TRANSFORMER is one of the most vital equipment in electrical transmission system. Power transformer is a static electric machine which transforms electric energy from one voltage level to another voltage level at a constant frequency. There is no moving part in transformer & hence it is called static machine. Grid operation and Power transmission can be reliable only if power transformers and transmission lines perform well. The transformers are very costly device. They are such an important part of a generation, transmission and distribution of an electrical system. So, one can say that transformer is a heart of an electrical system and hence they are required to be protected against any kind of faults occurring. If a fault is allowed to be persisted the equipment will get damaged causing a loss of corers of rupees. Even a micro volt discharge can disrupt mega units of system and hence causing losses, hence it is necessary to take the transformer out of service as soon as possible so that the damage is minimized and hence loss is minimized.
Traditionally the protection of transformers has been relegated to the application of transformer differential and back up over current relays to provide short circuit protection. But nowadays due to advent of multifunction digital and numerical relays, the term protection of transformer has gained a whole new level.
A transformer is a device that
i Transfer electric power from one circuit to another.
ii It does so without a change of frequency.
iii It accomplishes this by electromagnetic induction.
iv Where the two electric circuits are in mutual inductive influence of each other.
Main part of Transformer:-
1) Main Tank
2) O.L.T.C.
3) Conservator Tank
4) Breather
5) Radiator
6) Explosion vent
7) Cooling device
Basic information about Power Transformer Protection:
Large power transformers belong to a class of very expensive and vital components in electric power systems. If a power transformer experiences a fault, it is necessary to take the transformer out of service as soon as possible so that the damage is minimized. Traditionally, the protection of transformers has been relegated to the application of transformer differential and back up over current relays to provide short circuit protection. With advent of modern multifunction transformer packages, differential and over current protection are only two of many protective and logic functions that can be incorporated into transformer packages. Transformer protection requirements also vary depending on location of transformer in power system. Since Transformer protection requirements vary depending on application, users typically want only those functions that are needed for specific applications. In addition, the use of programmable logic functions extends the benefit of digital multifunction transformer protection.
Figure 1: Modular and interfacing structure of Relay Micom P632…
The costs associated with repairing a damaged transformer may be very high. The unplanned outage of a power transformer can also cost electric utilities millions of rupees. Consequently, it is of a great importance to minimize the frequency and duration of unwanted outages. Accordingly, high demands are imposed on power transformer protective relays.
The operating conditions of power transformers do not make, however, the relaying task easy. Protection of large power transformers is perhaps the most challenging problem in the power system relaying area.
Faults in transformers
Faults can be divided into three main classes:
- a) Faults in auxiliary equipment:
The incipient faults can develop major faults. Some of them are as follows:
1) Transformer oil:
2) Gas cushion:
3) Oil pumps and Forced Air fans:
4) Failure of insulation between Laminations of core and core bolt insulation failure:
5) Badly made joints and connections:
6) Inter turn faults:
- b) Winding Faults:
When the insulation between windings and between the winding and core fails, electrical faults are said to have taken place.
- c) Through Faults:
The through faults can occur due to overloads or external short circuits.
Types of protection
- Differential protection
• Definite Time Over Current protection
• Inverse Definite Minimum Time(IDMT) protection
• Under Voltage
• Over Voltage
• Over Fluxing
• Restricted Earth Fault
Specification of transformer used
I implement a single phase transformer protection scheme by numerical relay MiCom P632 manufactured by ALSTOM; in laboratory so that student can understand its real aspects and how protection scheme is developed in real field so for that I took 1KVA core type transformer.
– 1 kVA TRANSFORMER
– 220/110 VOLTAGE
– Core type transformer
– Single phase transformer
– Frequency=50Hz
– Turns ratio 1:1
– Cooling – Natural air cool
– INS CLASS-B
Specification of CTs:
– Burden 15kVA Class 5P
– Ratio 10/5 Frequency=50Hz Voltage =0.66kV
– H.V WINDING10/5 AMPERE
– L.V WINDING 5/10 AMPERE
Table 1: Function group of Micom P63* Relay group…
Fig. 2: Terminal diagram of Relay Micom P632…
Introduction of Micom P632 Relay
The P63x differential protection devices are designed for the fast and selective short-circuit protection of transformers, motors and generators and of other two-, three- or four winding arrangements. Four models are available.
The P631 and P632 are designed for the protection of two-winding arrangements, the P633 and P634 for the protection of three- or four-winding arrangements, respectively.
Main functions of Relay MiCom P632:
Fig. 3: Tripping contact diagram of Relay Micom P632…
- Three-system differential protection for protected objects with up to four windings.
• Amplitude and vector group matching.
• Zero-sequence current filtering for each winding may be deactivated.
• Triple-slope tripping characteristic.
• Inrush restraint with second harmonic, optionally with or without global effects may be deactivated.
• Over fluxing restraint with fifth harmonic component may be deactivated.
• Through-stabilization with saturation discriminator.
Fig. 4: Tripping characteristic of the differential protection…
- Ground differential protection.
• Definite-time over current protection.
• Inverse-time over current protection.
• Thermal overload protection.
• Over-/ under frequency protection.
• Over-/ under voltage protection (time-voltage protection).
• Limit value monitoring.
• Programmable logic.
Global functions
In addition to the features listed above, the P63x models provide comprehensive self monitoring as well as the following global functions:
- Parameter subset selection.
• Operating data recording (time-tagged signal logging).
• Overload data acquisition.
• Overload recording (time-tagged signal logging).
• Fault data acquisition.
• Fault signal recording (time-tagged signal logging with fault value recording of the phase currents for each winding).
• Extended fault recording (fault recording of the neutral-point current for each winding as well as the voltage).
Introduction of transformer differential
Fig. 5: Power circuit…
Fig. 6: Control circuit…
protection
On the basis of the primary transformer currents, the differential protection devices can be flexibly adapted to the reference currents of the protected object. Amplitude matching is by means of a straight-forward input of the reference power common to all windings plus the nominal voltages and the nominal transformer currents for each winding. Zero-sequence filtering may be deactivated separately for each winding in case of an operational grounding within the protected zone. The tripping characteristic of the differential protection device has two knees. The first knee is dependent on the setting of the basic threshold value Id> and is on the load line for single-side feed. The second knee of the tripping characteristic is defined by a setting. Above the user- selected differential current level Id>>>, the restraining current is no longer taken into account. Up to a certain limit, stability in the event of external faults is ensured by means of the bias. Due to the triple-slope tripping characteristic, the stabilization is particularly pronounced for high currents.
Fig. 7: settings of function parameter<parameter subset1<diff (this is a screen shots of micom software based setting program for which user have to install micom software given by ALSTOM)…
Fig. 8: settings of function parameters<parameter subset1<DTOC1 (this is a screen shots of micom software based setting program for which user have to install micom software given by ALSTOM)…
However, as an additional safeguard for through currents with transformer saturation, the MiCOM P63x series differential protection devices are provided with a saturation discriminator. Particularly the start-up of directly switched asynchronous motors represents a problem in differential protection due to transient transformer saturation caused by a displacement of the start-up current for relatively high primary time constants. Even under such unfavorable measurement conditions, the MiCOM P63x series differential protection devices perform with excellent stability. Stabilization under inrush conditions is based on the presence of second harmonic components in the differential currents. The ratio of the second harmonic component to the fundamental wave for the differential current of the measuring systems serves as the criterion. Optionally, tripping is blocked either across all three measuring systems or selectively for one measuring system. However, from a user-selected differential current level Id>>, the blocking criterion is no longer taken into account. For application as differential protection device for motors or generators, the harmonic restraint can be deactivated.
Fig. 9: fault recording graph of transformer DTOC over current protection scheme…
Fig. 10: fault recording graph of transformer IDMT over current protection scheme…
Power circuit diagram:
As shown in circuit diagram we have used 1 transformer of 1 kVA, 239/110V, two CTs having ratio of 10/10 Amp (primary side) and 10/5Amp (secondary side). Two rheostat (1 for load and another for internal fault purpose) having a ratio of 185ohm,1.5Amp.
Control circuit diagram
For energizing transformer we push ON button and it will close the contact C1, C2 of the contactor coil. As internal fault occurs contact of relay closes thus energizing auxiliary relay. The Ax1 thus opens up from its normally closed position to normally open. This de-energizes the contactor coil and hence circuit gets de-energized.
Terminal Diagram
X091-RELAY CONTAC
X032-PRIMARY SIDE CT INPUT X052-SECONDARY SIDE CT X031-VOLTAGE INPUT FROM PT X093-AUX- POWER SUPPLY (7,8)
Software settings done in Relay MiCom P632 for differential protection scheme:
(This flow chart is shown for easy understanding when you are working on Relay and is given for only one protection scheme.)
>CONFIIGURATION PARAMETER
->DIFF-WITH
>CONFIGURATION PARAMETER
->OUTP
->>GENERATION TRIP SIGNAL1
>FUNCTION PARAMETER1
->GLOBAL
->>MAIN
->>>DIFFERENTIAL TRIP SIGNAL1
>CONFIGURATION PARAMETER
->LED X7>FUNCTION PARAMETER
->GENERAL FUNCTION
->>DIFF-Yes
Fig. 11: settings of function parameters<parameters subset1<IDMT1 (this is a screen shots of micom software based setting program for which user have to install micom software given by ALSTOM)…
Calculations of set values in the reference of 1 KVA transformer:
We have value of Sref=0.1 MVA, Vnom=54.4kV
Current Iref=(Sref)/(√3 * Vnom)= 1 Amp
Fault resistance is placed at 45ohm.
So that current I=2.55 Amp
Differential current Idiff=1.55 Amp
Procedure for checking transformer differential protection scheme:
For a creation of fault there is a rheostat with a switch connected in a series with it and in open position. This is connected on a primary side as shown in a power diagram. Now as the switch is closed, thus the primary is short through the rheostat. So now Idiff passes through the relay as there is no current in the secondary side. The value of this diff current is above the pickup and hence the diff protection will be operated. So the relay will operate when Idiff is more than a set value and generate the trip signal and giving trip command.
Fig. 12: settings for over voltage (this is a screen shots of micom software based setting program for which user have to install micom software given by ALSTOM…
Fig. 13: settings for under voltage (this is a screen shots of micom software based setting program for which user have to install micom software given by ALSTOM…
Introduction and scheme of Definite time and inverse time over current protection of Transformer
Both the definite-time and the inverse-time over current protection operate with separate measuring systems for the evaluation of the three phase currents, the negative-sequence current and the residual current. Three stages each are provided for the three protections. The inverse-time over current protection offers a multitude of ripping characteristics for the individual measuring systems
Software settings done in Relay MiCom P632 for definite time over current protection scheme(This flow chart is shown for easy understanding when you are working on Relay and is given for only one protection scheme.)
>CONFIGURATION PARAMETER
->DTOC-1-WITH
>CONFIGURATION PARAMETER
->OUTP
->GENRAL TRIP SIGNAL1
>FUNCTION PARAMETER1
->>MAIN
->>>DTOC-1 TRIP SIGNAL1
>CONFIGURATION PARAMETER
->LED X8
>FUNCTION PARAMETER
->GENERAL FUNCTION
->>DTOC-1-Yes
Procedure for checking transformer over current protection scheme:
Push the green push button and energize the circuit. Now select the IDMT1 function in parameter subset1 and select “NO” value in enable feature so that this function is temporarily bypassed. This is done to show the individual test function of DTOC. Now, increase the load current by decreasing the rheostat load. As the current goes above specified amp the relay will send trip signal after the set time 5sec as the stated above and circuit gets de-energized.
Fig. 14: fault recording graph of transformer over voltage protection scheme…
In the P632, two three stage Inverse-time over current protection. Functions (IDMT1 and IDMT2) are implemented and can be assigned to the two transformer ends. It works on the principle that, if the current measured by the CT goes above the threshold then according to selection of operation characteristic the relay will send the trip signal. For each IDMT function, a setting parameter is provided for this assignment by the user.
That means as the value of current goes above 0.10A then the protection feature gets enable and the relay send the trip signal according the characteristics that has been selected.
Introduction and scheme of under and over voltage protection of Transformer:
The two stage voltage-time protection function of the p63x evaluates the fundamental wave of the pulse voltage. V<> protection is ready when it is enabled and measuring circuit monitoring has not detected a fault in the voltage measuring circuit. The p63x checks the voltage to determine whether it exceed or falls below set threshold. The triggers are followed by timer stages that can be blocked via appropriately configured binary signal inputs. If the decisions of under voltage monitoring are to be included in the trip Commands, then it are recommended that transient signals be used. Otherwise the trip command would always be present when the system voltage was disconnected, and thus it would not be possible to close the circuit breaker again.
Here we do not have a three transformer or a PT connected to transformer. Hence we have used auto-transformer for simulation purpose.
Here the setting is multiple of Vnom voltage.
This voltage is set by software, means we can set this value according to the PT secondary available. Thought here we do not have a PT so we have set this standard value to 110 volt. Here there is setting of Vnom PT sec. this setting has to be changed to 110 volt. We can change it according to the option available in that menu and hence make our further setting accordingly for trip command. So this means, if we set the threshold of 0.7 Vnom for V>setting then the relay will send the trip signal as soon as the voltage goes above 77V.
Fig. 15: settings of function parameters<parameter subset1<v/f (this is a screen shots of micom software based setting program for which user have to install micom software given by ALSTOM…
Similarly for under voltage feature, we have set the threshold of 0.4 Vnom in V<menu available in the same menu. This means if anyhow the voltage dips beyond 44 volt then the relay will give out the trip signal after the set time or even instantaneously.
The operating time can be set in the same menu under tV>for overvoltage and tv<for under voltage. For both the features we have kept the time as 5 seconds.
Procedure for checking transformer over and under voltage protection scheme
Connect the auto-transformer output between pin 1 and pin 4 of X032. Now before energizing the circuit, energize the auto-transformer and set the voltage anywhere between 44V to 77V. now push the green push button and energize the circuit. You will notice the relay and circuit in healthy state. Now for testing over voltage feature, slowly increase the voltage knob such that it crosses 77V.
As soon the threshold is crossed after 5 second, the relay will send the trip signal and visual indication will also be seen.
Similarly follow procedure from first step after clearing the fault and test for under voltage.
Introduction and scheme of over fluxing protection of transformer:
The P63X checks the voltage to detect whether it exceeds or falls below set threshold. The frequency is determines from the difference in a time between the zero crossing of the voltage. The voltage is the one that is measured by PT and send to relay at its terminal. As for simulation purpose we are using a single phase transformer, the frequency is constant. An introduction of frequency changer is not feasible so far over fluxing we have kept the frequency constant and we will change the voltage and thus the ratio of V/f will change. The input for this is also the same terminal where we have given supply for under voltage protection. The PT secondary voltages are tested for the given protection and will acts accordingly.
Procedure for checking transformer over fluxing protection scheme:
First of all to avoid any conflict in simulation of protection, we disable the over- under voltage feature. This is done as here also it is required to change the voltage level by auto- transformer to change the ratio of V/f after disabling the over under voltage feature, set the voltage level at 110V. Now push the green button and the circuit energize. If the voltage level is increased by 1 volt, immediately we can see the visual indication of protection of function and the alarm LED flashing. Now further if we increase the voltage then the value of ratio of V/f increases. When this crosses the threshold for trip signal then the relay sends the trip command exactly after the pre set delay time.
Fig. 16: fault recording graph of over fluxing protection scheme of transformer…
We have set Vnom as110V and fnom as 50 Hz. Normally the ratio of V/f for this simulation should be equal to 110/50=2.2 so we have set the alarm value for this protection as 1.00*(Vnom/fnom).
Meaning above 2.2 thresholds, the relay will not trip but will give alarm and visual indication.
Now if the value of the ratio goes above 2.8 times, then the relay will give trip command after a delay of 5 sec.
Conclusion
In relay MICOM P632, many protections are provided for the transformer such as differential protection, under-voltage protection, definite time over current protection, and inverse time over current protection, thermal overload protection, over/under frequency protection and many more.
The major operating challenge to transformer differential protection is maintaining security during CT saturation for external faults while maintaining sensitivity to detect low magnitude internal faults. CT saturation reduces the secondary output from the CT, and cause a false differential current to appear to relay. We have also faced a problem i.e. the transformer is of 1KVA and the lowest range in MICOM P632 is 0.1 MVA.
Relay calculates differential current in multiples of Ire which is based on MVA rating. So we have to set Ire equal to 1 and we have to set Vmin to 57.73 KVA.
As a result we always get better results for protecting the transformer by using numerical relay MICOM P632 rather than using an electromechanical relays.
Reference: Manual of Micom P632 Relay provided by ALSTOM
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