Selection Scheme of Current Transformer in Low Voltage Distribution System

Abstract: Analyze the principle of low-voltage current transformers, introduce the concept of accurate level and accurate level limits, and on this basis, combine engineering examples. Low-voltage current transformers are used for low-voltage measurement, metering, relay protection, system monitoring, and grounding protection.
Keywords: low-voltage distribution system, low-voltage current transformer, working principle, accuracy level, accuracy level limit, selection
1 Introduction With the transformation of China's power industry in the city network and rural network, as well as the continuous improvement of the low-voltage power distribution system automation, current transformer as an important electrical component in the low-voltage distribution system has been widely used in measurement , Measurement, relay protection, system monitoring, grounding protection, and various power system analysis, this article is a preliminary discussion of this.
2 The working principle of the low voltage current transformer The working principle of the low voltage current transformer is shown in Figure 1. The primary winding of the current transformer is connected in series in the measured line. I1 is the primary current of the current transformer, which is the line current, and N1 is the current transformer. The number of turns, secondary current of I2 current transformer (usually 5A, 1A), N2 is the secondary number of turns of the current transformer, Z2e is the impedance of the secondary loop device and the connecting wire. When the primary current flows in from the current transformer P1 end, P2 end, in the case of the second Z2e is connected, by the principle of electromagnetic induction, the secondary winding of the current transformer has a current I2 flowing from S1 through Z2e to S2, Form a closed loop. This results in an ideal current I1×N1=I2×N2, so that I1/I2=N1/N2=K, and K is the ratio of the current transformer.
3 Selection of low voltage current transformers
3.1 Measuring current transformer
3.1.1 Current transformers for measurement are current transformers for current supply for indicating instruments, integrator instruments and other similar appliances. Current transformers for measurement are widely used for the measurement of currents in low-voltage distribution systems. They are mainly accurate (for current transformers The rated levels are: 0.2, 0.5, 1, 3, 5, etc. The current measurement transformers that are widely used are mainly bus-type current transformers, which are easy to install and have a wide range of models and specifications. They can be based on different specifications. The busbars or cables use the most economical and reasonable current transformers. Table (1) uses AKH-0.66 current transformers to analyze the use and characteristics of current transformers for measurement.

3.1.2 Problems and application examples of current transformers for measurement in low voltage distribution systems The selection of secondary current output 5A and 1A for measuring current transformers in low voltage distribution systems is a problem frequently encountered by electrical engineers.
In December 2009, a chemical plant in Liaocheng, Shandong Province, the environment of the production workshops was mostly an explosive environment. The electrical control rooms of each workshop were not installed in the workshop, but were installed in public electric control rooms farther from the workshops to realize the system current. Concentrated collection of information, the distance between the current transformer and the control room on site is about 200 meters, some are even 300 meters, the secondary transmission wire is 2.5 square millimeters, and the current transformer used is AKH-0.66/30I 200/5A 0.5 class. For many specifications such as 5VA core 1 and many others, the current meter used is CL72-AI. This project is relatively large. When the project is completed and part of the project is commissioned, it is found that all ammeter display and field current are completely inaccurate.
After analysis, the rated capacity of the current transformer is the rated secondary current I2e of the current transformer, and the apparent power S2e consumed when passing through the secondary loop rated load Z2e, that is, S2e=I2e2Z2e; the apparent power consumed by the factor table is only 0.05VA is very small, so we can not consider, Z2e=ρ.2L/S=0.0176Ω. mm2/m×2×200m/2.5=2.82Ω, S2e=I2e2Z2e=5A2×2.82Ω=70.5VA, far It is much larger than the rated capacity of the current transformer 5VA, so the current transformer of 200/1A should be selected at this time. In February 2010, this project replaced all 5A current transformers, and at the same time, since the ammeter is a digital display, the ratio can be changed. Reset to 200/1 to make the entire system back to normal.
From this example, we can get the current transformer connected digital display ammeter, the transmission distance, such as table (2)

3.2 Measuring current transformer
3.2.1 Current transformers for metering are current transformers that are used in conjunction with metering energy meters and metering devices. The main accuracy levels are: 0.2, 0.5S, 0.2S.
3.2.2 Problems and Application Examples of Current Transformers for Metering in Low-voltage Distribution Systems Metering Current Transformers In low-voltage power distribution systems, the accuracy level of 0.2 and 0.2S is often a problem encountered by users and errors. The effect of wiring (inverted polarity) on metering.
3.2.2.1 The difference between accuracy class 0.2 and class 0.2S is shown in Table 3

3.2.2.2 Improper wiring (polarity reversed) of metering current transformer Impact on metering (1) Metering wiring method Three-phase three-wire correct active power when wiring: P=Pa+ Pc=UabIa.cos(30°+ Φa)+ Ucb.Ic.cos (30°-φc);
When the three-phase circuit is balanced, Uab=Ucb=√3U, Ia=Ic=√3I, ie, P=3UI cosφ
If phase A current transformer is reversed polarity, see wiring diagram (a) and phasor diagram (b)
Thus we can conclude that the current Io of the common line is √3 times the phase current;
The current hysteresis voltage of the meter 1 is: 30°+φa+180°=210°+φa;
The second lagging voltage of the energy meter is: 30°-φc;
Therefore, the active power at the wrong wiring is:
P′=Pa′+Pc′=Uab.Ia.cos(210°+φa)+ Ucb.Ic.cos(30°-φc)=UIsinφ;
If the power factor cosφ = 0.9, then when the A-phase measuring transformer is reversed in polarity, the metering power is actually measured:
P/P'-1=3UIcosφ/UI sinφ-1=3×0.9/0.4359-1=5.19 times;
(2) Metering wiring method The active power when three-phase four-wire is correctly wired is: P=Pa+Pb+Pc=UaIa.cosφa+Ub.Ib.cosφb+Uc.Ic.cosφc;
When the three-phase circuit is balanced, Ua=Ub=Uc=U, Ia=Ib=Ic=I, ie, P=3UIcosφ
If phase A current transformer is reversed polarity, see wiring diagram (c) and phasor diagram (d)
In this way we can conclude that the current Io of the common line is twice the phase current and the phase A current is -Ia;
Therefore, the active power at the wrong wiring is:
P′=Pa+Pb+Pc=−UaIa.cosφa+Ub.Ib.cosφb+ Uc.Ic.cosφc= UIcosφ;
When the polarity of the A-phase measuring transformer is reversed, the metering energy is the actual metering energy:
P/P'-1=3UIcosφ/UIcosφ-1=2 times;
3.3 Protection Current Transformers
3.3.1 Current transformers for protection are current transformers that provide current for protection relays. They are used with current relays and other similar electrical appliances. They are mainly used for low-voltage power distribution system current overload protection and short-circuit protection. The main accuracy levels are: 5P, 10P, 5, 10 means that the compound error is 5%, 10%, and the accurate limit coefficient is also called the limit coefficient, which is the rated current with the accurate limit value (the error at this time does not exceed 5%, 10 %) ratio to rated primary current, accurate limit factors are 5, 10, 15, 20,
3.3.2 Problems and examples of application of current transformers for protection in low-voltage distribution systems Protection of current transformers In low-voltage power distribution systems, the selection of accuracy levels and accurate limit factors is a problem frequently encountered by users.
In a factory in Hebei Province in 2003, due to the expansion of the new plant, a 250KVA (10/0.4kV) distribution transformer was installed near the new plant at 1500m. The distance between the plant and the main transformer was 2500m, and the load was small (rated current 80A). Taking into account the measurement accuracy, the transformer rated primary current is close to the maximum load current, the current transformer ratio 100/5A is selected, and the current transformer for protection is selected 100/5A, 10P10. In the construction of the sewer, the cable was cut off, causing a large area of ​​blackout near the factory.
Calculated by the power grid parameters: the main transformer short-circuit reactance is X1 = 0.07Ω, the transformer short-circuit reactance is X2 = 0.5Ω, the short-circuit reactance before the short-circuit point is X3 = 0.4Ω, the calculated reactance X*∑ = X1 + X2 + X3 = 0.97 . When the phase is short-circuited, the effective value of the sub-transient short-circuit current component of the TA is: I" = 1 / X * ∑ × 5.5 = 5.67 (kA). At this time, the short-circuit current is 56.7 times of the rated current, which is far greater than the accurate limit value. If the coefficient is 10 and the composite error exceeds 10% at the same time, the relay action will be affected and 500/5A 10P15 should be selected.
3.4 Residual Current Transformer
3.4.1 Grounding protection in low-voltage power distribution system includes: zero-sequence protection (both medium and high voltage can be used) and residual current protection (also called leakage current protection). The basic working principle of both is the same based on Kirchhoff. The current law, but the use of the occasion depends on the system grounding and different. Residual current transformers are mainly used in combination with relays, and are often used in conjunction with electrical fire monitoring systems. They have high sensitivity.
3.4.2 The difference between the remaining current transformers and the zero-sequence current transformers in the low-voltage power distribution system, Xiang (Table 4), and the application of residual current mutual inductance in different grounding systems, Xiang (Table 5).

3.5 Precautions in use of current transformers (1) When connecting current transformers, the same name must be consistent, namely P1, S1; P2, S2.
(2) When the current transformer is in normal operation, it must not be opened twice at the second time to prevent the secondary open circuit from generating high voltage and affect the safety of people and equipment.
4 Conclusion This article gives a brief overview of different types of current transformers in low-voltage power distribution systems. It is recommended for all power system experts and electrical engineers for reference, which is beneficial to the wide application of different types of low-voltage current transformers in low-voltage intelligent power distribution systems.
Article from: "Electrical Switch" 2010 4 period.
references
[1] Jiangsu Ankerui Electric Manufacturing Co., Ltd. Power Sensor Selection Manual, 200903.
[2] Ren Zhiyuan, Zhou Zhong. Principle and Application Guide for Power Electronic Measurement Digital Instrument, China Electric Power Press, 2007.

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