Selection and Application of Tapchangers to Transformers

Abstract

In this chapter we consider the selection criteria and applications of tapchangers to transformers. The transformer is a flexible component in the power network. It can take many forms and shapes to satisfy the user’s requirements. Its companion the tapchanger follows suit. In this chapter we consider many “standard” applications, and some unusual executions of tapchangers.

Age cannot wither her, nor custom stale.

Her infinite variety.

W. Shakespeare, Antony & Cleopatra

Appendix: Commutation of Current Between Coarse and Fine Tapping Sections

5.1.1 Loop Inductance

The derivations in Chap. 3 of interrupted current and recovery voltage show they are in phase. This is because the inductance in the local commutating loop, contributed by one tap is very low. This phase relationship is propitious for arc extinction. A more difficult switching condition is encountered in the position where the coarse and fine are interchanged into the effective circuit of the transformer. Briefly, in this position, the leakage reactance between the coarse and fine is included in the commutating loop, and causes a phase shift between the interrupted current and recovery voltage. To examine if the diverter switch is capable of performing this duty we need to know the magnitude of the interrupted current, the recovery voltage, and the phase angle between.

5.1.2 Commutation When Coarse and Fine Have Equal Voltage

To study the effect of commutating current between coarse and fine-tap sections, we shall consider a tapchanger with flag cycle diverter switch. We first analyse the case when the induced voltages of the fine and the coarse are equal. In Fig. 5.42 the tapchanger is about to switch from tap No. 13 to tap No. 4, i.e. transition contact X is about to break current. This commutates current from fine to coarse. To derive the switching current of contact X we need to find the current through it before the contact breaks. To derive the recovery voltage, we have to establish the voltage across the parted contacts after the current is interrupted. The two conditions are shown in Fig. 5.42a, b, respectively. The through current I splits into the parallel paths through the fine and the coarse in the inverse proportion of their impedances in Fig. 5.42a. Ignoring winding resistances, which are too small, the current through the fine section is

Fig. 5.42

Commutating current between coarse and fine

$$ I_{\text{f}} = I(R + jX_{\text{c}} )/(2R + j(X_{\text{f}} + X_{\text{c}} )) $$

(5.1)

The phase angle of the interrupted current relative to the reference vector is

$$ \o_{1} = \o + \arctan \,X_{\text{c}} /R - \arctan \left( {X_{\text{f}} + X_{\text{C}} } \right)/2R $$

(5.2)

Where R is the transition resistance, and X_f and X_c are the leakage reactances of the fine and coarse windings.

From Fig. 5.42b, the recovery voltage is

$$ V_{\text{r}} = I\left( {R + jX_{\text{c}} } \right) $$

(5.3)

The phase angle of V_r is

$$ \o_{2} = \o + \arctan \,X_{\text{c}} /R $$

(5.4)

The phase shift between the interrupted current and recovery voltage is ø₁ − ø₂. While these are exact equations, the significance of the special switching involved is not transparent. To get realistic relative magnitudes of the parameters and to appreciate the problem, we consider the following numerical example.

• 1. The magnitude and the phase angle of the interrupted current and recovery voltage are affected only by the transition resistance, the leakage reactance of the coarse and fine, and the relationship between the current and the transition resistance. Here we take R = E/I_fl where I_fl is the full load current.

• 2. For further simplification the two leakage reactances are taken as equal.

• 3. In most practical applications, the transition resistance is about 4–8 times the individual leakage reactances. We shall assume R = 6X_f or 6X_c.

Substituting the assumed figures in Eqs. 5.1–5.4 results in the following values:

• 1. Interrupted current = I/2

• 2. Recovery voltage = 1.01E

• 3. The phase angle 9.5°

The interrupted current and the recovery voltage are within the capabilities of the tapchanger but there exists a high angle between them. This is unlike the situation when tapchanging within the fine, when the phase angle is taken as zero.

5.1.3 Possible Failure to Interrupt at Contact X

The phase angle may mean that X fails to interrupt at the first or even the next few current zeros after contact parting. The arc may still persist until the Z contact also closes (Fig. 5.42e). This by itself is not a failure to switch. It is necessary that with the new conditions that prevail with the Z contact closed, the diverter should extinguish the arc at X. The current through the arc and the fine taps is now

$$ I_{\text{f}} = jX_{\text{c}} I/[R + \left( {X_{\text{f}} + X_{\text{c}} } \right)] $$

(5.5)

Applying the same parameters as before to the new switching condition, from Eq. 5.5 the phase angle between the interrupted current and the through current is 90°-arctan (X_f + X_c)/R. When the arc is extinguished (Fig. 5.42e), the recovery voltage V_r that appears across X and Y is jX_cI. The phase angle from the through current vector is 90°. The phase angle between the interrupted current and recovery voltage is the difference, 18.5°. Applying the same parameters as before to the new switching condition, from Eq. 5.4

• 1. I_f = I_int = 0.158 I in magnitude

• 2. V_r = 0.16 E in magnitude

• 3. The phase angle between them is about 18.5°.

The conditions are much ameliorated as compared to the earlier break (Fig. 5.42b). It would be the tap changer manufacturer who should confirm the ability of the diverter to clear these duties. Commutation of current from coarse to fine is similar to fine to coarse due to the symmetry of the circuit.

5.1.4 Case where there is a step voltage difference between the coarse- and fine-induced voltages

In this case the transition from fine to coarse and vice versa is performed in one step. The circuit is shown in Fig. 5.43a in the bridging condition. There is now also a circulating current

Fig. 5.43

a–f Page 1: Commutating current from fine to coarse. g–l Page 2: Commutating current from coarse to fine

$$ I_{\text{c}} = (V_{\text{c}} - V_{\text{f}} )/[2R + j(X_{\text{f}} + X_{\text{c}} )] $$

(5.6)

A rigorous analysis of the circuit is presented in [7], Sect. 4.4. For the present purpose, we shall only consider the case when the arc does not go out in the situation corresponding to Fig. 5.43b but the situation proceeds to Fig. 5.43c, where the Z contact closes while the X contact is still arcing to the Y contact. This shorts the transition resistance on the coarse side. However there is no short circuit, as the circulating current driven by the voltage difference between the coarse and fine is limited by one transition resistance on the fine side together with the leakage reactances of the coarse and fine. The revise circulating current

$$ I_{\text{c}} = E/(R + j(X_{\text{f}} + X_{\text{c}} )) $$

(5.7)

Where E = (V_b − V_f). The main current splits into the fine side as

$$ I_{\text{f}} = jX_{\text{c}} I/(R + j(X_{\text{f}} + X_{\text{c}} )) $$

(5.8)

The circulating current subtracts from component of the main in the fine taps, so that I_f − I_c which is the current to be finally interrupted is

$$ I_{{\text{int} }} = \left[ {I\left( {\cos \o - j\sin \o} \right)jX_{\text{c}} - E} \right]/\left[ {R + j\left( {X_{\text{f}} + X_{\text{c}} } \right)} \right] $$

(5.9)

Figure 5.43f shows the vector diagram. The recovery voltage after the arc clears is

$$ V_{\text{r}} = jX_{\text{c}} I(\cos \o - j\sin \o) - E $$

(5.10)

5.1.4.1 Numerical Example

Using the same parameters as in the previous section for a load power factor of 0.8

• 1. I_int = 0.816I in magnitude. The high interrupted current is practically all the circulating current set up by the voltage difference within the loop. See the small other component I_f shown in Fig. 5.43f. The phase angle of I_int with the reference vector is approximately 53.2˚

• 2. Recovery voltage = 1.103 E

• 3. The recovery voltage (Eq. 5.10) is the interrupted current (Eq. 5.9) times R + j(X_f + X_c). Therefore the phase angle between the two is arctan (X_f + X_c)/R. The voltage leads the current by this angle. The magnitude is 8.7°.

5.1.5 Commutation of Current from Coarse to Fine

Figure 5.43i–k show current commutation in the reverse direction, from coarse to fine. In Fig. 5.43j must clear the arc, with W closed. Frotcher current splits into the coarse side as

$$ I_{\text{co}} = I(\cos \o - j\sin \o)jX_{\text{f}} /(R + j(X_{\text{f}} + X_{\text{c}} )) $$

(5.11)

The circulating current now adds to the component of the main. The interrupted current is now

$$ I_{\text{int}} = I\left( {\cos \o - j\sin \o} \right)X_{\text{f}} + E/(R + j(X_{\text{f}} + X_{\text{c}} )) $$

(5.12)

The recovery voltage is

$$ V_{\text{r}} = jX_{\text{f}} I + E $$

(5.13)

Figure 5.43l shows the vector diagram.

5.1.5.1 Numerical Example

From Eq. 5.11, the interrupted current in the numerical example is

$$ I_{\text{int}} = 1.00I $$

From Eq. 5.12, V_r = 1.03E

By the same reasoning as with the interruption of the fine side, the phase angle between the interrupted current and the recovery voltage is 18.5°.

5.1.5.2 Note on Current Commutation

Commutation of current between the coarse and fine is rendered difficult due to the phase angle between the interrupted current and the recovery voltage. The closure of the main contact on the closing side helps in reducing the switching severity. There is no provision in the Standards to demonstrate the successful commutation between coarse and fine.