Renewable Interconnection Standard & Experimental Tests. Yahia Baghzouz UNLV Las Vegas, NV, USA

Transcription

1 Renewable Interconnection Standard & Experimental Tests Yahia Baghzouz UNLV Las Vegas, NV, USA

2 Overview IEEE Std 1547 Voltage limitations Frequency limitations Harmonic limitations Expansion of IEEE Std through Anti-islanding Passive methods Active methods Islanding tests on local inverters

3 Publication Year: 2003

4 IEEE Std scope and purpose Scope: The standard establishes criteria and requirements for interconnection of distributed resources (DR) with electric power systems (EPS). Purpose : The document provides a uniform standard for interconnection of distributed resources with electric power systems. It provides requirements relevant to the performance, operation, testing, safety considerations, and maintenance of the interconnection.

5 Interconnection System Response to Abnormal Voltages

6 Interconnection System Response to Abnormal Frequencies

7 Maximum Harmonic Current Distortion

8 Synchronization Parameter Limits for Synchronous Interconnection

9 Maximum Voltage Distortion for Synchronous Machines

10 Expansion of IEEE Std ( Recommended Practice for Establishing Methods and Procedures that Provide Supplemental Support for Implementation Strategies for Expanded Use of IEEE Standard 1547 Pending

11 Medium Voltage Standard in Germany (10 kv-110 kv)

12 Medium Voltage Standard in Germany (10 kv-110 kv)

13 Medium Voltage Standard in Germany (10 kv-110 kv)

14 Inverter Response to Power Outage - Test Islanding occurs when the disconnected part of the power network is sustained by the connected PV systems for a significant period of time. Islanding is not desirable for several reasons: Creation of a hazard for utility line workers by touching a line that is supposed to be de-energized, lack of control over voltage and frequency in the island. Interference with restoration of normal service.

15 Expected inverter response PV inverter manufacturers market inverters that are expected to meet current interconnection standards (i.e., IEEE Std. 1547): They are expected to disconnect within 10 cycles if the voltage drops below 50% or rises above 120% of its nominal value. disconnect within 10 cycles if the supply frequency drops below 59.3 Hz or rises above 60.5 Hz. disconnect within 2 seconds cycles If the voltage drops to a value between 50%-88%, disconnect within 1 second if the voltage rises to a value between 110%-120% of the nominal value.

16 Inverter islanding detection Standard protection of grid-connected PV systems consists of four relays that will prevent islanding under most circumstances. over-voltage relay, under-voltage relay, over-frequency relay, under-frequency relay. However, if the local load closely matches the power produced by the inverter, the voltage and/or frequency deviations after a power outage may be too small to detect, i.e., fall within the non-detection zone. In this case, additional schemes are required to minimize the probability of an island to occur.

17 Voltage and frequency deviations Let the ratio of P S /P D = α, and Q S /Q D = β. Before disconnect, P (1 ) V / D 1 V ' V 1 2 R After utility disconnect, Q D 2 (1 ) V / L 1 ' 1

18 Possible Cases Case A: P S > 0 and Q S > 0: The voltage decreases. The frequency depends on the values of α and β. Case B: P S > 0 and Q S < 0: Both the voltage and frequency decrease. Case C: P S < 0 and Q S > 0: Both the voltage and frequency increase. Case D: P S < 0 and Q S < 0: The voltage increases. The frequency depends on the values of α and β. Case E: P S = 0 and Q S 0: The voltage remains constant, while the frequency changes (decreases if Q S < 0 or increases if Q S > 0). Case F: P S 0 and Q S = 0: The frequency remains constant, while the voltages changes (increases P S < 0 or decreases if P S > 0). Case G: P S = 0 and Q S = 0: Both the voltage and frequency remain constant.

19 Common Active Anti-Islanding Techniques Voltage harmonic monitoring: inverter monitors voltage total harmonic distortion and shuts down if this parameter exceeds some threshold. Phase jump detection: phase between inverter's terminal voltage and its output current is monitored for sudden jumps. Slide-mode frequency shift: the voltage-current phase angle of inverter is made a function of system frequency. Impedance measurement: perturbation periodically applied to inverter current. This will force a detectable change in voltage if the utility voltage is disconnected. Active frequency drift: inverter uses a slightly distorted output current to cause the frequency of the voltage to drift up or down when utility is disconnected.

20 PV System A: 18 kw Fixed Array (installed in 1999) Inverter Manufacturer: Trace Technologies Inc., Rating: 30 kva, 120/208V, 3-Phase. Anti-islanding technique for critical case: unknown

21 Schematic diagram of Test Circuit

22 Test Procedure and Apparatus Connect the transient recorder, load bank, and meters for reading current or power flow into the load and utility grid as shown in Figure. Adjust the load bank to the desired fraction of load relating to generated power. Open the utility disconnect while recording the voltage and current waveshapes. Repeat the two steps above for different generation-load power mismatch levels.

23 PV System A Switching Events Event No. Case P S (kw) P D (kw) Q S = -Q D (kvar) A.1 D A.2 B A.3 E Case B: P S > 0 and Q S < 0: the voltage decreases and frequency 0. Case D: P S < 0 and Q S < 0: The voltage increases, and frequency 0. Case E: P S = 0 and Q S < 0: The voltage remains constant, and frequency 0.

24 Event A.1 (α = -0.66, β = -1)

25 Event A.2 (α = +0.32, β = -1)

26 Event A.3 (α 0, β = -1)

27 PV System B: 25 kw 2-Axis Tracking (installed in 2003) Inverter Manufacturer: Advanced Energy Systems, LTD Inverter Rating: 30 kva, 120/208V, 3-Phase. Anti-islanding technique for critical case: unknown

28 PV System B Switching Events Event No. Case P S (kw) P D (kw) Q S = -Q D (kvar) B.1 D B.2 B B.3 E

29 Event B.1 (α = -0.26, β = -1 )

30 Event B.2 (α = +0.26, β = -1)

31 Event B.3 (α 0, β = -1)

32 Test Summary Both systems shut down in less than 5 cycles after the utility outage in compliance with the Interconnection standard that allows up to 10 cycles. The over-voltage and under-voltages relays isolated the inverter when there was mismatch between the PV power and local load power. The under-frequency relay isolated the inverter when there was a match between the PV power and local load power. Although small, the reactive power generated by both inverters will ultimately drive the frequency of the islanded system to zero, thus triggering the under-frequency relay under all resistive load conditions. An adjustable reactive load (in addition to the resistive load) would be needed to match both real and reactive powers and test for islanding under zero deviation in both voltage and frequency.

33 SIMPLE TEST ON 2 KW PV SYSTEM PV Array Size: 2 kw (peak) DC-side measurement AC-side measurement Response to an overvoltage ( V < 1.2 pu) Response to a large overvoltage (V > 1.2 pu)

34 DC-SIDE VOLTAGE AND CURRENT The inverter utilizes the Perturb-and-Observe method for MPPT. The voltage is perturbed by nearly 4 V, or 1.5% of the nominal value every 2 seconds.

35 AC-SIDE VOLTAGE AND CURRENT The AC current THD measures nearly 4% (the limit is 5%).

36 INVERTER RESPONSE TO 14% OVERVOLTAGE The inverter shut down after 56 cycles. The inverter is in compliance with IEEE Std which allows up to a maximum of 60 cycles for 1.1 < V < 1.2 p.u.

37 INVERTER RESPONSE TO 33% OVERVOLTAGE The inverter shut down within 8 cycles. The inverter is in compliance with IEEE Std which calls for a maximum of 10 cycles for V > 1.2 p.u.

38 Break!