm-in consideration of the beautification.
Has a pipe type in order to increase the strength, is characterized by the absence of Amutai.

Tension insulator does not need the strap can be directly attached.

Wednesday, December 31, 2014

DC High-Potential Test

The DC hi-pot test is applied at above the rated voltage of a transformer to evaluate the condition of winding insulation. The DC high-voltage test is not recommended on power transformers above 34.5 kV; instead the AC hi-pot test should be used.
Generally, for routine maintenance of transformers, this test is not employed because of the possibility of damage to the winding insulation. However, this test is made for acceptance and after repair of transformers.
If the hi-pot test is to be conducted for routine maintenance, the AC test values should not exceed 65% of factory AC test value. The routine maintenance AC voltage value should be converted to an equivalent DC voltage value by multiplying it by 1.6, that is, 1.6 times the AC value for periodic testing (i.e., 1.6 × 65 = 104% of AC factory test value). The DC hi-pot test can be applied as a step-voltage test where readings of leakage current are taken for each step. If excessive leakage current is noticed, voltage can be backed off before further damage takes place. For this reason, the DC hi-pot test is considered to be a nondestructive test.
Some companies conduct the AC hi-pot test at rated voltage for 3 min for periodic testing instead of the 65% of factory test voltage. The hi-pot test values for DC voltages are shown in Table 1.1.
The procedure for conducting this test is as follows (refer to Figure 1.1a and b for test connections):

FIGURE 1.1
Transformer high voltage (hi-pot) test connection: (a) high winding hi-pot test connection and
(b) low winding hi-pot test connections.

Transformer must have passed the insulation resistance test immediately
prior to starting this test.
• Make sure transformer case and core are grounded.
• Disconnect all high-voltage, low-voltage, and neutral connections,
low-voltage control systems, fan systems, and meters connected to the transformer winding and core.
• Short-circuit with jumpers together all high-voltage bushings and all low-voltage bushings to ground as discussed under “Insulation resistance measurements.

Connect hi-pot test set between high-voltage winding and ground.
Gradually increase test voltage to the desired value. Allow test voltage duration of 1 min, after which gradually decrease voltage to zero.
• Remove low-voltage to ground jumper and connect hi-pot test set
between low-voltage winding and ground. Also connect the shortcircuited
high-voltage winding to ground. Gradually increase test voltage to desired value. Allow the test voltage duration of 1 min, after which gradually decrease voltage to zero.
• If the preceding two tests do not produce breakdowns or failures, the transformer is considered satisfactory and can be energized.
• Remove all jumpers and reconnect primary and secondary connections and other system equipment that may have been disconnected.
The following are some cautions and considerations in performing hi-pot
tests:
In liquid-filled transformers two insulation systems are in series, that is, solid insulation with oil or synthetic fluid. When AC or DC hi-pot test voltage is applied, the voltage drops are distributed as follows:

Table 1.1

When using DC hi-pot test voltage on liquid-filled transformers, the solid insulation may be overstressed.
Insulation that may be weakened near the neutral may remain in service due to lower stress under operating conditions. However, when subjected to hi-pot test voltage, it may break down and require immediate repair. The weakened insulation may usually be detected by the measurement at lower voltages.
If a hi-pot test is to be conducted for routine maintenance, consider the following in advance: (1) assume that a breakdown will occur, (2) have replacement or parts on hand, (3) have personnel available to perform work, and (4) is the loss of the transformer until repairs are made beyond the original routine outage.

Thursday, December 25, 2014

Transformers Insulation Resistance Measurement

This test is performed at or above rated voltage to determine if there are low resistance paths to ground or between winding to winding as a result of winding insulation deterioration. The test measurement values are affected by variables such as temperature, humidity, test voltage, and size of transformer.
This test should be conducted before and after repair or when maintenance is performed. The test data should be recorded for future comparative purposes. The test values should be normalized to 20°C for comparison purposes. The general rule of thumb that is used for acceptable values for safe energization is 1 MΩ per 1000 V of applied test voltage plus 1 MΩ. Sample resistance values of good insulation systems are shown in Table. The test procedures are as follows:

Typical Insulation Resistance Values for Power and Distribution Transformers

1. Do not disconnect the ground connection to the transformer tank and core. Make sure that the transformer tank and core are grounded.
2. Disconnect all high-voltage, low-voltage, and neutral connections, lightning arresters, fan systems, meters, or any low-voltage control systems that are connected to the transformer winding.
3. Before beginning the test, jumper together all high-voltage bushings, making sure that the jumpers are clear of all metal and grounded parts. Also jumper together all low-voltage and neutral bushings, making sure jumpers are clear of all metal and grounded parts.
4. Use a megohmmeter with a minimum scale of 20,000 MΩ.
5. Resistance measurements are then made between each set of windings and ground. The windings that are to be measured must have its ground removed in order to measure its insulation resistance.
6. Megohmmeter reading should be maintained for a period of 1 min.
Make the following readings for two-winding transformers:
a. High-voltage winding to low-voltage winding and to ground
b. High-voltage winding to ground
c. Low-voltage winding to high-voltage winding and to ground
d. Low-voltage winding to ground
e. High-voltage winding to low-voltage winding

The connections for these tests are shown in Figures 1.1a through e and 1.2a through e for single-phase and three-phase transformers, respectively.

Megohmmeter readings should be recorded along with the test temperature (°C). The readings should be corrected to 20°C by the correction factors shown in Table. If the corrected fi eld test values are one-half or more of the factory insulation readings or 1000 MΩ, whichever is less, the transformer insulation system is considered safe for a hi-pot test.

Figure 1.1 Test connections for insulation resistance of a single-phase transformer. Note: In figure (e) reverse the L and E leads to measure from high-winding to low-winding.

For three-winding transformers, test should be made as follows:
High to low, tertiary and ground (H-LTG)
• Tertiary to high, low and ground (T-HLG)
• Low to high, tertiary and ground (L-HTG)
• High, low, and tertiary to ground (HLT-G)
• High and tertiary to low and ground (HT-LG)
• Low and tertiary to high and ground (LT-HG)
• High and low to tertiary and ground (HL-TG)

Do not make the megohm test of the transformer winding without the transformer iquid because the values of insulation resistance in air will be much less than in the liquid. Also, do not make the insulation resistance test of the transformer when it is under vacuum because of the possibility of flashover to ground.
The test connections shown in Figure 1.1a, c, and e are most frequently used. The test connections in Figure 1.1b and d give more precise results.
The readings obtained in the connections in Figure 1.1a and b are practically equal to readings in test connections in Figure 1.1c and d, respectively.

DC voltage testing of electrical equipment

FIGURE 1.2
Test connections for insulation resistance of a three-phase transformer: (a) connection for high winding to low winding to ground; (b) connection for high winding to ground and low winding guarded; (c) connection for low winding to high winding to ground; (d) connection for low winding to ground and high winding guarded; and (e) connection for high winding to low winding.

Acceptable insulation resistance values for dry and compound-filled transformers should be comparable to those for Class A rotating machinery, although no standard minimum values are available.
Oil-filled transformers or voltage regulators present a special problem in that the condition of the oil has a marked influence on the insulation resistance of the windings.
In the absence of more reliable data the following formula is suggested:

where
IR is the minimum 1 min 500 V DC insulation resistance in megohms from winding to ground, with other winding or windings guarded, or from winding to winding with core guarded
C is a constant for 20°C measurements
E is the voltage rating of winding under test
kVA is the rated capacity of winding under test

This formula is intended for single-phase transformers. If the transformers under test is one of the three-phase type, and the three individual windings are being tested as one, then E is the voltage rating of one of the single-phase windings (phase to phase for delta connected units and phase to neutral or star connected units) kVA is the rated capacity of the completed three-phase winding under test.

Wednesday, April 23, 2014

Power System Protection Course- TRANSFORMER PROTECTION

TRANSFORMER PROTECTION

CONTENTS

TRANSFORMER PROTECTION.....................................................................

GENERAL..............................................................................................................

DIFFERENTIAL PROTECTION FOR TRANSFORMERS.................................

RESTRICTED EARTH FAULT PROTECTION FOR TRANSFORMERS.........

SPECIAL TRANSFORMER PROTECTION........................................................

'Qualitrol' Protection (Q).........................................................................................

Buchholz Relay.........................................................................................................

Interlocks and lntertrips............................................................................................

Coolant Level...........................................................................................................

Sealing Monitor........................................................................................................

Over-temperature Protection...................................................................................

EARTHING............................................................................................................

TRANSFORMER PROTECTION

GENERAL

All main transformers which transmit bulk power between the generators and the low-voltage distribution system of an offshore installation, and between the Supply Authority's system and the low-voltage equipment in onshore installations, have their own individual protective systems. This is to protect the transformer against damage due to electrical faults arising both outside and inside it.

A typical transformer protection scheme is shown in Figure 8.1 , which also shows associated instrumentation. Many of the general protection measures described earlier are applied also to transformers, but in addition there are some more specific ones.

FIGURE 8.1 - TYPICAL TRANSFORMER PROTECTION

Points worthy of note in Figure 8.1 include the following:

· Overcurrent protection is on the HV side only. It is provided by two inverse- time elements combined with an earth-fault element (2OCIT/E) together with two instantaneous high-set overcurrent elements (2OC), all in the same case. The relay operates to trip the HV circuit-breaker directly and both the HV and the LV breakers through the lock-out relay (TH). The time and current settings will be determined by the overall discrimination plan. Overcurrent on the LV side causes corresponding overcurrent on the HV side, which therefore takes care of both overloading and LV short-circuits.

· Restricted earth-fault protection is used on the secondary side (it is the only secondary-side protection), with four protective type CTs. The relay operates instantaneously to trip both the HV and the LV breakers through the lock-out relay.

· Lock-out hand-reset relay (TH).

· There is interlocking and intertripping from the HV to the LV circuit-breakers (but not in reverse).

· Instrumentation includes a maximum-demand ammeter with an alarm contact.

DIFFERENTIAL PROTECTION FOR TRANSFORMERS

It is explained in Section 7 that differential protection must be provided for generators because an internal fault is self-fed and would not be cleared by the generator supply breaker. Such differential protection, not forming part of the discrimination ladder, is arranged to operate instantaneously.

In the case of transformers however there is a circuit-breaker upstream of the unit, and this can clear an internal fault by removing the supply that feeds it. If the upstream circuit- breaker protection has an instantaneous 'high-set' relay (as here), the clearance can be immediate.

Therefore it is not usual practice to provide differential protection to offshore, or to smaller onshore, transformers, but to rely on the HV protection to clear any internal primary or 'through' fault. Internal earth faults on the secondary side are within the protected zone and are dealt with by the REF protection.

Nevertheless large onshore transformers are often provided with full differential protection using three primary side and three secondary side current transformers. This gives the same benefits as restricted earth-fault protection and, in addition, rapid protection against inter-phase faults in the transformer as well as earth faults on the primary (delta) winding. In these respects it is far superior to REF protection.

The difference between the primary and secondary currents in a transformer because of its turns ratio does not prevent the necessary balance in the differential relay circuits so long as the current transformer ratios are in inverse proportion to that of the power transformer. Where, as is usually the case, the power transformer has delta/star windings, which introduce a phase shift between primary and secondary currents, a star/delta arrangement of the CT secondary windings is necessary to achieve balance in the secondary circuit.

Allowance has to be made, in differential protection schemes for transformers, for the magnetising inrush currents which flow only in the primary windings when the transformer is switched onto the supply; they are not reflected in the secondary windings and therefore appear similar to primary fault currents, which may falsely operate the differential protection. The simplest solution is a short time delay in the relay - an induction disc relay may be used - although there are more subtle solutions available in cases where a delayed response is not desirable.

RESTRICTED EARTH FAULT PROTECTION FOR TRANSFORMERS

It should be noted that, although restricted earth-fault protection will operate satisfactorily for internal solid-earth faults on most parts of transformer secondary windings, a high-impedance fault to earth may not give rise to sufficient fault current to operate the relay, even though it is given a light setting.

FIGURE 8.2

PROTECTION OF TRANSFORMER WINDING BY RESTRICTED EARTH FAULT PROTECTION

Another point to be noted is that, if the fault occurs near the star-point, the voltage at that point may not be sufficient to cause a fault current high enough to operate the relay. This situation is shown in Figure 8.2. Thus, although restricted earth-fault protection is usually installed for transformer secondaries, it cannot be regarded as one hundred per cent certain to operate.

SPECIAL TRANSFORMER PROTECTION

In addition to the protection listed above, whose purposes have already been explained, there are the following additional features special to transformers:

'Qualitrol' Protection (Q)

Qualitrol protection is fitted only on sealed transformers such as those used on offshore installations. It is a proprietary device fitted at the top of the transformer. It detects over- pressure within the transformer and, if it exceeds a certain preset level, trips both HV and LV circuit-breakers simultaneously through a flag relay (FG) and the lock-out relay (TH). The device has a spring-loaded discharge disc to relieve pressure immediately if it builds up too quickly.

On large oil-filled grid and similar transformers internal pressure is normally relieved into the conservator. Nevertheless it is customary to fit such transformers with a pressure relief diaphragm on the tank top.

Buchholz Relay

Although termed a 'relay', this is in reality a mechanical device named after its inventor.

FIGURE 8.3 - BUCHHOLZ RELAY

The device is fitted in a horizontal section of the pipe running between the main tank and the conservator in large oil-filled transformers.

It consists of two parts as shown typically in Figure 8.3, a gas trap and a surge section. If an insulation weakness begins to develop under oil in any part of the transformer winding, small discharge currents start and create tiny bubbles of gas. As the breakdown slowly progresses, the rate at which gas is evolved increases. The bubbles rise slowly to the tank top and pass on, through the connecting pipe, towards the conservator. On the way they pass through the Buchholz relay and are caught in the gas trap. Over a period of time enough gas is accumulated to cause the oil remaining there to have a free surface, and a float gradually lowers until, on reaching a preset level, it actuates a mercury switch. This is usually arranged to give an alarm, since the process is gradual and has not yet reached breakdown stage calling for immediate disconnection.

The lower part is the surge section. Here a vane is suspended vertically across the flow of oil between the tank and conservator and is held firmly against a stop by a counterweight. Normally the oil flow is very slight, depending only on temperature changes in the transformer, and the vane does not move. But if there is a complete electrical breakdown in any winding under the oil a power arc will develop inside the tank, causing an expanding, high- pressure bubble of oil vapour round the arc. This will rapidly displace oil from the tank into the conservator, causing a surge of oil past the vane, which will swing against the action of the counterweight and actuate another mercury switch. Because an actual breakdown will have occurred, this contact is always arranged to trip the supply side of the transformer.

The above describes the operation of a typical Buchholz relay in principle. Different manufacturers have added many refinements to this basic design.

Interlocks and lntertrips

Interlocking and intertripping is provided between the HV and LV breakers. If the HV breaker opens for any reason, whether tripped by a fault or operated manually, the LV breaker (if closed) trips in sympathy and cannot be reclosed until the HV breaker has been closed first.

It will be seen from Figure 8.1 that a fault, whether on the HV or LV side, operates through the lock-out relay and trips both the HV and the LV circuit-breakers simultaneously. This is to ensure that, after such a fault, not only is the transformer isolated from its normal supply side but also that it cannot be back-fed from the LV side.

The intertrip acts as a back-up for this, but it is also needed to ensure sympathetic opening of the LV breaker when the HV breaker is opened by hand, as distinct from by a fault.

Coolant Level

A sight-glass is provided to check the coolant level within the tank of a sealed transformer. The level varies with temperature, and allowance must be made for this; level marks for 15C and 45C may be given.

Conservators of large oil-filled transformers usually have a sight-glass to indicate oil level.

Sealing Monitor

A centre-zero pressure/vacuum gauge may be provided to indicate pressure in the vapour space over the liquid coolant of a sealed transformer. The transformer is filled to a level marked on the sight-glass and sealed at a specified temperature - say 45°C. In service any variation above or below this temperature, due either to change of ambient temperature or to transformer loading, causes the liquid level to fall or rise slightly and a consequent small vacuum or pressure to be indicated on the gauge.

If the pressure shown by the gauge moves over a range less than its normal one, it may indicate a failure of the tank sealing allowing air to be 'breathed' in and out. Such a situation should be investigated.

Over-temperature Protection

Whereas winding temperature can be monitored by normal temperature-sensing, a special arrangement is sometimes used in large liquid-filled transformers.

In this application Negative-Temperature-Coefficient (NTC) thermistors are used in temperature-monitoring instruments. They are suspended in the oil in a housing with a heating element and employ the technique of 'thermal imaging'. The thermistor is connected into a resistance bridge, whose output may operate indicating instruments as well as actuating alarms and trips through an electronic detector circuit.

Whereas NTC thermistors can operate over a range of temperatures by adjustment of the associated measuring circuits, a PTC thermistor is made to change its resistance at a particular temperature, subject to a small tolerance. It is more suitable for detecting overtemperature at particular locations in equipment - for example, at hot spots in generator or motor windings into which they can be embedded during manufacture. As the PTC thermistor passes through its critical temperature, the sudden change of resistance can be made to actuate an alarm or even to give a trip signal.

EARTHING

On all offshore and onshore installations the transformer secondary star-point is usually solid-earthed either through a link or through the neutral bar of the LV switchboard which it feeds as a 4-wire system. The earth connection can be isolated when desired (for example when megger-testing the secondary) by means of a link at the switchboard, or, where the earth connection is made through a link in the 3-pole circuit-breaker, by withdrawing and isolating the circuit-breaker unit itself.

Care must be taken, after opening an earth link for any reason, to ensure that it is replaced immediately after the test. The whole protection of the transformer may depend on it.