Innovative hardware is key to maintaining voltage quality and containing system reinforcement costs.
To Minimise the Investment Required to Maintain the Voltage Within Specified Limits, particularly on low-voltage (LV) networks in sparsely populated areas, Electricité de France (EDF; Paris, France) designed hardware to reduce the voltage drop between the LV feeder source and the remote end of the circuit. Providing that the technical and economic conditions are satisfied, this new technology offers a more-affordable alternative to the traditional methods of network reinforcement.
THE VAS AND TMC VOLTAGE REGULATORS
The statutory voltage levels in France are 400 V between phases, thus the declared voltage for single-phase services is 230 V. The EDF design standards specify the use of single-phase services for customer demands up to 18 kVA; above this value, three-phase services are installed. To maintain voltage within statutory limits, EDF now installs one of two new voltage regulators, the Voltage Adapter Slipper (VAS) and the Tri/Mono Converter (TMC).
The VAS, of which EDF is co-owner with the manufacturer Optarel (Bègles, France), is an electronic control load regulator that maintains the output voltage within the range 230 V +6% to -10%. The unit, being installed in Fig. 1, draws part of the upstream or source voltage by means of the second winding of a transformer placed in parallel. This second winding has switched connectors controlled by solid-state relays, which are piloted by a microprocessor. Fig. 2 represents a single-phase unit, but for a three-phase system, the three phases are identical, each being equipped with independent control. The upstream-voltage fraction is injected into the primary winding of a second transformer, the secondary winding of which is in series on the downstream (remote end) network. Digital control and the use of solid-state relays allow for fast response to the voltage gradients in the network source voltage or from the effects of demand or load shedding at the remote end.
The technical specification for the VAS unit is presented in Table 1, and Fig. 3 shows the ground-mounted VAS regulator.
The TMC, manufactured by Thuillier (France) and distributed by Optarel, is installed on a voltage-controlled LV network. The TMC serves to reduce the voltage drops due to the load unbalance that is attributable to a single-phase customer at the remote end of the circuit. The TMC is designed to distribute single-phase load current across all three phases and the neutral of the LV feeder. The ground-mounted converter is shunt connected to the network and positioned just upstream of the single-phase customer, creating the unbalance on the voltage-controlled network. The design specifications of the TMC provide voltage improvement for a single-phase customer, where the initial voltage is above the minimum voltage (Table 2).
The TMC comprises the following components:
An autotransformer with a voltage ratio = 2, which (between phase one and neutral) supplies a U1 voltage with a nominal value of 115 V.
A transformer, also with voltage ratio = 2, which (between phase two and phase three) supplies a U2 voltage with a nominal value of 200 V.
The single-phase customer is supplied by the U3 voltage resulting from the series use of voltages U1 and U2. As the U1 and U2 phases are perpendicular, the nominal value of U3 is indeed 230 V. Fig. 4 shows how the combination of U1 and U2 improves the customer's voltage.
The improvement therefore provided by the TMC is as follows:
Without TMC, phase one supplies the customers load I amperes and the neutral conductor -I amperes
With the TMC in circuit, four conductors supply the customer's load instead of just two conductors, thereby reducing the voltage drop between the feeder source and the customer's terminals. The load current in phases one and three is I/2 amperes, and the load current in phase two and the neutral -I/2 amperes.
The VAS and TMC units do not require any preventive maintenance.
DECISION-MAKING CRITERIA FOR VAS INSTALLATIONS
To optimise the application and benefits associated with the VAS, EDF has developed decision-making criteria for system design technicians. The VAS is installed in close proximity to the customers whose voltage is below statutory limits in order to reduce the network voltage drop between the feeder source and the customers. The installed capacity or rating of the VAS must be greater than the total maximum demand of the downstream customers.
The introduction of a VAS results in an increase in current and therefore an additional voltage drop between the source and point of connection. On a network with a nominal voltage of 230 V, the maximum voltage drop that can be compensated is 20% relative to the nominal voltage. In locations with a larger voltage drop, conventional network reinforcement measures are required. Voltage drops generated upstream become significant when the downstream voltage drop to be compensated is more than 12% of the nominal voltage. Depending on the maximum voltage drop to be corrected, EDF has derived a simple criteria, relative to the number of customers upstream and downstream of the VAS, to ensure the overall operating quality of the “LV feeder + VAS” system:
If the maximum voltage drop to be corrected is less than 12%, the feeder shall contain no more than four customers upstream of the VAS.
If the maximum voltage drop to be corrected is between 12% and 20%, there must be no customers on the feeder upstream of the VAS (unless situated very close to the station).
In all cases, a single-phase VAS will supply only one customer; a three-phase VAS will supply only two customers.
To simplify the technician's decision, and to avoid developing specific software, the criteria only considers customer numbers. To achieve this, the criteria considers safety margins and thus is less optimised than a calculation that takes into account different customer parameters.
VAS COST-BENEFIT ANALYSIS
The installation of a VAS increases the current on the upstream section of the network, thus resulting in additional losses. A pessimistic evaluation of the economic repercussion of these losses is 20% of the capital cost of the VAS. It is possible to evaluate the number of years (N) that the investment associated with installing the VAS must be remain in situ to make the installation cost-efficient using the following expression:
where C is the capital cost of the solution, and I is the cost of the avoided reinforcement. The constant 1.08 takes into account the 8% discount rate used by EDF in investment studies
When N is less than 10, the VAS solution should usually be adopted, however the decision needs to be confirmed depending on local considerations. When N is greater than 10, the risk of a reduction in the efficiency of the VAS must be considered according to:
The capitalised cost of the internal losses of the VAS could render this strategy less profitable than a classical reinforcement.
The risk of changes in the number of connected customers and summated customer demands.
If this risk is judged to be too high, the more usual method of network reinforcement is preferable.
CRITERIA FOR TMC INSTALLATIONS
A similar approach is developed to give design engineers decision-making guidance in respect of installing TMC units (Fig. 5). The aim is to minimise the voltage drop attributable to a single-phase customer at the end of the network; there may be three-phase customers in close proximity but no other single-phase customers. If more than one mono-phase customer is located at the remote end of the network, a redistribution of customers between phases could reduce the unbalance so that a TMC is not required.
The economic benefits are studied in a similar manner (using the prior equation) to that used for the VAS; the number of years (N) need to be determined to ensure the investment in the VAS is cost efficient. This occurs for the lowest vale of N evaluated from:
where C is the cost of the TMC solution, and I is the cost of the avoided reinforcement
The installation of the TMC increases losses (e.g., transformer and autotransformer losses), while the reinforcement strategy reduces network losses. A conservative estimate is that the additional losses are equivalent to 20% of the cost of the TMC. Hence, this is the justification for the 1.2 factor included in the evaluation formula.
When N is greater than 10, it is probable that because of the losses, the TMC solution becomes less cost-effective than the reinforcement alternative. In these situations, network reinforcement is planned. When N is less than 10, the TMC solution is applied.
LOW-VOLTAGE NETWORK DESIGN MODELLING TOOL
The first step taken by EDF is to integrate into the distributor's information system details of the type, connection phase and nominal power ratings of the VAS and TMC.
For network nodes downstream of the VAS, in order to simplify modelling, EDF considers that the difference between the nominal and remote end voltages is equal to the voltage drop from the VAS's second winding. In simple terms, this pre-supposes perfect control of the voltage at the VAS's second winding. Therefore, the peaks and valleys of the voltage drop due to changes in transformation ratio in the VAS are not shown.
EDF's LV design tool has been in use in the operational centres since 1996; it evaluates the unbalanced system using a statistical approach. It uses an unbalance coefficient which, when multiplied by the voltage drop in a balanced system, estimates the maximum voltage drop among the three phases. It takes into account the objective to distribute the single-phase customers equally between the three phases.
To simulate the effect of the TMC, the tool's main functional modification is therefore to consider the single-phase customer equipped with a TMC as being a three-phase customer. This approximation would appear to be in line with the function that assumes three-phase customers have perfectly balanced load demands. By assuming that the single-phase customer equipped with a TMC is the same as a three-phase customer, the unbalance coefficient is reduced.
VAS AND TMC IN-SERVICE APPLICATIONS
Because of the technical and economic constraints, which specify the installation of the VAS and TMC units, these devices are primarily intended for use on rural networks. With the pressure to reduce investment costs, the EDF operational centres have shown great interest in the VAS and TMC equipment, recording satisfactory operational performance. Currently, there are more than 150 VAS and 50 TMC units installed on the LV networks operated by EDF.
Philippe Loevenbruck (firstname.lastname@example.org) graduated from the Ecole Centrale de Paris and joined Electricité De France in 1987. He successively worked on transmission network issues (i.e., frequency regulation) and power-system restoration after a general collapse, and on distribution network issues, including low-voltage network development planning and modelling of the customer's demand curves. In 2002, Loevenbruck started managing the R&D project “Load Profiling” in readiness for the opening of the French Electricity Market in July 2004.
|Nominal Power of VAS units||Iron losses Pf (W)||Copper losses Pc (W)|
|12-kVA single phase||240||720|
|18-kVA single/three phase||360||1080|
|36-kVA three phase||729||2160|
|Up-stream three-phase load||Voltage Vmin = %Vn||Minimum Voltage Vmin|
|Where Vmin is 230 V|
|Nominal Rating of the TMC||Iron losses Pf (W)||Copper losses Pc (W)|