Figure 1: Thyristor Controlled Reactor (TCR).

Working Principle and Waveforms of Thyristor Controlled Reactor (TCR)

A basic TCR is shown in figure (1). It consists of two main components, thyristor switch (T_y) and linear reactor ‘L’. Thyristor switch comprises of two back to back thyristors which conduct on alternate half cycles of the supply frequency. If a gate pulse is applied to the thyristors, it results in conduction of thyristor valve or switch. The current in the reactor can be controlled from maximum to zero value by varying the firing delay angle α.

Figure 2: Waveforms of Thyristor Controlled Reactor (TCR)

The duration of current conduction intervals is controlled by delaying the closure of the thyristor switch with respect to the peak applied voltage once in each half cycle as shown in figure (2).

When α = 0°. the thyristor switch (T_y) gets closed at the peak of the applied voltage, the amplitude is maximum and hence the resulting current in the reactor is equal to the steady state current. When α = 90°. the amplitude is zero and hence there is no current flow during the corresponding half cycle. The TCR current as a function of angle, (a) can be expressed as,

\[{{I}_{LF}}(\alpha )=\frac{V}{\omega L}\left( 1-\frac{2\alpha }{\pi }-\frac{\sin 2\alpha }{\pi } \right)\]

Figure 3.

Where.

V – Amplitude of applied voltage

L – Inductance of TCR

ω – Angular frequency of applied voltage.

The amplitude variation of the fundamental  TCR current with the delay angle a is shown in figure (3). From figure (3), it can be observed that the TCR can control the current continuously from zero to a maximum value.

Characteristics of Thyristor Controlled Reactor (TCR)

Figure 4: V-I Characteristics of Thyristor Controlled Reactor (TCR)..

The TCR ratings are decided as per the operational requirement. It can be operated in specified V-I region. The boundaries of this region is determined by maximum voltage, current and admittance which are shown in figure (4).

Where,

V_L(max) – Maximum voltage

I_L(max) – Maximum current

Y_L(max) – Maximum admittance

If TCR operates at fixed delay angle, α = 0° (say), then it act as Thyristor Switched Reactor(TSR). When TSR is fed from A.C supply it gives fixed inductive admittance and its reactive current is directly proportional to the supply voltage.

The post What is Thyristor Controlled Reactor (TCR)? Working Principle & Diagram appeared first on Electrical and Electronics Blog.

Figure 1.

Design of SVC possesses thyristors without gate tum-off capability. Separate apparatus for leading and lagging VAR are incorporated in SVC. To absorb reactive power, thyristor-controlled or thyristor-switched reactors are used and to supply reactive power thyristor-switched capacitor is used. Figure 1 shows a basic model of SVC.

Fixed Capacitor-Thyristor Controlled Reactor (FC-TCR)

Figure 2.

As shown in figure (2), an FC-TCR consists of a fixed (permanently connected) capacitor with a thyristor controlled reactor. In this the method of firing delay angle control is employed to vary the current in the reactor. A filter network with required capacitive impedance usually replaces the fixed capacitor fully or partially at the fundamental frequency. This is done so as to generate the required reactive power. But at certain frequencies it provides a low impedance in order to avoid the dominating harmonics generated by TCR.

Characteristics:

The V-I characteristic of Fixed Capacitor-Thyristor Controlled Reactor is defined by maximum admittance of inductor and capacitor and by their voltage and current rating. The VI characteristic of FC-TCR is shown in below figure (2),

Where,

I_CM = Maximum capacitive current

I_LM = Maximum inductive current

V_CM = Maximum capacitor voltage

V_LM = Maximum TCR voltage

Y_C = Capacitor admittance

Y_LM = Maximum Inductor admittance.

Advantages of Static VAR Compensator (SVC)

1. SVC is simple in operation.
2. It improves the steady state and transient stability
3. It has higher voltage capacity
4. It gives faster and reliable response.
5. It is less expensive.

Applications of Static VAR Compensator (SVC)

1. SVCs are employed in long transmission lines to increase the power transfer capability.
2. These VAR compensators are also employed at sub transmission and distribution system levels for balancing the three individual phases of the system supplying unbalanced loads.
3. Fluctuations in the supply voltage can be minimized by employing static var compensators.
4. Control of dynamic over voltage is also possible.

Difference between SVC and STATCOM

The post What is Static VAR Compensator (SVC)? Working Principle, Diagram & Advantages appeared first on Electrical and Electronics Blog.

Figure 1: Voltage Source Converter.

Symbolic representation of a voltage sourced convener is as shown in figure 1. The symbol has a box with a gate tum-off device paralleled by a reverse diode, and a D.C capacitor as its voltage source.

In a voltage source converter, devices undergo sequential switching to present unidirectional D.C voltage of a D.C capacitor, as A.C voltage to the A.C side. The A.C output voltage can be varied in magnitude and also in any phase relationship to the A.C system voltage, using suitable converter topology. The power reversal here involves only the reversals of current but not the voltage. In case the storage capacity of the D.C capacitor reduces and if the D.C capacitor has no other source of power, the converter can no more impart or consume the real power for more than a cycle. A converter imparts or absorbs the reactive power alone, as long as the A.C voltage and A.C current (leading or lagging) are at 90º phase angle, where the A.C current is taken as reference.

Basic Operating Principle of a Voltage Source Converter

Figure 2: Voltage Source Converter working principle.

The basic operating principle of a voltage source converter generating reactive power is comparable to that of a conventional rotating synchronous machine (see Figure 2). Single line diagram for a basic voltage source convener scheme for reactive power generation is as shown in figure. The convener gives a set of controllable three phase output voltage with system frequency by the charged capacitor C_S when a D.C input voltage is given. Each output voltage is in phase with and coupled to the respective A.C system voltage through a small tie reactance. The tie reactance is of the order 0.1 to 0.15 p.u and it is provided by the per phase leakage inductance of the coupling transformer. The reactive power exchange between the converter and the A.C system is controlled by changing the amplitude of output voltage produced. This means that when the output voltage is increased than that of the A.C system voltage, current flows via the tie reactance from the converter to the A.C system. Thus reactive (capacitive) power for the A.C system is generated by the convener. Now if the output voltage is decreased then the reactive current flows to the converter from the A.C system. Thus reactive (inductive) power is absorbed by the converter. In case if the output voltage and the A.C system voltage are equal in amplitude, the reactive power exchange is zero.

Circuit Diagram of a Voltage Source Converter

Figure 3: Circuit Diagram of a Voltage Source Converter

The circuit diagram of single-phase full wave bridge converter is shown in figure (3).

Working of a Voltage Source Converter

Voltage source converter generates A.C voltage from D.C voltage. A single phase full wave bridge converter consists of 4 valves, namely valve-1, valve-2, valve-3 and valve-4 and each valve consists of a turn-off device T, and diode D, connected in series with each other. On the D.C side, as the voltage is unipolar, it is supported by a capacitor. The capacitor is used to handle the current that accompanies the switching sequence of the converter valve and shifts in phase angle of the switching valves without change in D.C voltage and two ac connection points A and B.

Conversion of AC voltage to D.C voltage is possible by changing the turn-ON and turn-OFF sequence of valves. In one cycle the single phase full wave bridge operates in four different operating modes as given below,

1. T₁ and T₄ ON, T₂ and T₃ OFF (Inverter)
2. T₁ and T₄ ON, T₂ and T₃ OFF (Rectifier)
3. T₁ and T₄ OFF, T₂ and T₃ ON (Inverter)
4. T₁ and T₄ OFF, T₂ and T₃ ON (Rectifier)

When the turn-off devices T₁ and T₄ are turned ON, voltage becomes positive i.e., +V_d for one half cycle and with T₂ and T₃ turned ON, V_AB becomes negative. The interaction of the converter generated A.C voltage with the A.C system voltage and impedance results in A.C current, which is generally a sinusoidal wave form.

From the 1^st operating mode time t_a to t_b with the turn-off devices T₁ and T₄ ON and T₂ and T₃ off, V_AB is positive and current i_AB is negative. Here power flow is from D.C to A.C. Hence inversion action takes place. In 2^nd operating mode time t_b to t_c, the current i_ab is positive and flows through diodes D₁ and with power flow A.C to D.C. Hence, it acts as a rectifier. Similarly 3^rd and 4^th operating modes are conducted. The operations of all the modes are shown below in comparison table.

The output waveforms of single phase converter is shown in figure (4).

Figure 4.

Figure 5.

Figure 6.

Figure 4 gives the output waveforms of voltage and current as per the given table. Figure 5 is the voltage across the valve 1 i.e., V_T1-D1. It is also known as lost waveform. Figure 6 shows the power flow from A.C to D.C with a power factor lagging. It gives the relationship between A.C voltage and current phasors.

Disadvantages of a Voltage Source Converter

The following are the disadvantages of voltage-source converters,

1. The output current limitation is low and it cannot control the capability of the semi-conduction device system faults.
2. In voltage-source converter, it is very difficult to protect converter against internal faults.
3. They have high-short circuit current and the transformer connections are complex compared with CSC.
4. Rapid increase in rise of capacitor discharge current results in the damage of valves.

The post What is Voltage Source Converter? Working Principle & Circuit Diagram appeared first on Electrical and Electronics Blog.