Smoothing reactors (JBS ready)

The direct current that is coming from the rectifier in d.c. systems has superimposed harmonic components, also called ripple. The smoothing reactor is connected in series with the

Figure 2-49

rectifier (convertor) and the whole load current, including the d.c. current and small a.c. harmonic currents, flows through it.

The purpose of the reactor is to provide high impedance to the flow of the harmonic currents, reduce their magnitude and thus making the d.c. current more smooth. The higher reactor inductance the smaller remaining harmonic currents (ripple), but at the same time higher reactor costs and losses. The voltage drop across the reactor is the notches in the voltage from the rectifier.

A smoothing reactor has no nominal power rating in the same sense as a.c. reactors. However, a size comparison can be made on the base of the stored magnetic energy. In this respect the size of a smoothing reactor in HVDC systems can be much higher than even the largest shunt reactors, which also is reflected in the physical dimensions. Sometimes it is necessary to share the winding (which naturally is single-phase) on two core limbs in order to keep the outer dimensions within the transport profile.

Besides reducing the current ripple, the smoothing reactor has other functions to cover, like: • preventing commutation failure in the inverter by limiting the rate-of–rise of current during

commutation in one bridge (the transfer of current from one valve to another in the same row in the bridge) and during collapse of voltage in another bridge;

• reducing the rate-of-rise of current if failures occur on the d.c. system; • improving the dynamic stability of the transmission system;

• reducing the risk of commutation failure during a.c. system voltage drop.

The design may be dry-type or oil-immersed, with or without gapped iron core or magnetic shield. The magnetic characteristic may be linear or non-linear.

In large HVDC systems the smoothing reactors is operating at a high d.c. potential to earth. When dry-type air-core reactors have been used, they have been placed on platforms, which have a high insulation level to earth. Dry-type air core reactors will probably in many cases, depending on the

Oil-immersed smoothing reactor in the Rihand - Dehli HVDC transmission

required inductance and the service voltage, be cheaper and lighter than oil-immersed reactors. However, even dry-type air core reactor weight may amount

to 25 – 50 ton, so an insulating platform must be of a mechanical robust design.

Dry-type air core reactors have a linear inductance characteristic, while oil-immersed reactors may have a non- linear inductance characteristic due to saturation in ferromagnetic core or shield, depending on the chosen flux density when designing the reactor.

The direct current flowing through smoothing reactors causes a magnetising bias where the a.c. magnetisation is superimposed. The magnetic flux will then not oscillate symmetrically around zero but around a flux value determined by the d.c. magnetisation. In the part of the cycle when the d.c. flux and the a.c. flux have the same direction, the iron core may be saturated.

Figure 2-50 shows an example where the vertical red line indicates the bias d.c. magnetisation caused by the d.c. current flowing through the reactor. The two dotted horizontal lines indicate the

Figure 2-50

range of linked flux variation caused by the superimposed harmonic a.c. voltage. They are located symmetrically around the d.c. linked magnetic flux. The two dotted vertical lines indicate the limits for the corresponding harmonic a.c. current. The latter limits are located asymmetrically in relation to the d.c. magnetising current.

The inductance L of the reactor is defined as:

di d

L= Ψ (2.5.8.1)

This is identical to the slope of the magnetisation curve, which varies with the magnetising current. The lower part of the curve is linear, and in this range L is constant. Where the core is completely saturated, the curve is also linear and L is also constant, but here the slope of the curve corresponds to the inductance of an air core reactor, as if the iron core does not exist. Between these two linear ranges there is a range where the curve is non-linear. In this range L is not constant but varies during the cycle of the a.c. voltage and current. The resulting L is called the incremental inductance, which is lower than the inductance in the low linear range of the curve. Consequently the reduction of the current ripple will also be lower compared to the reduction that would be achieved if the reactor were operated in the low linear range of the magnetisation curve. On the other hand, however, this would cause a more expensive reactor.

Magnetising current L in ked m ag n et ic f lu x

i

ψ

Air-core smoothing reactor in the FennoSkan HVDC transmission

Transformer handbook. Draft. Rev. 02Q Page 58 of 197

Smoothing reactors in large HVDC transmission systems manufactured by ABB are nowadays oil- immersed and designed with gapped iron core, just like large shunt reactors. For such reactors the incremental inductance is an essential parameter. It can be measured during the delivery test provided there is a sufficiently large d.c. source available in the test laboratory. Alternatively the incremental inductance can be calculated based on a recording of the magnetisation curve of the reactor.

Smoothing reactors in HVDC links are subject to special dielectric stresses when the direction of power flow in the link changes. To verify the ability of the reactor to withstand such stresses a polarity reversal test is performed before delivery from the factory. Figure 2-51 shows a voltage versus time diagram for such a test. A period with negative polarity is followed by a period of positive polarity and finally a period back to negative polarity. To demonstrate that there is a satisfactory safety margin the test voltage Upr should be higher than the rated d.c. voltage in operation, for example 25% or other value, according to agreement.

Figure 2-51

The polarity reversal test is followed by an a.c. voltage test of 1 hour duration with PD measurement.

The dielectric testing includes also a withstand test with d.c. voltage equal to 1,5 times the rated service voltage of 1 hour duration and with PD measurement.

Supplementary information is found in:

• IEC 289 (1988) Reactors, which is in the process of being revised and will be issued with the number IEC 60076-6;

• IEEE Std 1277-2000 IEEE Standard General Requirements and Test Code for Dry-Type and Oil-Immersed Smoothing Reactors for DC Power Transmission.