Power quality, including voltage and frequency stability, is a critical factor for the safe and reliable operation of electrical and electronic equipment. Grid frequency is typically maintained to within 1%, while voltage may vary by as much as 10%. In the UK, energy suppliers are legally required to supply electrical power at 230V +10% -6%, which gives a minimum of 216.2V and a maximum of 253V. Variations in voltage can cause equipment to malfunction or even damage to the machinery itself. Some common power-quality issues include voltage variation, power-outages, frequency instability and noise. Issues may originate at the electrical generation plant, or be introduced more locally, such as with noise from a motor propagating through the electrical system. In order to fully understand the different types of power quality issues, it is useful to consider how they are caused and interconnected with each other.
CAUSES OF VOLTAGE INSTABILITY
Electricity is conventionally generated by turbines, which rotate an electromagnetic coil (the rotor) within an array of copper bars (the stator). As the magnetic field of the rotor passes a stator bar, an electrical current is induced, which changes direction as the positive and negative poles of the rotor sweep past the stator, resulting in an alternating current (AC). A generator rotating at 3,000rpm produces an AC frequency of 50Hz. Standard generators have three sets of stator bars and therefore produce 3-phase AC, with a 120° phase shift between each phase, the standard throughout the electrical transmission and distribution system. The voltage induced by a conductor moving through a magnetic field is a function of the strength of the magnetic field and the speed of movement. As the speed of rotation increases, so the voltage induced also increases, which, in turn, causes more current to flow. Therefore, achieving frequency and voltage stability across the national grid conventionally means synchronisation of every generator, so they are all rotating at the same speed and with rotors passing stators at the same time.
As electrical demand rises and falls across the grid, the changing electromagnetic load alters the power required to keep the generator rotating at the same speed. The mechanical power input must therefore be controlled to maintain the speed of rotation. This problem is aided considerably by the inertia of the rotating generators, shafts and turbines, which strongly resists rapid changes in rotational velocity, allowing time for the input power to be controlled. This damping effect means that conventional thermal power plants, including biomass and fossil fueled ones, provide a valuable utility to the grid.
Although inertia can smooth out short-term changes in the load, the power generators ultimately need to respond. Conventional thermal power plants can respond well to demand, but outages can cause mismatches. Variable renewable sources, on the other hand, are dependent on wind and sun, making it much more difficult for them to follow the load. Increased use of wind and solar power is greatly increasing the challenge of maintaining frequency stability. Although it is possible to switch generators on and off to give some degree of frequency response, the large number of small operators makes it much harder for this to be coordinated at grid level. There is also little inertia in wind and solar, meaning that expensive ancillary frequency stability services are increasingly required such as battery, flywheel or even superconducting loop storage.
Reactive power is an important concept in frequency and voltage stabilisation. If the load is entirely resistive, although the direction of current reverses, the direction of energy transfer doesn’t reverse and always flows from the generator to the resistive load (an active load). This is not the case for reactive loads, which store and release energy, either through inductance or capacitance. For reactive loads, energy can flow back and forth without being dissipated by the load, causing increased resistance losses in transmission lines. In practice, the load on the grid causes a mixture of active and reactive power to flow through the grid. To prevent too much reactive power causing heating, power loss and overload in generators and transmission lines – and to assist with voltage regulation – reactive power is managed within the network.
VOLTAGE VARIATIONS
A ‘surge’ is an imprecise term that can encompass fluctuations in voltage over various time periods. IEEE 1100-2005 defines voltage instability in terms of transients, swells and dips. Transients last from microseconds to a few milli seconds. Impulsive transient spikes rise very rapidly, as occurs due to a lightening strike or a large load being turned off. Oscillatory transients are periodic rising and falling of voltage. Swells and dips last longer than transients, typically for a few cycles and with voltage differing from nominal by 5% to 10%. Swells refer to overvoltage conditions and dips, undervoltage.
Brownouts are undervoltage conditions that last for longer periods of minutes or hours. Brownouts are sometimes deliberately initiated to conserve power and prevent a complete blackout or loss of all power. This works because many loads draw less power at lower voltages.
Resistance devices such as heaters and incandescent lights draw power that is directly proportional to the voltage. An incandescent light will glow brighter as voltage increases and dim as the voltage drops. Similarly the output of a heater will increase and decrease with the voltage. These devices are not in harmed by undervoltage conditions but may burn out if the voltage is too high for a prolonged period of time.
POWER QUALITY PROTECTION
Foundations for a quality power system are isolation, grounding and power converters. Proper grounding provides a reference voltage, from which all other voltages are measured, as well as a return path for electrical current. Greater energy efficient and compactness can be achieved by non-isolated power supplies, but they cannot provide the same level of protection against potentially harmful voltages reaching the output. Isolation provides an effective way to to protect operators from dangerous voltages, while also shielding equipment from transients and swells. Isolation may involve electrical insulation and inductive coupling through a transformer. Surge suppression removes transients and swells, protecting electrical equipment from the effects of these overvoltage conditions. Filters smooth the voltage, removing noise and harmonics.
Surge protection can be achieved by damping out sudden changes in current using inductors or, more commonly, short circuiting current back into the power distribution lines when an overvoltage is detected. A very rapid shorting response can be achieved using a spark gap, tuned to spark at a predetermined voltage. Alternatively, discharge tubes or semiconductor devices can be used. Capacitors may also be used to damp out sudden changes in voltage, and surge protectors may combine a number of different measures.
Uninterruptible Power Supplies (UPSs) can provide the highest level of protection, with an instant response to power outages and voltage fluctuations. The power is not actually supplied directly from the grid to the load, but rather the grid is just used to continuously charge a bank of batteries, which are then used to supply the load.