NB/T 41008-2017Power supply technical guidelines for AC electric arc furnace - Power quality assessment (English PDF)

3.1 Improvement ratio

Improvement rate improvementratio Used to describe the comprehensive compensation effect of power quality control equipment or technical measures. Its value is equal to before or after the control equipment is put into operation or technical measures The relative rate of change of the power quality indicators measured at the nodes to be evaluated before and after implementation, see equation (1). Xo-xt Xo × 100% (1) In the formula. K --- improvement rate;

Xo---the evaluation value of the power quality indicator before the equipment is operated;

Xt---The evaluation value of the power quality indicator after controlling the operation of the equipment.

3.2 Monitoring assessment

The measured power data is compared with the allowable limits to evaluate various power quality parameters. [GB/T 32507-2016, definition 3.19]

3.3 Predicted assessment

A model is established for the evaluation object, and various power quality parameters are evaluated by calculating the obtained pre-estimated data. [GB/T 32507-2016, definition 3.20]

3.4 Point of common coupling

The connection of more than one user in the power system. [GB/T 32507-2016, definition 2.1.19]

5.1 Evaluation of the scope of application

5.1.1 Pre-assessment

The pre-assessment is mainly applicable to the planning, design, or commissioning operation of the AC arc furnace power supply system.

5.1.2 Monitoring and evaluation

Monitoring and evaluation is mainly applicable to the operation phase of AC arc furnace operation and the effect of power quality control equipment or the implementation of technical measures. Evaluation.

5.2 Applicable standards and their allocation of indicators

5.2.2 Evaluation of power quality of downstream power supply at PCC point For the electric power quality assessment and its limit distribution within the AC arc furnace power supply enterprise, it should be based on GB/T 17626 (all departments) Sub-).

5.2.1 PCC point power quality assessment

Power quality assessment (including limit allocation) for PCC points should be based on national and industry power quality standards.

5.3 Evaluation point selection principle

The principle of selecting the power quality assessment point for AC arc furnace is.

a) In principle, the electrical power quality assessment requirements for AC arc furnaces are to be carried out at the PCC point;

b) If necessary, you can choose to evaluate the power quality of the relevant nodes in the AC arc furnace enterprise.

5.4 Design or measured data

Evaluate the required design or measured parameters, including.

a) short-circuit capacity to be evaluated, maximum short-circuit capacity for normal operation, and minimum short-circuit capacity for normal operation;

b) capacity of the power supply equipment;

c) One-time wiring diagram of the electric arc furnace power supply system, including electrical equipment parameters, such as transformer nameplate parameters, wiring (cable)

Model and length, nameplate parameters of the loop series reactor, etc.;

d) AC arc furnace design parameters and operating parameters, including leads and their electrode parameters, short net (high current system) impedance, melting period work Rate factor, power factor during refining period, nameplate parameters of electric arc furnace, process parameters, etc.

e) range of reactive power variation, design harmonic generation;

f) nameplate parameters of all reactive power compensation equipment;

g) equipment parameters of the self-contained power plant including the generator;

h) Motor nameplate parameters.

6.1 Circuit model used for pre-assessment

The pre-evaluation recommended AC arc furnace circuit model is shown in Appendix A.

6.2 Power quality interference calculation method

6.2.1 Assumptions

The pre-assessment calculation of power quality indicators and other electrical quantities is carried out under the following assumptions.

a) The three-phase supply voltage is symmetrical and is a pure sine wave;

b) the electrical parameters of the system power supply are symmetrical;

c) The electric arc furnace and its electrical electrical parameters are symmetrical.

6.2.2 Power shock and its corresponding voltage change estimate

6.2.2.1 Calculation method

The reactive power impact during AC arc furnace operation and its corresponding voltage variation estimation method are shown in Appendix B.

6.2.2.2 Voltage change assessment method

Voltage change assessment should be performed in accordance with GB/T 12326.

6.2.3 Pre-evaluation of three-phase unbalance

6.2.3.1 Calculation method

This standard recommends the use of the symmetrical component method to calculate the level of imbalance caused during the operation of AC arc furnace; mature short circuit can be used Flow calculation program.

6.2.3.2 Calculation conditions

Set the evaluation point to operate at the rated voltage. Calculate the sequence current and its sequence voltage caused by the single-pole floating and two-phase short-circuit operating conditions.

According to GB/T 15543, the serious ones are taken as the final evaluation results.

6.2.3.3 Evaluation method

According to GB/T 15543.

6.2.4 Harmonic pre-assessment

6.2.4.1 Determination of harmonic current level of electric arc furnace

The amount of harmonic current generation should be determined based on the design of the electric arc furnace or related design manual. Appendix C is the harmonic level of some types of electric arc furnace Experience value.

6.2.4.2 Harmonic calculation

The node harmonic current injection method should be used to calculate the harmonic voltage distortion level of the evaluation point. See Appendix D for node harmonic current injection.

6.2.4.3 Evaluation method

Should be carried out in accordance with GB/T 14549.

6.2.5 Power supply frequency impact assessment

6.2.5.1 During the operation of the AC arc furnace, the active impact will have an impact on the operating frequency of the grid. About AC arc furnace operation for power supply Refer to Appendix E for the frequency impact.

6.2.5.2 When it is necessary to evaluate the influence of the power supply frequency, consider applying the formula (E.1) in Appendix E for calculation and evaluation.

6.3 Evaluation report and conclusion

The evaluation report should include the following.

a) background and process description;

b) design or measured data;

c) assumptions;

d) based on standards and their limit indicators;

e) evaluate the circuit model used;

f) a simulation curve of active, reactive, apparent power, power factor as a function of arc current;

g) the level of reactive power fluctuations and their voltage change levels;

h) conclusion of voltage change evaluation;

i) Harmonic evaluation conclusions;

j) conclusion of three-phase unbalance evaluation;

k) Other evaluation indicators.

7.1 Test instrument technical requirements

The test instrument should meet.

a) General requirements for GB/T 19862 power quality monitoring equipment;

b) GB/T 17626.30 Electromagnetic compatibility test and measurement technology Power quality measurement method.

7.2 Evaluation criteria

The assessment method should follow the following national standards.

a) GB/T 12325;

b) GB/T 15945;

c) GB/T 15543;

d) GB/T 14549;

e) GB/T 24337; f) GB/T 12326.

7.3 Test cycle

The test cycle should be determined in consideration of the following requirements.

a) should cover all possible combinations of operating conditions for AC arc furnaces, including packing, arcing, oxidation, reduction, etc.;

b) Minimum test period. continuous 24h test under normal operating conditions;

c) If there are reactive power compensation and power quality control equipment, these equipments should be put into normal operation.

Note: When evaluating the effect of power quality control equipment, the test period should include. the minimum period of test is 24h when the equipment to be evaluated is not running;

Under the normal operation of the equipment, the minimum period of testing is 24h.

7.4 Evaluation method

7.4.1 According to the corresponding national standards, give specific evaluation results to the following power quality indicators, and give conclusions on whether or not to exceed the standard, provide 24h curve.

a) voltage deviation;

b) frequency deviation;

c) flicker;

d) harmonic voltage and current;

e) interharmonic voltage; f) Three-phase unbalance.

7.4.2 Give the average of the following electrical parameters and give the curve.

a) active power;

b) reactive power;

c) power factor;

d) Fundamental power factor.

Note: The performance evaluation of power quality control equipment should be carried out under the following conditions.

a) In the case that the power quality control equipment is not operating, evaluate according to 7.4 requirements;

b) Re-evaluate according to the requirements of 7.4 under the operation of the power quality control equipment;

c) Comparing the evaluation results of the above two cases, giving the final evaluation of the equipment management effect, mainly including. the amount of indicators concerned before and after the compensation equipment is put into operation Change in value, improvement rate, and whether it exceeds the standard.

7.5 Evaluation Report Requirements

See Appendix F.

A.1 AC arc furnace pre-evaluation model

a) before compensation b) after compensation Figure A.1 AC arc furnace pre-evaluation model In Figure A.1, the upstream of the PCC point is the power supply system, U is the power supply voltage, Xs is the system inductive reactance; the downstream of the PCC point is the internal power supply of the electric arc furnace.

Wiring, Xt is the short-circuit impedance of the steel plant total step-down transformer or step-down transformer (referred to as. total drop), Xf is the electric arc furnace transformer (referred to as. furnace change)

Short-circuit impedance, Xd, r are the inductive reactance and resistance of the lead and the electrode. The compensation device can be an impedance change type (eg SVC) or an active type (eg SVG).

A.2 Simplified analysis circuit

The simplified analysis circuit of Figure A.1 is shown in Figure A.2. Figure A.2 Figure A.1 simplified circuit

In Figure A.2, U is the power supply voltage (phase voltage), X=X1 X2, where X1 is the integrated impedance seen upstream from the evaluation point.

(taking into account the influence of the compensation device), X2 is the comprehensive impedance of the evaluation point looking downstream (taking into account the influence of the compensation device), the arc is variable Resistor R1 represents, and is set to. R = R1 r. According to the electrician theory, there is a formula (A.1)~ formula (A.7). Flow through the evaluation point current RMS. (R2 X2) (A.1) Evaluation point power factor. R2 X2 (A.2) inspecting power. Active power. R2 X2 (A.4) Arc power. R2 X2 (A.5) Electrical efficiency. P × 100% (A.6) Reactive power. R2 X2 (A.7)

According to the above formula analysis, only when X=R, the electric arc furnace power is the largest, and the power factor is 0.707.

A.3 AC arc furnace pre-evaluation calculation example

The evaluation point is the electric arc furnace power supply bus, the line voltage is 700V, the rated power of the electric arc furnace design is 60MW, X=4.04mohm, false Let r=0. The curve of each electrical parameter obtained according to the formula in A.2 with the arc current (arc resistance) is shown in Figure A.3. Visible. when electricity When the arc current is too large, the reactive current component accounts for the main part, and the power factor is very low. Description. P --- active power in watts (W); Q --- reactive power, the unit is spent (Var); S --- apparent power in volt-amperes (V·A).

Figure A.3 Schematic diagram of the simulation of the electric parameters of the electric arc furnace with the change of the arc current

B.1 Power impact prediction method

This method still uses the circuit model of Figure A.2.

Let Sd=3×Upcc2/X2 (Sd refers to the short-circuit capacity of the electric arc furnace, and Upcc is the phase voltage of the PCC). In theory, the electric arc furnace absorbs The power P and the reactive power Q satisfy the formula (B.1), as shown in Figure B.1. P2 (Q-Sd/2) 2 = (Sd/2) 2 (B.1)

Set the power factor of the electric arc furnace short net design to cosphid (point B in Figure B.1), and the rated design power factor is cosphin (Figure B.1, A)

Point), then the longitudinal distance between A and B in Figure B.1 indicates the maximum reactive power shock that may occur during the operation of the EAF. deltaQmax=Sd(sin2phid-sin2phin) (B.2)

If the short-circuit capacity of the PCC point is SN, the voltage change (deltaV) corresponding to the reactive power surge is. deltaV=deltaQmax/SN (B.3)

C.1 Ultra high power AC arc furnace harmonic current experience value

The summary of EAF harmonic current calculations used by different companies is shown in Table C.1.

Table C.1 Summary of EAF Harmonic Current Calculations Used by Different Companies Harmonic number Harmonic current content rate (HRIh) Western Europe (BBC) Japan (Japan New Motor)

British Soviet ABB Shanghai Baosteel Anda Max 95% 2 5.6 4~9 (maximum 30) 9~12 10 5.1~9.5 5.0 17.40 4.21 3 6.5 6~10 (maximum 20) 15~20 11 4.4~11.2 6.0 20.15 6.69 4 3.3 2~6 (maximum 15) 5~7 3 2.0~4.8 3.0 6.48 2.58 5 6.4 2~10 (maximum 12) 4~6 9 2.6~8.9 4.0 10.63 6.84 6 1.7 2~3 (maximum 10) 1 1.5 4.31 1.36 7 2.5 3~6 (maximum 8) 2~3 4 0.7~5.7 2.0 5.05 3.40 8 1 1.0 1.47 0.63 9 1 0.3~1.4 1.0 2.16 0.82 10 and above < 1 < 1 < 1.24 < 1

Note: This table is the empirical data of the melting period of ultra high power AC arc furnace.

C.2 LF harmonic current experience value

The empirical value of LF harmonic current is shown in Table C.2. Table C.2 LF harmonic current calculation

Harmonic number 2 3 4 5 6 7 8 9 11, 13 10, 12, 14~25 Hth harmonic current Content rate (HRIh)

4.0 7.0 3.0 3.5 1.0 1.5 1.0 0.8 0.4 0.2

E.1 Estimation of frequency deviations caused by the entire system

Because the active power fluctuation period of the electric arc furnace is short, the power supply network can sufficiently utilize its power-frequency static characteristic when subjected to such an impact.

One adjustment of the sex and frequency (ie, regardless of the frequency regulator function of the generator set), the frequency fluctuation caused by the active power impact can be Equation (E.1) calculation. deltaP SG × rhoKG KL (E.1) In the formula. deltaf---frequency fluctuation caused by active power impact, in Hertz (Hz); fN --- system frequency nominal value, in Hertz (Hz), fN = 50Hz; deltaP---impact active power in megawatts (MW); SG --- system total power generation capacity in megawatts (MW);

rho --- spare capacity factor, rho is equal to the ratio of SG to the total active load of the system; KG --- Power-frequency static characteristic coefficient of generator. Turbine generator set KG=16.6~25 (standard value); Hydrogenerator set KG=25~50 (standard value);

KL --- The frequency adjustment effect coefficient of the system load, KL=1~3 (standard value).

Under normal circumstances, rho >1, indicating that the system has spare capacity. If the generator is fully loaded or the prime mover has no governor device, KG=0;

When considering the influence of the active impact of the electric arc furnace, according to conservative estimation, KG=0 can be made, so that it can be known from the formula (E.1) that the impact of the active load on the frequency The effect is only related to the impact load accounted for the total capacity of the system and the frequency adjustment effect coefficient KL of the load. KL needs in the actual system Tested or estimated based on load data analysis. Using equation (E.1), in the most severe case, let KL=1, KG=0, then when active When the impact load does not exceed 0.4% of the total power generation capacity of the system, the frequency variation can be guaranteed to not exceed 0.2 Hz, which can be used as a project pre-assessment. Practical reference criteria.

E.2 Estimation of damage to generator sets in near-area power grids

When the power system is disturbed by a load (active power) somewhere, all the machines in the system are caused by the sudden change of the bus voltage phase of the impact point.

The group has the same angular increment for the power angle of the impact point, and the slope of the initial operating point of each unit according to its power angle characteristic curve (the whole step power) is large.

Small share the impact load. An electric power system with m generators, the active power disturbance amount deltaPL occurs at node k when t=0, Then it can prove the impact power of the i-th generator. Psik Psik Êê Úú deltaPL (E.2) In the formula. The impact power received by the ith generator at deltaPi(0)---t=0;

Psik -- the full step power between the -i and k points, ie dPik ddeltaik deltaik0 =E'iUkBikcosdeltaik0 (E.3)

Where E'i is the constant electromotive force after the transient reactance of the i-th generator; Bik is the transfer susceptance between the two points i and k; when deltaik0 is t=0, i, k voltage phase difference between two points; Pik = E'iUkBiksindeltaik is the active power transmitted between i and k.

It can be seen from equation (E.2) that in the moment of impact at node k, the generator set in the system will mainly be electrically connected to node k according to their respective electromotive forces.

Distance (ie Bik is the reciprocal), ie the full-step power factor Psik/sum for each k point Psik to share the disturbance amount deltaPL of the load, away from k The closer the electrical distance is, the greater the impact of sharing. This process is instantaneous and rapid. When the generator is subjected to the disturbance, However, the original electromagnetic power output is changed, and due to the mechanical inertia relationship, the mechanical power cannot be suddenly changed, which causes the power unevenness.

The balance will inevitably cause a change in the speed of the generator. […]