Integration of Wind Power in Medium Voltage Networks - Voltage Control and Losses

Since some years ago greenhouse gases and especially carbon dioxide have become one of the most widely debated issues. To reduce the emission of carbon dioxide and nuclear waste, renewable energy sources as for example wind power plants have become very successful. These generation units are normally small compared to common thermal power plants and they are often placed distributed and connected to the distribution network. Therefore they are also called distributed generation (DG) units. For the distribution network the connection of generation units is a challenge since distributed networks were planned and built and dimensioned for the connection of load. Customers are normally connected to the distribution network and therefore voltage quality is an important issue for distribution networks. Medium and low voltage distribution networks are quite passive today with none or only somewhat communication and voltage control. In many cases the only feasibility to control the network voltage is the on-load tap changer at the high voltage/medium voltage transformer. Voltage variations and varying network losses are two important aspects when connecting generation units to the distribution network. To reinforce an existing network by building new lines to cope with the voltage variations caused by DG units and in such a way increase the DG capacity in a distribution network is always a solution but it is an expensive one. In this thesis other solutions which allow an increase of the DG capacity without network reinforcement are considered. Three different methods for voltage control in medium voltage distribution networks are mentioned in this work: coordinated control of the on-load tap changer, reactive power consumption and active power curtailment. A voltage control algorithm to maintain the voltage within the limits at all network nodes has been developed within this work. The voltage control algorithm is able to perform three different control strategies: local on-load tap changer (OLTC) control, DG control and coordinated OLTC control. Local OLTC control is often used in medium voltage networks today. With DG control the voltage is controlled at the connection point of the DG units by the use of reactive power consumption and active power curtailment. In the case of coordinated OLTC control the voltage is controlled by a coordination of the on-load tap changer, reactive power consumption and active power curtailment. In addition to previous work done within this area voltage measurement values from already installed electricity meters are used as feedback for the OLTC control. The voltages obtained by the control algorithm and the network losses are simulated within this work. Simulations of the network voltages and the losses in the network were done on a generic network with three typical kinds of feeders. It is shown that the DG capacity in this generic network can be increased if the network becomes more active and the DG units are participating in the network voltage control as in the case of local DG control and coordinated OLTC control. Furthermore a more fair distribution of the curtailment has been tested but that increases the totally used curtailment. To exemplify on a real existing network, the network of Svalöv substation (Sweden) was analysed and transferred into the simulation tool. The Svalöv area has already today a lot of wind turbines connected and more are planned. The simulations from the generic network were repeated on this existing network and also here the DG capacity could be increased significantly by a more active voltage control in the network

Increasing DG Capacity of Existing Networks through Reactive Power Control and Curtailment

Renewable energy sources (RES), especially wind turbines, have become more important during the last years. An increasing number of distributed generation (DG) units are connected to weak medium voltage distribution networks in rural areas where they have a large influence on the voltage and the line losses. Voltage rise is in this case often a limiting factor for the maximum amount of DG capacity. Already current wind turbines with a capacity of 2 MW can often not easily be connected to existing 10 kV feeders. To increase the DG capacity of existing networks without reinforcement DG units can be controlled. This paper proposes abandoning unity power factor used today and letting the converters used as network interface of many new wind turbine generators absorb reactive power to reduce the voltage level. Since reactive power has great influence on losses in the network the use of reactive power is limited. Line losses due to the transfer of reactive power are taken into account in this study. Furthermore the use of curtailment is analysed. Simulations of voltage change and line losses when using reactive power control by the connected wind turbines and curtailment in a simple test system are presented. Without reinforcement of the network it was possible to increase the DG capacity from 2;7MW to more than 4MW in the test network without violating voltage limits. Line losses increase but to a reasonable extent and lost energy due to curtailment is insignificant

Efficient Integration of Distributed Generation in Electricity Distribution Networks - Voltage Control and Network Design

Distributed generation (DG), i.e. generation connected to the low and medium voltage distribution network (DN), has been increasing a lot during recent years. Thus the traditional assumption of a unidirectional power flow and a voltage decrease along the distribution feeders is no longer valid in all operation conditions. Voltage control in these networks is often limited to the on-load tap changer at the high voltage/medium voltage substation. Thus keeping the voltage at the customer connection point, which is an important quality criterion for electricity supply, within the limits may become a challenge. Since most of the available voltage band is assigned to the voltage decrease caused by the load, only a small part is available for a voltage rise from DG power injection. To overcome this limitation, traditionally the network has to be reinforced, which is always a solution but quite expensive. Coordinated voltage control is introduced as an alternative to avoid or postpone network reinforcement. The proposed algorithm receives actual voltage measurements from electricity meters at the customer connection points. The voltage setpoint at the substation and the reactive and active power output of the DG units are then adjusted to keep the voltage within the limits. Thereby the voltage band is used more efficiently and as a last option, the active power output from the DG units may temporarily be limited and some energy spilled. The voltage control scheme has been verified by power flow simulations of an existing DN in Sweden using real time series for consumption, photovoltaics and wind generation. It turned out that the need for active power curtailment is low even for large DG penetration if applying coordinated voltage control. Next, a passive DN has been turned into an active DN by introducing coordinated voltage control in a field test. The main objective has been to test the effect of asynchronous measurements from electricity meters and DG units and the impact from the communication. Control with asynchronous measurement turned out to be possible and curtailment has been reduced considerably. As coordinated voltage control uses active power curtailment as a last option to keep the voltage within the limits, it is, especially for the DG developer, important to estimate, to what extent curtailment will be utilised. Based on this data DG developers have to decide, if they would prefer a more expensive connection, which is able to always transfer the maximum DG output, i.e. a firm connection, or if they prefer to accept some temporary restrictions, if it is at a lower cost and faster available. Power flow simulations could be used to determine the expected curtailment. They are exact but they require a lot of input data and are time consuming, especially for calculations over large time series. Therefore a 5-Step-Method, which is fast, simple-to-apply and needs only a reduced set of input data, has been developed. The 5-Step-Method can be applied to calculate the expected curtailment for a DG unit with a predefined nominal output at a given location. However, the method could also be applied to determine the maximum nominal DG output at a given location, if a predefined amount of curtailment can be accepted. To verify the 5-Step-Method, it is applied on DG connections in a generic test system. The obtained results are quite close to the ones from power flow calculations for the considered scenarios. The results for the expected curtailment calculated by the 5-Step-Method are however not conservative compared to power flow calculations, i.e. showing a larger amount of curtailment, for all scenarios. Finally the necessary steps for implementing coordinated voltage control and non firm DG connections are summarized both for distribution network operators and DG developers

Electricity Meters for Coordinated Voltage Control in Medium Voltage Networks with Wind Power

During the last years the amount of electricity generated by Distributed Energy Resources (DER), especially wind turbines, has been increasing a lot. These Distributed Generation (DG) units are often connected to rural distribution networks, where they have a large impact on the voltage and the network losses. The network voltage at the customers point of connection is an important quality criteria and has to follow different standards as e.g. EN 50160. Therefore the voltage change caused by the integration of production units in the distribution network is an important aspect when integrating more DG in distribution networks and often a limiting factor for the maximum DG capacity which is possible to integrate into an existing network without reinforcement. Using the available voltage band more efficient by applying coordinated voltage control is a possibility to increase the hosted DG capacity in an existing distribution network without reinforcement of the network. To get the actual network status the new generation of electricity meters, which have the feasibility to communicate real time voltage measurements from the customers side to a network controller, give some benefits to a more flexible and coordinated voltage control in the network. The voltage range in the network will be used adapted to the actual load and generation situation instead of using worst case assumptions as it is good practice until now. A main part of the voltage control in medium voltage distribution networks is done by the on-load tap changer (OLTC) which takes the voltage at the consumers point of connection into account. A generic 10 kV distribution network with three typical types of feeders, as pure load, pure generation and mixed load and generation feeder, has been outlined. Coordinated voltage control is implemented by a central voltage controller. Simulations on the voltage and the network losses have been done and will be presented in this paper. The maximum DG capacity in the test system increases most when introducing coordinated control of the OLTC but also the use of reactive power adds some benefit. Further increase of the DG capacity by more extensive use of curtailment is always possible but due to economical aspects not favoured

Coordinated voltage control in medium and low voltage distribution networks with wind power and photovoltaics

Distributed Generation (DG) installations have been increasing during the last years. Wind power and photovoltaics are two of the most common renewable energy sources for DG typically connected to the distribution network (DN) originally planned and built to supply loads. DG units connected to the DN impact the voltage where customers are connected. Network voltage is an important quality criterion in DN. Voltage rise caused by DG units may become one of the limiting factors for the hosting capacity of wind power and photovoltaics in DNs. Increasing the hosting capacity by network rebuilding is possible but it is expensive and time consuming. Coordinated voltage control has been proposed to increase network capacity without the need of reinforcement. Simulations based on an existing medium and low voltage DN with wind power and photovoltaics are presented. It is shown that coordinated voltage control can increase the hosting capacity and avoid network reinforcement

Minimization of Reactive Power Exchange at the DSO/TSO interface : Öland case

A rising penetration of renewable energy sources in electric power grids is both a challenge and an opportunity to utilize its potential to stabilize operation of future power systems. Bi-directional flows between distribution and transmission systems can cause significant problems with keeping the voltages and reactive power in grids within permissible levels. This paper addresses the problem of reactive power exchange between the distribution system of Öland and the mainland Swedish electricity grid. Wind turbine generators with the capacity that highly exceeds total demand in Öland, are used to minimize absolute reactive power exchanged at the point of connection. This is done by applying droop control functions for reactive support to the wind turbines. Results indicate that the controllability of the reactive power support from wind turbine generators can keep the reactive power flow minimized at the point of connection and simultaneously diminish the active power losses in the system. The analysis in this paper has been done using the PSS/E software.QC 20191220</p