1 Introduction
In recent years, with the transformation of China's power industry mid-city network and agricultural network, and the increasing automation of power supply systems, current transformers have been widely used in relay protection as an important electrical equipment for power systems. System monitoring and power system analysis.
As a communication component between the primary system and the secondary system, the current transformer plays a small current that converts the large current of the primary system into a secondary system, and supplies power to the current coil of the measuring instrument and the relay respectively, correctly reflecting the normality of the electrical equipment. Operating parameters and fault conditions, so that the secondary side of the measuring instrument and relay and the primary side of the high-voltage equipment are electrically isolated to ensure the safety of the staff. At the same time, the secondary side equipment is standardized and miniaturized, the structure is light, the price is cheap, and the screen is easy to install, and the low-voltage small-section control cable is convenient to realize long-distance measurement and control. When a short-circuit fault occurs in the system, it can protect secondary equipment such as measuring instruments and relays from large currents. Let's take a shallow discussion about the selection and use of current transformers to help readers.
2 principle of current transformer
Transformer, generally W1 ≤ W2, visible current mutual influenza is a "converter", the basic principle is the same as the transformer, the working condition is close to the transformer short-circuit state, the primary side symbol is L1, L2, the secondary side symbol is K1, K2 . The primary side of the transformer is connected to the main line. The measured current is I1, the primary side turns to W1, the secondary side is connected to the current meter with low internal resistance or the current coil of the power meter, and the secondary current is I2. For W2. The primary secondary electromagnetic quantity and the prescribed positive direction are prescribed by electrical engineering.
It can be known from the principle that when the secondary side is open, only the magnetizing current I0 for establishing the main magnetic flux Φm is present in the primary current I1. When the secondary current is not equal to zero, a demagnetizing magnetizing force I2W1 is generated, which attempts to change Φm. However, when U1 is certain, Φm is basically constant, that is, I0W1 is kept unchanged. Because I2 appears, the primary current I1 must be increased to offset the demagnetization of I2W2, thus ensuring that I0W1 does not change, so there are:
I1Wl=I0Wl+(-I2W2) (1)
I0=I1+W2I2/Wl (2)
In the ideal case, the resistance of the coil, the core loss and the leakage flux can be obtained:
I1W1=-I2W2
Yes: Il/I2=-W2/W1
3 Current transformer selection
3.1 Principle of current transformer selection and inspection 1) The rated voltage of the current transformer is not less than the rated voltage of the installation point line;
2) Calculate the current transformer change according to the primary current calculation current IC;
3) Select the accuracy of the current transformer according to the requirements of the secondary circuit and verify the accuracy;
4) Verify the dynamic stability and thermal stability.
3.2 Current transformer variable current ratio The ratio of the current rated current I1n and the secondary rated current I2n of the current transformer is called the rated converter current ratio of the current transformer. Ki=I1n/I2n≈N2/N1, N1 And N2 is the number of turns of the primary and secondary windings of the current transformer.
Current transformer primary side rated current standard ratio (such as 20, 30, 40, 50, 75, 100, 150 (A), 2Xa / C) and other specifications, the secondary side rated current is usually 1A or 5A. 2Xa/C means that the same product has two current ratios. It is realized by changing the connection mode of the connecting piece outside the oil conservator at the top of the product. When connected in series, the current ratio is a/c, and the current ratio in parallel is 2Xa/C. In general, the current transformer current ratio of the metering should be chosen such that the primary rated current I1n is not less than the load current in the line (ie, the calculation IC). If the load calculation current in the line is 350A, the current transformer's converter ratio should be 400/5. The current transformer for protection can be selected to be larger in order to ensure its accuracy.
3.3 Current transformer accuracy selection and verification The so-called accuracy refers to the maximum error when the primary current is rated within the specified secondary load range. The accuracy and error limits of current transformers in China should use different current transformers for different measuring instruments.
The principle of accuracy selection: the current transformer for billing measurement has a accuracy of 0.2 to 0.5; the ammeter used to monitor the load current in each incoming and outgoing loop should be 1.0-3.0. Current Transformer. In order to ensure that the accuracy error does not exceed the specified value, the secondary load (volt-amperes) of the current transformer is generally verified, and the secondary load S2 of the transformer is not greater than the rated load S2n, and the selected accuracy can be guaranteed. Accuracy check formula: S2 ≤ S2n.
The load of the secondary circuit is l:. Depending on the value of the impedance Z2 of the secondary loop, then:
S2=I2n2—Z2—≈I2n2 (∑—Zi—+ RWl+RXC)
Or S2V1≈∑Si+I2n2 (RWl+RXC)
In the formula, Si and Zi are the rated load and impedance of the instrument and relay coil in the secondary circuit, and RXC is the contact resistance of all the joints and contacts in the secondary circuit, generally taking 0.1Ω, and RWL is the secondary loop wire resistance. ,
The calculation is formulated as: RWL=LC/(r×S).
Where r is the conductivity of the wire, copper wire r = 53 m / (Ω mm 2 ), aluminum wire r = 32 m (Ω mm 2 ), S is the wire cross-sectional area (mm 2 ), and LC is the calculated length (m) of the wire. Set the unidirectional length of the transformer to the meter to L1.
then:
L1 transformer is star-connected LC=L1 two-phase V-shaped wiring 2L1 one-phase wiring The accuracy of current transformer for relay protection is commonly used 5P and 10P. The accuracy of the protection stage is nominally based on the maximum compound error ε% at the rated current limit at the primary current (eg ε% = 5% for 5P). The so-called rated accuracy limit primary current, that is, the primary current is a multiple of the rated primary current (n = I1/I1n), also known as the rated accuracy limit coefficient. That is, the current transformer for protection is within the range that may occur, and the maximum compound error does not exceed the ε% value.
Current transformer ε% error curve verification steps:
(1) Calculate the primary current multiple of the current transformer flowing according to the type of protection device;
(2) Determine the allowable secondary load of the current transformer on the 10% error curve according to the type, ratio and primary current multiple of the current transformer;
(3) Calculate the actual secondary load of the current transformer according to the type of short circuit that has the most serious secondary load on the current transformer;
(4) Compare the actual secondary load with the allowable secondary load. If the actual secondary load is less than the allowable secondary load, it means that the error of the current transformer does not exceed 10% error:
1) increase the cross section of the connecting wire or shorten the length of the connecting wire to reduce the actual secondary load;
2) Select a relatively large current transformer, reduce the primary current multiple, and increase the allowable secondary load;
3) Connect the secondary windings of the current transformers in series to allow the secondary load to be doubled.
3.4 Current transformer dynamic stability and thermal stability verification The manufacturer's product technical parameters are given the dynamic stability multiple Kes and the thermal stability multiple Kt, so the dynamic stability and heat stability can be verified according to the following formula. .
1) Dynamic stability check Kes × I1N ≥ iSh;
2) Thermal stability check (KtI1n) 2t≥I(3)∞tima
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