DISTRIBUTED GENERATION FROM SMALL HYDRO PLANTS. A CASE STUDY OF THE IMPACTS ON THE POWER DISTRIBUTION NETWORK.

Transcription

1 DISTRIBUTED GENERATION FROM SMALL HYDRO PLANTS. A CASE STUDY OF THE IMPACTS ON THE POWER DISTRIBUTION NETWORK. N. Lettas*, A. Dagoumas*, G. Papagiannis*, P. Dokopoulos*, A. Zafirakis**, S. Fachouridis**, Κ. Manousaridis** * Aristotle University of Thessaloniki / Greece ** Public Power Corporation S.A., / Greece ABSTRACT Distributed Generation (DG) is gaining the research interest as it can offer an alternative way to cover the needs for more electric power in a deregulated electricity market under certain financial and environmental constraints. Several operational problems arise by the operation of DG in the distribution networks. Scope of this paper is to investigate several operational cases in a 20 kv distribution network including 8 small synchronous and asynchronous hydro generators. The voltage variation along the distribution network is examined under variable loading conditions, variable generation modes and by implementing voltage regulator and compensation capacitors. Results show that the DG can act as a voltage supporting mechanism in case of maximum loading conditions, while the voltage can be kept within limits in cases of low loading or when the generators are disconnected. Index terms. Distributed generation, voltage profile, voltage regulation 1. INTRODUCTION The traditional model in electric power generation consists of centralized plants, requiring high voltage transmission networks and medium and low voltage distribution networks to transfer the power to the points of consumption. Fossil fuel power plants comprise the bulk of this generation model. Environmental issues and the exhaustion of fossil fuels began to pose new problems for the power industry. The stabilization in demand growth in most industrialised countries together with the liberalization in the electricity market, have led to a new situation, where major investments in new power plants and in the expansion of the high voltage transmission network are examined with increasing scepticism. As a result of all these factors and the evolutions in power generation technologies, a shift has emerged in the power industry generation model from centralised massive generation to a smaller and more modular scheme of distributed generation (DG), Griffin et al (1). DG provides electric power closer to the consumer, eliminating the unnecessary transmission and distribution costs. DG includes the application of small generators, typically smaller than MW, which are not centrally planned and dispatched and which are usually connected to the distribution network, (2), (3), Hatziargyriou (4). Most types of DG use traditional power generators like diesel engines, combustion turbines, combined cycle turbines or hydro-power. Moreover, DG technology includes the implementation of fuel cells and renewable energy sources like wind, solar, or small hydro generation. DG is emerging as a promising generation technology for a number of reasons. Distributed energy resources comply with the European Directives requiring to act on the demand side. It improves efficiency, ensures security of supply of clean energy and contributes in the development of new sustainable energy generation and transformation technologies, such as renewable energies and fuel cells. The White Book on Renewable Energy Sources of EU and other national directives (5), (6) are encouraging the development of renewables, including small hydro plants. In addition, almost all EU countries have started deregulating their electricity markets and encouraging private investments in the energy sector. The level of deregulation affects greatly the distributed generation possibilities. At the same time, the increased needs in system generating capacity is the moving force for the development and installation of more modular power plants to meet the demand. However there is a trade-off point between the benefits of DG and the adverse grid effects at distribution and even at transmission levels that delay the wide penetration of DG in Europe. The presence of DG can fundamentally alter the operation of distribution networks due to a number of significant impacts, Hatziargyriou and Zervos (7), Harrison and Wallace (8), Wallace (9), Wallace and Harrison (10), Masters (11). The introduction of distributed generators in a distribution system invokes the possibility of critical changes in the usual mode of operation, affecting the system protection and fault clearing capability. In cases where the DG capacity is comparable or larger than the local demand, especially at rural areas with low load densities, bi-directional power flow is likely to occur exceeding the equipment thermal ratings. The voltage at the connection bus of the generators rises and voltage regulation within specified limits may prove to

2 be difficult. Furthermore the connection of DG increases the short circuit and fault levels of the distribution system and may influence the network stability during transient conditions. Finally, depending on the type of DG plants and the topology of the network, the power quality is also affected. Scope of this paper is to present results from a case study including the connection of 8 small hydro generators, synchronous and asynchronous, to the distribution network at the level of 20 kv. The capacities of generators range from 0.35 MW to 1.92 MW. The operation of the generators is totally random as there are no rules or constrains concerning the connection to the grid. Different situations were examined for cases of minimum and maximum network loading when coupling or uncoupling the hydro plants to the grid. 2. CASE STUDY SYSTEM DESCRIPTION The power system studied in this work is a part of the distribution network of Public Power Corporation of Greece in North Greece. It consists of 2 substations with two main transformers 150/20 kv, with a rated power of 40/50 MVA each, connected to the transmission system and a 100bus distribution network at 20 kv. The total circuit length is around 100 km, consisting of Aluminum Coated Steel Reinforced (ACSR) overhead distribution lines, with crosssections 95 mm 2 and 16 mm 2. The total load of the network is about 20 MVA, covering small urban and rural settlements. 17 small hydro power generators, synchronous and asynchronous are connected to this network. Their rated capacities vary from 0.22 MW to 4.1 MW. 2.1 Lines The case study power system consists of two independent distribution lines and two lines under construction, for the future connection of further distributed generators. In this paper only one of these lines (line A) of the network, which is illustrated in Figure 1, is analytically examined. More specifically, the line A has a length about 40 km, contains 7 asynchronous and one synchronous generator and its load vary from 1.84 MVA to MVA. The ratings of the generators are given in Table 1. The generators are connected to the grid at 20 kv with step-up transformers. The asynchronous generators have their own capacitor units in order to compensate the reactive loads locally. The resistance and the reactance of the overhead lines are: Crosssection Resistance (Ω/km) Reactance (Ω/km) 95mm mm On Load Tap Changers Each of the main transformers of the substations is fitted with an on load tap changers (OLTC). These are motorized mechanical switching arrangements that adjust the transformer turns ratios, while the Ga-4.1 Ga-3.2 TRa-3 Ca-4.1 Ga-4.2 Ca-3 La-14 TRa-4 Ca-4.2 Ga-3.1 Ga-5.1 Ca-5.1 Ga-2 Ga-5.2 Ca-2 TRa-2 La-12 La-11 La-10 TRa-5 Ca-5.2 Ca-1 La-9 TRa-1 La-8 Ga-1 Feeder REGa La-6 La-1 La-2 La-3 La-4 Ca-6 La-5 Fig.1. General layout of the case study network

3 transformer is in operation. Each OLTC has an associated automatic-voltage-control relay which, in the simplest case, monitors the voltage of the transformer secondary and instructs the OLTC to tap up or down as required. The specific OLTC has 17 steps, giving the ability of secondary voltage regulation in area of -12.5% +7.5%, in terms of the nominal voltage of 20 kv. A TABLE 1 - Generators in line A Rated Rated Generator Power Voltage (MW) (Volts) Ga-1 (Synchronous) Ga-2 Ga-3.1 Ga-3-2 Ga-4.1 Ga-4.2 Ga-5.1 Ga-5.2 Rated cosφ (lag.) Voltage regulators and reactive compensators In the middle of the line there is also a voltage regulator, to keep the voltage at a remote node constant. The regulator is equipped with a solid-state voltage sensing circuit (VSC) that has a voltage balance point and that causes the regulator to change taps to maintain a constant base voltage at its input terminals. This voltage equals to the line voltage divided by the control winding primary-to-secondary ratio. The voltage drop along the line up to the remote controlled node is simulated using a compensation circuit. The compensator is essentially an artificial line, containing adjustable resistances and reactances. It can be adjusted to represent the actual voltage drop on the line up to the node, where the voltage is to be controlled. The voltage drop on this miniature line is subtracted from the reference voltage in the VSC. The VSC balances the voltage differences by causing tap changes until the line voltage is set to a value that balances the calculated voltage drop. The line A has three single-phase regulators in a delta connection. The turn ratio changes by 0.625% at each step while the range of the tap changer in the regulator is from -16 to +16 steps. So, the voltage can be regulated in the area of kv. The voltage regulation is adjusted for a remote voltage reference node, where the voltage must be equal to 1.0 p.u. At the examined system, the reference node is the node where load La-10 is connected. 2.4 Line reactive compensation Another approach, which is used by the Distribution Network Operator (DNO) to keep the voltage between the permissible limits along the line, is the placement of proper capacitor banks at selective points of the network. The capacitors produce reactive power at the point of connection and consequently support the voltage. At the line A, there are two points of connection of static compensators. The first point is in the input of the voltage regulator and the rated reactive power of the capacitors is 1800 kvar. The second point is in a remote node of the line, with the rated capacitors reactive power being 1800 kvar. 2.5 Loads The dominant use of the electric power is either domestic in small towns or rural mainly for water pumps. The demand fluctuates greatly, depending on the time of day and the season of the year. The maximum demand is recorded at summer noon, when the needs for air-conditioning at domestic and for irrigation in rural areas are high. On the contrary, the minimum demand is recorded during the nights of winter, when there is no need for water pumps and the domestic loads are at minimum. Also, moving towards the end of the line, the line impedance rises as the conductors cross-section becomes smaller. In order to ensure that consumers at the remote end of the network are supplied within the specified voltage limits, the DNO regulates the substation output voltage, with OLTC, a few percent above the nominal voltage. Because of the load variation during the day and season, at maximum loads the secondary voltage of each transformer is regulated at 1.07 p.u., while at the minimum loads the output voltage is regulated at 1.03 p.u. Analytically, the maximum and the minimum loads of the line, which are recorded during the year, are given in Table 2. The power factor for all loads is assumed to be equal to 0.90 lagging. 3. SIMULATION METHODOLOGY The above network was simulated using the NEPLAN software (12). The power flow module calculates, among the others, the active and reactive power flow at each branch, the current through the lines and the voltages at each node.

4 TABLE 2 Maximum and minimum loads LOAD MAXIMUM VALUE (kw) MINIMUM VALUE (kw) La La La La La La La La La La La La Test Cases A set of cases for the line A, with different initial conditions and ways of operation, have been investigated. The cases which are examined are: the loads are at minimum or at maximum, the network capacitors are connected or disconnected from to the network, while the voltage regulators are in operation and out of operation. All the above combinations can create eight marginal operational situations of the network, while any other case is included among these limiting cases. All the above cases are summarized in Table 3. Initially, it was examined the case at which the total load is supplied by the network feeder and all the distributed generators are out of operation. In Fig. 2 the voltage profile along the line A for the above eight operational situations is given. In the same diagram the upper and the lower voltage limits, as set by DNO, are also shown. Fig.2. Network operation without generators In the second test case, it is considered that all the generators are operating simultaneously at their rated power, meaning the maximum power that can be delivered by distributed generators. For every one of the above eight marginal situations, the calculation of voltage variation along the network has been done. The results of these calculations are shown in Figure 3. Lower voltage Fig.3. All generators connected at full load TABLE 3 - Cases for Figures 2 and 3 CASE Voltage Capacitor Regulator Bank Loads CASE #1 OFF OFF MAX CASE #2 OFF ON MAX CASE #3 ON OFF MAX CASE #4 ON ON MAX CASE #5 OFF OFF MIN CASE #6 OFF ON MIN CASE #7 ON OFF MIN CASE #8 ON ON MIN Except from the above extreme cases with all distributed generators to be either disconnected or to operate at full load, several intermediate operational situations are considered. In the following test case each generator is assumed to be connected to the network individually and independently from the others. The influence of the independent connection of the 2 greater generators, namely the Ga4 and Ga1, on the line voltages is shown for the following operational states Voltage regulators are not connected in the line (Fig.4) Voltage regulators are operating in the line (Fig.5) The cases shown in these 2 diagrams are summarised in Table 4. One other set of simulations investigates the case where the generators are connected in sequential steps. Initially, in the first step, generators Ga1, Ga3 and Ga5 whose total rated power is about 2 MW are connected. In the second step, the generator Ga-1 with 1.3 MW is connected successively, without disconnecting the previous set. In the third step the rest generators are connected.

5 Fig.4. Individual connection of generators with inactive voltage regulator Fig.6. Sequential connection of generators and inactive voltage regulator Fig.5. Individual connection of generators and voltage regulator in operation Fig.7. Sequential connection of generators and voltage regulator in operation TABLE 4 Cases for Figures 4 and 5 CASE Capacitor Generator Loads Bank ON CASE #1 OFF MAX Ga4 CASE #2 OFF MAX Ga1 CASE #3 ON MAX Ga4 CASE #4 ON MAX Ga1 CASE #5 OFF MIN Ga4 CASE #6 OFF MIN Ga1 CASE #7 ON MIN Ga4 CASE #8 ON MIN Ga1 Once again the voltage profiles along the line are given, for the two different cases, as above. These cases correspond to the cases where the voltage regulator is inactive (Fig. 6) and to the case where the voltage regulator is active (Fig. 7). All the above cases are summarized in Table 5. TABLE 5 - Cases for Figures 6 and 7 CASE Capacit or Bank Loads Generators ON CASE #1 OFF MAX Step 1 CASE #2 OFF MAX Step 1+Step 2 CASE #3 ON MAX Step 1 CASE #4 ON MAX Step 1+Step 2 CASE #5 OFF MIN Step 1 CASE #6 OFF MIN Step 1+Step 2 CASE #7 ON MIN Step 1 CASE #8 ON MIN Step 1+Step 2 Step 1: 2.21 MW total power. Step 2: 1.3 MW total power. Step 3: 1.72 MW total power 4. DISCUSSION OF THE RESULTS Starting from Fig. 2, it is clear the necessity of the voltage regulator and the network capacitor banks, when the loads are at maximum. When the regulator is

6 in operation and the capacitors are connected, then the voltage is between the permissible limits at loads to (case #4). At the rest cases the voltage is out of the limits. On the contrary, at minimum loads, the voltage is between the permissible limits, independently from the presence of capacitors or the operation of the voltage regulator. At the cases where all the generators are in operation and the loads are at maximum, from Fig. 3 it is shown that if the voltage regulator is inactive, this results in the violation of the voltage limits. When the regulator is functioning properly, then the voltage across the line is in the permissible limits, independently from the presence of the capacitors. At the rest of the cases, where the loads are at minimum (cases #5 - #8), the voltage is among the permissible limits, without the presence of the capacitors or the regulator. Comparing Fig. 2 and 3, an upward parallel shifting of the corresponding voltages can be observed, when the generators are in operation. The synchronous generator is working in Automatic Voltage Control mode (1 p.u. at the connection node of the network) and each of the asynchronous generators has its own compensation capacitors. As a consequence, the voltage is supported along the line and the upper and lower voltage limits are not violated in any case. From Fig. 4, where the regulator is inactive, it is evident that, even after the independent connection of the two biggest generators, the voltage is below the permissible limits, when the loads are at maximum. At minimum load the voltage drop along the line is very small and the capacitors can be disconnected from the network, without any impact at the voltage levels. In the cases where the regulator is in operation, it can be seen from Fig. 5, that the cases where the voltage is lower than the limits at the remote end of the line, are those with the capacitors disconnected from the network and the load at maximum. In all other cases, the voltage is between the limits. On the contrary, at minimum loads, the voltage is in the permissible limits, independently from the presence of the capacitors or of the regulator. In Fig. 6, the voltage along the line is recorded, when the generators are connected sequentially, at three steps and the voltage regulator is not working. It is shown that in all four cases of maximum loads, the effect of the connection of the generators in the voltage profile is small and the voltage is lower than the permissible limit. In the minimum loads the voltage remains almost constant along all the length of the line, equal to 1.03 p.u., independently from the presence of capacitors. In the cases where the regulator is in function, it can be seen from Fig. 7, that the two steps of generator connections are adequate to support the voltage along the line. In cases #5 and #6, where the capacitors are disconnected from the network and the loads are at minimum, the voltage regulator does not make any voltage adjustment, as the voltage drop is compensated by the voltage rise due to the generators. Moreover, at cases #7 and #8, where the capacitors are coupled to the network and the loads are at minimum, the voltage regulator reduces the output voltage, in order to maintain the reference node voltage to 1.0 p.u. 5. CONCLUSIONS Scope of this paper is to present results from a case study in a 20 kv radial distribution network including 8 small synchronous and asynchronous hydro generators. The voltage variation along the line is recorded for various loading conditions and operational scenarios including the presence of a voltage regulator and of reactive compensation capacitor banks. The NEPLAN software was used as the simulation tool. Results show that the voltage variation in the network exceeds significantly the lower limit in the case of maximum loads without the use of auxiliary voltage support systems such as voltage regulator, capacitors and generators. The voltage is between the permissible limits, only when both the voltage regulator and the capacitors are connected. On the other hand, at minimum loads, the voltage is between the permissible limits independently from the presence of the voltage regulator and the capacitors. The introduction of the small hydro generators, alone or sequentially in groups, shows that the generators can support the voltage without violating the upper limit. It is shown that with the parallel operation of the regulator, the voltage may be maintained between the limits for all cases. But in a case of a fault, when one or more generators are disconnected from the network, the voltage remains between the permissible limits only when the capacitors are connected. On the contrary, at minimum loads, the operation of the generators or their sudden disconnection does not affect the voltage variation. In all cases examined the power flow resulted always from the feeder to the loads, regardless of the presence of the generators. Bi-directional power flow affects the operation of the voltage regulator greatly and is still under investigation. 6. ACKNOWLEDGEMENTS The contribution of Ms E. Rountou and Ms K. Statiri, in the preparation of this work is greatly acknowledged.

7 7. REFERENCES [1] Griffin T., Tomsovic K., Secrest D., Law A., 2000, Proceedings of the 33rd Hawaii International Conference on System Sciences. [2] CIRED Working Group 04, 1999, "Dispersed Generation". [3] CIGRE Working Group 37-23, 1997, "Impact of increasing contribution of dispersed generation on the power system". [4] Hatziargyriou N., "Distributed Generation", Chapter 13. [5] White Paper for a Community Strategy and Action Plan, 1997, "Communication from the European Commission: Energy for the Future, Renewable Sources of Energy", 559. [6] European Commission, 2000, "Directive of the European Parliament and of the Council on the Promotion of Electricity from Renewable Energy Sources in the Internal Electricity Market". [7] Hatziargyriou N., Zervos, A. 2001, Proceedings of the IEEE, 89, [8] Harrison G., Wallace R., 2003, Proc. IEEE/PES Transmission and Distribution Conference & Exposition. [9] Wallace R., 1999, IEE Colloquium on Protection and Connection of Renewable Energy Systems. [10] Wallace R., Harrison G., 2003, CIRED 2003, 17 th International Conference on Electricity Distribution. [11] Masters C. L, 2002, Power Engineering Journal, 16, [12] NEPLAN Power Systems Engineering Software,