two
Protection function and principle
2.1 Differential protection (D-type configuration)
The ratio differential protection mainly protects the internal fault of the transformer. The device directly collects the current on both sides of the transformer, and the existing phase difference is determined by the software
The differential current and braking current are calculated after automatic correction according to the field winding wiring mode.
The conventional ratio differential principle is adopted, and its action equation is as follows:
Id ＞ Icd
（Ir ＜ Ir0）
Id - Icd ＞ Kcd ²(Ir- Ir0)
(Ir ≥ Ir0)
When the above two equations are met at the same time, the ratio differential element acts. Where, Id is the differential current, Ir is the braking current, and Kcd is the ratio
Braking coefficient, Icd is the fixed value of differential current threshold, Ir0 is the current value of inflection point.
It is recommended to set the inflection point current Ir0 in the element to 0.7 times the rated current Ie at the high voltage side, so as to ensure that the braking current is less than
When the rated current is Ir ＜ Ie, there is no braking effect.
The threshold criterion of differential current should not be too small, and Icd ＝ (0.4 ～ 0.8) Ie is recommended.
The ratio braking coefficient can be adjusted according to the following formula:
Kcd = Kk ²(Ktx²Fwc △U △Fph)
Wherein, Kk is the reliability coefficient, taken as 1.3~1.5; Ktx is the homomorphic coefficient, 1.0;
Fwc is the allowable error of current transformer, taken as 0.1
△ U is the error caused by transformer voltage regulating tap, taking half of the voltage regulating range
△ Fph is the relative error caused by current imbalance caused by current transformer, taken as 0.05
The recommended value range of the ratio braking coefficient Kcd is 0.3~0.7.
For double turn transformer:
Id ＝∣
²
IH
²
I L ∣
Ir ＝∣
²
IH -
²
I L ∣/ 2
Where,
²
IH 、
²
I L refers to the current at the high voltage side and low voltage side respectively, both of which flow into the transformer in the positive direction.
In the differential protection device, the phase difference of the current at each side of the transformer is automatically corrected by the software. Current mutual inductance on each side of transformer
All transformers are star connected and have the same polarity as the directional transformer. The differential current and braking current of each differential element can be in the "operating state"\
The "calculation quantity" menu is displayed in real time, or displayed in the main menu in real time cycle, so as to facilitate device detection and operation monitoring.
2.1.1 Phase correction method
The device software corrects the current phase at Y side with Y/- 11 Wiring as an example, the method is as follows:
)/ 3
-
' (
A
B
A I
I
I
?
?
?
?
)/ 3
-
' (
B
C
B I
I
I
?
?
?
?
)/ 3
-
' (
C
A
C I
I
I
?
?
?
?
2.1.2 Second harmonic braking
The second harmonic of three-phase differential current is used as the criterion of inrush current lockout. The action equation is as follows:
Max[Ida2, Idb2, Idc2] ＞ Kxb ·
Technical Specification of Max [Ida, Idb, Idc] PST 645UX Transformer Protection Measurement and Control Device
sixty-nine
Where Ida2, Idb2 and Idc2 are the second harmonic content of A, B and C three-phase differential current, Ida, Idb and Idc are A, B and C three-phase differential current,
Kxb is the second harmonic braking coefficient. When the above conditions are met, the three-phase ratio differential protection will be locked. Kxb is generally 10%~20%
The recommended value is 0.15, and the error range is ± 5% or better error is 0.05.
The effective value of the second harmonic of each differential element can be displayed in real time in the menu of "operation status calculation quantity" or circulated in real time in the main menu
Ring display for device detection and operation monitoring.
2.1.3 TA disconnection
It has TA disconnection locking, alarm function and differential current out of limit alarm function.
The starting condition of TA disconnection is that the differential current is greater than 33% of the differential starting current. When any of the following conditions is met, the device exits the instantaneous TA disconnection
Line discrimination:
a) If the better phase current of a side is less than 20% of the rated current of the side before starting, the side will not be judged;
b) After starting, any phase difference current is greater than 120% of the rated current on this side;
c) The current on either side increases after startup compared with that before startup.
If the above three conditions are not met, if one side meets the following conditions at the same time, it is judged as TA disconnection:
1) Only one phase current is zero;
2) The other two phase currents are equal to those before startup.
If the differential current is greater than 15% of the differential starting current, the differential current will be reported to exceed the limit through the 10s delay device, but the differential protection will not be locked. This function can
It also serves as the monitoring function of the AC sampling circuit of the protection device.
2.1.4 Differential quick break
When the differential current of any phase is greater than the setting value of differential quick break, it acts on the main outlet relay. It is used for serious problems in transformer differential zone
Switch off the transformer quickly in case of fault. The differential quick break setting value shall be able to avoid the better unbalanced current of external fault and the excitation when the transformer is air dropped
The magnetic inrush current is generally 6-12 times of the rated current.
2.2 Phase to phase overcurrent protection
When the device executes the three section overcurrent discrimination, the discrimination logic of each section is consistent, and the action conditions are as follows:
1) I? ＞ Idn; Idn is the current setting of section n, and I? Is the phase current;
2) T ＞ Tdn ； Tdn is the n-segment delay setting.
Its low-voltage locking element and negative sequence voltage locking element are enabled and disenabled by corresponding control word.
2.2.1 Low voltage locking element
The low-voltage element acts when any of the three line voltages is lower than the low-voltage fixed value, and opens the locked protection element. With this element,
It can ensure that the device will not malfunction under non fault conditions such as motor feedback.
2.2.2 Negative sequence overvoltage locking element
When the negative sequence is greater than the negative sequence overvoltage setting value, the locked protection element will be opened. Composite voltage locking can be formed together with low-voltage locking elements
Component and overcurrent component constitute composite voltage overcurrent protection.
2.2.3 Inverse time delay element of interphase current
The inverse time protection element is a protection element whose action time limit naturally matches with the current in the protected line. By translating the action curve
It is very convenient to realize the cooperation of the whole line. The device provides three common inverse time characteristics, namely standard inverse time, very inverse time and extreme inverse time,
The inverse time characteristic is adjusted by the corresponding "inverse time index" in the setting value. The inverse time characteristic formula is as follows:
1) General inverse time limit
zero point zero two
zero point one four
( ) 1
tp
t I
Ip
?
? Technical Manual of PST 645UX Transformer Protection Measurement and Control Device
seventy
2) Very inverse time limit
thirteen point five
( ) 1
tp
t
I
Ip
?
?
3) Extreme inverse time limit
two
eighty
( ) 1
t I tp
Ip
?
?
Where: tp is the time coefficient, the range is (0.05~1); Ip is the current reference value; I is fault current; T is the tripping time.
The inverse time limit of the setting value is the product of the numerator in the above expression, in seconds, and the setting range is (0.005~127).
It can also be realized for long time inverse time limit. The expression of long time inverse time limit is as follows:
one hundred and twenty
( ) 1
tp
t
I
Ip
?
?
For the interphase current protection of the device, the definite time limit or inverse time limit can be selected by setting the relevant bits of the control word. When the inverse time limit mode is selected,
Automatically exit the fixed time overcurrent section III protection, and the function strap of the interphase current section III is the interphase current inverse time limit function enabling/disenabling strap.
2.3 Overload elements
Overload element monitors three-phase current, and its action conditions are:
MAX (I?)> Ifh， Wherein, Ifh is the fixed value of overload current.
It can be set as alarm or trip through corresponding control word.
2.4 Zero sequence overcurrent protection
The device provides three sections of zero sequence overcurrent protection at the high and low voltage sides. The zero sequence III section at the high voltage side can be set as alarm through the control word
When the side zero sequence current is used as tripping or alarm, the zero sequence current can be selected from the KG1.1 bit of control word 1 in the operation parameter by external special
The zero sequence CT used is introduced or generated by the software itself. The device defaults to the external connection of the zero sequence current at the high-voltage side.
The zero sequence section III protection at the low-voltage side can be set as inverse time limit protection through the control word, and the inverse time limit characteristic of the zero sequence protection is the same as the inverse time limit element of the interphase current
Pieces.
2.5 Zero sequence overvoltage protection
The zero sequence overvoltage element acts when the following conditions are met:
1) The zero sequence voltage is higher than the setting value of zero sequence overvoltage;
2) The circuit breaker is in the closing position;
3) The delay time is up.
It can be set as alarm or trip through the corresponding control word. When the zero sequence overvoltage is selected as alarm, the device uses its own 3U0 zero sequence voltage for judgment
No.
2.6 Low voltage protection
When the system voltage is lower than the setting voltage, judge whether to cut off the load. Low voltage protection is blocked by current to prevent low voltage in case of PT disconnection
Voltage protection maloperation.
The criteria for low-voltage components are:
1) The voltage between the three phases is lower than the fixed value of low voltage; Technical Manual of PST 645UX Transformer Protection Measurement and Control Device
seventy-one
2) Three phase current is less than a certain setting value;
3) The circuit breaker is in the closing position;
4) The delay time is up.
2.7 Acceleration function
The device is equipped with the acceleration function after manual closing, which is enabled or disabled by the soft pressing plate. The starting conditions are:
1) The time of the circuit breaker in the opening position exceeds 30s;
2) The circuit breaker changes from opening to closing, and the allowable acceleration time is widened for 3s;
The device is set with separate current acceleration section current setting value and corresponding time setting value.
2.8 Non electric quantity protection
The device provides four non electric quantity protection inputs, which can be tripped or alarmed by the soft strap. When the soft strap is put into operation, the non electric action jumps
Gate; When the soft strap exits, non electric quantity only sends signals.
For the sake of flexibility, the signal lamp on the device panel only shows four kinds of signals: non electric quantity 1, non electric quantity 2, non electric quantity 3, and non electric quantity 4. Each non
The specific meaning of electricity shall be determined according to the specific terminal access of each project. Generally, the default non electric quantity 1 is the heavy gas input terminal,
Non electric quantity 2 is the light gas input terminal, non electric quantity 3 is the pressure release input terminal, and non electric quantity 4 is the high oil temperature input terminal.
2.9 Trip logic matrix
Each protection tripping logic of the device adopts the fixed value setting mode. After the action of each protection element, the exit group is set by the exit tripping password
X3:3-4 is the standby remote control point, X3:13-14 is the manual tripping out, and does not participate in the setting of the tripping matrix. The tripping password is 16 bit binary code, and its setting
The range is 0000~FFFFH. After the protection element acts, when a bit of its tripping password is set to 1, the corresponding exit will be tripped, and when it is 0, the corresponding exit will be tripped
The exit does not act.
