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Research On Neutral Grounding Mode Of High Voltage 10KW Diesel Generator

Date: 2020/28/07

Research On Neutral Grounding Mode Of High Voltage 10KW Diesel Generator


0 Introduction

The Electric Connection between the neutral point and the earth of the distribution network is called the neutral point earthing mode, also called the neutral point operation mode. What kind of grounding method is used for neutral point is a very wide technical and economic problem. Different grounding modes will directly affect the overvoltage, the insulation level of electrical equipment, the reliability of power network, the selectivity and sensitivity of relay protection, and the interference of communication lines.


1 neutral point common ground way

There are four types of common system earthing in power systems:

(1) the neutral point is directly grounded: The neutral point of high voltage AC power system is directly grounded, when the single-phase-to-ground fault occurs in the system, the single-phase-to-ground short circuit will be formed and the short-circuit current is very large, which is very beneficial to the relay protection, non-fault relative to the ground voltage does not increase, will not cause intermittent arc over-voltage.


(2) ARC SUPPRESSION COIL GROUNDING: High Voltage AC Power System neutral arc suppression coil grounding, between the neutral point and grounding in series with a reactor to offset the capacitive current, limit the single-phase grounding fault short-circuit current.


(3) the neutral point is grounded by resistor (high resistance, low resistance) : the High Voltage AC power system is grounded by high resistance at the neutral point, and a high impedance resistance is connected in series between the neutral point and the grounding point, the short-circuit current of single-phase grounding fault is limited to 5 ~ 20A, the neutral point of high-voltage AC power system is grounded with low resistance, and a low impedance resistance is connected in series between the neutral point and the grounding point, the short-circuit current of single-phase grounding fault is limited to 100 ~ 1000A.


(4) neutral ungrounded: The neutral point of high voltage AC power system is ungrounded, the single-phase grounding current is capacitive current when the system has single-phase grounding fault, when the single-phase grounding current is small (not more than 10a) , the system can run for 1 ~ 2 hours with fault, the disadvantage is that when the single-phase grounding fault occurs, the ARC can not self-extinguish when the grounding current is large, which leads to the gap arc over-voltage, endangers the equipment, destroys the insulation and even causes the multi-phase short circuit.


1.1 factors affecting selection of grounding method

The factors influencing the selection of grounding mode are: 1 Power Supply Reliability; 2 personal and Equipment Safety; 3 overvoltage factor; 4 relay protection; 5 investment.


1.2 comprehensive comparison of common neutral grounding modes

A comprehensive comparison of common neutral grounding modes is shown in Table 1.

Table 1 comprehensive comparison of common neutral grounding modes

comprehensive comparison of common neutral grounding modes


2 High Voltage 10KW diesel generator

When the neutral point is ungrounded, if the single phase to ground fault is located outside the high voltage 10 kw diesel generator, if the capacitance current is greater than a certain value, the fault arc can not be extinguished by itself, often accompanied by the intermittent arc, the built-up phase produces over-voltage, causing insulation damage, and the generator delta-connected high-order harmonic will form a loop heating, so the general use of y connection. The withstand voltage standard is 1.5 times rated line voltage, I. E. 2.6 u phase.


In order to ensure the safe operation of the high voltage 10KW diesel generator system, it is necessary to ensure that the over voltage of the non fault phase is less than 2.6 u phase in case of the single phase to ground fault of the system, and that the over voltage of the non fault phase is less than 2.6 u phase in case of the single phase to ground fault of the high voltage 10KW diesel generator system, if there is no gap arc voltage, the resistance current IR of the grounding point should be larger than the total capacitance current I ∑ C, generally IR ≥(1.0 ~ 1.5) I ∑ C, which can further suppress the over-voltage multiple less than 2.5 u phase, when IR ≥3.0 I ∑ C (or Xc/r ≥3.0) , the internal overvoltage suppression effect can be satisfied, as shown in Fig. 1.


High resistance grounding mode only when the grounding current is small, the effect of automatic arc extinguishing is obvious, generally applicable to grounding capacitance current less than 10a high voltage system, the capacitance current I ∑ C of 10KW diesel generator system in Shanghai center is 6A.


Through analyzing the mechanism of generator neutral point resistance grounding, in order to meet the principle requirement of generator grounding type, high resistance grounding type should be adopted in electric generator grounding type.


Advantages: 1 to ensure the continuity of power supply, when the single-phase-to-ground fault current is lower than the high-voltage 10 kw diesel generator safety tolerance, with fault operation (generally 2h) . 2 The gap arc overvoltage is suppressed, which makes the overvoltage lower than 2.6 u phase. 3. Simple relay protection.


DISADVANTAGES: 1 suitable for small-scale high-voltage system grounding capacitance current (less than 10a) , limited the use of the scope. When the short-circuit current of single-phase-to-ground fault is large, the current flowing through the resistance will cause the heating problem, and the resistance will be burned out if the time is too long.

Diagram of system impedance and transient overvoltage multiple

Fig. 1 Diagram of system impedance and transient overvoltage multiple


3 Selection of neutral grounding resistance for emergency high voltage 10KW diesel generator in Shanghai center

3.1 selection of neutral grounding resistance

Each 10KW diesel generator company has different selection and control methods for the 10KW diesel generator neutral grounding resistance, and there is no uniform regulation for this in China.


(1) the grounding current is controlled at 100 ~ 150A, 10 ~ 30a and 10% of the rated current.


(2) multiple 10KW diesel generator + multiple contactors + one resistance earthing scheme, and multiple 10KW diesel generator + multiple contactors + multiple resistance earthing scheme. The former has low cost and small area, but its control logic is complicated, the latter, on the contrary.


(3) there are several 10KW diesel generator running with one contactor closing to control circulation, and there are several contactor closing with circulation controller.


The Shanghai Center Emergency High Voltage 10 kw diesel generator neutral grounding resistance is

The Shanghai Center Emergency High Voltage 10 kw diesel generator neutral grounding resistance

Therefore, 4 sets of 2500kw (standby power)/10.5 KV, 1 set of 303 high voltage resistance box with temperature alarm and 4 sets of high voltage AC contactors are selected.


4 calculation of short-circuit current of single-phase-to-ground fault with high resistance

The schematic diagram of the short-circuit current of the cable with single-phase-to-ground fault is shown in Fig. 2.

The schematic diagram of the short-circuit current of the cable with single-phase-to-ground fault

Fig. 2 schematic diagram of short-circuit current of cable with single-phase-to-ground fault

The short-circuit current of single-phase-to-ground fault with complete metallic property of cable is

The short-circuit current of single-phase-to-ground fault with complete metallic property of cable

ICX is the capacitance current of the protected line, the capacitance current of the shortest cable in Shanghai Center is 0.15 A, and the capacitance current of the longest cable is 0.7 a.


5 The fault point of the cable passes through the transition resistance RG = (30 ~ 300) , and the short-circuit current of the single-phase-to-ground fault is

The fault point of the cable passes through the transition resistance RG = (30 ~ 300) , and the short-circuit current of the single-phase-to-ground fault

The total short circuit current is 20.7 a when a single phase to ground fault occurs in the emergency high voltage 10 kw diesel generator system. The maximum operating mode is 


5.2 a for each of the four 10 kw diesel generator, the minimum operating mode is 310 kw diesel generator, each current load 6.9 a, does not affect the 10 kw diesel generator operation.


5 Calculation of zero sequence current protection for high resistance grounding line and zero sequence current protection for high voltage resistance box


5.1 Idz0x calculation of zero sequence current protection transmission lines

The zero sequence current protection setting is Idz0x = KIcx according to the capacitance current condition of the protected line


Under: K —— Reliability Coefficient, 4 ~ 5.

The sensitivity is: Km = ID/IDZ0X ≥2


Km ≥2 for main protection and Km ≥1.5 for backup protection.

LINE ZERO SEQUENCE CT conversion ratio is 100/1, Zero Current 1 current protection setting (secondary side) isIdz0x=5×0.53/100=0.0265 A


At 0.06 a/0.8 s, the sensitivity is Km=20.7/(0.06×100)≈3.4≥2


The set value of zero current 2 current protection (secondary side) is Idz0x= 4×0.53/100=0.0212 A


At 0.03/1.6 s, the sensitivity is Km=11.3/(0.03×100)≈3.8≥2


Idz0R calculation of zero sequence current protection for 5.2 high voltage resistance box

It is 1.1 ~ 1.2 times larger than the set value of the lower zero-sequence protection, and the time limit is 0.3 s.


The zero sequence CT ratio of the high voltage resistance box is 50/5, and the value of zero current 1 current protection (secondary side) is Idz0R= 1.1×0.06×100/10=0.66 A


At 0.7 a/1.1 s, the sensitivity is Km=20.7/(0.7×10)≈2.9≥2


The set value of zero current 2 current protection (secondary side) is Idz0R= 1.1×0.03×100/10=0.33 A


At 0.4 a/1.9 s, the sensitivity is Km = 11.3/(0.410)≈2.8≥2 to ensure personal safety, the contact voltage should meet the National Standard: When the protection operation time is more than 1s, the contact voltage value is 50v.


The fault point to ground voltage is Ugz= IdRjd=20.7×1=20.7 V<50 V


Among them, RJD is the value of grounding resistance, ≤1.0.

 

6 High Resistance Grounding Operation Strategy

(1) when single-phase-to-ground fault occurs in the system, the short-circuit current is too small to alarm the temperature of the high-voltage resistance box, and it does not affect the operation of the 10KW diesel generator. Four 10KW diesel generator are connected to the high-resistance ground at neutral point.


(2) when single-phase-to-ground fault occurs in the system, the short-circuit current is larger, which makes the high-voltage resistance box temperature alarm, and the 10KW diesel generator control system controls four high-voltage AC contactors to cut off the high resistance of neutral grounding, four 10 kw diesel generator operate in a neutral ungrounded mode.


(3) four 10KW diesel generator are operated in a neutral point ungrounded mode, and the capacitance current of the system cable to the earth I ∑ cL is calculated as shown in Table 2. As can be seen from table 2, the Line capacitance current of Shanghai center is about 4A.

calculation of the Earth capacitance current I ∑ cL of the system cable

Table 2 calculation of the Earth capacitance current I ∑ cL of the system cable


The capacitance current of the distribution cabinet and cable-to-ground forming system is I∑c= α(1+β)I∑cL=1.3×(1+15%)×4=6 A


The formula is as follows: (1.2 ~ 1.8) as the Coefficient of freedom, (15% ~ 20%) as the influence rate of power distribution equipment.


(4) the sensitivity of zero sequence current protection (backup protection ≥1.5) is

Km= (I∑c-Icx)/Idz0x=(6-0.53)/3≈1.8≥1.5


(4) the influence of neutral point ungrounded operation mode on 10KW diesel generator. The single phase to ground fault current is 6A, the maximum operating mode is 1.5 a for 410kw diesel generator, and the minimum operating mode is 2.0 a for 310kw diesel generator, doesn't affect the 10KW diesel generator.


7 System Diagram of neutral grounding resistance

The Shanghai Center Emergency High Voltage 10KW diesel generator neutral grounding resistor system is shown in figure 3.

emergency high voltage 10 kw diesel generator neutral grounding resistor system

Fig. 3 emergency high voltage 10 kw diesel generator neutral grounding resistor system


8. Conclusion

High Voltage 10KW diesel generator neutral grounding is a very wide range of economic issues. Different countries and cities in a country have different neutral grounding methods, which are mainly based on their own operating experience and tradition.