Interaction of Distributed Energy Resource (Solar, windmill and generators) and its effect on power quality and reliability of the power system.

Interaction of Distributed Energy Resource ( Solar, windmill and generators) and its effect on power quality and reliability of the power system. ?

Standards:IEEE-1547

Power quality is the study of compliance with Various DER systems, for operation of?the electrical system smoothly and seamlessly. Power Quality disturbances include Power Outages, Voltage Variation, and Distortion of Voltage and Current.

For Various reasons, a shift is taking place, or expected to take place in future, from largescale power generation units towards small-scale generation connected to distribution networks. The term "distributed generation" (DG) or "distributed energy resources" (DER) is being used to refer to this small-scale generation. The existing situation is referred to as "centralized generation". Distributed energy resources have advantages over centralized generation in a number of applications, for example cheaper total energy bills in the case of combined-heat-and-power; saving on fossil fuel in case of renewable energy sources, improvement of local reliability in case of weak or unreliable public grids, deferring of distribution system investment in case of heavily-loaded grids, costs security in case of high volatility in electricity prices. No matter what the application, the integration of sources of electrical energy at distribution level will have impacts on design and operation of the distribution system. A much-discussed impact is the expected deterioration of power quality due to the large-scale deployment of distributed energy resources. On the other hand, the existing power quality may have an adverse effect on the distributed energy resources. This article presents the relation between power quality and distributed energy in a systematic way.

?Power Quality without and with distributed energy resources:

?The term power quality refers to the electrical interaction between the electricity grid and its customers or equipment connected to it. Power quality will be considered here as covering all deviations from the ideal voltage and current waveform (a constant-magnitude non-distorted sinewave of nominal frequency and magnitude, with the current in phase with the voltage). Interruptions are considered as part of power quality, even though they play a minor part in the remainder of this paper. ?Power quality would cover almost any aspect of power systems. But this definition has shown its value in that there are no longer any boundary areas and that it allows an integrated study of all aspects that involve the customers. Power quality concerns the electrical interaction between the network and its customers.

?The situation is different for DER for a number of reasons. The most important difference is probably that the distribution system is not intended for connecting generator sources. It is the task of the network operator to supply its customers with reliable electricity of sufficiently high (or relevant) quality. Generator sources connected to the distribution network may interfere with this task. Another indirect aspect of the relation between DER and power quality is that the tripping of a DER unit may have adverse consequences on the system. Especially when large numbers of DER units trip simultaneously, this can have an adverse impact on the reliability and security of the system. Such units have not been designed to contribute to the control and stability of the system. In many cases, it is even better than the DER units disconnect from the system as fast as possible during a disturbance. This prevents the DER units interfere with the existing control and protection systems. With a large penetration of DER, such a criterion would lead to severe problems for the system. The impact of DER on power quality depends to a large extent on the criteria that are considered in the design of the unit. When the design is optimized for energy production only, massive DER penetration will probably adversely impact quality.

?Impact of voltage quality on DER :

?Variations and events Power quality disturbances (voltage disturbances or current disturbances) are deviations from the ideal sinewave that potentially affect the network or the customer equipment. In most studies on power quality, a distinction is made between “variations” and “events” (although not always explicitly and not always using this terminology). Variations are small deviations from the sinewave that occur during normal operation. Examples of variations are (harmonic) waveform distortion and voltage (magnitude) variations. Events are large deviations that occur e.g. during switching operations and faults. The distinction between events and variations becomes clearest when considering measurements. Events are those disturbances that require triggering: they are detected when a measurement value exceeds a threshold. For example: a voltage dip is detected by the dropping of the RMS voltage below a voltage-dip threshold. Variations are those disturbances that can be measured at any instant or over any time window. The IEC power-quality measurement standard, IEC 61000-4-30, clearly indicates methods of doing this: starting from a 10-cycle window through aggregated 2-hour values and weekly statistics. This whole procedure assumes that it is possible to exactly predefine the measurement interval, which is only the case for variations.

Normal and abnormal events In design studies it is further appropriate to distinguish between “normal events” and “abnormal events”. Normal events are due to changes in voltage and/or current that are a normal consequence of the operation of a power system. An example is the rapid voltage change due to tap-changer operation. Such rapid voltage changes occur several times a day. Equipment connected to the grid that cannot tolerate these, will suffer a mal-operation several times a day. Therefore all equipment should be immune to these events. Another example of a normal event is the current taken from the grid when a television or computer screen is plugged in. The peak value of this current can be up to 50 Ampere or higher, albeit only for a few milliseconds. The fuse or circuit breaker in the supply to domestic customers should be able to tolerate this current. Other equipment connected in the neighborhood should tolerate the voltage disturbance that results from this current. Abnormal events are events due to abnormal circumstances, either in the grid or with the customer. A long interruption is the most extreme example, but also a lightning overvoltage or a short-circuit fault falls into this category. It is somewhat difficult to decide what is normal and what is abnormal. We decided to use the following criterion: switching actions that proceed as planned are normal events; faults and unintended consequences of switching actions are considered abnormal events. Thus capacitor energizing is a normal event. But re-strike during capacitor de-energizing is an abnormal event. The immunity of equipment against abnormal events is a matter of trade-off between costs and immunity. The impact of the event is a very important criterion. An example from power system design is the way in which short-circuit faults are removed. At the transmission level the loss of supply to customers is considered unacceptable, therefore the design is such that a fault does not lead to a supply interruption. At distribution level however, such a design would become too expensive. The consequences of an interruption are also less here because less end-customers are affected. Therefore the design is such that a fault will lead to an interruption for a small number of customers. For most abnormal events an immunity limit will be chosen. For any event with a severity less than the immunity limit, the equipment should not trip. We will go into more detail in this when discussing voltage dips below.

Voltage quality and DER The distinction between variations, normal and abnormal events can be used as basis for the design of DER units connected to the grid. Normal operation; variations Power-quality variations are the small disturbances in voltage and current as they occur during normal operation of the power system. The design of DER units should be such that existing levels of voltage variations do not lead to premature failure or disconnection of the unit. A higher level of disturbance will typically lead to faster component ageing. A very high level may lead to immediate disconnection of the DER unit from the grid or even to equipment damage. For large installations, the local level of variations may be used in the design. Such an approach runs the risk however that the disturbance level increases beyond the design criterion. For smaller, off-the-shelf equipment a global criterion should be used. The design should be such that it can tolerate the disturbance level at any location. The only available guide on this is the European voltage characteristics standard EN 50160. An additional margin has to be considered to cope with the well-known fact that 95% levels are given for most variations instead of 100% levels. ?A long-term flicker severity equal to 1.0 is given as a limit. But this level is exceeded at many locations in the system, as the authors know from experience. The good news is that the fast voltage fluctuations that lead to light flicker rarely have an adverse impact on equipment. In short, the design of equipment immunity against variations is the same as for any other end-user equipment.

Normal Events: Normal events are mainly due to switching actions in the power systems or within customer installations. Examples of normal events that may lead to problems with equipment are voltage dips due to motor starting or due to transformer energizing and capacitor energizing transients. It is important that DER units are immune to all normal events to prevent frequent tripping of the installation. Whereas EN 50160 gives reasonable guidelines for variations. The best design rule available is that the equipment should tolerate the worst normal event. Similar design rules are needed for end-user equipment like adjustable-speed drives. The regular tripping of such drives is due to capacitor energizing transients. shows that it is not straightforward to implement these rules. ?The second harmonic could be a problem for some equipment (leading to dc current) and it may interfere with some protection and fast control algorithms. ?The interface between the DER unit and the grid should be designed such that it can tolerate voltage dips due to transformer energizing. The main concern will be the tolerance of power-electronic interfaces against the high levels of even harmonic components.

The design issues are the same with DER units as with end-user equipment, but the consequences of tripping are different. A difference from the viewpoint of the unit operator is that tripping of the unit could pose a safety issue. Tripping implies that the energy flow from the unit to the grid is interrupted immediately. The energy input on the chemical or mechanical side of the unit may take some time to stop. The result is a surplus of energy in the unit that expresses itself in the form of high overvoltages or Overspeed in the case of rotational machines. Safety measures are obviously needed especially for mass-produced equipment. But as any safety measure has a finite failure risk, the number of trips should be limited in any case. As normal events may have a high frequency of occurrence, it is of utmost importance that the units are immune against these events.

Abnormal Events:

?The immunity requirements of DER equipment against abnormal events like frequency swings and voltage dips will depend on the severity of those events. The consequences of abnormal events may be very severe and it is not feasible to design DER units that can cope with any such event. The design should be such that the unit is not damaged, but it may have to be disconnected to prevent damage. In that case the unit will no longer supply electrical energy to the network. This will give a loss of revenue to the operator of the DER. The loss of a large (conventional) generation unit will lead to a frequency swing in the whole system. The second group of abnormal events of importance to DER units are short-circuits and earth faults. Their main immediate impact on DER units is the voltage dip at their terminals.

?Impact of DER on Power Quality:

DER units may have an adverse influence on several power-quality disturbances. The most discussed issue is the impact on voltage variations. Also increased levels of harmonics and flicker are mentioned as the potential adverse impact of DER units. But DER units can also be used to mitigate power-quality variations. This especially holds for power-electronic interfaces that can be used to compensate voltage variations, flicker, unbalance and lowfrequency harmonics. The use of power-electronic interfaces will however lead to highfrequency harmonics being injected into the system. These could pose a new power-quality problem in the future.

Hosting Capacity: To quantify the impact of increasing penetration of DER on the power system, the hosting capacity approach should be adopted. The basis of this approach is a clear understanding of the technical requirements that the customer places on the system (i.e. quality and reliability) and the requirements that the system operator may place on individual customers to guarantee a reliable and secure operation of the system. The hosting capacity is the maximum DER penetration for which the power system operates satisfactorily. The hosting capacity is determined by comparing a performance index with its limit. The performance index is calculated as a function of the DER penetration level. The hosting capacity is the DER penetration level for which the performance index becomes less than the limit

?Low-frequency harmonics: The power-electronic interfaces of DER units contribute to waveform distortion. The current waveform contains frequency components at integer multiples of the power-system frequency and at integer multiples of the switching frequency. We will refer to the former as "low- frequency harmonics" and to the latter as "high-frequency harmonics”

The level after connection of the DER unit should not exceed the local planning level. The planning level is less than or equal to the voltage characteristic as in EN 50160. The actual choice is made by the network operator and/or the authorities. The concept of hosting capacity can be applied here to obtain the amount of DER units that can be connected to the grid without exceeding the limits. The hosting capacity will depend on the existing disturbance level (sometimes referred to as the "background level").

The concept of hosting capacity is explained for harmonic distortion in Figure 8. The hosting capacity gives the amount of DER that can be integrated into the system without exceeding any performance limit. In this case, the limit is the planning level for harmonic distortion. This assumes however that every DER unit has the same distortion (relative to its rated power), which is not the case in practice. When assuming high-distortion interfaces the hosting capacity would be reduced significantly. The current practice is often that a new customer is allowed to connect as long as the distortion level after the connection is below the limit. As low distortion interfaces are more expensive it is likely that the units that are initially connected have a high-distortion interface whereas low-distortion interfaces will only be used when the existing distortion level is high already. The result is a significant reduction in hosting capacity.

A potential problem with increasing levels of DER is the occurrence of resonances due to the increased amount of capacitance connected to the distribution grid. The interface with distributed generation often contains a capacitor. This capacitor may be involved in series or parallel resonances that cause amplification of harmonic distortion produced elsewhere. Manufacturers, as well as consultants, recommend that capacitor banks be installed with small ac reactors and be tuned to resonance frequency lower than the lowest harmonics in the actual grid. This problem with the occurrence of resonances occurs for induction machines, but not for VSC-based interfaces and for synchronous machines when they provide reactive power themselves so that capacitor banks are unnecessary. The increased capacitance is however NOT the source of the harmonic distortion. The harmonic currents may be generated by the DER units themselves, by other local equipment, or by equipment elsewhere. As the current generated by DER units does not contain significant low-frequency harmonics, the source is most likely found in other local equipment or in equipment elsewhere. Despite this, the potential increase of harmonic voltage distortion should be treated as an impact of increasing levels of DER units.

Low Frequency Harmonics

High-frequency harmonics:?Voltage-source converters are known as a source of high-frequency harmonics. The switching frequency and multiples of the switching frequency (1 kHz and up) appear in the spectrum of the current. Pulse-width modulation leads to groups of frequency components around the integer multiples of the switching frequency. Hysteresis control, used in smaller converters, leads to a noise-like frequency spectrum around an "average switching frequency" determined by the design of the converter. If the switching frequency is close to a large high-frequency ripple on the voltage. An increasing penetration of DER with power-electronic interfaces, will lead to an increasing level of high-frequency harmonics.

Voltage Variations: The introduction of DER units will generally lead to an increase of the voltage magnitude experienced by the customers. With highly variable sources of energy (like wind and sun) the voltage magnitude will also show a higher level of changes over a range of time scales. ?The strength of the hosting-capacity approach is that it relates the limits to be posed on the amount of DER to the limits posed by other customers on voltage quality and reliability.

Voltage fluctuations: The term "voltage fluctuations" is used to cover a wide range of changes in the voltage magnitude. There is a significant overlap with the term "voltage variations". Here we will however limit the use of the term "voltage fluctuations" to those changes in voltage magnitude that (potentially) lead to light flicker with incandescent lamps, as defined in the IEC flicker standard (IEC 61000-4-15). The severity of voltage fluctuations is quantified through the "short-term flicker severity" (symbol Pst) and the "long-term flicker severity" (symbol Plt). due to the tower.

?The short-term flicker severity due to individual wind installations, from measurements and simulations presented in different publications, is up to Pst=0.2 with SCR=20 (SCR is the ratio between the rated power of the wind power installation and the short-circuit power of the grid). In case there are several installations connected close to each other, the flicker level will be higher than with one turbine. But fortunately they will not simply add. According to IEC 61400-21 [15] the Pst levels should be added by using the following expression: ∑= = N i Pst Pst i 1 , (1) with Pst,i the Pst contribution from each individual installation. With identical turbines, the contribution from N units is N times the contribution from one unit. With a contribution of Pst=0.2 for SCR=20, Pst=0.4 is reached for SCR=5.

Faults and Voltage dips: An increase in the amount of DER units most likely is associated with a reduction in the number of conventional generation stations being in operation. This replacement will lead to a reduction of the fault level in transmission systems. This makes that voltage dips due to faults are spread over a larger area.

?The presence of generation units at distribution level may lead to a reduction in voltage drop when a dip propagates from transmission to low-voltage. For a given location in the network this will result in a reduced dip frequency. For three-phase faults, only synchronous machines give a significant continuous fault contribution. Induction machines only contribute during the first few cycles of the fault. For non-symmetrical faults also induction machines contribute to the fault, thus reducing the dip frequency. The contribution of power-electronic converters depends on the current-limitation settings, the control algorithm, and the protection used. In general the mitigation effect is larger for non-symmetrical faults than for symmetrical faults.

The presence of generation units at distribution level may lead to a reduction in voltage drop when a dip propagates from transmission to low-voltage. For a given location in the network this will result in a reduced dip frequency. For three-phase faults, only synchronous machines give a significant continuous fault contribution. Induction machines only contribute during the first few cycles of the fault. For non-symmetrical faults also induction machines contribute to the fault, thus reducing the dip frequency. The contribution of power-electronic converters depends on the current-limitation settings, the control algorithm, and the protection used. In general the mitigation effect is larger for non-symmetrical faults than for symmetrical faults.

Responsibility sharing: Moderate dips should be tolerated by equipment as these kinds of events occur between 10 and 100 times per year. Severe ones should be rare in a well-designed and operated system. It is important that agreements between the network operator and the customers (DER operators) include information on what kind of dips and an estimated frequency of occurrence that can be expected. Such a responsibility sharing should form the base for immunity requirements on both industrial-process equipment and DER units.,

Load flow studies and short circuit and protection Studies:

A load flow study of the island needs, needs to be completed in order to evaluate the match between generation and load, It should be in addition to any studies done with the DER operating in Parallel with the system. The load flow studies needs to include voltage profile for all significant load conditions. The person performing the load flow analysis should be aware of the unexpected interactions with voltage control devices that may exist.

Short circuit studies should be performed to ensure clearing of faulted conditions, the studies should be performed for the parallel case and the island case. The identification of ground source is extremely important for ground fault protection coordination.?

?Conclusions

?The concept of power quality being the sum of voltage quality and current quality can be applied to DER units. Voltage quality is at first a concern for the operator of the DER unit and should be treated in the same way as voltage quality for industrial and commercial customers. Current quality concerns the way in which DER units impact the system and other customers supplied from the same system. The hosting-capacity method allows a quantitative assessment of the impact of DER units. This method can be used to determine which level of DER penetration is acceptable, locally as well as globally. The method has been developed for current quality and voltage quality studies but is also appropriate to study the third aspect of power quality and DER: the tripping of large amounts of DER units due to voltage disturbances in the system. The main disturbances of concern are frequency swings due to the loss of a large generation unit, and voltage dips due to short circuits and earth faults. A large overall penetration of DER may lead to frequency instability, especially after the loss of a large generation unit. Local and regional concentrations of DER may lead to stability and security problems at transmission level and under and overvoltages at distribution level.

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