A Proposed Hybrid Method For Islanding
Detection
SMITA SHRIVASTAVA*, S. JAIN**, R. K. NEMA***,
* Hitkarini College of Engineering and Technology Jabalpur-India **, *** Moulana Abul Kalam Azad National Institute of Technology, Bhopal-India
Abstract:
In this paper Islanding detection methods for distributed resources are reviewed. The mainly used islanding detection techniques may be classified as active and passive techniques. Passive techniques don’t perturb the system but they have larger nondetection zones, whereas active techniques have smaller nondetection zones but they perturb the system. In this paper a new hybrid method is proposed for detecting islanding of inverter based distributed generators. This method is a combination of active frequency drift method and change in proportional power spectral density. Active frequency drift method is very effective in finding islanding with various modifications. Its drawbacks are overcome by change in proportional power spectral density method.
Key words: Islanding, Distributed generation, Islanding detection, active techniques, passive technique.
I. Introduction
The deregulation of electric supply and the emergence of a new league of generators, e.g. wind turbines, photovoltaic cells and fuel cells have led to a new field of study, distributed generation. Distributed or dispersed generation may be defined as generating resources, other than central generating stations, that is placed close to load being served, usually at customer site. The number of DG in distribution system is rising, as DG can avoid transmission and distribution (T&D) capacity upgrades, reduce transmission and distribution line losses, improve power quality, improve voltage profile of the system, etc [1]. In fact, many utilities around the world already have a significant penetration of DG in their system. But there are many issues to be taken into account with the DG and one of the main issues is islanding.
When a distributed generation system with some loads is disconnected from the utility power system, the distributed generation is going to supply the loads and, although this is
rare, continue an islanded operation of power system. The islanded operation should be avoided because of safety reasons for maintenance man and power quality reasons of distributed lines. To solve these problems, islanding detectors are used to detect an islanded operation and trip the circuit breaker between the power system and the distributed generation [2]. Islanding detection methods are based on the type of generator used in distributed generation. Classification of DGs based on type of generators:
Fig.1 Classification Of Distributed Generators
Depending on the type of generators different types of islanding detection methods are used. Here in this paper special emphasis is given on islanding detection method for inverter based distributed generators.
Distributed Generator
Synchronous Generator
Induction Generator
Fig.2 Islanding Detection Techniques
II. Islanding
Islanding is a situation that occurs when part of a network is disconnected from the remainder of EPS but remains energized by a distributed resource (DR). Failure to trip islanded DR can lead to a number of problems for this resource and the connected loads. The current industry practice is to disconnect all DRs immediately after the occurrence of islands. The main concerns associated with such islanded systems are: 1) the voltage and frequency provided to the customers in the islanded system can vary significantly if the distributed resources do not provide regulation of voltage and frequency, 2) islanding may create a hazard for the utility workers by causing a line to remain energized, 3) the distributed resources in the island could be damaged when the island is out-of-phase reclosed to the EPS, and 4) islanding may interface with the manual or automatic restoration of normal service for the neighboring customers.
III. Islanding Detection
Various techniques have been developed for detecting islanding [2]-[19]. Recent developments in islanding detection are reviewed in details in [20]. The core concept of most of the islanding detection techniques remain the same that some of the system parameters (like voltage, frequency, etc.) change greatly with islanding but not much when the distribution system is grid connected. Islanding detection techniques can broadly be divided into remote and local techniques.
Remote islanding detection techniques are based on the communication between utilities and DGs. Supervisory Control and Data Acquisition (SCADA) [2] or power line signaling scheme [3]-[5] can be used to determine when the distribution system is islanded. Remote techniques are better reliable but are expensive to implement especially for small systems. Therefore, local techniques are widely used to detect islanding. Further these can be classified into passive and active techniques. Passive methods continuously monitor the system parameters such as voltage, frequency, harmonic distortion, etc. Rate of change of output power of DG [6], rate of change of frequency [7], rate of change of frequency over power [8], change of source impedance [9],[10], voltage unbalance [11],[12], harmonic distortion [11],[13],[14] and frequency monitoring with reconfiguration of frequency relay [15] are a few examples of passive islanding detection techniques. The main problem with the passive detection techniques is that, it is difficult to detect islanding when the load and generation in the islanded system closely match. The limitation of the passive detection techniques can be overcome by active techniques, which can detect islanding even under a perfect match of generation and load in the islanded system. Active methods directly interact with the power system operation by introducing perturbations. These small perturbations will result in a significant change in system parameters when the DG is islanded, whereas the change will be negligible when the DG is connected to the grid. Impedance measurement method [9], slip-mode frequency shift algorithm (SMS) [16], active frequency drift (AFD) [17], active frequency drift with positive feedback (AFDPF) [17], automatic phase-shift (APS) [18] and adaptive logic phase phase-shift (ALPS) [19] are a few examples of active islanding detection techniques. The problems with these techniques are that they introduce perturbations in the system and detection time is slow as a result of extra time needed to analyze the system response of the perturbations. Active methods based on impedance measurement introduce high frequency signals, AFD injects a distorted current waveform, and SMS, AFDPF, APS and ALPS shifts the phase of output current. This will often lower the quality of power. Therefore, there is a need to develop an efficient methodology to detect islanding of the distribution system with DG, without adverse effects to the system.
IV. Proposed Islanding detection method
A new method is proposed here to detect islanding. These methods is hybrid of a passive method and an active method and detect islanding for inverter based distributed generators like PV Cell or micro CHP.
Fig.5 A waveform using AFD
Fig.6 Pulsation of chopping fraction cf.
period tends to cause a transient at the instant the DG is disconnected from the grid as a result of the filter. This transient causes a distinct low-frequency signature on the PPSD. The PPSD may be an indication of loss of mains.
Fig. 3 PPSD of voltage periods in grid- connected operation
Fig. 4 PPSD of voltage periods in islanding operation
The grid frequency is very stable most of the time. However, frequency oscillations could occur due to mass load switching and faults. This inevitably will cause large PPSD in the low-frequency band of grid periods, leading to a nuisance trip for islanding. One way of solving this problem is to classify the spectral properties in islanding and power system oscillation. Another way is to construct a more sophisticated hybrid solution which combines PPSD and an effective active anti islanding technique. In this hybrid solution, PPSD can be used as an islanding indicator to activate another active anti islanding scheme; then, a logic decision can be made based on the effect of the active scheme. By doing this, we can avoid the nuisance trip and reduce the excessive disturbance caused by the active method.
Among the active islanding methods, the active frequency drift method (AFD) has received recent attention [17]. AFD is implemented by adding a short period of zero time into output current of the inverter, as shown Fig.5 The ratio of the zero time Tz , to half of the period of the voltage waveform, Tgrid , is referred to as the “chopping fraction” (cf): The chopping fraction enables the islanding detection to drift up (or down) the frequency of the voltage in the islanding situation. But due to the fixed value of cf, the conventional AFD has a large NDZ and too slow to meet the islanding detection time limit. As a method of overcoming the weaknesses of AFD, a novel AFD scheme, in which pulsation of chopping fraction can deviate the frequency instantly away from nominal, is proposed. This scheme is referred to as AFD with pulsation of chopping fraction (AFDPCF) [22]. It can be modeled as below:
cf max if Tcf max_on cf = cfmin if Tcf min_on
0 otherwise
where cfmax and cfmin are the maximum and minimum values of cf respectively. Figure 6 depicts the proposed method. The critical gains of the AFDPCF anti-islanding algorithm are values of cf and its on-time Tcf_on. The
detection method passes UL 1741-test’s time limit of 2 s for
islanding detection and the allowable limit of current harmonics produced by the inverter Total Harmonic Distortion (THD) is below allowable values required by IEEE Std. 929-2000 [1]. The drawback of this method is its dependability. Some time it leaves some islanding events
No
Yes
No
Yes
Zero crossing detection
Obtain 64 periods in a moving time window
Mean removal
Add 64 zeros to form 128 point moving time window
FFT and PSD Computation
PPSD Computation
Greater than Presetting Value
Initiate AFDPCF
Frequency is beyond the set point
Send Signal to Relay
The algorithm based on combination of two methods is an answer for the problem. Whenever PPSD indicates islanding situation AFDPCF activates and checks the islanding event and send the trip signal to the relay and DG gets disconnected.
Fig.7 Proposed Islanding Detection Algorithm
V. Conclusion
The method is proposed on the basis of literature survey. The two methods compensate the short comings of each other. As PPSD is a sensitive indicator of islanding, it is highly dependable but it does not have security. It requires certain modifications that make this method more secure. On the contrary AFDPCF is highly secure but theoretically less dependable. It is expected that combination of these two will prove as a perfect method of islanding detection. The advantage of this method is perturbation due to active method is not exit permanently in the system; it comes into effect for very short period after detecting islanding from passive method. This will reduce THD and improve power quality. It is possible that time taken for detection may be higher then other active methods, it can be acceptable on the cost of accuracy of the method.
VI. References
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