GB2290636A - Line conditioner for the reduction or elimination of disturbances - Google Patents

Abstract

A line conditioner has active current 4 and voltage 3 filters for reducing disturbances in an electric power system caused in the actual system or in the customer's installation. The active filters comprise electronic converters with static circuit-breakers switched at high frequency, and reactive elements. One of them 3 corrects voltage disturbances originating from the power system. The other 4 reduces harmonics required by the customer's installation. The two filters are interconnected by the actual line and direct current bars to which energy storage elements C7', C8' are connected. In the event of power system failure, a static circuit-breaker S'1, S'2 isolates the equipment from the power system, leaving the less critical load 2 connected to it by means of a second static circuit-breaker, while the active current filter 4 begins to operate as an uninterrupted supply system for the more critical part of the load 1 supplied by the storage elements. <IMAGE>

Description

LINE CONDITIONER FOR THE REDUCTION OR FLIMINATION OF DISTURBANCES The low voltage electric distribution system, in the absence of consumers, will demonstrate a voltage wave form having a quality which will very occasionally be disturbed mainly by faults in the lines and transforming centres, switching, and by atmospheric electric discharges. Consumers subject the power system to the influence of a plurality of loads which, even though they operate correctly, can affect the voltage wave form with excessive permanent or transient breakdowns, delivery of harmonic currents, peaks of current during start-ups and surges during stoppages, among other disturbances. Furthermore, the loads can deteriorate and produce anomalous consumption and short circuits which have to be isolated by protective systems prescribed by legislation.

If the defective load is not isolated, it can cause significant disturbances in the neighbouring points of the power system.

Interruptions of less than one second are the disturbances which most frequently impair the quality of the power system.

Uninterrupted supply systems are powerful electronic devices currently intended to eliminate prolonged power system failures.

Their range is normally based on the storage of energy in lead and nickel-cadmium batteries. Emergency periods usually last between 8 minutes and several hours. For shorter periods, rapid nickel-cadmium batteries are most suitable but they behave badly if a significant proportion of their energy is to be discharged in periods of less than 5 minutes. On the other hand, electrolytic capacitors are not currently adequate for periods in the range of more than some 500 ms, even though they have recently improved a lot with regard to stored energy/volume.

Attempts are being made nowadays to fill the gulf between ranges of 0.ls and several seconds by storing energy in superconducting coils. In fact, uninterrupted supply systems with these storage means which are intended to eliminate disturbances in very critical loads or installations are already being marketed. The complexity of the cryogenic system which nowadays demands the superconducting coil enables this system to solve problems of quality of supply in extreme random cases.

The present patent proposes a conditioner which is suitable for an electrical voltage energy storage means (battery, capacitor) operating as an uninterrupted supply system in the event of a power system failure.

The present invention relates to a line conditioner based on active current and voltage filters for reducing or eliminating disturbances in the electric power system caused in the actual power system or line or in the customer's or subscriber's installation. It comprises two active filters formed by electronic converters with static circuit-breakers which are switched at high frequency and reactive elements (capacitors and coils). One of them is dedicated to the correction of voltage disturbances originating from the power system. The other to the reduction of intensity harmonics demanded by the customer's installation. The two are electrically interconnected by the actual line and some direct current bars to which an energy storage element (capacitor or battery) is connected.In the event of a breakdown of the power system, a static circuitbreaker isolates the equipment from the power system, leaving the less critical load connected to it by means of a second static circuit-breaker while the current-filtering converter reconfigures its operating strategy and begins to operate as an uninterrupted supply system for the more critical part of the load, supplied by the storage element. The storage element charges itself during normal operation, that is with the power system present.

The present invention is further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an embodiment of the invention with a three-phase configuration with its critical load (1) and non-critical load (2).

Figure 2 shows an embodiment of the invention as a corrector with a single-phase configuration.

The active voltage filter consists of the voltage-injecting transformers T1, T2 and T3, the a.c. filters Ll-Cl, L2-C2 and L3 C3 and the circuit-breaker bridges QR1-DR1 to QT4-DT4. Each bridge of four circuit-breakers (QRl-DRl to QR4-DR4; QSl-DSl to QS4-DS4 and QTl-DTl to QT4-DT4) supplies one of the filters operating as an autonomous inverter supplied by the storage element C7-C8. The changeover of the circuit-breakers takes place at high frequency (some 10 KHz) with a criterion which will be desribed hereinafter.Each single phase filter Ll-Cl to L3-C3 acts independently and has the object of reducing the voltage harmonics generated by its corresponding inverter bridge at the frequency close to the changeover frequency thereof, allowing the fundamental frequency and the harmonics to pass to an order of about 20.

The suppressors SP1 to SP3 reduce the surges which may originate from the primaries of T1 to T3 when there are high overloads or short-circuits at the output of the conditioner.

The current-correcting active filter is formed by the bridge of circuit-breakers QR'l-DR'l to QT'1-DT'2. The limiting element in each phase is a coil (L4, L5 and L6) and the storage element is the set of capacitors C7 to C8 . The capacitors C4 to C6 with this mode of operation are in parallel with the load and merely operate as a filter of the high harmonics generated by the load and the actual current-correcting filter.

In the event of damage to or a fault in the power system, the static circuit-breaker S1 to S6 closes simultaneously with the delivery of an order to close the contact-maker COR-COT in parallel therewith, making the load uncritical in direct power system supply. At the same time, the static circuit-breaker S'1 to S'6 opens and isolates the current-correcting active filter from the power system and from the uncritical load. This filter starts to operate as an inverter or uninterrupted supply system exclusively supplying the critical load from the energy accumulated in the storage element C7-C8'.

The magnetothermic circuit-breaker IlS-IlT protects the equipment in the event of a general failure.

NORMAL OPERATION AS CORRECTOR OF VOLTAGE, CURRENT AND REACTIVE POWER The contact-maker COR and COT is open. The static contact-maker S1 to S6 is open. The static contact-maker S'1 to S'6 is closed.

The detailed operation will be described for the sake of simplicity with reference to the single-phase diagram in Figure 2. The active voltage filter in series (3) is the pulse width modulation controlled inverter constituted by the circuitbreakers S1 to S4 and the filter L1, C1. It is supplied by the capacitor C17 and C'9 at a voltage u1 of some 700 V of direct current. The voltage generated by the inverter is connected in series with the line by means of the compensating transformer T1.

The ratio of turns n of the transformer is approximately 11.

This value may easily be deduced from the value selected for u (700 V) and the design specifications of the equipment. (The previous value has been selected for a permanent variation in the input voltage of + 18% and a permanent variation in the output voltage of + 3%.) The input voltage to the filter L1 C1 (u)p can assume three values: -u1, 0, u1. The objective of the control strategy is to generate in the minimum amount of time the voltage < l/n which exactly compensates the instantaneous difference between the power system voltage and its ideal value. A simpler inverter may be obtained by eliminating S3 and S4 and connecting the coil L to the mid-point of C', and C'8. The value zero for up therefore disappears, causing a greater harmonics rate in UF which, in turn, tolerates a lower fidelity response and a greater filter L1 C1.

The active current filter in parallel (4) is a coil-limited highfrequency rectifier. The circuit-breakers S5 and S6 are modulated so as to satisfy two main conditions: to generate the harmonic currents demanded by the load and to keep ul equal to 700 V (the voltage required by the voltage-correcting active filter). The coil L2 acts as a filter at the high frequencies generated by the changeover of S5 and S,. As mentioned in section 3, C4 is not strictly necessary when the circuit is functioning as a current filter as it is connected directly to the output of the conditioner.However, its presence during this mode of operation helps to reduce the current harmonics having frequencies close to and higher than the power system through the series active filter of voltage which, in turn, would cause rippling of voltage in the power system owing to its non-zero short-circuit impedance.

As described with reference to the three-phase circuit in Figure 1, the contact-maker COR and the static circuit-breaker S1-S2 short circuit the conditioner in the event of breakdown and the static circuit-breaker S'1-S'2 isolates the power system from the critical load and from the conditioner in the event of a power system failure. Under these circumstances, the voltagecorrecting filter does not act and the current filter acts as an inverter with supply in mid pick-up (mid-point between C'7 and C'8) changed over to high frequency with some technical pulse width modulation which is useful for this purpose. The capacitor C4 now acts as part of the output filter L4 C4 of the inverter.

By selecting a changeover frequency for S5 and S6 of the order of 20 kHz, the capacitor C4 does not have to have a reactive power at the fundamental frequency higher than 10% of the nominal power. This condition complies with the influence which C4 has in the operation of this circuit as a current filter as described hereinbefore.

The static circuit-breaker S'1-S'2 opens during power system failures, isolating the conditioner and the critical load from it. To avoid surges, opening is carried out when the current passes through zero. At the same time, and as mentioned in the detailed description, the control of the current filter changes the changeover strategy and starts to operate as a pulse width modulated inverter. 4 C4 acts as an output filter of said inverter in such a situation.

The design of the active intensity filter for some typical requirements as power system conditioner does not allow all the load to be supplied during a power system failure since said filter operates as an inverter. However, this is theoretically and technically possible with the proposed topology even though it economically penalises the components of the intensity filter.

Without excessively penalising the design of the intensity filter, a critical load of the order of 30% of the nominal load can be expected during operation as an inverter.

The static circuit-breaker S'1-S'2 has to be very rapid to isolate the uncritical load in the least possible time in the event of a power system failure. A circuit-breaker with thyristors may be used. Power transistors which are easier to control may also be used nowadays.

The three-phase variation described in Figure 1 allows independent control of each phase of the voltage-correcting filter and of each phase of the current-correcting filter on condition that the design limits established for the components are not exceeded and the voltage of the d.c. bars is kept constant.

For its part, the claimed equipment can carry out independent control of the respective output and input phase voltage modules and angles. This possibility may be employed to compensate for voltage imbalances in the power system, both modular and angular, and to bring them about in a controlled manner to compensate for any other anomaly and to facilitate management of the power system.

Similarly, the current-correcting filter, in addition to compensating harmonic currents and reactive current, can compensate imbalances in the active currents of the fundamental frequency or can produce them in a controlled manner to compensate for any other anomaly and facilitate management of the power system.

Therefore, the topology of the claimed circuit allows the operation of a genuine universal conditioner.

OPERATION AS AN UNINTERRUPTED SUPPLY SYSTEM The active current filter acts as an independent inverter in the event of a power system failure. It extracts the energy from the capacitors of the d.c. connection and supplies the loads considered as critical with a nominal voltage as 220 V between phase and neutral.

For correct operation, it is necessary that, on detection of a power system failure, said active current filter is isolated from it and manages through the circuit-breaker S'1-S'2, Figure 2.

The period of power supply under nominal conditions is guaranteed for 0.5s in the case of critical loads.

In the transition during the passage from operation as an active filter to an inverter, the moment of the voltage wave form when the voltage wave form disappears is taken into consideration, and the effects of this disturbance on the load are reduced in this way.

Among the proposed claims, the electronic equipment for line conditioning may be designed for different input and output voltages, single or three-phase configuration, for different powers, sizes, shapes, components, protective devices and control criteria, without these variations and without any variations in details of presentation and organisation affecting the essence of the invention since the equipment manufactured within the general described idea with any of these modifications will only be variations which are also covered and protected by this patent.

Claims (6)

1) Line conditioner for the reduction or elimination of disturbances, essentially characterised in that it consists of a voltage-generating active filter in series between the power system and the load and a current-generating active filter in parallel with the load which are connected by a d.c. voltage storage element, the active voltage filter comprising an inverter (preferably d.c.-a.c. convertor) supplied by said storage element which in turn supplies a low pass LC filter of which the output voltage is delivered in series between the power system and the load by means of a reduction transformer.

2. Line conditioner for the reduction or elimination of disturbances according to Claim 1, characterised in that the active intensity filter consists of a coil connected between each output phase of the critical load and the mid-point of a branch of circuit-breakers which are bidirectional in current and unidirectional in voltage and of which the positive and negative ends are connected to the d.c. voltage storage element, the neutral of the power system being connected to the mid-point of the d.c. voltage storage element.

3. Line conditioner for the reduction or elimination of disturbances according to Claim 1, characterised in that, between each output phase of the critical load and the neutral there exist capacitor paths which act as a filter both during operation as a current corrector and during operation as an inverter.

4. Line conditioner for the reduction or elemination of disturbances according to Claim 1, characterised in that the d.c.

voltage storage element may optionally be divided into two interconnected blocks with a ripple-reducing coil.

5. Line conditioner for the reduction or elimination of disturbances according to Claim 1, characterised in that, between the output terminals for critical load and the corresponding output terminals for non-critical load there is arranged a static circuit-breaker for forced blocking which opens in the absence of the power system, the output terminals for critical load remaining isolated from the power system and from the uncritical load and connected directly to the current-correcting filter which works as an inverter or uninterrupted supply system in such a situation.

6. Line conditioner for the reduction or elimination of disturbances according to Claim 1, characterised by the arrangement of an electromechanical contact-maker in parallel with a static one which short circuit the delivery transformer of the voltage corrector in the event of a power system failure, connecting the power system directly to the non-critical load and also in the event of an overload.