*98823*

UDC 621.313.333.1

PROTECTION SYSTEM OF SEMI-CONDUCTING RECTIFIER WITH APPLICATION OF MAGNETSENSITIVE INSTRUMENTS

S.S. Issenov, е-mail: isenov_sultan@mail.ru

Saken Seifullin Agricultural University, Kazakhstan

At present about 1/3 of produced electric energy around the world is transformed (with the help power electronics equipment) of before utilization. Power electronics represents nowadays an electrotechnical sector. Its output plays a vital role in electrotechnical and electric-power industries. To prove that the following data are given below [1]:

1) Annual usage volume around the world is (8… 12) *10¹² K.W.H. Annual energy production expenses amount 400… 500$ billion. 72… 78$ billion amount real loss of generative, transmitting and consuming objects;

2) The main electric energy users are electric drives of various functions (51%), lighting (19%), heating/cooling (16%), and telecommunication (14%);

3) At present less than 25% of electric energy is used optimally to implement work required (meaning loss minimization). This is achieved by application of high-performance methods of electric energy network system’s controlled transformation into energy of object control. The majority of such methods are based on high-performance energy converters utilization.

The progress in semiconductor electronics and transforming technique has influenced greatly the electric-power industry development. Application of semiconductor control systems and converters has made it possible not only improve some technical and economic indexes of the systems exist, but also apply new principles of control, the realization of which used to be unreasonable economically or impossible technically. To supply different loads noncontrolled rectifier (NR) is used in the number of direct-current and alternating-current drives. Strong reliability demands are made of the system of noncontrolled rectifier – load. That is why in order to protect the given system there are used quick-break fuses and quick-operating catalyst and noncatalyst switching units and protectors, which are installed at the point of entry and outlet of the rectifier.

To protect elements and optimize operating regime of noncontrolled rectifier - load electrotechnical system improved in its characteristics (lack of contacts, climatic impact resistance, and diminutiveness) a differential protection (DP) is applied on basis of ferreed appliances. The high sensibility of semiconductors’ electrical characteristics to different external actions (magnetic field, temperature) makes it possible apply them as sensors that measure and transform the corresponding impact magnitude.

Providing overall protection is one of the solving ways in creation and perfection of noncontrolled rectifier protection. The overall protection maintains the rectifier elements and the whole system itself against possible emergency state. A differential protection of semi-conducting rectifier (SR) underlies the overall protection, taking into account the most dangerous state that brings to overcurrent leaking.

There is known the differential protection of NR [2]. One of its features includes utilization of direct-current and alternating-current transformers as current sensors. Here the direct-current transformer (DCT) is functioning as current matching node at the NR indirect point of entry and outlet. This may be the basis of the overall protection. However, it is necessary to perfect its technical and economical indexes first: to exclude DCT, which doesn’t fully meet the demands made to semi-conducting devices protection. It also requires the break of output circuit of NR while examining, setup, or checking the DCT and matching node. Moreover, while incrementing the noncontrolled rectifier – load system the DCT mass and size indexes grow nonlinearly.

DP is more economical when DCT functions are implemented by magnetic diode, which is placed close to the NR output line or inside the resistor networked at the outlet point of the NR. Application of high-speed protection system that forms the basis of differential protection with direct-current and alternating-current sensors is one of the solving ways of the problem set (picture 1).

While connecting semi-conducting rectifier according to the bridge circuit VS1÷VS6 to ac network through the switching unit QF and a transformer TV, a load is linked up at its outlet point. At the transformer point of entry TA1÷TA3 alternating-current sensors are placed, to their entry point the VS1÷VS6 noncontrolled rectifier is networked. NR goes out through the RU nonlinear element, R1 resistor, VB1 controlling bundles of magnetic diode, KV1 sealed switch, trough the ILI gating circuit and is connected with the effector (E). At the transformer outlet point a velocity sensor of current change is placed on the basis of VD7÷VD9 magnetic diodes. The velocity sensor is networked with the second entry point of the ILI element through the single-limit comparator with feed back (VD10, VD11) and C1 condenser, which compares the velocity sensor of voltage build-up with voltage reference U_reference. The ILI element outlet point is networked through the effector (E) with the outlet point of nonautomatic disengage unit and the controlling entry point of the commutation unit disengagement. VB1 magnetic diode is placed close to the ac busbar. During the normal operating conditions of SR there is a signal at the outlet point of the nonautomatic engaging unit (NEU) that provides the starting of the commutation unit. So the SR is connected with the ac network through TV transformer and to the load by means of ac busbars. At the same time rectified current proportionate to current at the SR entry point is running along the VB1 magnetic diode gate winding and KV1 sealed switch. VB1 magnetic diode is in off condition as 2 magnetic fields are applied to it: one from the gate winding and another one from SR ac busbar. The magnetic flow of the VB1 magnetic diode’s gate winding is regulated by current change in the winding with the help of R1 resistor. Magnetic field created by ac busbar is regulated by the distance change between the busbar and the magnetic diode or by the angle change between them. Under the action of the magnetic flow of KV1 sealed switch’s gate winding the latter flips (front contact seals in and back contact breaks) thus effector (E) input circuit is prepared to act, and signaling unit’s (SU) input circuit breaks.

Picture 1 – Differential protection of the semi-conducting rectifier on magnetic diodes

The SU signalizes the continuity of differential protection control circuit. There is no signal at the first entry point of ILI element as VB1 magnetic diode is locked. There is also no signal at the second entry point of ILI element because of the comparator base, which does not act till the SR operates normally and current velocity change di/dt at SR’s point of entry and consequently at the outlet point of the current change velocity sensor is correspondent to the normal operating condition of SR (di/dt<di/dt_steady). That is why there is no signal at the entry point of ILI element and SR, and the commutation unit remains engaged.

When short circuit takes place in SR, at its entry and outlet points between the ac busbars and in the load short-circuit current appears in the circuit between the network and the short circuit point. At the same time short-circuit current velocity exceeds greatly current velocity change controlled by the velocity sensor of current change in the ac circuit at SR. That is why the signal at the outlet point of the velocity sensor of current change is strong enough to make the comparator base act (di/dt>di/dt_steady) and at the outlet point of the latter a signal appears, which comes in the second entry point of the ILI element.

At the outlet point of ILI element a signal appears that comes in the entry point of the effector (E). The effector (E) acts and gives signal to the commutation unit’s controlling entrance. The latter acts and disengages the SR power circuit from the ac network.

DP also acts at overcurrent. Current increases proportionally at the SR points of entry and outlet. However, magnetic fluxes influencing the VB1 magnetic diode increase proportionally to the current of protection act, when the RU nonlinear element goes into action. Then the RU nonlinear element limits the voltage at the NR outlet point and therefore limits the current and magnetic flux of the VB1 magnetic diode’s gate winding, it opens; at the first entry point of the ILI element a signal appears and at the outlet point of the latter a signal appears also. This signal sets the effector (E) going, and the commutation unit disengages the SR from the ac network.

To disengage the SR a signal is made by the nonautomatic disengaging unit (NDU). The signal goes to the controlling entry point of the commutation unit disengagement, which disengages the SR from the ac network.

Conclusion: application of DP of semi-conducting rectifier helps cut maintenance charges. There are several advantages of the DP application: the opportunity of full interconnection of entry and outlet circuits, non-contact transformation of little mechanical motions into electrical signals, magnitude detection and direction of magnetic field induction with high locality, the “non-sparking” mechanical commutator creation in circuits, and non-contact current test.

Literature

1. G.S.Zinoviev. Theory of power electronics. Tutorial. 3^rd edition revised and augmented. – Novosibirsk. Publication of NSTU (Novosibirsk State Technical University).2004.-672 p.

2. V.S.Kopyrin, V.P.Markowskiy. Differential protection of the non-controlled rectifier – drive winding system//Science and new technology in power industry of Pavlodar-Ekibastuz region: thesis report. – Almaty, 1994. 68-71 p.