Tеst Models for Explosion Protection of High Voltage Oil Filled Electrical Equipment

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The MAIN JOURNAL for POWER GRID SPECIALISTS in RUSSIA


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34

August 25–29, France, Paris

Equipment Protection

Tеst Models for Explosion 

Protection of HighVoltage 

OilFilled Electrical Equipment

Leonid DARIAN (

Леонид

 

ДАРЬЯН

),

 Vladimir POLISCHUK (

Владимир

 

ПОЛИЩУК

),

 Aleksey SHURUPOV (

Алексей

 

ШУРУПОВ

),

CJSC “Technical inspection UES”,

 Russian Federation Joint Institute for High Temperatures of RAS

INTRODUCTION

Life time of transformers or another HV oil-

fi

 lled 

electrical equipment (HV OFEE) is about several decades. 
The gradual degradation of paper-oil insulation occurs 
under the in

fl

 uence of partial discharge, heating, cavitation 

and other factors in service. In time the deterioration of 
insulation characteristics exceed a critical level, that’s 
why untimely out of service may cause arc discharge 
(AD) due to internal short circuit (ISC). Electric power 
of discharge may range from tens to hundreds MW. Large 
amount of hydrogen, oxides of carbon and hydrocarbon 
gases is formed due to decomposition of transformer oil 
(TO) under action of AD. Due to TO incompressibility, 
formation of gases causes the raise of pressure that quite 
often ends by explosion of HV OFEE body. Mixture of 
atmospheric air and hot hydrogen, oxides of carbon and 
hydrocarbon gases can ignite the in

fl

 ammation. In this 

case the damage from explosion increases many times, 
and the work of electrical power substation is suppressed 
for a long time. According to data from foreign and 
native sources, the possibility of early out of service is 
10

-2

, means every 1 from 100 transformers will be out of 

service due to explosion. The possibility of in

fl

 ammation 

after HV OFEE explosion is about 15%.

In case of severe accidents the losses determined 

by cost of the replacing equipment can amount to tens 
of millions dollars. Therefore, the improvement of 
explosion protection for HV OFEE is important for 
the electrical power industry. As time goes, the lack of 
appropriate technical and organizational solutions will 

make this problem worse. Firstly, there is a general trend 
of increasing equipment capacity, and secondly, it’s 
not always possible to provide an adequate renewal of 
equipment.

The necessity of explosion protection improvement 

for HV OFEE is coming from the project "Basic 
principles (Concept) for Technical policy of Russian 
electric power industry for the period up to 2030" and the 
"Regulations of technical policy for JSC "FGC of UES". 
According to HV OFEE operating conditions, we cannot 
completely exclude the possibility of internal short circuit. 
Nevertheless we can achieve reduction of accident risks as 
well as loss reduction by improving HV OFEE design and 
its protection systems.

The destruction degree of HV OFEE is mainly 

determined by energy 

Q

a

 released in AD. The energy 

Q

a

 

depends on AD duration (or action time of protection 
devices), point of ISC origin, characteristics of external 
circuit. According to the literature data range of 

Q

a

 

possible values for industrial HV OFEE exceed by two 
orders of magnitude. For example, 

fi

 xed 

Q

a

 energy values 

in 735 kV power transformers are in a range of 1 to 147 
MJ. [4] 735 kV single-phase transformer tank exploded 
at AD energy level of more than 8 MJ, but 

fi

 re started at 

energy level of more than 14 MJ. However, even at 

Q

a

 

energy level of more than 20 MJ not every transformer 
explosion was accompanied by 

fi

 re.

Q

a

 energy level of 330 kV instrument transformers is 

about 0.3—1 MJ, Qa energy level of 100 MVA distribution 
transformers can be varied in the range of 3—10 MJ. 

Q

a

 


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energy level of more powerful transformers and boxes of 
bushings under AD can be tens of MJ.

Loss reduction can be achieved in several ways 

including:
•   improvement of diagnostic techniques for degradation 

level of TO and POI insulating characteristics in order 
to take an equipment out of service for repair and 
maintenance activities;

•   reduction of protection devices action time;
•   using of alternative insulating 

fl

 uids with improved 

performance instead of mineral transformer oils;

•   creation of a non-explosive HV OFEE design and 

improvement of protection systems.
In response to this problem needed an effective test 

model for equipment under the impact of high pressure 
pulse which occurs in AD. The standard test model for 
explosion protection of HV OFEE based on method of 
electric arc ignition in the internal volume of HV OFEE.

Methods [5—9] present the results of researches which 

justi

fi

 ed an alternative test model for explosion protection 

of HV OFEE. In this method a high pressure pulse which 
occurs after ISC was simulated by chemical energy of 
explosive materials (EM). The new method allows us to 
carry out tests directly on place of manufacture or on site 
of HV OFEE instead of expensive test facilities. Figures 
show that the alternative tests will cost much cheaper than 
standard tests, besides this method would be cheaper for 
larger HV OFEE.

Joint Institute 

о

 High Temperatures of RAS as per JSC 

“FGC UES” order developed an arcless source of pulse 
pressure (ASPP) for explosion protection tests of HV 
OFEE. Tests of protection system prototypes were carried 
out in HV OFEE models at energy effect up to 5 MJ by 
now.

In this study we present test results of protection system 

prototypes for HV OFEE. This work also summarizes 
the research results of AD in transformer oil, which was 
the basis for developed ASPP, and application of this 
method for explosion prevention tests of serial instrument 
transformers 110 kV/330 kV.

According to accepted de

fi

 nition of explosion-proof 

electrical equipment — it is electrical equipment, where 
may occur structural damage under ISC, however all its 
components must be inside the normable safety area close 
to the equipment, which is calculated as the diameter 
(width) of the equipment increased by two of its height 
but not less than 1.8 m.

ARC DISCHARGE IN TRANSFORMER OIL 

Our research results of AD in test model of HV 

OFEE are described in detail in studies[5—8]. The AD 
basic parameters resulting from these studies are below. 
Experiments [5—8] were carried out under conditions 
which are similar to conditions after ISC occurrence in 
industrial HV OFEE, where discharge current increases 
up to 10—30 kA in 3—10 ms. In our experiments the 
critical discharge current reached 30 kA at the rise time of 

1—3 ms. The total discharge duration was 3—20 ms. The 
maximum heat release in the arc 

Q

a

 reached 0.1 MJ. The 

capacitive storage acted as energy source with maximum 
voltage of 5 kV. Used electric circuit allows to simulate 
two half-wave of heteropolar current, however basic 
experiments were carried out with one half-wave voltage. 
The maximum of AD average power is reached during the 

fi

 rst half period, hereafter discharge voltage and power are 

reducing due to resistivity degradation of insulating 

fl

 uid.

AD was ignited between two parallel brass electrodes 

about 20 mm in diameter. The distance between electrodes 
was varied from 17 to 30 mm. The electrodes were located 
in a body of 310 mm internal diameter and of 61 liters 
volume. The liquid volume was 35 liters. The remaining 
volume (26 liters) was 

fi

 lled with nitrogen at atmospheric 

pressure. The electrodes were close to body axis. The 
distance from point of discharge origin to “liquid-
nitrogen” interface was 100 mm.

AD current and voltage, the pressure in the body, the 

pressure of gas bubble above liquid were measured during 
the experiments. Response time of pressure sensor was 
less than 0.5 ms. One pressure sensor (PS) was installed 
close to body foot (PS1), the other was installed at 
50 mm from upper level of the liquid (PS2). We used high-
speed shooting of the discharge with a time resolution of 
0.1 ms and point of discharge origin of “liquid-nitrogen” 
interface with a resolution better than 0.8 ms. Shooting 
was carried out through a side window of 150 mm 
diameter. The amount of hydrocarbon gases formed due 
to decomposition of transformer oil (TO) was calculated 
from pressure value.

The experiments were carried out using different 

liquids: mineral transformer oil of GC brand, vegetable 
oils and distilled water. The research data of AD in TO is 
below.

The arc discharge was initiated by applying voltage 

(~ 3 kV) to a copper wire of 0.1 mm diameter connecting 
electrodes. Fig. 1 shows oscillograms of current and 
voltage in AD. The discharge duration (~ 7,5 ms) is close 
to half-wave voltage duration at power frequency. At start 
time the voltage oscillogram is on a sharp rise, and then 
there is a rapid decline. Hereafter the discharge voltage 
was varied in a small range. The estimated high voltage 
peak duration (~ 20 ms) is equal to electric explosion time 
of copper initiator.

There are some voltage pulsations at current fall time, 

which are probably associated with arc movement on 
the surface of electrodes. Arc movement rate is about 
20 m/sec. Analysis revealed that AD column extends 
under action of its magnetic 

fi

 eld with the result of AD 

voltage increase which causes shunt breakup with further 
voltage decrease. Figures show that typical electric 

fi

 eld 

value in AD column is 0.1—0.3 kV / sm.

High-speed shooting of the discharge showed that 

plasma glow was concentrated close to electrodes at the 
beginning. At this point the glow area started to expand at 
a rate of 0.3 km/s, however in 0.5 ms the rate decreased 


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August 25–29, France, Paris

Fig. 1. Current and voltage oscillograms (current vs. time)

approximately threefold. Thus, the 
plasma expansion rate was much 
lower than the speed of sound in 
TO, which is about 1.4 km/s [10]. 
Radiation 

fl

 ashed over the electrode 

spacing in about 1 ms after AD 
occurrence.

Fig. 2 and 3 show liquid pressure 

"oscillograms" from the same 
experiment as current and voltage 
oscillograms in Fig. 1. As Fig. 2 
and Fig. 3 show that variation of 
pressure in TO is repetitively-pulsed. 
It could be seen quite distinctly 
especially in the beginning of arching 
which is about 3 ms. The 

fi

 rst  six 

pressure extremums (maximum and 
minimum) were registered with PS1 
and PS2. The time difference between 
them were approximately 0.1 ms. 
This time delay was the same as time 
difference of sound propagation in 
TO from AD burning area to pressure 
sensors location. Herewith PS1 signal 
was ahead of time from PS2 signal 
which is associated with location of 
sensors. Correlation from different 
pressure sensors started to decrease in 
about 3 ms after the AD occurrence. 
Apparently, it’s associated with 
arc movement on the surface of the 
electrodes. AD is moving to body 
foot towards PS1 under the action of 
ponderomotive force.

Fig. 2 shows that the 

fi

 rst pressure 

extremums followed at 0,8 ms 
interval, then one-step transition was 
down to 

 0,6 ms. There is some 

correlation between the signals from 
pressure sensors and the discharge 
voltage oscillogram. Thus, the 

fi

 rst 

PS1 pressure maximum is equivalent 
of the "smeared" voltage maximum. 
There was a voltage step up to 
2.2 kV in 3.64 ms after arc ignition 
(Fig. 1) before the absolute pressure 
maximum in the oil, which was 

fi

 xed 

in 3.71 ms after AD occurrence and 
amounted to 

 1,7 MPa (Fig. 2). 

Apparently, there were sound waves 
in a liquid under a sharp voltage 
decrease (breakdown).

Fig. 4 shows a pressure 

oscillogram measured in nitrogen. TO 
rises under the in

fl

 uence of expanding 

gas-vapor bubble, which leads to gas 
compression and pressure increase. 

Fig. 2. Pressure in TO close to the foot (pressure vs. time)

Fig. 3. Pressure in TO close to “liquid-nitrogen” interface

 (pressure vs. time)

Equipment Protection


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According to high-speed shooting the 
liquid level rises uniformly up to 0.1 m 
of height, than vapor-gas mixture 
ruptured to nitrogen area. Typical liquid 
rise rate is approximately 10 m/s, which 
is signi

fi

 cantly less than the speed of 

sound in nitrogen, that’s why the process 
of compression is adiabatic. Fig. 4 shows 
the calculations of Poisson adiabatic 
pressure in gas at two values of liquid 
rise rate, which determine gas volume 
rate. As 

fi

 gure shows, the calculated and 

measured pressure values are consistent 
with each other.

The data of liquid rise rate allows to 

calculate components of energy balance 
in AD. In the experiment 

Q

a

 energy 

value was 64 kJ and the liquid maximum 
kinetic energy was 3.5 kJ. Thus, the 
liquid kinetic energy transferred 5—10% 
of total energy released in the discharge, 
and the main part of the energy was needed for TO heat 
and its decomposition.

After discharge in TO the overpressure in nitrogen 

“blanket” was at 10—50 KPa level, which is proportional 
to gas volume released due to TO decomposition. The 
process of gas formation under AD in TO is characterized 
by ratio of gas formation 

Bg

, which is the ratio of 

released gas volume to AD energy. According to our data 

Bg

 = 0.11 liters / kJ.

According to our tests approximately the same value 

of gas formation under AD (0.1—0.11 liters / kJ) was 
calculated for insulating 

fl

 uids from vegetable oils (rape, 

soya, castor). The electric 

fi

 eld in AD column burning in 

vegetable oils is 0.1—0.3 kV/cm.

 Carried out experiments allow us to de

fi

 ne qualitative 

features of AD dynamic effect in TO to HV OFEE body, 
the main of which is lack of shock waves in liquid. Perhaps 
a shock wave occurs at the moment of initiator explosion, 
but it quickly degenerates into a sound wave. The average 
pressure rise rate in liquid is 0.3—0.5 MPa/ms. At the 
background of increasing pressure of body walls there 
are intensive sound waves. According to our data the 
maximum pressure of body wall in our experiments was 
about 2 MPa. The pressure in the arc burning zone was 
slightly higher. The rate of “liquid- nitrogen” interface 
due to expansion of vapor-gas bubble was 10—20 m/s. 
Estimated that pressure in vapor-gas bubble at such 
boundary 

fl

 uid rate should be 5—10 MPa.

ARCLESS SOURCE OF PULSE PRESSURE 

(ASPP)

The research results of AD de

fi

 ned the requirements 

for arcless source of pulse pressure (ASPP) using for 
simulating AD effect in HV OFEE. The pressure pulse in 
ASPP is generated under the expansion of jet of powder 
gases (JPG) produced due to combustion of explosive 

Fig. 4. Overpressure in nitrogen (pressure vs. time)

materials (EM). It is important that duration of pulse 
pressure effect should be about 50 ms. This fact excludes 
the possibility of EM using for JPG with the necessary 
parameters such as hexogen or trotyl. Therefore, in our 
experiments we used gunpowder as EM, because it burns 
much slower than trotyl. Gunpowder ef

fi

 ciency is 3.8 

kJ/g, speci

fi

 c gas production — 0.9 l/g.

SPG generator was a high-pressure body, where EM 

combustion products 

fl

 ew out from Laval nozzle. The 

pressure pulse magnitude and duration can be controlled 
by changing nozzle area, EM mass, EM allocation in 
combustion body and ignition methods. Experiments 
were carried out in the same body as AD experiments. 
JPG generator was attached to one of the lower windows, 
so that the area of JPG in

fl

 uence was the same as under 

arching. TO and water were used as working 

fl

 uids. Heat 

rate under EM combustion 

Q

 was varied in the range of 

10—50 kJ.

Pressure was measured at characteristic points of the 

body, high-speed shooting of the liquid was recorded 
under JPG in

fl

 uence. According to measurements jet 

pressure at the entrance of the liquid was about 10—
20 MPa for approximately 1 ms. Duration of jet exposure 
to the liquid was varied from 20 to 60 ms. A typical 
pressure value of body walls was about 1 MPa. Motion 
state of "liquid-nitrogen" interface under SPG in

fl

 uence 

immersed in the liquid was the same as under AD under 
the same energy effect. The interface, while remaining 

fl

 at, was rising at a rate of 10—20 m/s. It should be noted 

that there were no big differences between water and 
transformer oil response for JPG impact. 

The experiments proved the possibility of hydraulic 

similarity of liquid 

fl

 ow under the JPG and AD effect. The 

equivalence of the liquid under JPG and AD effect was 
achieved by energy equations and duration of exposure. 
In this context, JPG generator (ASPP) can be used in 


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August 25–29, France, Paris

explosion protection tests for simulating AD effect in HV 
OFEE body.

Fig. 5 presents ASPP calculated for energy of 5 MJ 

before the test of "shooting" from ASPP to controlled 
container 

fi

 lled with air. Fig. 6 shows an "oscillogram" 

of pressure in the controlled container. This 

fi

 gure  also 

shows the calculated pressure values. As it seen, the 
experiment is coherent with the calculation. After reaching 
the maximum measured pressure value is reduced due 
to powder gases cooling. In the model this effect was 
not taken into account. This fact explains the difference 
between theory and practice over the time.

ASPP can be used for series of tasks aimed to improve 

the explosion protection of HV OFEE:
•   Test of HV OFEE production samples;
•  Examination of explosion protection systems and 

devices ef

fi

 ciency;

•   Elaboration of HV OFEE new constructions with high 

level of explosion protection, model testing;

•   Basic data acquisition for development and veri

fi

 cation 

of numerical methods for HV OFEE perspective 
constructions and explosion protection systems.

EXPLOSION PREVENTION TESTS FOR 

INSTRUMENT TRANSFORMERS 

The explosion protection tests using ASPP were carried 

out with serial current transformers (CT) and voltage 
transformers (EVT). Test results were described in detail 
in studies [9, 11, 12]. A brief summary is presented here. 
Transformer models, characteristic values of internal short-
circuit currents and the expected AD energy maximum 
are shown in Table 1. The calculations of AD energy 
maximum were based on fault currents and transformer 
design data from manufacturing plants. During the tests, 
pressure sensors were installed at different distances from 
JPG input inside the transformer. This test was recorded 
by high-speed shooting from two mutually perpendicular 
directions with a time resolution of less than 3.3 ms. 
Area of SPG effect was consistent with most probable 
occurrence of AD area. Transformers were 

fi

 lled  with 

transformer oil of GC brand during the tests.

The design of tested transformers was considerably 

different. In CT manufactured by Russian Federation 
the tank with windings are installed in the bottom 
where located the porcelain insulator with the pressure 
compensator in bellows form (Fig. 4). Bellows provides 
compensation of oil volume temperature changes and 
protection of internal insulation from humidifying. In 
instrument transformers manufactured by Ukraine the 
tank with windings are installed in the top. Protective 
bellows are located under the tank with windings. CT 330 
is equipped with protective membrane instead of bellows.

110 kV CT 

According to test results, exposure time at transformer 

overpressure of 0.5 MPa was approximately 60 ms. By 

means of high-speed shooting was de

fi

 ned 

that basic deformation of the body has 
occurred during the 

fi

 rst 20—25 ms after 

ASPP ignition, when overpressure inside 
CT was about 1 MPa. The transformer 
body has undergone considerable plastic 
deformations, thus its volume has increased 
approximately up to 8.5 liters (12%). 
Deformation of the body did not caused 
TO leakage. 

110 kV Voltage transformer (EVT)

Exposure time of ASPP at energy of 

1MJ was 60—70 ms. EVT overpressure 
was at 0.5 MPa. In 10—15 ms after ASPP 
start the 

fl

 ange of protective membrane 

began to move and 

fi

 nally torn off. The 

bellows has started to move approximately 
in 15 ms after ASPP start and it was 
completely opened approximately in 20 ms. 

Fig. 5. ASPP calculated for 5 MJ

Fig. 6. Pressure in controlled container  versus time response 

characteristics (pressure vs. time)

Equipment Protection


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Transformer 

model

Producer, 

factory

Fault 

current 

range, k

А

Energy 

Q

a

, MJ

Exposure 

time of 

ASPP, ms

1

110 kV CT  

Russian 

Federa-

tion

1.5—3

0.4 60

2

330 kV CT    Ukraine

5—10

1

60

3 110 kV EVT Ukraine

15—20

1

70

Table 1. Instrument transformers tested by ASPP

The internal pressure maximum was about 4 MPa, 
pressure growth rate was about 1 MPa/ms. The protective 
membrane was torn off and TO stream level was risen up 
to 10 meters. Thus radius of TO disorder was less then 
radius of standard safety zone. There were no visible 
deformation of transformer metalware and baseinsulator 
as opposed to bellows and top guard vessel. 

330 kV CT

Tests of two CT models has been carried out. Essential 

design defect of the high-voltage block of the tested 
model have been revealed during 

fi

 rst test. The jet of 

TO with velocity of 20 m/s started to 

fl

 ow from service 

openings located in this part of CT after ASPP start. 
Jet range was about 40 meters. This design of CT has 
been rejected. Further results of tests using modernized 
design of the high-voltage block are considered. Service 
openings in new block have been eliminated, and bellows 
instead of a protective membrane has been established. 
The considerable plastic deformations of body after tests 
have been observed. The maximum deformation of the 
transformer body was up to 30 mm, a de

fl

 ection of top 

fl

 ange edges was 20 mm and a swelling of the transformer 

top cover was 40 mm. Insulation of transformer windings 
has been considerably broken down. 

According to accepted de

fi

 nition these three tested 

transformers including modi

fi

 ed CT of TFRM type can be 

recognized as explosion-proof and 

fi

 reproof at AD energy 

values from Table 1. 

SPECIAL ASPECTS OF TRANSFORMER 

EXPLOSION 

According to analysis of published studies, the most 

vulnerable elements of internal faults occurrence in 
power transformers are bushings, oil-

fi

 lled cable box and 

tap changers. AD develops close to the point of a short 
circuit origin between transformer body ("the ground") 
and construction elements under high potential. The 
length of arc column de

fi

 ning AD voltage depends on HV 

OFEE construction may range from 0.1 to 0.3 meters. 
AD continuously and randomly transfers along internal 
surfaces of the transformer due to ponderomotive forces 
and convective currents. Since the AD characteristic 
average rate is about 10 m/s and the time of its 
“lifetime” — ~ 50 ms, the surface area of the transformer 
under AD effect is ~ 0,1 m

2

. Therefore, AD transfers inside 

the volume of 10-30 liters. This fact gives a reason for 
shock wave absence in HV OFEE despite the high power 
of AD.

Pressure equalization period inside transformer tank 

is estimated as sound wave double transmission time 
of maximum distance between opposite walls of the 
transformer. Pressure equalization period in 330 kV 
instrument transformers which are similar to previously 
discussed transformers is ~ 1 ms. This period is less than 
arc duration. Pressure equalization period in distribution 
transformers is ~ 15 ms. This means that there is a high 

pressure differential in large transformers during arching. 
The maximum value is achieved in arching zone. These 
calculations show that small transformers should be 
destroyed uniformly across the surface under the pulse 
pressure in

fl

 uence. Such damages were recorded during 

our tests of instrument transformers. Failure mode of large 
transformers is local where the damaged area is less than 
10% of total surface area. An example of such damage 
is shown in Fig. 7 — Transformer in Western Siberia 
Substation.

The maximum overpressure for transformer body 

depends on design, location and pulse duration. 
According to general requirements for transformer 
body deformation should be in elastic zone with static 
overpressure of 0.05 MPa. There is tank rupture under 
dynamic loading of the transformer at overpressure 
above 0.5 MPa for more than 5 ms.

Apparently most probable conditions for transformer 

explosion are in the range of 10—30 ms after AD ignition. 
At early stage of AD burning for about 10 ms, internal 
pressure of the transformer does not reach critical values. 

Fig. 7. Transformer in Substation after explosion


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August 25–29, France, Paris

At later stages of AD in about 30 ms, the probability 
of explosion is lower. Firstly, AD electric power is 
signi

fi

 cantly reduced due to increase of insulating 

fl

 uid conductivity. As a result there is a decrease of gas 

formation rate in AD and changes in pressure growth 
rate. Secondly, there is an effect of internal volume of the 
transformer tank due to elastic and plastic deformation 
of the walls under high pressure in

fl

 uence by this point 

of time. The additional volume partially compensate for 
pressure growth due to TO decomposition.

In the view of foregoing considerations, we can lay 

down basic requirements for explosion protection systems 
of HV OFEE:
•   the response time for internal pressure increase should 

not exceed 5 ms;

•   the system should limit the pressure maximum in the 

transformer tank at the level of 0.3—0.5 MPa.
Protection system should be installed close to problem 

areas of the transformer in case it is not possible to protect 
the entire internal surface.

MODEL TESTS FOR EXPLOSION 

PROTECTION OF HV OFEE

Well-known protection methods of HV OFEE focus 

on formation of additional volume 

Δ

V for expansion of 

TO in case of internal pressure growth under AD. The 
effectiveness of the protection system can be estimated 
from:

k

 = 

Δ

V/ B

g

Q

a

   

(1)

The 

k

 value so-called a protection system reliability 

parameter is the ratio of the additional volume of TO and 
gas volume released due to TO decomposition under AD. 
Approximately we can take as follows
•  

k

 > 0.7—0.8 tank deformation is elastic and the 

equipment is explosion safe as well as explosion proof;

•   0.3 < 

k

  <0.7 should be considerable plastic deformation 

of the transformer body;

•  

k

 < 0.1—0.3 should be expected explosive destruction 

of the body.
Tentatively we can take typical values of protection 

system reliability as 

k

~ 0.7—0.8 

и

 

k

2

 ~ 0.1—0.3.

There are two ways of additional volume formation 

for TO in literature. The 

fi

 rst method is based on 

using porous coverings on the internal surfaces of HV 
OFEE body [13]. It is expected that porous material 
is compressed under high pressure in

fl

 uence  resulting 

formation of available volume, which is 

fi

 lled  with 

expanding hydrogen, carbon oxides and hydrocarbon 
gases due to decomposition of TO. In consequence 
pressure growth inside the body is limited. Additional 
protective effect can be achieved in case compression 
of material is enough to spend a signi

fi

 cant portion of 

kinetic energy of TO 

fl

 ow for compression. "Porous 

covering" method may be effective in case substantial 
compression of porous material takes place at a relatively 
low overpressure — approx. 0.3—0.5 MPa. Up to the 
present day this method of protection has not passed the 
tests and it’s not used in practice.

The second method of protection is so-called "protective 

membrane" method. Principle of method is installation 
of protective membranes on HV OFEE body which are 
destroyed under action of AD pulse pressure and used for 
TO 

fl

 ow to special container. [14] It is considered that this 

way internal pressure of HV OFEE can be kept within 
acceptable limits. The second protection method is widely 
used, for example in SERGI Transformer Protector system.

Porous covering method 

In the experiments of this protection method we used 

HV OFEE breadboard model which looks like steel 
cylindrical tank of 0.95 m

3

 volume and 1.45 m height 

with a conical nozzle at the bottom. Tank diameter was 
1 m, thickness was 7 mm. The cover has been screwed 
to a sidewall by 24 bolts with thread diameter of 12 mm. 
The plate of polyfoam of 50 mm thickness has been glued 
on a steel cover of a breadboard model. The polyfoam 
was made from extruded crumb with 0.04 kg/dm

3

 density. 

The tank was 

fi

 lled with water. ASPP was installed at a 

distance of 0.2 m from the top cover. Estimated value of 
ASPP pulse was 0.35 MJ.

Figure 8 shows a shot with breadboard model in 80 ms 

after ASPP start, when the cover has lifted in 0.8 meters. 
After experiment only 3 bolts from 24 has survived, the 
cover de

fl

 ection has amounted to 50 mm, the polyfoam 

has crumbled into small fractions.

Hence the discussed protection method of porous elastic 

covering cannot protect OFEE body from considerable 
deformation under action of a high pressure pulse. This 
result was expected. Indeed for the effectiveness of this 
protection system needed volume increase which is 
accessible for the liquid due to compression of porous 
material for its compensation in 3—5 ms — the time of 
pressure rise in the liquid. This is possible either at slow 
pressure rise rate of 0.1 MPa/ms, or at small sizes of the 

Fig. 8. Test of HV OFEE breadboard model with 

porous covering 

Equipment Protection


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41

info@eepr.ru,   www.eepr.ru

protected model of 0.1 meter. The 
main impulse load took only a part 
of tank working surface under AD 
and internal volume increased due to 
compression of porous material was 
not enough to prevent an explosion.

These tests were carried out using 

polyfoam as a damper material. 
It’s possible to use other damper 
materials with different properties. 
However, the ef

fi

 

ciency of the 

protection system in general cannot 
be suf

fi

 ciently high. This fact can be 

illustrated by following calculations. 
If the typical size of transformer tank 
is 

a

, then the possible increase of 

available oil volume under covering 
of transformer internal surface 
with damper material of maximum 
compression 

h

 will be about:

Δ

V

 ~ 

6a

2

h

                  (2) 

The porous covering regularly 

compresses under action of pulse 
pressure only in case tank size is under  

~ 0,5 m. Taking for calculations that 

a

 ~ 0,5 m and 

h

  ~ 0,02 m, we 

fi

 nd by 

formula (2), 

Δ

V

  ~ 30 liters. It may be 

enough to protect the transformer from the explosion at 
AD energy of 0.5 MJ (

Vg

  ~ 55 liters).

At the increasing size of transformer tank after 

short circuit occurrence the covering can be effectively 
compressed only close to the short-circuit origin with total 
area of 1 m

2

. In this case, the additional volume will be 

Δ

V

 ~ 20 liters. The effectiveness of this protection system 

under AD energy 

Q

a

  ~ 1 MJ (

Vg

  ~ 110 liters) will be 

k

  ~ 0.2, i.e. even at a relatively low AD energy we can 

expect explosive destruction of tank.

In summary, porous coverings which are compressed 

at pulse pressure of 0.3—0.5 MPa, having an effective 
Young's modulus of 0.5 MPa, may be used in explosion 
proof transformers with the tank size up to 0.5 meters if 
expected AD energy does not exceed 0.5 MJ. In addition 
this porous covering material has to maintain its properties 
during continuous operation.

Method of a protective membrane 

Fig. 9 shows a test of OFEE breadboard model using 

this protection method. The HV OFEE model looks like 
steel cylindrical vessel of 1,4 m diameter 

fi

 lled with water. 

The thickness of breadboard model steel cover is 12 mm. 
The air volume has been separated from the water by an 
aluminum membrane with the thickness of 0.2 mm and 
diameter of 200 mm. The concrete blocks of 70 kg have 
been installed at a distance of 300 mm from the cover to 
simulate transformer windings. Area of ASPP effect was 
between concrete blocks and cover at 0,2 m distance from 
roof opening — opposite the PS2. PS1 detected pressure 

in air bubble behind the membrane, PS3 and PS4 detected 
pressure in 

fl

 uid from far 

fi

 eld of ASPP effect. The pulse 

action for HV OFEE body was recorded by high-speed 
shooting. Pulse energy of high pressure was 1MJ, pulse 
duration was 50 ms.

Membrane contact sensor recorded its rupture in 3 ms 

after ASPP start. The water 

fl

 ow rate through membrane 

calculated due to air pressure changes in air bubble 
behind the membrane was 20 m/s. Pressure maximum in 
the liquid has reached 1.8 MPa. High-speed shooting has 
shown that the deformation of HV OFEE body had lasted 
for 10—15 ms. After the test we found out that the residual 
deformation of steel cover was about 40 mm and concrete 
blocks were moved for 50 mm. The conditions for the 
discussed protection method in this test were optimal: a 
thin membrane of a large diameter was installed right in 
front of the epicenter of the pressure rise. However the 
present protection method was not effective enough.

The tank rupture prevention system in Fig. 9 is a 

simpli

fi

 ed version of SERGI Transformer Protector (TP) 

system. This protection system is used in energy utilities of 
Russia in recent years, but the experience of its operation 
is not encouraging. On September 22, 2009 there was an 
explosion of AT-1 — 330 kV tank due to ISC at substation 
"Mashuk", where this system was installed. SERGI gave 
an explanation of this rupture in the report [15]. According 
to this report there was a peak current of 10 kA and arc 
duration of 60 ms at the time of rupture. In the analysis 
of TP system SERGI experts assumed the AD voltage 
of 37 kV, so that the energy released in AD was about 

Fig. 9. HV OFEE breadboard model for tests using protective 

membrane method


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42

August 25–29, France, Paris

11 MJ. This AD voltage value seems conservative, because 
it was calculated without taking into account the voltage 
loss in inductance. According to our estimates, the AD 
voltage was signi

fi

 cantly low, so that the total AD energy 

was about 4 MJ. GPFD volume (gaseous products of 

fl

 uid 

decomposition) was about 0,45 

м

3

 under this 

Q

a

 

energy.

According to data [15] the protective membrane of 

8 inches diameter (~ 250 mm) was destroyed in 4,5 ms 
after short-circuit occurrence at pressure of 0.08 MPa. 
There was TO 

fl

 ushing through the opening which 

caused "Depressurisation" of transformer tank in 112 ms. 
According to this report [12], even though the TP system 
did not protect the HV OFEE body from explosion it 
prevented the 

fi

 re occurrence. However, the data in the 

report [15] raise some doubts. According to this data the 
maximum oil 

fl

 ow rate through the destroyed membrane 

does not exceed 20 m/s. According to calculations based 
on report values of TO 

fl

 ow rates, approximately 25 liters 

of TO were leaked through the diaphragm under AD. 
Therefore, the reliability coef

fi

 cient of the protection 

system (1) is 

~ 0.07, that’s why TP couldn’t protect HV 

OFEE tank from explosion. As for the lack of 

fi

 re,  the 

probability of its occurrence after rupture does not exceed 
20%, so we cannot give a credit to TP system especially as 

Q

a

 energy was relatively small.

For information, the autotransformer tank rupture after 

explosion on "Mashuk" substation was local, total damage 
area was 1 

м

2

.

TESTS OF DYNAMIC PROTECTION 

SYSTEM 

The analysis shows that well-known explosion 

prevention systems of HV OFEE are not effective enough 
and it’s a necessary to develop new protection systems. 
This section brie

fl

 y describes tests results of dynamic 

protection system (DPS) developed in Joint Institute 
for High Temperatures, Russian Academy of Sciences. 
The main elements of DPS are spring-loaded running 
blocks (Fig. 10). The blocks were installed on the side of 
transformer body close to the most probable occurrence 
of short circuit area. The maximum displacement of the 
blocks under in

fl

 uence of pulse pressure was ~ 0,3 m. 

Protection of bushings was carried out 
using special diaphragms.

The tests were carried out in 

autotransformer (AT). The autotransformer 
was out of service, but all the elements 
have been preserved inside the body. Fig. 
11 shows a photo of the transformer with 
established DPS elements (guard vessels are 
painted blue). There were 16 running blocks 
under circular guard vessel and 35 blocks 
under rectangular guard vessel (Fig. 10). 
There was an ASPP inside the protective 
chamber from the left. DPS wasn’t installed 
at the back side of the transformer, it was 
installed at one of three bushings.

ASPP with energy of 1 to 3 MJ and 

exposure duration of 30 to 50 ms was 
used in the tests. High-speed shooting 
(up to 2000 frames per second), four PS 
and displacement sensors were used for 
diagnostics of the tank deformation. The AT 
tank was 

fi

 lled with water.

A series of experiments were carried 

out. The pressure pulse was supplied to 
most likely points of short circuit origin 
from both sides of transformer, including 
the bushing area. Plastic deformation of 
the tank with partial destruction of the 
structural elements but without leaks was 
recorded under pulsing of the back side of 
autotransformer without DPS. High-speed 
shooting has recorded that blocks started to 
move in 5 ms after pulsing.

The study highlights:
• pressure maximum in autotransformer 

breadboard model increases approximately 

Fig. 10. Dynamic protection system (working valve 

block without guard vessel)

Fig. 11. AT with DPS before tests 

Equipment Protection


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info@eepr.ru,   www.eepr.ru

in proportion to ASPP energy: under the energy of 1 MJ 
pressure maximum is about 0.5 MPa, under the energy of 
3 MJ the pressure exceeds 1 MPa.
•   The basic body deformation without DPS starts in 

20—30 ms after the pulsing.

•   Displacement rate of blocks with DPS increases due 

to ASPP energy increase: maximum velocity of blocks 
reaches 30 m/s at ASPP energy of 3 MJ.

•   DPS has much lower response time in comparison with 

the factory explosion protection system of protective 
membrane.

•   DPS installed in front of the pulse pressure bushings 

protects the body from plastic deformation up to 3 MJ 
of pulse energy.
It is estimated that DPS with tested con

fi

 guration has 

reliability coef

fi

 cient 

k

 ~ 0,5. The reliability of explosion 

protection can be increased from 30 up to 50% in case 
DPS is installed both on sides of the transformer and all 
high-voltage bushings.

CONCLUSIONS

1. Presented research results of transformer oil 

fl

 ow 

and liquid 

fl

 ow under action of AD and jet of powder 

gases (JPG). The study shows that the transformer oil 

fl

 ow and liquid 

fl

 ow under action of AD and JPG impact 

are similar at equal values of energy duration and energy 
exposure.

2. Developed an arcless source of pulse pressure 

(ASPP) for 

fi

 re and explosion prevention tests of HV 

OFEE under energy of 5 MJ.

3. Developed test method for HV OFEE using ASPP. 

The proposed method is recommended for model tests 
and for evaluation of explosion prevention system 
performance for all types of HV OFEE. This method is 
used as an alternative to existing method based on AD 
initiation inside the equipment.

4. Described experience of explosion tests for serial 

instrument transformers.

5. Experimentally shown that the protection method 

of porous coverings which can be compressed under 
high pressure of expanding vapor-gas bubble on internal 
surfaces of HV OFEE cases — cannot be effective for 
large transformers.

6. Test models of “protective membrane” method 

showed that this method didn’t protect HV OFEE body 
from plastic deformation to its explosion. Reliability 
parameter of this protection method doesn’t’t exceed 0.1. 

7. Presented dynamic protection system (DPS) of 

explosion prevention for HV OFEE. Tests of DPS installed 
inside autotransformer showed that DPS prevents the 
explosive destruction of the autotransformer body at least 
at the energy of 3 MJ.

LITERATURE

1. Darian L.A., Arakelian V.G, Gas-resistance of 

insulation liquids. Electrotechnika (rus) 

 2, 1997, p. 

45—49.

2. Vanin B.V., Lvov U.N., Neklepaev U.N. etc, About 

faults of 110—500 kV transformers in service // Power 
stations. 2001. 

 9. p.53.

3. Petersen A.,«The Risk of transformer 

fi

 res  and 

strategies which can be applied to reduce the risk» // 
Session CIGRE-2010. France. Paris. 2010. Report A2-
101.

4. Foata M., Dastous J.B.,«Power transformer tank 

rupture prevention // Session CIGRE-2010. Paris. 
2010. Report A2-102.

5. Darian L.A., Dementiev Yu.A., Efremov V.P., 

Polistchook V.P., Shurupov A.V., Alternative method 
of estimation of explosive safety of high-voltage oil-

fi

 lled electrical equipment, Electro, N 5, P. 43—46, 

2009. 

6.  Daryan L.A., Kozlov A.V., Luzganov S.N., Povareshkin 

M.N., Polistchook V.P., Shurupov A.V., Shurupova N.P. 
Experimental study of a 

fl

 ow of liquid under action of 

an arc discharge and jet of powder gases// Physics of 
Extreme States of Matter- 2010. Chernogolovka. 2010. 
P. 112.

7. Daryan L.A., Kozlov A.V., Kotov A.V., Povareshkin 

M.N., Polistchook V.P., Shurupov A.V., Shurupova 
N.P., Pulse arc discharge in mineral and organic oils// 
Proceedings of Int. Conf. on Physics of Extreme States 
of Matter-2012. 1—6 March, Russia, Elbrus. Institute 
of Problems of Chemical Physics. Chernogolovka. 
2012. P. 168.

8.  Darian L.A., Kozlov A.V., Luzganov S.N., Povareshkin 

M.N., Polishuk V.P., Shurupov A.V. Shurupova N.P., 
Pulsed arc discharge in mineral and organic oils // 
Proceedings of Low Temperature Plasma Physics — 
2011. Petrozavodsk. 2011. Publisher Petrozavodsk 
State Universitety. Vol. 1, p. 106—111

9.  Darian L.A., Kozlov A.V., Povareshkin M.N., Polishuk 

V.P., Shurupov A.V., Son E.E., Fortov V.E., Arcless Tests 
of the High Voltage Oil-

fi

 lled Electrical Equipment on 

Explosion-proof // News of RAS. Energetics. 2011. 

5. 

p. 74.

10. Physical quantities. Catalogue edited by Grigorieva I.S. 

and Melikhova M.: Energoatomizdat. 1991. 1260 p.

11. Darian L.A., Polishuk V.P., Shurupov A.V., Arcless Tests 

of the High Voltage Oil-

fi

 lled Electrical Equipment on 

Explosion-proof. Energo-info (rus), 2012, 

 9, p.54—

58.

12. Darian L.A., Kozlov A.V., Povareshkin M.N., Polishuk 

V.P., Shurupov A.V., Arcless Tests of the High Voltage 
Oil-

fi

 lled Electrical Equipment on Explosion-proof // 

Electro. 2011. 

 5. p. 23.

13. Mishuev A.V., Kazennov V.V., Gromov N.V., Fire and 

explosion protection device for transformer under AD. 
Patent RU 2334332. 

14. Manie F., Explosion protection device for transformers. 

Patent RU 2263989. 

15. 

SERGI company. Substation «Mashuk». Activation 
of Transformer Protector on 22/09/ 2009. Reference 
Are61rr00.03e 

от

 09/10/09.


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Life time of transformers or another HV oil-fi lled electrical equipment (HV OFEE) is about several decades. The gradual degradation of paper-oil insulation occurs under the infl uence of partial discharge, heating, cavitation and other factors in service.

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