Some Considerations in the Thyristors Reactive Effects on the Power Supply

Transcription

1 Journal of Advanced Tiansportation, Vol. 21, Spring Some Considerations in the Thyristors Reactive Effects on the Power Supply Andrew Jakubowicz Silicon Controlled Rectifier (SCR) drives have become a standard technique for transit systems. Their utilization meets the most demanding specifications and results in an effective decrease in power consumption which translates in decreased costs for energy. The regenerative DC power may be inverted to 3-phase AC power and returned to the line as usable power. However, regenerative capability is at a cost penalty. Introduction Rope-propelled people movers are an alternate to self-propelled systems for short to medium distance urban mass transit applications for point-topoint operations. A fully-automatic system utilizes, as its principle, wellproven ropeway technology together with state-of-the-art electronic computer controls. The modern trend in rope propelled people mover automation is toward utilization of SCR drives. A silicon controlled rectifier or SCR is a solidstate switch. It is sometimes referred to as a thyristor. The SCR has an anode, cathode and control element called the gate. The device provides controlled rectification since it can be turned on at will. The SCR can rapidly switch large currents at high voltages. They are small in size and low in weight. The advanced features incorporated in the thyristors power conversion include variable speed control capabilities over the entire speed range in a stepless, jerk-free manner. Figure No. 1 shows a typical travel diagram taken by use of a brush recorder at one of the Circus Circus :Enterprises, Inc. Metro-Shuttles locations in Las Vegas and Reno, Nevada. Figure No. 2 shows an example of a typical speed and power diagram made according to the design calculations. Mr. Jakubowicz is project engineer, VSL Transit Systems.

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3 Thyristors Reactive Effects 87 SVEED IN WS 5, v-5 MIS I 7.8 % M, 503M ~ POWER 200 IN K W i A 140 Figure 2. Example of Typical Speed and Power Diagrams The conventional SCR drive usually provides a display with indication of test functions at strategic points in the circuit. Memory-type fault detectors aid in the diagnostics of motor overload, armature overvoltage, instantaneous overcurrent, tachometer overspeed, phase loss and other critical malfunctions. The new digital drives have also been designed to interface easily with computers and controllers. All of the strategic parameters within the drive and control circuit can be continuously monitored and recorded. This recording can be suspended at the instant of failure. When demanded, the monitored failures can be displayed for the last 20 firing cycles before failure. This log can be displayed on the screen or printed. The SCR is able to block current until a small signal (gate pulse) is applied to the gate cathode and until the SCR becomes forward biased (anode positive with respect to the cathode). When these conditions occur,

4 88 Andrew Jakubowicz 3 \ \ \ ASS E M BLY LOGIC FILTER ~ N D EOMPENSATI ON (OPTIONAL) Figure 3. Typical SCR Drive System \ A r) TACHOMETER FEEDBACK MECHANICAL IOPTIO N AL ) POWER SECONDARY SYSTEM the SCR will conduct (rectify the AC voltage) and continue conducting until it is reversed biased (anode becomes negative with respect to the cathode) (Sabina, 1982). Figure No. 3 is a circuit diagram of a typical SCR application in ropepropelled people mover drive systems. The isolation transformers provide secondary power to the SCR power converter, secondary system and optional filter and compensation unit. AC power is converted to variable DC power which is supplied to the DC drive motor. The output mechanical power operates the drive machinery. The optional tachometer generator is recommended for speed feedback control. The drive generally does not require an 1 R compensation circuit then because the tachometer control will inherently compensate for the loss in speed. The SCR provides the advantage of reverse power flow, an economical feature of an electrical means of braking. This reversible power flow is used effectively in M ETRO-S H UTTLE drive systems. However, the associated reactive effects are not usually recognized which may cause misunderstanding of SCR operational principles. Although there are no code or standard requirements for filter and compensation units, SCR related electrical power disturbances, noise transients and system power factor effects do need to be addressed, especially in downtown or airport applications.

5 Thyristors Reactive Effects 89 Regenerative Power as an Inherent Ability of the SCR Drives Regeneration occurs when the motor is running in one direction and the polarity of the voltage is reversed at the armature. In regenerative braking, the high inertia of driven machinery is dissipated during braking by causing the drive members to become generators and to pump power back into the AC supply. When decelerating, controls turn the DC-motor into the generator that inverts the DC-voltage back into the power feed. This is possible because the motor-generated voltage is greater than the applied reverse line voltage. This smooth and rapid reversing of the motor for regeneration is accomplished through the full wave, four-quadrant operation, three-phase silicon controlled rectifier. The thyristors can be gated such that power is returned to the AC line (inverted) as usable power rather than supplied by the AC line. The AC lines act as a load. The result is an effective decrease in power consumption. So the question is not Can power distribution accept regenerative power?, but Does the power having a distorted wave from regenerative SCR circuit cause unacceptable line pollution and what are the reactive effects on the electric power supply lines? It is well known that the operation of a thyristor power rectifier, due to its physical properties, will not be without certain effects on the electrical supply mains (Zimmer, 1975). The reason for concern lies in the overloading of the mains and harmonic backeffects as regards reactive power. The rectifier may be likened to a generator superimposing harmonics of a definite frequency and amplitude on the supply mains. The harmonics flow back into the supply mains causing voltage drops due to the impedance of the circuit. These distort the sinoid shape of the mains voltage. Harmonics can also be accompanied by resonances caused by low circuit capacity, long cables or capacitive consumers. There is a possibility of AC line disturbances that can cause malfunctions in other equipment connected to the line when an SCR fires. When the drive is started the regulator will provide a speed error signal (voltage for armature feedback units) to the current control driver circuits. The output of the driver circuit is applied to the input of the forward or reverse SCR logic. The output signal of the firing circuit varies as the input signal to the firing circuit varies. The firing circuit is synchronized to a particular AC phase. The input signal to the firing circuit is therefore phase synchronized and converted to a series of pulses to the gate driver circuit. The actual output signal to the gate/cathode of the SCR is many high-energy

6 90 Andrew Jakubowicz pulses during the period of SCR conduction. This method of gate firing helps to reduce interference from other equipment operating on the same power line by providing a burst gate signal over the entire SCR firing cycle (Sabina, 1982). If the AC line supply should fail even momentarily during an SCR regenerative cycle, the DC line fuses would be lost and require replacement to restore operation. Considering the thyristor rectifier as a primary electrical consumer in these applications, particular care must be observed when operating regulating circuits from AC lines connected to SCR circuits. Depending upon the construction of the mains network, the type of switching employed, and the method of operating the rectifier unit, there is a mains feedback to contend with. As described above, this is caused by the active and reactive power and by the harmonic reactive power. Some of the thyristor rectifiers offered for transit systems are designed in such a way as to reduce such feedback effects to a reasonable minimum. They usually comply with recommendations for the maximum admissible value of the harmonic voltage vibrations in the system voltage. If more stringent demands regarding the power factor (reactive power) and the harmonic load at the mains supply point exist, a means of compensation can be included. In cableway operations, the consistent reactive power occurence can be compensated simply by using capacitive fundamental oscillation reactivepower from parallel condensors. As a rule, approximately 10% of the transformer capacity can be connected on the low-voltage side as condensor power without any difficulties due to change in the mains resonance frequency (Zimmer, 1975). If needed, additional compensation may be connected to the high-voltage side via filter circuits (condensors arranged in series with dampeners). These are adjusted to the low ordinal number of rectifier harmonics. According to German Standard VDE0875 for high-frequency interference, the amount of harmonic vibrations of the voltage should not exceed lo%, or every individual vibration should be less than 5% of the electric voltage. The admissible values in Switzerland, Austria and France are within the limits of the German regulations (Ott, 1975). The reactive power in a rectifier unit arises in the form of commutation 'reactive power (approx. 5%) and controlled reactive power (approx. 95%). As long as the reactive power remains constant, it poses no real problem, such as power fluctuation; it merely causes an inferior power factor.

7 Thyristors Reactive Effects 91 Variation of Power Factor The system power factor varies with operating conditions (acceleration/ deceleration). The power factor of a phase-controlled SCR drive varies principally with motor speed. The motor feed is controlled by phasing back the firing angle (see the preceeding firing circuit description) of the SCR s and, therefore, changing the average DC voltage (Duncan, 1977). The power factor may be very low at low motor speeds. If the rectifier drive is a significant portion of the line load, then we can observe measurable disturbances on the network. In certain cases, reasonance problems may occur, particularly in conjunction with power factor correction. Special filters for eliminating these drawbacks must be used (Landur, 1974). The major factor determining the apparent power factor is the motor EMF, which is linearly related to motor speed. This phase variation with speed and conditions of motoring or braking with its attendant power factor change is known as displacement power factor. It is so called to distinguish it from power factor changes caused by actually varying the ratio of reactance to resistance in a circuit. High quality SCR units usually provide a displaced power factor of 0.88 at full load and full speed. Conclusion The well established static converter has the advantage of higher efficiency, an unequalled fast response, and less weight compared to rotating converters, and the proper recognition of their problems can protect users against unexpected negative effects. Particular care should be taken in design and selection of all associated electrical controls considering minimum sensitivity to electrical noise, emissions and line pollution generated by thyristors. The reduction of levels and frequencies that could initiate loss of control is critical (Wyss, 1984). Specifically, this applies to industrial computers and programmable logic controllers used frequently for monitoring purposes and data logging and acquisition. These devices are vulnerable to electrical power disturbances, and today s microcomputers are no exception. The careful grounding of main board chassis and power conditioning devices is very helpful. In practice, compromises are usually made in selecting the optimum drive

8 92 Andrew Jakubowicz (Sackman, 1970). However, special care can be taken in selecting the SCR units to reduce this problem. GLOSSARY OF TERMS (Allen-Bradley, 1983) Regenerative Braking Gate Four Quadrant Operation The technique of slowing or stopping a drive by regeneration. Regenerative Braking is similar to Dynamic Braking, but it is accomplished electronically. The generator power is returned to the line through the power converter. It may also be dissipated as losses in the converter (within its limitations). The control element of an SCR. When a small positive voltage is applied to the gate momentarily, the SCR will conduct current (when the anode is positive with respect to the cathode of the SCR). Current conduction will continue even after the gate signal is removed. The four combinations of forward and reverse rotation and forward and reverse torque of which a regenerative drive is capable. The four combinations are: 1. Forward rotation/ forward torque (motoring) 2. Forward rotation/ reverse torque (regeneration) 3. Reverse rotation/ reverse torque (motoring) 4. Reverse rotation/ forward torque (regeneration)

9 Thyristors ' Reactive Ejject s 93 Power Factor Filter I R Compensation Isolation Transformer A measurement of the time phase difference between the voltage and current in an AC circuit. It is represented by the cosine of the angle of this phase difference. Power factor is the ratio of Real Power (kw) to a total kva or the ratio of actual power (W) to apparent power (volt-amperes). A device that passes a signal or a range of signals and eliminates all others. A way to compensate for the voltage drop across resistance of the AC or DC motor circuit and the resultant reduction in speed. This compensation also provides a way to improve the speed regulation characteristics of the motor, especially at low speeds. A transformer that electrically separates the drive from the AC power line. An isolation transformer provides the following advantages: 1. In DC motor application, it guards against inadequate grounding of plant power lines through grounds in the DC motor armature circuit. 2. Enhances protection of semiconductors from line voltage transient. 3. Reduces disturbances from other solid state control equipment such as drives without isolation transformers, time clock systems, electronic counters, etc. References I. Duncan, Edward T., Commutation of Power Factor Considerations in the Applications of SCR Drive Systems to Elevator Hoisting Machinery, Elevator World 2 (1977). 2. Landur, Franz, Mine Hoists with Thyristor Converters, Lifi, 1 /2, (1974).

10 94 Andrew Jakubowicz 3. Ott, Walter, Reactive Effects on the Electric System in Case of Ropeway Drives with Converters. International Congress of Transportation, Vienna, (1975). 4. Sabina Electric and Engineering Co., Installation and Operation Manual, Type RG-86/ 8800 Three Phase, Regenerative DC-Drives (1982). 5. Sackman, William, Adjustable Speed Drives, Machine Design, (1970). 6. Wyss, Paul, Tentative Standards for Funiculars-prepared for adoption into ANSI B (1984). 7. Zimmer, Wilfred, Concerning the Application of Rectifiers in Modern Cableways, international Congress of Transportation, Vienna, ( 1975).