SIMULTANEOUS CONTROL OF VOLTAGE AND POWER OF MICRO-TURBINE IN DISTRIBUTION NETWORK

Transcription

1 SIMULTANEOUS CONTROL OF VOLTAGE AND OWER OF MICRO-TURBINE IN DISTRIBUTION NETWORK B. Yousefpour G. Gharehpetian R. Noroozian M. Nafar Electrical ering Department, Amirkabir University of Technology, Tehran, Iran ABSTRACT Distributed Generation (DG) is expected to play an increasingly significant role in power generation in coming years. In Iran, more attention is recently paid to the Micro- Turbines (MTs), since 20% of the world s gas resources are believed to exist in Iran. A MT unit consists of a gas turbine engine, a permanent magnet generator, a 3-phase AC/DC rectifier and a DC/AC power inverter. In this paper this combination has been modeled by SCAD/EMTDC. It is shown that the well-known voltage control strategy of MT can result in operational problems for MT and distribution system. A new method is presented in this paper, to overcome these problems. Simulation results show that the proposed control strategy can limit the active and reactive power flow of the MT and can limit the short circuit capacity of the system too. Introduction Restructuring, environmental and fuel considerations and technological successes are generating a new market opportunity for the energy industry known as Distributed or Dispersed Generation (DG). DG can be seen in the from of Gas or Diesel-fired Turbine, Reciprocating s, Micro-Turbine (MT), small Hydro Induction Generations, Wind Turbine, Fuel Cell and hotovoltaic anels. They can be installed on the distribution primary and secondary feeders as commercial and residential active power sources, respectively. DG can be used in the distribution substations and can be connected to the subtransmission system too. These relatively small generating units (usually under 30MW) are sited at or near customer locations and can improve power security which encompass both system reliability and power quality [1]. Distributed Generation requires more active distribution networks which allow electricity to flow in two directionsto the electricity user for consumption in homes or businesses, and on to the network when the user is exporting excess generation capacity [1,2]. It must be noted that an end user can generate both thermal and electrical energy in a single combined heat and power (CH) generation system located at or near its facility. CH (or cogeneration) system can deliver cost-effective energy with efficiencies exceeding 90% [1]. Considering the above mentioned points, DG can provide a multitude of services to both utilities and consumers, including standby generation, peak shaving capacity, load sharing, based-load generation, or CH. Less well understood benefits include ancillary services (e.g., VAR support, voltage support, network stability, black start and spinning reserve). This may be of more economic benefit than simple energy for the intended load. Table (1) lists different type of DG technologies and their important properties [1, 2]. Technology. Size Range kw Efficiency (%) ackage cost ($/kw) O&M cost ($/kw) Table 1: Different DG roperties Diesel Gas Micro Turbine Fuel Cell For these technologies, there are two basic generator/utility operating modes [3]: Isolated operation with single or multiple generators and arallel operation with utility standby. In the both methods, DG can have following interfaces with utility: Rotating machine interfaced generators and Electronically interfaced generators. Table (2) summarizes common form of interfaces for different DG technologies [4]. Type of DG. Internal Combustion Combustion Turbine Micro turbine Small Hydro Fuel cell Wind Turbine hoto Voltaic Table 2: Typical Interfaces Typical Interface Device. Induction Induction or Induction Induction or Induction

2 Among different DG technologies, in Iran more attention is recently paid to Micro-Turbines. This technology is particularly applicable to Iran, since about 20% of the world s gas resources are believed to exist there. The country is wide-spread, three times bigger but less populated than Germany and the limitations associated with restricted access to long distance markets have substantially reduced the commercial viability of gas resources. Micro-turbine MT is small and simple-cycle gas turbine, generating electric power in the range of 25kW to 500kW [3]. They are similar to large gas turbines with improved performance and efficiency when operated at partial kilowatts (see Tab.1). Unlike traditional backup generators, MTs are designed to operate for extended period of time and require little maintenance. They can supply a customer s base-load requirements or can be used for standby, peak shaving, and cogeneration applications. They can be operated on natural gas, have propane or landfill gas. They are relatively small in size, have low emissions, low operating costs and can be connected to the power grid or operated independently (see Tab. 2). A micro-turbine as shown in Fig.1 has a combustion turbine engine integrated with an electrical generator that produces electric power while operating at a high speed, generating in the range of 50000rpm to rpm. Electric power is produced in the thousands of Hz, converted to DC voltage, and then inverted back to low frequency AC voltage by an inverter [2, 3]. Exhaust Heat to User Combuster Compressor Air inlet Recuperator Fuel Turbin e Fig.2: The thermal cycle of a Micro turbine ower Electronic Interface. The technical challenges associated with the MT are: The system interface and The control of the MT. The control of the output power of MT (and other DG sources) is achieved by controlling the power electronic interface realized by WM voltage sources inverters (VSI). As shown in Fig.3 the output voltage of MT is a high frequency voltage. A three phase diod rectifier is used to converge this high frequency voltage to a DC voltage. A WM GTO power inverter is used to invert DC voltage to 50Hz AC voltage [5,7]. Gas Turbi ne ermanent Magnet DC-AC Connection Bus to Distribution Network Fig.1: Micro-Turbine System. The Micro-Turbine, the compressor and the permanent magnet (M) generator are directly mounted on the single shaft. As a result there is no gear box. This is possible since the high-speed permanent magnet generator (HSMG), has no exciter on rotor [3]. Fig. 2 represents the thermal cycle of the Micro-Turbine [1-4]. Control Strategies Fig 3: MT power electronic interface. The control technique chosen for each DG will depend on the type of equipment installed. Control strategies of MT are divided into the two general categories: Voltage control ower control. Strategies. Control strategy selection, depends on MT installation place. Usually voltage control mode is selected if the MT is connected to low voltages system, and power control mode is chosen if the MT is connected to MV system [6, 9]. In this paper, a new control mode is suggested which can switches between the voltage and the power control modes. Voltage Control

3 Fig. 4 shows the voltage control block diagram. In this control method the main goal is to set the AC bus voltage. One of the I-controllers controls the inverter angle shift as shown in Fig. 4-a and another one, controls the modulation index, mi, as shown in Fig. 4b. Vbus Vref Vbus Vref Shift I- Angle Controller I- Controller modulation Index Fig. 4: Voltage control block diagram This control strategy is desirable for residential LV system but in the case of a shot circuit, this control method tries to set the AC voltage to the reference value. The faulted system current increase results in power increase and therefore MT must supply an additional power demand for the faulted LV distribution system. To overcome this problem, the technique of the power flow control, which has been used in MV system (for commercial systems), must be combined with the voltage control strategy. roposed Voltage and ower Control Strategy In the suggested method of this paper, the control strategy in the normal condition is the voltage control strategy, as shown in Fig. 4. During the fault condition, first of all, the fault must be detected. To detect the fault, the following index has been used: flow k = (1) ref Where flow and ref are the power flow from MT to the system and reference value of power flow, respectively. k must be greater than one to consider system temporary overloads. In case of a fault, k is greater than same special value, say k1 and the control strategy must be switched to constant power flow control strategy as shown in Fig. 5. Fig.5: ower flow Control diagram. The control rules can be expressed by the following equations: flow ref < K1 Voltage Control flow > K1 ower Control ref Simulation Results To predict the impacts of MT on distribution system control, protection and power quality conditions, it is crucial for the industry to have reasonable model representations and simulation results of these devices. The IEEE 13 bus system has been selected as a case study system and MT has been connected to Bus-634 of this system. The MT model is based on the generator and converter blocks of SCAD/EMTDC software [10]. The field of this model must be constant, to model the behavior of M generator field [8]. It is assumed that the MT has the following rated values: 60kW, 400 V, 2 poles and rpm. 1-phase, 2-phase, and 3-phase short circuits, sudden load increase and 1-phase, 2-phase and 3-phase feeder outages have been studied, for the voltage and suggested control strategies but only the important cases have been presented in this paper. Case A: Voltage control strategy is active- The voltage of bus 634 is about 0.8 pu before the connection of MT to this bus, as shown in Fig. 6. As it can be seen, after the connection of MT, the voltage control strategy has set the AC bus voltage to 1 pu. This simulation indicates the validity of control strategy. In the next simulation, at t=1.2s a fault has been simulated for 0.2 second near to the MT bus. As shown in Fig 7-a and 7-b, respectively, the active and reactive output power of MT increased rapidly. In this case, short circuit capacity has increased too much. Case B: roposed method has been used- Fig. 8 shows the AC bus voltage variations after the connection of MT t the AC bus at t=0.8 s and after fault occurrence at t=1.2s. As it can be seen for t>1.2s the voltage control is not active. During fault, i.e., 1.2 t 1. 4 s the suggested control is activated. The simulation results can be seen in the figures 9-a and 9- b. Respectively, the active and reactive power flows have been presented in these figures. It is obvious tat the power flows and short circuit capacity (SCC) that the AC bus has been successfully limited by the proposed control strategy. (2) (3)

4 Fig. 6: Bus 634 voltage variations, Case A. Fig.8: AS bus voltage variations, Case B Fig.7a) Active and b) reactive power flow form MT to system, Case A. Fig. (9): a) Active and b) reactive power flow, Case B.

5 Conclusion In this paper, Micro-Turbine with its power electronic interface has been simulated in the IEEE 13 bus distribution system. It is shown that the ordinary voltage control strategy of MT in LV system can result in short circuit capacity increase. To overcome this problem a new control strategy has been proposed. The simulation results demonstrate the capability of the suggested method to limit the short circuit capacity increase. Reference [1*]: E. M. etrie and H. Lee Willis, Distributed Generation in Developing Countries, ( [2*]: A Malmquits, O. Aglen, E. Keller, Micro turbine: Spreading the shift to distributed heat and power, Transmission and Distribution, April 2001, pp [3*]: Ann Chambers, Stephanie Hamilton, Barry. Schnoor, Distributed Generation, A Non technical Guide, Oklahoma, enn Well Corporation, [4*]:. Barker, K.Johnson, ower system Modeling Requirements for Rotating Machine Interfaced Distributed Resources, IEEE Transaction on power system, 2002, pp [5*]: E. Carroll, ower Electronics for very high power applications, ABB Review February 1999, pp [6*] S.van Zyl, C. T. Gaunt, Control strategies for Distributed s operating on Weak Distribution Networks, in roc IEEE Bologna ower Tech Conference, June 23 th- 26 th, Bologna, Italy. [7*]: Ann Chambers, Barry Schnoor, Stephanie Hamilton, Distributed Generation of electric power, ennwell, [8*]: A. Al-Hinai, A. Feliachi, Dynamic Model of a Microturbine used as a Distributed, IEEE [9*]: M. I. Marei, E. F. Saadany, M. M Salma, Flexible Distributed Generation, IEEE [10]: