THE PRINCIPLES OF RELAY PROTECTION IN PROTECTIVE METHOD

2.1.2 Method of protection  Main protection: Restraint-different relay protection and Buchholz relay - Function : main protection for transformers - Protection area: against all types o

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 :

IBI1max = kqt x √ 3 S U dmB Cdm = 1,4.√ 3 110 40 10 3 = 293,92 (A)

 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 = 300

- 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:

IBI1max=kqt x √ 3 S U dmB1 Hdm = 1,4.√ 3.22 40 10 3 = 1469,62 (A)

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 BI 5 = 1500

We choose BI3, BI6 same BI1 so the ratio is n BI 3 =n BI 6 = 300

METHOD OF PROTECTION

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.

- Short circuit on the system.

- One-phase short circuit in the system.

 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

For a two-winding transformer, install overcurrent protection at one end of the transformer near the power source, since overloading one winding will typically overload the other winding too For transformers with multiple windings, ensure an overcurrent protection set on each winding side (i.e., primary and secondary) so every winding is protected.

- Function : Ground cover (inner shell) inside the transformer

 Overload protection : Overcurrent or Thermal relay

Figure 2.1 Diagram of protection mode for transformers

METHOD PROTECTION FOR LINE

Line L is a medium-voltage line that requires robust short-circuit protection to safeguard against faults on or near the conductor The protection scheme uses a primary inverse-time overcurrent relay (50) as the main protection, providing coordinated tripping based on fault current magnitude and duration An instantaneous overcurrent relay (51) is included as redundancy to rapidly clear severe faults that exceed the instantaneous threshold Together, the 50 inverse-time and 51 instantaneous relays form a reliable protection scheme for Line L, enhancing selectivity, reducing damage, and improving system availability.

To detect and prevent ground faults on line L, use zero overcurrent relay (50N, 51N).

Figure 2.2 Diagram of protection mode for line

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):

Within differential protection, the protective zone is defined by the placement of two current transformers at the beginning and end of the protected element These CTs supply the current signals that are compared by the differential relay to detect faults inside the protected section Consequently, the boundary of the protection zone is determined by the CT locations, ensuring accurate fault detection by comparing the inlet and outlet currents for internal faults while avoiding disturbances from outside the protected element.

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 shows restraint differential relay protection, where the threshold current of the restraint differential relay changes with the current in the protection circuit branch The comparator part of the relay compares the absolute values of the two currents to determine whether a fault condition exists, enabling reliable differential protection of the protected equipment.

Relay impacts when ISL ≥ IH

During a short circuit outside the protected area, the two secondary currents IT1 and IT2 deviate only slightly, creating a small angle between them This small deviation causes the restraint current to be larger than the differential current, so the differential relay does not operate Consequently, the protection system may fail to trip for faults beyond the protected zone.

Figure 3.1.3 Deviation angle between IT1 và IT2 because error of BI

- When short circuit in the protected area: Two vectors IT1 and IT2 have large deflection angle so ISL > IH At that time the relay will work.

- When the power supply from one side: ISL = IH=> Relay works

Figure 3.1.4 The impact area of differential protection + Characteristic segment (a):

The low threshold current difference indicates a low current value triggering deviation protection due to unbalanced current during normal operation If line a is set too high, protection sensitivity is reduced; conversely, a very low setting on line a can cause incorrect protection actions.

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 transformer's UN% value The threshold is typically set at a level where the short-circuit current at the transformer output and the associated fault current exceed a defined multiple of the transformer's rated current.

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.

On the pipe connecting the MBA oil tank to the oil extension tank, a Buchholz relay is installed as a two-level safety device The relay comprises two glass-bulb metal buoys, each equipped with a mercury or magnetic contact In normal operation with the oil tank full, the floating buoys remain submerged in oil and the relay contacts stay in the open state.

When a gas leak occurs or a minor fault causes air bubbles to form, these bubbles accumulate under the lid of the Buchholz relay As the gas buildup grows large enough, the float sinks, closes the contact, and the relay issues a level 1 warning.

During a severe electrical fault that involves phase-by-phase contact in damaged wiring, gas generation can skyrocket, forming a continuous stream that travels through relays toward the expansion tank This surge can submerge the bottom float, causing the bottom valve to close and restrict flow The rapid gas buildup highlights the critical role of the expansion tank, relay protection, and float-controlled valves in maintaining system safety and preventing overpressure.

During minor impact incidents, the electrical contact is affected, which can disrupt signaling systems In severe impact scenarios, protective actions are triggered to immediately isolate and cut off the transformer, preventing further damage and ensuring a safe shutdown.

- Protection has two levels: light - signaling and heavy - cutting.

Oil flow relays protect transformers by using the same principle as Buchholz relays and are installed in the regulator/load box of the transformer When a fault occurs in this assembly, the transformer oil heats up and flows, triggering the relay to actuate and trip the transformer to prevent further damage.

TIME OVERCURRENT RELAY

Time overcurrent relays operate with a time delay (Δt) when the current through the protection device exceeds a defined threshold This behavior provides selectivity by coordinating the operating times across protection levels: downstream relays clear faults quickly, while upstream relays adopt longer delays to prevent unnecessary trips In a level-based protection scheme, the relay located closer to the source (upstream) has a greater operating time, ensuring proper protection coordination and stability of the electrical system.

- 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 Ilvmax passing through the protection element: with:

-kat : the safety factor, kat = 1,1 ÷ 1,2.

-kmm : the opening factor, kmm = 2 ÷ 5.

-ktv : the returning factor, kv = 0,85 ÷ 0,9 with electromechanical relays, ktv =1 with digital relays.

-Ilvmax : maximum working current of line

Thresold current of secondary side: with

-Ksd : Diagram factor of BI

-ni : 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 transformer raises its temperature, and sustained high overload can cause overheating, accelerating deterioration of the transformer's life To protect power transformers from overloads, conventional overcurrent protection is employed This protection uses current relays that monitor load current and respond to overload conditions, triggering protective actions to prevent damage and extend transformer life.

Thermal imaging-based protection for large power transformers monitors temperature rise at multiple test points to prevent overload Depending on the extent of the temperature increase, the system triggers sequential protection levels: a warning, followed by enhanced cooling—using air or oil—and a reduced transformer load If these actions do not bring temperatures within safe limits within the specified time, the protection scheme automatically trips the transformer from the system to safeguard equipment.

CALCULATION OF SHORT CIRCUIT

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.

Resistance of system: X ' 11 = X ' HT = S cb

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: XD23 = XD34 = XD45 = XD56 = 1 4 XD = 1 4 0,039 = 0,0098

Short-circuit types that need to be calculated apply the formula in the following table:

Type of short circuit X (n) Δ m (n) Calculate

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) 2 59,88 ,85 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:

Short circuit current in zero:

Current of phase A in positive:

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

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:

Short circuit current in zero:

Current of phase A in positive:

( 0,083 +0,084 ) 2 8,02,02 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

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) 2 43,54e,35 Three-phase shortcircuit current in zero:

Table of short circuit of min mode with 2 transformer working in parallel

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

0,087+ 0,176 ,41 Short circuit current in zero:

0,087+ 0,089 = 0,044 Current of phase A in positive:

Three-phase shortcircuit current in zero:

Table of short circuit of min mode with 1 transformer working

CALCULATE PROTECTION FOR LINE

SHORT-CIRCUIT CURRENT IN SOME CASES ON LINE L

Two transformers connected in parallel have an equivalent resistance that is half of a single transformer, which results in a higher short-circuit current Consequently, the maximum short-circuit current on the line occurs in the maximum system power mode with two transformers operating in parallel Conversely, the minimum short-circuit current on the line occurs in the minimum system power mode when only one transformer operates independently.

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: kat – safety factor, kat = 1,2

INngmax denotes the maximum external short-circuit current This value represents the largest short-circuit current and is usually equal to the short-circuit current value of the busbar at the end of the line.

-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

The protected area is defined as the length of the protected line, measured from the protection point at the start of the line to the point where the short-circuit current equals the protection threshold.

- The largest protection area: (Lmax)

Max shortcircuit currentMin shortcircuit currentThreshold current

MAXIMUM OVERCURRENT RELAY

Threshold current parameters include a safety factor kat = 1.2, an open factor kmm = 2, and a return coefficient for the digital relay kv = 0.95 The largest working current is Ilvmax = 238.57 A Converting the maximum working current to a per-unit (relative) system yields Ilvmax_pu = 238.57 / Ibase, where Ibase is the chosen base current This per-unit value enables consistent relay configuration and cross-equipment comparison.

I ¿ −1 xTMS with with : TMS: constant time set of relay (s).

I ¿: Short-circuit current through relay.

- Short circuit point N6: IN6 max = 9,01

Impact time characteristics at N6 t = I 13,5 ¿ −1 xTMS

- Short circuit point N5: IN5 max = 9,88

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

CALCULATE ZERO PROTECTION (TTK)

-Threshold current of protection with: k0 - adjustment coefficient, k = 0,3

IdmBI - 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

CHARACTERISTICS OF THE TIME OF OVERCURRENT PROTECTION t max mode t min mode

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.

The largest short circuit current go through protection in every transformer is N (1,1) in the max power system, 1 transformer work independently

I (1,1) N 2 BVmax = I (1.1) N 2 ,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 IHtt is calculate restrain current

Straight line ISL = 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

During a short circuit within the protection zone, the supply forces the ISL differential current to equal the IH damping current, ensuring that the theoretical relays operate reliably To validate relay performance, we assess the relay’s sensitivity and confirm its proper functioning under these conditions.

With: Ikd – 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

We have ISL = IH = INmin = 57,74

Straight line IH= 57,74 cut characteristic line (d)

So, protection ensure to cut safely when having short circuit at N1’

According to the calculation described in Chapter 4, the smallest short-circuit current that passes through the short-circuit protection at N2 is a two-phase fault that occurs when the system operates at minimum power with two transformers connected in parallel This two-phase short-circuit condition represents the minimum fault current through the N2 protection during parallel transformer operation Understanding this scenario helps determine protection settings and ensures reliable protection for low-power, dual-transformer parallel configurations.

We have ISL = IH = INmin = 16,98

Straight line IH= 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 MBA

7.1.2 Check the work of over fast cutting current

Sensivity of protection is defined by formula: k N = I N min

I kd with condition Sensivity of protection : 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 N min

I kd with condition Sensivity of protection : k N = I Nmin

 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 N = I N min

I kd with condition Sensivity of protection : 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 N min