IEC 61000-2-1 defines various types of voltage fluctuations, type (d) voltage fluctuations have characterized as a series of random or continuous voltage fluctuations.
Loads that can exhibit continuous, rapid variations in the load current magnitude can cause voltage variations that are often referred to as flicker. The term flicker is derived from the impact of the voltage fluctuation on lamps such that they are perceived by the human eye to flicker. To be technically correct, voltage fluctuation is an electromagnetic phenomenon while flicker is an undesirable result of the voltage fluctuation in some loads. However, the two terms are often linked together in standards. The flicker signal is defined by its rms magnitude expressed as a percent of the fundamental. Voltage flicker is measured with respect to the sensitivity of the human eye. Typically, magnitudes as low as 0.5 percent can result in perceptible lamp flicker if the frequencies are in the range of 6 to 8 Hz.
Typically, flicker occurs on systems that are weak relative to the amount of power required by the load, resulting in a low short-circuit ratio. This, in combination with considerable variations in current over a short period of time, results in flicker. As the load increases, the current in the line increases, thus increasing the voltage drop across the line. This phenomenon results in a sudden reduction in bus voltage.
Depending upon the change in magnitude of voltage and frequency of occurrence, this could result in observable amounts of flicker. If a lighting load were connected to the system in relatively close proximity to the fluctuating load, observers could see this as a dimming of the lights.
A common situation, which could result in flicker, would be a large industrial plant located at the end of a weak distribution feeder.
Whether the resulting voltage fluctuations cause observable or objectionable flicker is dependent upon the following parameters:
- Size (VA) of potential flicker-producing source
- System impedance (stiffness of utility)
- Frequency of resulting voltage fluctuations
A common load that can often cause flicker is an electric arc furnace (EAF). EAFs are nonlinear, time-varying loads that often cause large voltage fluctuations and harmonic distortion. Most of the large current fluctuations occur at the beginning of the melting cycle. During this period, pieces of scrap steel can actually bridge the gap between the electrodes, resulting in a highly reactive short circuit on the secondary side of the furnace transformer. This meltdown period can generally result in flicker in the 1.0- to 10.0-Hz range. Once the melting cycle is over and the refining period is reached, stable arcs can usually be held on the electrodes resulting in a steady, three-phase load with high power factor. Large induction machines undergoing start-up or widely varying load torque changes are also known to produce voltage fluctuations on systems. As a motor is started up, most of the power drawn by the motor is reactive. This results in a large voltage drop across distribution lines. The most severe case would be when a motor is started across the line. This type of start-up can result in current drawn by the motor up to multiples of the full load current.
Although starting large induction machines across the line is generally not a recommended practice, it does occur. To reduce flicker, large motors are brought up to speed using various soft-start techniques such as reduced-voltage starters or variable-speed drives.
In certain circumstances, superimposed inter-harmonics in the supply voltage can lead to oscillating luminous flux and cause flicker.
Voltage inter-harmonics are components in the harmonic spectrum that are noninteger multiples of the fundamental frequency. This phenomenon can be observed with incandescent lamps as well as with fluorescent lamps. Sources of inter-harmonics include static frequency converters, cycloconverters, subsynchronous converter cascades, induction furnaces, and arc furnaces.
Many options are available to alleviate flicker problems.
Mitigation alternatives include static capacitors, power electronic-based switching devices, and increasing system capacity. The particular method chosen is based upon many factors such as the type of load causing the flicker, the capacity of the system supplying the load, and cost of mitigation technique.
Flicker is usually the result of a varying load that is large relative to the system short-circuit capacity. One obvious way to remove flicker from the system would be to increase the system capacity sufficiently to decrease the relative impact of the flicker-producing load. Upgrading the system could include any of the following: reconductoring, replacing existing transformers with higher kVA ratings, or increasing the operating voltage.
Motor modifications are also an available option to reduce the amount of flicker produced during motor starting and load variations.
The motor can be rewound (changing the motor class) such that the speed-torque curves are modified. Unfortunately, in some cases this could result in a lower running efficiency. Flywheel energy systems can also reduce the amount of current drawn by motors by delivering the mechanical energy required to compensate for load torque variations.
Recently, series reactors have been found to reduce the amount of flicker experienced on a system caused by EAFs. Series reactors help stabilize the arc, thus reducing the current variations during the beginning of melting periods. By adding the series reactor, the sudden increase in current is reduced due the increase in circuit reactance. Series reactors also have the benefit of reducing the supply-side harmonic levels. The design of the reactor must be coordinated with power requirements.
Series capacitors can also be used to reduce the effect of flicker on an existing system. In general, series capacitors are placed in series with the transmission line supplying the load. The benefit of series capacitors is that the reaction time for the correction to load fluctuations is instantaneous in nature. The downside to series capacitors is that compensation is only available beyond the capacitor. Bus voltages between the supply and the capacitor are uncompensated. Also, series capacitors have operational difficulties that require careful engineering.
Fixed shunt-connected capacitor banks are used for long-term voltage support or power factor correction. A misconception is that shunt capacitors can be used to reduce flicker. The starting voltage sag is reduced, but the percent change in voltage (ΔV /V ) is not reduced, and in some cases can actually be increased.
A rather inexpensive method for reducing the flicker effects of motor starting would be to simply install a step-starter for the motor, which would reduce the amount of starting current during motor start-up.
With the advances in solid-state technology, the size, weight, and cost of adjustable-speed drives have decreased, thus allowing the use of such devices to be more feasible in reducing the flicker effects caused by flicker-producing loads.
Static var compensators (SVCs) are very flexible and have many roles in power systems. SVCs can be used for power factor correction, flicker reduction, and steady-state voltage control, and also have the benefit of being able to filter out undesirable frequencies from the system.
SVCs typically consist of a TCR in parallel with fixed capacitors.The fixed capacitors are usually connected in ungrounded wye with a series inductor to implement a filter. The reactive power that the inductor delivers in the filter is small relative to the rating of the filter (approximately 1 to 2 percent). There are often multiple filter stages tuned to different harmonics. The controls in the TCR allow continuous variations in the amount of reactive power delivered to the system, thus increasing the reactive power during heavy loading periods and reducing the reactive power during light loading.
SVCs can be very effective in controlling voltage fluctuations at rapidly varying loads. Unfortunately, the price for such flexibility is high. Nevertheless, they are often the only cost-effective solution for many loads located in remote areas where the power system is weak.
Much of the cost is in the power electronics on the TCR. Sometimes this can be reduced by using a number of capacitor steps. The TCR then need only be large enough to cover the reactive power gap between the capacitor stages. Thyristor-switched capacitors (TSCs) can also be used to supply reactive power to the power system in a very short amount of time, thus being helpful in reducing the effects of quick load fluctuations. TSCs usually consist of two to five shunt capacitor banks connected in series with diodes and thyristors connected back to back. The capacitor sizes are usually equal to each other or are set at multiples of each other, allowing for smoother transitions and increased flexibility in reactive power control. Switching the capacitors in or out of the system in discrete steps controls the amount of reactive power delivered to the system by the TSC. This action is unlike that of the SVC, where the capacitors are static and the reactors are used to control the reactive power. The control of the TSC is usually based on line voltage magnitude, line current magnitude, or reactive power flow in the line. The control circuits can be used for all three phases or each phase separately. The individual phase control offers improved compensation when unbalanced loads are producing flicker.