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

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

Transient stability of distributed generation in MV-ring networks

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 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. To guarantee after fault clearing stable operation of the DG-units, the protection scheme and settings have to be changed. In this paper conventional protection schemes of MV- ring networks are discussed as well as the additional measures that have to be taken to protect a MV-ring network including DG properly. It is found that it is not possible to find a proper relay setting to protect the MV-ring network in such a way that the fault clearing process is fast enough to maintain stability of the DG-units. In the paper it is demonstrated that the fault clearing process can be sped up by adding communication to the protection scheme

Harmonic distortion and oscillatory voltages and the role of negative impedance

This paper focuses on two power quality effects that are noticed with power electronic appliances. One effect is a harmonic oscillation by inverters of photo voltaic systems because they bring a high parallel capacitance to the grid and negative output impedance. The other effect is a sub-harmonic oscillation due to the negative differential impedance of constant power loads. This paper discusses both effects and proposes counter measures to mitigate these effects. For harmonic oscillations due to photo voltaic inverters, an extra feature for these inverters is discussed. For the sub-harmonic oscillatory voltages due to appliances with negative differential impedances, a solution can be found in the voltage control systems of the power generators connected to the networ

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

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

Photovoltaic energy in power market

Photovoltaic (PV) penetration in the grid connected power system has been growing. Currently, PV electricity is usually directly sold back to the energy supplier at a fixed price and subsidy. However, subsidies should always be a temporary policy, and will eventually be terminated. A question is raised whether grid-connected PV generation will be more beneficial by making biddings in power markets than by supplying at a fixed price. An economic model of profit maximization for PV generation when joining power markets is proposed to answer the question. A simplified model is applied to simulate a case study of PV biddings in the Amsterdam Power Exchange (APX) spot market, using PV generation data from a standardized neighborhood PV installation. A Monte Carlo method is used to calculate penalty costs due to over-predicted irradiation. Also a Monte Carlo simulation is applied to survey a number of random imbalance capacities and corresponding prices within a Gaussian distribution by repeating the calculation loop. The sensitivity for prediction errors is examined by simulations with different unpredictability levels of solar irradiation. The outcome of the simulations is a value for the difference between the two revenues of PV generation when joining power markets and when supplying at a fixed price

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