🗊Презентация Current and voltage transformers

Включение

Измерительные трансформаторы тока и напряжения применяют в качестве преобразователей больших переменных токов и напряжений в относительно малые величины, измерение которых возможно стандартными приборами с относительно небольшими пределами измерений. Измерительные трансформаторы тока и напряжения применяют в качестве преобразователей больших переменных токов и напряжений в относительно малые величины, измерение которых возможно стандартными приборами с относительно небольшими пределами измерений.

secondary windings are rated for 5 A and 1A Also 2, 2.5, 10, 20 A CT equivalent circuit and its simplification

the leakage impedance of the primary winding Zx1 has no effect on the performance of the transformer, and may be omitted. the leakage impedance of the primary winding Zx1 has no effect on the performance of the transformer, and may be omitted. The load impedance Zb includes the impedance of all the relays and meters connected in the secondary winding

CT phasor diagram CT phasor diagram the voltage Em across the magnetizing impedance Zm is given by Em = Eb +Zx2I2 The magnetizing current Im is given by Im =Em/ Zm The primary current I1 (referred to the secondary winding) is given by I1 = I2 +Im

The per unit current transformation error defined by The per unit current transformation error defined by The ratio correction factor R is defined as the constant by which the name plate turns ratio n of a current transformer must be multiplied to obtain the effective turns ratio Although ∈ and R are complex numbers

Example Consider a current transformer with a turns ratio of 500 : 5, a secondary leakage impedance of (0.01+j0.1) and a resistive burden of 2.0 . If the magnetizing impedance is (4.0+j15) , then for a primary current (referred to the secondary) of Il, find the error and the correction factor.

Since the magnetizing branch of a practical transformer is nonlinear, Zm is not constant, and the actual excitation characteristic of the transformer must be taken into account in determining the factor R for a given situation. Since the magnetizing branch of a practical transformer is nonlinear, Zm is not constant, and the actual excitation characteristic of the transformer must be taken into account in determining the factor R for a given situation.

The magnetizing characteristic of a typical CT is shown in this Figure The magnetizing characteristic of a typical CT is shown in this Figure This being a plot of the r.m.s. magnetizing current versus the r.m.s. secondary voltage, Im for each Em must be obtained from this curve, and then used in equations to calculate the ratio correction factor

Из анализа полученных уравнений можно сделать следующие выводы: При возрастании сопротивления вторичной обмотки или ее разрыве (I2=0) происходит возрастание МДС Imw1 до I1W1, это в свою очередь вызывает резкое увеличение потока Фm, сопровождающееся а) ростом потерь в сердечнике и его перегрев, б)ростом ЭДС Е2, что может вызвать аварийную ситуацию пробоя Увеличение сопротивления нагрузки вторичной цепи, например, за счет включения большого числа приборов, приводит к росту Im и тем самым к росту токовой и угловой погрешностей. Im будет тем меньше, чем выше магнитная проницаемость сердечника и чем меньше магнитные потери, а также при уменьшении индукции до ~0,05-0,15 Тл Увеличение индуктивного сопротивления нагрузки приводит к увеличению угла и следовательно к увеличению токовой погрешности и уменьшению угловой погрешности.

Polarity markings on CT windings Example Consider the CTs shown in Figures (a) and (b) If the primary current is 1000 A, and the two CT ratios are 1000 : 5 and 1000 : 5 respectively, the current in the burden impedance ZL is 10 A Figure (a) If the CT secondaries are connected as shown in Figure (b), the burden current becomes zero.

Consider the CT connections shown in Figure The wye connection shown in Figure (a) produces currents proportional to phase currents in the phase burdens Zf and a current proportional to 3Io in the neutral burden Zn. No phase shifts are introduced by this connection. Consider the CT connections shown in Figure The wye connection shown in Figure (a) produces currents proportional to phase currents in the phase burdens Zf and a current proportional to 3Io in the neutral burden Zn. No phase shifts are introduced by this connection. The delta connection shown in Figure (b) produces currents proportional to (Ia −Ib), (Ib −Ic) and ( Ic −Ia) in the three burdens Zf. If the primary currents are balanced, (Ia −Ib) =√3|Ia|exp(jπ/6), and a phase shift of 30◦ is introduced between the primary currents and the currents supplied to the burdens Zf. By reversing the direction of the delta windings, a phase shift of −30◦ can be obtained.

Principle of the magneto-optic current transformer (MOCT)

This angular shift is electronically converted to a voltage, which is proportional to the instantaneous value of the magnetizing force around the current-carrying conductor, and hence to the instantaneous value of the current. This voltage may then be suitably amplified and filtered to provide a replica of the current in the primary conductor. Alternatively, the voltage may be sampled at a suitable rate to provide a sampled-data representation of the primary current. It should be clear that such an electronic CT is most suited to relays and meters which can utilize low-power signals, or sampled data of the signals. As will be seen later, this type of signal source is particularly suited for electronic relays and computer relays. Electronic CTs are linear, and have a very wide dynamic range, i.e. they are able to measure accurately currents at light loads as well as those corresponding to very heavy faults. This angular shift is electronically converted to a voltage, which is proportional to the instantaneous value of the magnetizing force around the current-carrying conductor, and hence to the instantaneous value of the current. This voltage may then be suitably amplified and filtered to provide a replica of the current in the primary conductor. Alternatively, the voltage may be sampled at a suitable rate to provide a sampled-data representation of the primary current. It should be clear that such an electronic CT is most suited to relays and meters which can utilize low-power signals, or sampled data of the signals. As will be seen later, this type of signal source is particularly suited for electronic relays and computer relays. Electronic CTs are linear, and have a very wide dynamic range, i.e. they are able to measure accurately currents at light loads as well as those corresponding to very heavy faults.

Voltage transformers Voltage transformers – also known as potential transformers – are normal transformers with the primary winding connected directly to the high-voltage apparatus, and with one or more secondary windings rated at the standard voltage of 69.3 V for phase-to-neutral voltages or 120 V for phase to-phase voltages. Their performance, equivalent circuit and phasor diagrams are similar to those of a power transformer. The error of transformation of such a transformer is negligible for all practical purposes in its entire operating range – from zero to about 110% of its normal rating. We may consider such transformers to be error-free from the point of view of relaying. Voltage transformers are rather expensive, especially at extra high voltages: 345 kV or above. Consequently, they are usually found on low-, medium- and high-voltage systems. At extra high voltages, capacitive

Are the more usual sources for relaying and metering. Are the more usual sources for relaying and metering. In passing, we may mention a possible problem with voltage transformers when used on ungrounded (or high-impedance grounded) power systems. As shown in the Figure,

Coupling capacitor voltage transformers CCVT Connections and equivalent circuit

The Thevenin voltage is given by The Thevenin voltage is given by and the Thevenin source impedance is a capacitance of (C1 +C2) Since the Thevenin impedance of a CCVT is capacitive, the nonlinear magnetizing branch of the connected transformer may give rise to Ferro resonant oscillations, especially under light loads. Zf, is usually provided to damp these oscillations.