2. External characteristics

U_dα

δ > 0

δ = 0

II

α = 120⁰

β = 60⁰

α = 90⁰

β = 90⁰

α = 0

α = 60⁰

I

I_d

Ограничительная характеристика

Figure 2.3

I - a rectifier conditions;

II - inverter condition;

ΔU_x has the as same sing as a no-load voltage.

As

then in case

δ=0 will be β=γ.

The range of a modification β is limited by an angle δ, allowable by the given type of a thyristor.

β_min = δ₀ + γ,

when δ₀ - classification parameter for the given type of a thyristor.

Usually δ₀ = 0.2 degree at f _c= 50 Hz.

Principle of operation for more complex inverter’s schemes is one and the same. Consequently, all circuits of the rectifying may be used for a current inverting. For this purpose it is necessary to change a sign opposite - EMF and a commutation delay angle to make more than 90 degrees .

All equations of dependences I_d = f(α, γ, X_а) and an external characteristics U_dα =f(I_d ) of rectifiers are fair to dependent inverters if to replace on α to π-β.

Then for the considered scheme

The same one is for other circuits of rectifying and inverting.

The dependent inverters are widely applied for an electrical haulage, for the electric drive to braking DC for current recuperation to a supply network, for the electric drive of an alternating current by the rectifier cascade scheme, for systems of DC power lines etc.

3. Equipment and characteristics

3.1 Transformers for converting sets

Functions of transformers:

- Transformation of magnitude of voltage;

- A galvanic outcome (the power circuit are separated from a circuit of a load);

- Transformation of phase number of a voltages system;

- Shift of phases of primary and secondary voltages.

The operating conditions of a transformer for converter differ from the operating conditions of transformers for linear load and standard frequency in 50 Hz.

The nonlinear loads are a reason of nonlinear currents through windings of the transformer and nonlinear magnetic fluxes in a core. The heightened frequency causes additional losses in the core and windings.

Constructive differences:

- Owing to heavier emergency operation it is necessary to augment inductance of windings for limitation of emergency currents, and also safely to fix windings and core for the reason that dangerous electrodynamic forces arise at emergencies;

- For series of schemes a total power of secondary windings S₂ is greater than a total power of primary windings S₁;

• The dispersed magnetic fluxes essentially influence to character of electromagnetic processes

in the converter. They determine inductive reactance of dispersion Х_s, which, in basic, is determined by inductance L_a). The character of distribution of dispersed magnetic fluxes depends on a core shape, type and windings disposition each from other, their construction, etc.

The resistance of windings is determined r_a.

For low power converters r_a >> Х_a and dispersed fluxes is neglected at such convertersdesigning.

For high-power converters there are Х_а >> r_a (as a rule, ), and r_a is neglected, but commutation stage of rectifiers influences to a marked degree to operation and converters designing.

For converters of average power it is necessary to take into account as r_a as Х_а.

There are two types of transformers for rectifiers and dependent invertors.

1) The transformer for the single-cycle schemes (half-wave, three-phase zero, full-wave three phase with LR), where exists direct -current component of a secondary winding, and for compensation of a forced magnetization flux it is necessary to arrange two secondary windings on a same rod (for example, "«zigzag" for a zero scheme). For an identical disposition of secondary windings in relation to primary they execute as rigid each other winded spirals, that the dispersed fluxes between secondary windings would be minimum ones.

2) The transformer for the two-stroke schemes differs from usual only by higher values Х_s (single-phase and three-phase bridge-schemes). At compiling equivalent circuits of resistance of a primary winding reduce to secondary, because parameters of a load I_d and U_d are data for calculation of converter.

where r '₁ -active resistance reduced to a secondary winding from a primary winding,

r₂ - active resistance of a secondary winding.

Х_а = Х ' _1S+X_2S,

where Х ' _1S - dispersed inductance of a primary winding reduced to a secondary winding ,

X_2S - dispersed inductance of a secondary winding.

The parameters of the equivalent schemes of the transformer define by practical consideration with the help of experiences short circuit and open-circuit conditions.