New generation types in Europe versus Future Power Quality Needs

This paper aims to give an overview on main PQ development in Europe. It focuses basically on recent legislative, regulatory, standardization, voltage quality monitoring and education initiatives summarizing the latest outcomes

Power quality and energy efficiency in smart cities

DURING the last two decades, PQ related problems have increased all over the world. The uses of electronic appliances, computers, data processing equipments, variable speed drives, electronic ballasts, etc. have increased enormously. These devices are quite vulnerable to supply voltage disturbances. In contrast, they produce current emissions in the network because of their non-linear operating characteristics and in this way influence the quality of the network voltage

Power flow management in active networks

This paper proposes a new method to manage the active power in the distribution systems, a function under the framework of the active network (AN) concept. An application of the graph theory is introduced to cope with the optimal power generation (DGs/Cells dispatch) and interarea power flows. The algorithm is implemented in a distributed way supported by the multi-agent system (MAS) technology. Simulations show how the method works in cases of optimal operation, congestion management, and power generation cost change

The effect of inrush transients on pv inverter's grid impedance measurement based on inter-harmonic injection

This paper addresses a cause for false tripping of photovoltaic inverters with antiislanding protection based on impedance measurement with inter-harmonic injection. Earlier discussions about tripping problems happening when several devices are doing the measurement at the same time are supplemented with a problem caused by inrush transients of nearby devices. A series of experiments was conducted in the Power Quality laboratory of the TU/e, on a PV inverter which complies with the DIN VDE 0126 standard. Impedance measurement was done in parallel with the inverter and measurement results are presented. A criterion for false tripping caused by transients is explored. Also, influences of network impedance and grid harmonic pollution on false tripping were analyzed. In the end, some signal processing techniques are proposed to avoid this problem

Effect of grid disturbances on fault-ride-through behaviour of MV-connected DG-units, in especially CHP-plants

In the near future a significant amount of the consumed electrical energy will be generated by distributed generation (DG). Because of the small size these units are normally connected to the local distribution grid [1]. Connection of DG changes the operation of the distribution grid. In order to minimize the effect of DG during grid disturbances some standards oblige immediate disconnection of DG-units. With an increasing number of DG-units this strategy can lead to a disconnection of a large amount of DG-units and jeopardize the security of supply. To prevent large unbalances between generation and load because of disconnection of DG-units some grid operators have defined fault-ridethrough requirements. These fault-ride-through requirements are especially applied for large wind parks which are connected to the transmission grid. In this paper the effect of disturbances in the transmission grid on the fault-ride-through behaviour of medium voltage connected DG-units is discussed. The examined grid is an existing transmission and distribution grid. To the distribution grid a large amount of combined heat and power (CHP) plants are connected. The CHP-plants are equipped with an under-voltage protection which switches-off the CHP-plant at a voltage level (dip) of 0,8 p.u.~100 ms. In the paper it is determined what amount of CHP-plants is disconnected due to transmission grid disturbances. It can be concluded that multi-phase faults as well as single-phase-to-ground faults result in a disconnection of a significant amount of CHP-plants. Voltage dips in the transmission grid below 0,6 p.u. lead to a disconnection of all CHPplants, between 0,6 and 0,7 p.u. lead to a partly disconnection of CHP-plants and between 0,7 and 0,8 p.u. to no disconnection at all. It is demonstrated that with a setting of 0,8 p.u.~200 ms most CHPplants stay connected and will not loose stability. Dynamic simulations have shown that the CHP-plants start to consume reactive power directly after fault clearing. This is due to armature reaction of the synchronous generator. The amount of consumed reactive power is related to the fault duration. The voltage recovery in medium voltage grids is delayed due to this reactive power consumption. This can lead to a disconnection of CHP-plants even when the fault is cleared within the clearing time of the under-voltage protection. The delay in voltage recovery has to be taken into account when fault-ride-through requirements for transmission grid faults are defined. © CIGRE2009

Assessment of the impacts of voltage dips for a MV customer

Voltage dips problems often cause large financial losses to sensitive industrial and commercial customers. Worldwide many industrial customers, connected to the high and medium voltage networks, often complain about voltage dips to their network operators. A voltage dip problem becomes critical in case of the incompatibility between the power supply and the immunity of the connected devices at customer's installation. Improving the network's supply performance to reduce voltage dips would lead to huge amount of investment; whereas a process outage at a customer's installation often involves significant amount of financial losses. Therefore, an optimized solution is to be found out based on the network's yearly dip statistics and the sensitivity of the customer's installations to restrict this problem. In this paper, an industrial customer connected to a typical MV network is considered. The annual voltage dip frequency of the Dutch network is estimated and its impact on the considered customer is analyzed. Further, a methodology is proposed to define responsibilities of the different parties involved to minimize voltage dip problems

The interconnection in active distribution networks

The active distribution network (AN) has been mentioned recently to adapt with a large-scale implementation of distributed generators. One of its enhancements is increasing interconnections to provide more than one power flow path among local control areas. These parallel physical connections might cause several problems for the network such as congestion and loop flow. Considering the characteristics of the AN, this paper proposes a decentralized approach to control power flow which has some analogies to the telephone networks. The implementation of this control mechanism is based on a multi-agent system (MAS) technology. A simulation of the power system and MAS is created to illustrate the possibility of the proposed method

Influence of protection on transient stability of medium voltage grids including distributed generation

Due to the increase of distributed generation (DG) in the future it can become important to keep DG connected to the grid in order to maintain the balance between consumed and generated electrical power. Keeping DG-units connected to the grid during a disturbance, the dynamic behavior of the DG-units becomes important. Protection schemes based on definite and inverse over-current are simulated and transient stability of DG-units is investigated. The simulations have shown that there is no major benefit to apply inverse over-current relays in MV-grids with short feeder length and high short circuit power. With conventional grading of the protection in most cases the DG-units become unstable. The paper shows that reduction of the time grading is possible however, this reduction is not sufficient to guarantee stable operation. The best option is fast fault clearing of the complete feeder. It is shown that this is possible without loss of selectivity and with stable operation of the remaining DG-units

Power electronic loads with negative differential impedance in a low voltage distribution system

Today's domestic appliances are more and more adapted and controlled by power electronics and processors, and their number is growing. This development can bring both advantages and disadvantages for the power quality in local grids. An advantage for example is the growing number of power supplies that are equipped with a power factor corrector front-end. This type of power supply behaves as a resistive load and has a very good power factor. A disadvantage for example is the constant power load character that comes with a controlled AC to DC power supply with a constant loaded DC output, independent of the applied grid voltage. This type of load has a Negative Differential Impedance (NDI) and can cause voltage instability. This paper discusses the NDI effect that power supplies can bring, and their effect on voltage stability. (4 pages