STEP START SYSTEM
The idea behind a step-start system is simple: to reduce large current surges when energizing high power inductive iron core or large capacitive components from low impedance sources. Developing a system to accomplish this, however, can lead to problems. Sensing, timing, step impedance, reliability, and life of the system must all come together in the smallest and lightest package possible. The creative use of vacuum interrupters and energy storage closing and tripping work together to solve some of the problems of space and weight. Special sensing, timing and safeties help make the unit effective. But why use a step-start arrangement instead of a single contactor or circuit breaker? The answer is in the inherent characteristics of the devices to be energized.
Energizing a Device
Normally, a device such as a transformer or reactor with an iron core, or a capacitor will have retentivity or require charging energy. When the reactive device is energized, the only limit to initial charging current may be its series resistive component. This creates very low initial impedance for ½ to many cycles on AC or many milliseconds on DC. When being energized from a low impedance source, these momentary low impedance loads can create high current surges and oscillations, which can result in over-voltages, unnecessary trippings, or induced currents and over-voltages in surrounding objects or equipment. Unless these current surges and oscillations are reduced or eliminated, contacts in the controller, the windings or insulation of the load device, and surrounding electrical equipment could sustain damage.
Closing inrush currents can approach the same level as fault currents. In iron core magnetic components where resistive factors are generally less than 10% of the total impedance, normal charging inrush currents easily reach 5 to 7, even 10 times their steady state full load currents or 100 times steady state excitation currents. These will cause momentary fluctuations in the voltage levels and phase relations that can cause severe problems in the entire vicinity connected to the same power source. High frequency charging currents, particularly in bank to bank capacitor switching with low impedance and closely associated banks, can reach many thousands of amps. This especially occurs while switching on the second bank when the first bank is already energized. Since these currents generally are oscillatory at frequencies in the order of 10's of kilohertz, extreme currents are likely to be induced in surrounding equipment and even transmitted into the airwaves causing major problems in computers and other sensitive electronic apparatus. Switching pre-strikes and re-strikes causing repeated high inrush currents can also cause the same condition.
The Results of Energizing a Device
Accumulated load equipment deterioration occurs from repeated initial charging, inrush, or high frequency charging currents. These currents can also shorten the life of the switching device to less than 10% its expected operating life. Fault interrupting life will also be decreased to some degree due to this condition. Erosion from closing the contacts is usually 2 to 10 times greater than erosion from interrupting the same current. To reduce load equipment deterioration and increase the life of the switching device, initial charging, inrush, and high frequency charging currents must be minimized by some means.
Reducing Deterioration and Increasing Life
For iron core components, inrush currents can be minimized by closing on the opposite polarity from which the unit was interrupted. The proper retentive polarity will then be present, generating immediate back electromotive force. This eliminates the high charging current, which would continue until the field is up to the level of the previous interruption. Synchronizing is not easily done however.
Uncharged capacitors are also essentially zero impedance when waiting to be charged. If capacitors could be energized at the instant the source voltage was zero or the same polarity and at or lower than the voltage of any partial charge level remaining after previous interruption, capacitive inrush currents could be reduced. Capacitors may retain a charge for some time after interruption and if re-energized with a source of opposite polarity, can create tremendous inrush currents. Again, synchronization is complicated if not impossible.
The Step-Start Solution
Accomplishing reliable, repeatable timing requirements to minimize these inrush problems without a Step-Start is an almost insurmountable task where long life timing reliability would be required. The extreme complexity of the many electronic components and the exacting mechanical coordination make precise timing extremely difficult. The easiest and most reliable solution is to use a step-start system where precise timing is not a serious problem and inrush current levels can be set as required by inserted step impedance.
The basic unit consists of a main contactor or circuit breaker and a step contactor with a series current limiting device of a sufficient short time kilojoule capacity. The current limiting device generally is a low inductance resistance to minimize oscillatory tendencies. However, small air mize oscillatory tendencies. However, small air core reactors (with their reduced heat producing problems) are excellent for limiting high frequency inrush of bank to bank capacitors if the ringing frequency is not a problem. Due to their high price and size, air core reactors are generally not economical for the step limiting impedance requirements of power frequencies. They may also cause over voltages on interruption or pre-strikes. The two switching devices and the current limiting device with the addition of step timing, safeties (along with normal control circuitry), and an enclosure if required comprise the step-start system.
Operation of the System
To begin energizing, the step contactor is first closed, placing the inrush current limiting device in the circuit until the load is partially charged. Usually 0.1 second is sufficient unless it is in the primary of a power supply with a large DC capacitive load. If this is the case, it will need a longer time to reach a satisfactory level of charge. The main contactor or circuit breaker is then closed, bypassing the step unit. In some cases more than one step may be desirable.
In some systems the step impedance is in series with the main switching unit and the step contactor is used to bypass the limiting impedance. This is less desirable since it requires both switching units to have full momentary current ratings, a more expensive arrangement. In special applications, a two step interruption as well as closing may be desirable. Again the timing safeties must be used to protect the momentary rated current limits of the step unit.
Protecting the System
Safeties have been designed into the step-start system to prevent damage to the unit. Included are safeties to prevent the main contactor from closing if the step contactor has not been closed for the required time, and to prevent it from closing if the step contactor drops out at any time prior to the main contactor closing. Safeties will also automatically drop out the step system after a selected maximum time (usually 0.15 seconds) after the main breaker is told to close. This prevents overheating or destruction of the current limiting unit that usually does not have a continuous capability, if the mainbreaker has not closed.
Consideration must be made for the total momentary capability of the step system. Minimum cooling times between each operation must be established. When safeties and proper allowances have been made, the possibility of damage to the unit is minimal.
It is recommended that most step-start systems be protected by fused load-break air disconnects rather than the slower backup breakers. Fuses have some current limiting ability and their interrupting setting cannot be easily tampered with. They do not require control power for operation.
In many cases a more economical system can be supplied where the step-start system breakers are locked out above a stated fault current level allowing smaller breakers to be used. These breakers do not interrupt if the fault current is above that level but allow the backup systems to activate.
Fault sensing for the step-start system, in some cased, is done only on the load side, letting the backup handle any line side faults. With the possible exception of ground faults, most cases will use the high current lockout if line fault availability is above a stated level.
When the step-start system is installed and functioning, it will effectively reduce large current surges when energizing high power iron core or large capacitive components from low impedance sources. By reducing these currents, the step-start system effectively decreases deterioration in load equipment and in turn increases the life of the switching device. At the same time it reduces the disturbances to other equipment. Size, weight, and simplicity make the step-start a cost-effective alternative to much more complex and expensive devices for the same purpose.
Ross Engineering Corp. Step-Start Vacuum Contactors and Circuit Breakers, when used with air disconnects, are usually not drawn out unit since all components are relatively lightweight and have easy accessibility. Complete draw out units have been constructed where required, although the additional complications will increase size, complexity and expense considerably.
Specifications are for reference only and are subject to change.
Contact Ross Engineering Corp. for current information.