1.1 Select BI for line protection BI7The maximum current working through the line L : 1.2 Select BI for transformer protection 1.2.1 Select BI1 BI4 - Current transformer BI1 và BI4 are s
SELECT THE CURRENT TRANSFORMER
SELECT BI FOR LINE PROTECTION (BI7)
The maximum current working through the line L :
Select BI has the rated current 350A and the rated second current 5A, the rate voltage 22 kV The conversion ratio is n BI 7 = 350
SELECT BI FOR TRANSFORMER PROTECTION
- Current transformer BI1 và BI4 are select with the same the conversion ratio The maximum current flows through BI1 :
Select BI has the rated primary current 300A and the rated second current 5A, the rate voltage 110 kV The conversion ratio is n BI 1 =300
- Similar, the conversion ratio of BI4 is n BI 4 =
- Current transformer BI2 và BI5 are select with the same the conversion ratio
Considering the overload condition of the MBA, the maximun current flows through BI1 is:
Select BI has the rated primary current 1500 A and the rated second current 5A, the rate voltage 22 kV The conversion ratio is: n BI 2 =1500
- Similar, the conversion ratio of BI5 is : n
We choose BI3, BI6 same BI1 so the ratio is n BI 3 =n BI 6 = 300
METHOD OF PROTECTION 7
METHOD OF PROTECTION FOR TRANSFORMER
2.1.1 The type of faults and abnormal working mode
Fault types can be divided into two groups: internal faults and external faults:
- Touch ground (short circuit) and short circuit ground - Breakdown of voltage divider.
- Short circuit on the system.
- One-phase short circuit in the system + Abnormal working mode:
Main protection: Restraint-different relay protection and Buchholz relay - Function : main protection for transformers
- Protection area: against all types of faults inside the transformer.
+ Restraint differential protection : remove short circuit Single phase or Multi- phases inside transformer
+ Buchholz relay: remove winding faults and oil faults.
Back-up protection : time overcurrent protection and instantaneous overcurrent protection
+ Back-up protection for transformers
+ Remove short-circuit faults occuring inside and outside transformer - Protection area : inside in transformers and a part outside.
+ The impact time of back-up protections must be after the impact time of the main protections
+ Coordinating time with neighboring protections
+ If the transformer receives power from multiple sources, then put the power orientation at the connection to the source having smaller impact time
+ If there is a two-winding transformer then just put the overcurrent protection at one end of the transformer, near the source (because if one coil is overloaded then
7 the remaining coil of the transformer is overloaded too) If there is a transformer with multiple windings, each side must have one set
- Function : Ground cover (inner shell) inside the transformer
Overload protection : Overcurrent or Thermal relay - Function : Remove overload fault
Figure 2.1 Diagram of protection mode for transformers 2.2 Method protection for line
Line L is a medium voltage line that requires effective protection against short circuits and accidental contact To ensure safety and system reliability, a time overcurrent relay (50) is employed as the primary protection, utilizing inverse-time overcurrent coordination Additionally, an instantaneous overcurrent relay (51) serves as a backup to provide redundancy and rapid fault detection.
To detect and prevent ground faults on line L, use zero overcurrent relay (50N, 51N).
Figure 2.2 Diagram of protection mode for line
METHOD PROTECTION FOR LINE
3.1.1 The principle of restraint-different relay protection
- Directly compare the amplitude of the current at the two ends of the protected element
- Active when the current deviation between two protected elements exceeds a given value (threshold current):
The differential protection zone is defined by the placement of the two current transformers at both ends of the protected element These transformers supply the current signals used for comparison, ensuring accurate detection of faults within the designated area Proper positioning of these transformers is essential for the effectiveness and reliability of differential protection systems.
Figure 3.1.1 Differential relayprotection a) Diagram of the principle; b) Vector graph of current when short circuit outside the zone and in normal mode; c) short circuit in the area.
In theory =0 However, reality may be different 0 by the effect of unbalanced currents by some of the following reasons:
+ Due to the error of the current transformer has a small value
+ Due to magnetic circuit saturation of BI: occurs when the transformer is no loaded or short circuit outside has large value in the short time.
THE PRINCIPLES OF RELAY PROTECTION
RESTRAINT-DIFFERENT RELAY PROTECTION
3.1.1 The principle of restraint-different relay protection
- Directly compare the amplitude of the current at the two ends of the protected element
- Active when the current deviation between two protected elements exceeds a given value (threshold current):
The differential protection zone is defined by the placement of current transformers at both ends of the protected element, which detect the current signals required for accurate comparison This setup ensures precise fault detection within the specified area Proper positioning of these transformers is crucial for the effectiveness of differential protection systems, enhancing electrical safety and reliability.
Figure 3.1.1 Differential relayprotection a) Diagram of the principle; b) Vector graph of current when short circuit outside the zone and in normal mode; c) short circuit in the area.
In theory =0 However, reality may be different 0 by the effect of unbalanced currents by some of the following reasons:
+ Due to the error of the current transformer has a small value
+ Due to magnetic circuit saturation of BI: occurs when the transformer is no loaded or short circuit outside has large value in the short time.
The threshold current is defined as follows: with: =0,1
: homogeneous coefficient BI the same select =1
: the largest external short circuit
Restraint differential relay protection is differential relay has been added restraint element to increase the sensitivity and reliability of the protection.
(With conventional dimension from the busbar to the line)
Figure 3.1.2 illustrates restraint differential relay protection, highlighting how the threshold current varies based on the current flowing through the protection circuit branch The relay’s comparator evaluates the absolute values of these two currents to determine whether the system is experiencing a fault This comparison ensures accurate detection of abnormal conditions, enhancing the reliability of the protection system The restraint differential relay adjusts its threshold current dynamically, improving sensitivity and stability in fault detection.
When a short circuit occurs outside the protected zone, the secondary current vectors I T1 and I T2 exhibit a minimal deviation angle, resulting in a restraint current that exceeds the differential current As a result, the relay remains inactive, preventing false tripping and ensuring reliable protection operation.
Figure 3.1.3 Deviation angle between I T1 và I T2 because error of BI
- When short circuit in the protected area: Two vectors I T1 and I T2 have large deflection angle so I SL > I H At that time the relay will work.
- When the power supply from one side: I SL = I H => Relay works
Figure 3.1.4 The impact area of differential protection + Characteristic segment (a):
A low threshold current difference signifies a minimal current value for deviation protection, reflecting the influence of unbalanced currents during normal operation If the threshold (a) is set too high, it reduces the system’s sensitivity, while a too-low (a) setting may lead to incorrect tripping or false protections Properly adjusting the threshold (a) is crucial for maintaining reliable and accurate deviation protection in power systems.
This feature segment ensures relay operation when considering the BI error.
Characteristic for high braking, ensuring the working of relays when saturating the circuit from BI
Characteristic for high value deviation.
When the current deviates from this value, the protection will act regardless of the restraint current.
This feature segment relies on the percentage value of U N %, which is typically set at a threshold where the short circuit at the transformer's output and fault current become significantly higher than the transformer's rated current This setting helps in accurately detecting abnormal conditions, ensuring reliable transformer protection and safety Properly configuring this threshold is essential for effective fault detection and maintaining system stability.
BUCHHOLZ RELAY PROTECTION
- Relays operate based on the evaporation of transformer oil when there is a problem and the level of oil drop is too much.
The Buchholz relay is installed on the pipe connecting the oil tank to the oil extension tank of the MBA It features a two-level design with two glass-bulb metal buoys, equipped with either mercury contacts or magnetic contacts Under normal operating conditions with the oil tank full, the floating buoys remain submerged in oil, keeping the relay contacts in an open state.
Gas leaks or electrical faults, such as minor wire touches, produce air bubbles that collect in the lid of the Buchholz relay When enough gas accumulates, it causes the float to sink and close the contact, triggering a level 1 warning Proper detection of gas buildup in the Buchholz relay is essential for early fault diagnosis and ensuring transformer safety.
During electrical incidents involving significant contact with multiple wires, large volumes of gas are generated phase-by-phase, creating a stream that flows through relays to the expansion tank When this occurs, the bottom float becomes submerged, causing the bottom valve to close and prevent further gas escape.
When a minor impact occurs, the contact affects signaling systems, potentially causing disruptions In contrast, a severe impact triggers immediate action to disconnect the transformer, ensuring safety and preventing further damage This protocol helps maintain operational integrity and safety standards during electrical incidents.
- Protection has two levels: light - signaling and heavy - cutting.
Oil flow relays operate on the same principle as Buchholz relays, providing essential protection for transformers Positioned within the regulator load box, these relays detect irregularities by sensing oil movement caused by overheating or faults When a problem occurs, heated oil shifts and triggers the relay, leading to automatic transformer shutdown to prevent further damage Proper functioning of oil flow relays is crucial for maintaining transformer safety and preventing costly failures.
TIME OVERCURRENT RELAY
A time overcurrent relay functions with a time delay (∆t) when the current exceeds its threshold value, providing effective protection This relay ensures selectivity by adjusting its operating time based on the level, meaning the closer the relay is to the source, the longer its impact time Proper coordination of time overcurrent relays is essential for reliable electrical system protection and to prevent unnecessary outages.
- Impact when the current through the protection element exceeds a given threshold:
- Effect against all types of incidents
- Working on the principle of each level, the closer to the source the greater the impact time.
Figure 3.3 Time overcurrent protection Thresold current for protection:
- Thresold current of time overcurrent relay selected according to I lvmax passing through the protection element: with:
-k at : the safety factor, k at = 1,1 ÷ 1,2.
-k mm : the opening factor, k mm = 2 ÷ 5.
-k tv : the returning factor, k v = 0,85 ÷ 0,9 with electromechanical relays, k tv =1 with digital relays.
-I lvmax : maximum working current of line
Thresold current of secondary side: with
-K sd : Diagram factor of BI
-n i : the ratio of current transformers
Coordinate with neighborhood protections on the principle of step-by-step ladder.
OVERCURRENT ZERO SEQUENCE PROTECTION
Figure 3.4 Overcurrent Zero sequence protection
-Overcurrent Zero sequence protection also the overcurrent protection so the principle of action is also the effect when the zero sequence current does not exceed the thresold value :
- Overcurrent Zero sequence protection, must place zero sequence current transformer at the neutral of the transformer.
OVERLOAD TRANSFORMER PROTECTION
Overloading a power transformer causes its temperature to rise, and prolonged or high-level overloads can lead to overheating, significantly reducing the transformer's lifespan To safeguard against overloads, modern power transformers are equipped with overcurrent protection systems These systems typically utilize current relays designed to detect excessive current flow and trigger protective measures, preventing damage and ensuring reliable operation Proper implementation of overcurrent protection is essential for maintaining transformer health and preventing costly failures.
Thermal imaging is a vital method used to prevent overloads in large power transformers by detecting temperature rises at various test points Based on the detected temperature increases, the protection system initiates different response levels, including warnings and increased cooling measures such as air or oil cooling If these measures are ineffective and the transformer's temperature continues to exceed safe limits over a specified period, the system will automatically shut down the transformer to prevent damage.
CALCULATION OF SHORT CIRCUIT 16
CALCULATION OF REACTANCE VALUE
Select S cb = 25 MVA, calculation in the relative system.
Basic voltage equal to average voltage at voltage level:
4.1.1 Calculation of reactance value of element in maximum power system mode
Subtation has two parallel transformers:
The same positive sequence schema but don’t have E:
4.1.2 Calculate the resistances in the minimum system capacity mode Power of system: S HT = 1700 MVA
Transformer station operates with 1 transformer.
The same positive sequence schema but don’t have E:
CACULATE SHORT-CIRCUIT CURRENT
The all short circuit point:
The resistance of the part on the line in the modes and diagrams is determined as follows: X D23 = X D34 = X D45 = X D56 = 1
4 0,039 = 0,0098 Short-circuit types that need to be calculated apply the formula in the following table:
4.2.1 Calculate short-circuit current in maximum system power mode
Short-circuit types to consider:
N (1) – Line to ground short circuit
N (1,1) - Double phase short circuit with ground connection a) 2 transformers parallel
The total resistance positive, negative, zero when the short circuit at the point N1:
Current of phase A in positive:
Short circuit current in zero:
Current of phase A in positive:
(0,011+ 0,012) Three-phase shortcircuit current in zero:
The total resistance positive, negative, zero when the short circuit at the point N2:
Current of phase A in positive:
1 =3.7,04!,12 Short circuit current in zero:
(0,047+0,048) Three-phase shortcircuit current in zero:
The total resistance positive, negative, zero when the short circuit at the point N3
Current of phase A in positive:
Short circuit current in zero:
Current of phase A in positive:
Three-phase shortcircuit current in zero:
At point N4, N5, N6 Similar calculation we have:
Table of short circuit of max mode with 2 transformer working in parallel
I 0(1,1) 85,91 20,09 10,48 8,56 7,23 6,26 b) 1 transformer works independently Short cirtcuit current at N1, N1’
The same in 2 transformer mode works in parallel
The total resistance positive, negative, zero when the short circuit at the point N2 :
Current of phase A in positive:
2 =3.4 Short circuit current in zero:
Current of phase A in positive:
( 0,083+0,084) Three-phase shortcircuit current in zero:
Table of short circuit of max mode with 1 transformer working
4.2.2 Calculate short-circuit current in minimum system power mode
The type of short circuit:
N (1) – Line to ground short circuit
N (1,1) - Double phase short circuit with ground connection a) 2 transformers working parallel Short circuit current at N1
The total resistance positive, negative, zero when the short circuit at the point N1
Short cirtcuit current in zero:
Current of phase A in positive:
(0,015+0,017) Three-phase shortcircuit current in zero:
Table of short circuit of min mode with 2 transformer working in parallel
I 0(1,1) 61,22 19,11 13,56 10,51 8,58 7,25 b) 1 transformer works independently Short cirtcuit current at N1, N1’
The same in 2 transformer mode works in parallel
The total resistance positive, negative, zero when the short circuit at the point N2 :
- Short circuit N(1): XN’2∆= XN’22∑ + XN’20∑ = 0,087+0,089 = 0,176
Short circuit current in zero:
Current of phase A in positive:
Three-phase shortcircuit current in zero: X
Table of short circuit of min mode with 1 transformer working
CALCULATE PROTECTION FOR LINE 27
SHORT-CIRCUIT CURRENT IN SOME CASES ON LINE L
When two transformers operate in parallel, their combined resistance is halved compared to when a single transformer works independently, resulting in a higher short-circuit current The maximum system short-circuit current occurs in this parallel mode, representing the system's peak power capacity Conversely, the minimum short-circuit current takes place when only one transformer is functioning independently, indicating the lowest system power mode during a fault.
Table 6.1 Short-circuit currents in max system power mode, 2 transformers:
Table 6.2 Short-circuit currents in min system power mode, 1 transformer:
We calculte instantaneous overcurrent relay, maximum overcurrent relay, zero overcurrent relay prrotection for line L.
1 The threshold current of protection The threshold current : with: k at – safety factor, k at = 1,2
Nngmax represents the maximum external short-circuit current, which is typically equal to the short-circuit current at the end of a power line It is a critical parameter in electrical system design, ensuring equipment can withstand the highest possible fault current Understanding Nngmax helps in selecting appropriate protective devices and maintaining system safety and reliability.
- The external short-circuit current of line L:
The maximum external short-circuit current: 9,01
The minimum external short-circuit current: 7,39
- The threshold current for instantaneous overcurrent relay protection of line L :
Protect area of instantaneous overcurrent relay
Min shortcircuit current Threshold current 10
The protected area is defined as the length of the protected line, measured from the initial protection point to the location where the short-circuit current reaches the threshold current of the protection device This measurement ensures accurate identification of the critical segment of the line requiring protection Understanding the protected area's length is essential for effective fault detection and system safety Proper calculation of this length enhances system reliability and helps in optimizing protective relay performance.
- The largest protection area: (Lmax)
MAXIMUM OVERCURRENT RELAY
The threshold current : with: kat –Safety factor, kat = 1,2 kmm – Open factor, kmm = 2 kv - Return coefficient selected for digital relay: kv = 0,95
Ilvmax- The largest working current, Ictlvmax = 238,57 A
Convert maximum working current to relative unit system:
-Construction work time characteristics: Overcurrent protection definite time : t= xTMS with with : TMS: constant time set of relay (s).
- Short circuit point N6: IN6 max = 9,01
- Short circuit point N5: IN5 max = 9,88
10,29−1 Similar calculations for short circuits on the line we have
Table 6.3 Impact time of the max system
Table 6.4 Impact time of the min system
CHARACTERISTICS OF THE TIME OF OVERCURRENT PROTECTION t max mode t min mode 003
CALCULATE ZERO PROTECTION (TTK)
-Threshold current of protection with: k 0 - adjustment coefficient, k = 0,3
I dmBI - Rated current of the current transformer set for the line.
The working time of overcurrent protection does not have time to select according to independent characteristics:
Time characteristics impact of overcurrent protection TTK
CHECK THE WORK OF PROTECTION FOR
CHECK THE WORK OF THE TRANSFORMER PROTECTION
7.1.1 Check the work of the transformer protections compare offset current with braking a Checking safety factor in the brake when short circuit in external over current.
To checksafety factor in the brake when short circuit in external over current, we check when short circuit in enternal over current is the largest.
- Consider the largest current through short circuit at N2
The largest short circuit current go through protection in every MBA is N (3) short circuit current in the max power system, 1 transformer work independently.
2 =¿ 12,05 The largest short circuit current go through protection in every transformer is N (1,1) in the max power system, 1 transformer work independently
2 1) ,02 Conclusion, short circuit in external protection by:
I SL =I kcbtt max =f imax K dn K kck I N ngmax ¿ 0,1.1 1,8.12,05=2,17
Safety factor in the brake is defined by fomula:
With I Htt is calculate restrain current
Straight line I SL = 1,34 cut characteristic (c) so:
Protecting brake stady, not affected when short circuit outside the protection zone b Short-circuit sensitivity in the protection zone
In the event of a short circuit within the protection zone, the differential current (I_SL) is always equal to the damping current (I_H), ensuring that theoretical relays operate correctly To verify relay performance, it is essential to assess its sensitivity, ensuring reliable protection and system stability during fault conditions Proper testing of relay sensitivity guarantees prompt and accurate fault detection, enhancing overall electrical system safety and reliability.
With: I kd – starting current in protection
To check the sensitivity of the protection, we consider the smallest short-circuit current when a short circuit occurs in the protection zone (at N1’ and N2’)
Following to calculate in chapter 4, The smallest short-circuit current that passes through short circuit protection at N1 'is a two-phase short-circuit when the system power is minimum
Straight line I H = 57,74 cut characteristic line (d)
So, protection ensure to cut safely when having short circuit at N1’
Based on the calculations in Chapter 4, the smallest short-circuit current passing through the short-circuit protection at N2 is a two-phase short circuit during minimum system power when two transformers operate in parallel.
We have ISL = IH = INmin = 16,98
Straight line I H = 16,98 cut starting characteristic current line at characteristic segment (c)
So, protection ensure to cut safely when having short circuit at N2’
The impact characteristics of the braking differential protection applied to the
7.1.2 Check the work of over fast cutting current
Sensivity of protection is defined by formula: k = I N min
Sensivity of protection : with condition k N = I Nmin
So protection get requirement for sensitive level.
7.1.3 Zero sequence in earth fault ( TTK)
Sensivity of protection is defined by formula: k N = I
So protection get requirement for sensitive level.
CHECK THE WORK OF THE PROTECTIONS IN THE LINE
Check sensivity of protection. k N = I Nmin
So protection get requirement for sensitive level.
The smallest short circuit currenr TTK go through protecting is short circuit
N (1,1) current at N6 in the minimum power system.
=> So protection get requirement for sensitive level.
Sensivity of protection is defined by formula: k = I N min N
Sensivity of protection : with condition k N = I Nmin
=> So protection get requirement for sensitive level.
7.2.3 Zero sequence in earth fault (TTK)
Sensivity of protection is defined by formula: k N = I