Use only for doing work with or for BC Hydro. Complete Legal Acknowledgement is at

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4 Reviewed: Sergey Kryuchkov Distribution Engineering Scott Merriman Distribution Standards Valentina Dabic Distribution Planning Warren Quan Distribution Operations, FVO Raj Solanki Distribution Engineering Garry Walls Distribution Planning Steven Yau Distribution Standards Cheong Siew Distribution Planning Josh Patton Distribution Engineering Aaron Ellis Distribution Planning Travis Brown Distribution Engineering Page 2 of 12

5 Scope Definitions This standard defines the method for determining load unbalance emission limits for customers connecting to, or requesting connection to BC Hydro s public medium-voltage (MV) distribution network. It also describes the assessment process for verifying compliance with prescribed load unbalance emission limits. These terms are used in this section: Term Voltage Unbalance Electromagnetic Disturbance Public Network Private Network Emission Level Low Voltage (MV) Medium Voltage (MV) Cumulative Probability (CP[x]) Definition A condition in a poly-phase system in which the r.m.s. values of the line-to-line voltages (fundamental component), or the phase angles between consecutive line voltages, are not all equal. The degree of the inequality is expressed as the ratio of the negative sequence component to the positive sequence component. Any electrical or magnetic characteristic, either conducted or radiated, that affects the normal operation of equipment, or causes unwanted sensory experiences for people. Low or medium voltage networks are considered public when they serve, or are intended to serve, more than one customer from the same transformer. Low or medium voltage networks are considered private when they serve, and are only intended to serve, one customer from a dedicated transformer. The level of a given electromagnetic disturbance emitted from a particular device, equipment or system. Describes networks with a rated voltage of 0 V < U n 1,000 V. Describes networks with a rated voltage of 1,000 V < U n 35,000 V. The probability that the measured variable falls within a specified range [x]. For example, the CP95 value for a set of voltage measurements refers to the voltage quantity that was exceeded during 5% of the recorded period. Refer to Q1-02 for discussion about CP[x] statistical notation. Page 3 of 12

6 Application Q4 is intended for application by distribution planning engineers, regional engineers, designers and power quality engineers for: 1) Calculation of customer emission limits for approved operation on a BC Hydro-owned public MV network, and 2) Assessment of customer emission performance for confirmation of operational compliance with prescribed limits. Q4 emission limit determination is valid for all customers connected to, or requesting connection to, a BC Hydro-owned public MV network. Q4-07 specifically defines BC Hydro s emission limit requirements for load unbalance. What is Voltage Unbalance? Voltage unbalance occurs when the line voltages, phase voltages or phase angle separations in a three-phase system are not equal between all phases. The exact definitions, as found in IEC , are expressed as the ratios of the negative and zero-sequence components to the positive sequence component. BC Hydro considers only negative-sequence unbalance for network planning and customer emission limits (voltage and load), as it directly influences the safe and reliable operation of customer equipment. UU uuuuuuuuuu = UU 22 UU 11 (EQ 1) Where: UU 1 = pppppppppppppppp ssssssssssssssss voltage cccccccccccccccccc UU 2 = nnnnnnnnnnnnnnnn ssssssssssssssss voltage cccccccccccccccccc Where r.m.s. phase-to-phase magnitudes are known, voltage unbalance can be approximated using the algorithm represented below using both (EQ 2) and (EQ 3): Page 4 of 12

7 ββ UU uuuuuuuuuu = ββ (EQ 2) Where: ββ = UU aaaa 44 + UU bbbb 44 + UU cccc 44 ( UU aaaa 22 + UU bbbb 22 + UU cccc 22 ) 22 (EQ 3) Where r.m.s. voltage phasors are known, unbalance can be calculated directly by the determination of sequence components. This is accomplished using the algorithm represented below using both (EQ 4) and (EQ 5): UU uuuuuuuuuu = UU aa + aa 22 UU bb + aauu cc UU aa + aauu bb + aa 22 UU cc (EQ 4) Where: aa = = jj (EQ 5) Typical Causes The North American Equipment Manufacturers Association (NEMA) asserts that voltage unbalance at motor terminals should not exceed 1%, and that de-rating of motors should occur when that limit is violated. This is because the magnitude of the current unbalance may be 6 to 10 times as large as the voltage unbalance, causing increased heating and insulation breakdown, thereby leading to shorter life expectancy. Voltage unbalance on public utility networks is caused by load and/or impedance asymmetries. These can occur due to: voltage regulation equipment failure or mis-operation; unbalanced loads, particularly where use of long single-phase extensions is common practice; harmonic resonance conditions; undetected high-impedance fault conditions, or single-phase recloser or fuse operations. Page 5 of 12

8 Effects Voltage unbalance affects only three-phase customers. The primary effects of voltage unbalance are to shorten the life, decrease the efficiency, and degrade the performance of three-phase motors. Since a motor is a constant power device, if the voltage changes on any one phase then the current will compensate to maintain power output. In fact, the magnitude of the resulting current unbalance can be 6-10 times larger than the voltage unbalance (expressed as a percentage). For example, a 100 Hp motor running at full load with a 2.5% voltage unbalance will see a 27.7% current unbalance. Current unbalance causes additional heating in the motor windings. The temperature rise (ΔT + ) can be estimated using the following equation: TT + = TT bbbbbbbbbbbbbbbb UU uuuuuu(%) (EQ 6) Motor winding insulation life is reduced by 50% for each 10 C temperature rise. Using the formula above, it is estimated that a motor running at 80 C would see a temperature increase of 6.4 C when operating with just 2% voltage unbalance. Given the extreme relationship between voltage and current unbalance, the National Electrical Manufacturers Association (NEMA) recommends that motors be de-rated when voltage unbalance exceeds 1%. MV Customer Emission Limits Voltage unbalance limits for public MV networks are adapted for BC Hydro from assessment methods found in CAN/CSA This section describes the method for determining customer unbalance emission limits. BC Hydro s distribution network planning limits for negative sequence voltage unbalance (U unb2 ) are shown below in Table 1Error! Reference source not found.: Table 1 MV network voltage unbalance planning limits Network -Sequence (U unb2 %) Standard 2.0% Rural 3.0% Page 6 of 12

9 For assessment of emission limits the customer s location must first be determined to operate with either standard or rural planning limits. The following rules apply in sequence for determining the appropriate planning limit to use for subsequent calculations: 1) All customers served by substations found in Table 2 are allocated the rural planning limit. Table 2 - Rural distribution substations Rural Substations AFT CLB FRC HLD MTE SCM TDR AHM CLN FSR KAS MWN SEL TKD ASK CNL FST KWA MYE SMH UFR ATL DLK GDN LBH NAK SPN WAR AYH EDD GHL LYT NDR SPT WIN BBR ELT GVL MAS PSN SZM WOS BEL FM2 HBY MCB SBR TCK WWD 2) Customers served by substations not found in Table 2 that are located greater than 20 kms* from the substation are allocated the rural planning limit. 3) All remaining customers are allocated the standard planning limit. * Distance is measured by conductor length from the substation to the customer s MV point of interconnection. Step 1: Simplified Evaluation for Immediate Approval The simplified evaluation method involves comparison of the maximum single phase power equivalent of the load unbalance (SS uuuu ) with the short circuit power (SS ssss ) available at the customer s point of evaluation. 1) Immediate approval is granted if the following criteria is fulfilled: SS uuuu SS ssss 0.2% (EQ 7) SS uuuu Single phase power equivalent of the load unbalance at i. SS ssss Short circuit power at the point of evaluation See ES55 Q1-01. Page 7 of 12

10 Notes: 2) If immediate approval is not granted through application of the simplified evaluation method, then proceed to Step 2 for detailed limit calculations. 1) Available short circuit power (SS ssss ) refers to the calculated three phase fault level at the customer s point of evaluation. Step 2 Voltage Unbalance Emission Limit Calculation When customer installations do not qualify for immediate approval, their voltage unbalance emission limits must be calculated independently. Each customer on the MV network is assigned an emission limit in accordance with their share of the total capacity of the feeder to which they are connected. Limits for voltage unbalance are calculated using MV system planning levels, network capacity and the customer s load characteristics. Limits are presented as load current unbalance limits for ease of application. The equation below determines the negative sequence current unbalance emission limit for the installation, i. EE ll2ii = αα KK uuuu αα SS tt (EQ 8) ZZ 2 GG uuuuuu+llll SS ii EE ll2ii current unbalance emission limit for the installation (%). KK uuuu global contribution allocated for emissions from unbalanced installations in the MV and LV networks see Table 3 α summation law exponent - use 1.4. GG uuuuuu+llll network planning limit. SS ii SS tt ZZ 2 MV customer s agreed power (MVA). total supply capacity of the MV network (MVA). negative sequence impedance at the point of evaluation (Ω). Notes: 1) The customer s total current unbalance shall be limited to the lesser of: i. The emission limits prescribed by (EQ 8) of this standard; or ii. 50% negative sequence current unbalance. Page 8 of 12

11 The global contribution allocated for emissions from the unbalanced installations in the MV and LV networks is calculated considering line length, system configuration, and voltage levels. Values for KK uuuu and selection criteria are given in Table 3, below. Table 3 Global contribution allocation, KK uuuu. System Configuration High density load area with short lines or cables and meshed networks (Downtown Victoria). Mix of high density and suburban load areas with relatively short lines (< 20 km). Mix of medium and low density load areas with relatively long lines ( 20km). KK uuuu Step 3: Acceptance of higher emission levels Under some circumstances the BC Hydro MV system can tolerate emission levels higher than what might be calculated in Step 2. The following factors may provide some margin for allocating higher emission limits: Some customer installations do not produce significant voltage unbalance, thus some of that customer s voltage unbalance emission allocation could be re-assigned to others. Conditionally higher emission limits can be allowed when a customer is forced to operate under degraded network configurations through no fault of their own. If any of the above scenarios might apply to the emission limit assessment, consult a Power Quality Engineer for guidance. Performance Criteria Customer Emissions Assessment Customer emission performance with respect to current unbalance is assessed as follows: average negative sequence current unbalance shall be evaluated over 10 minute intervals for a minimum of 1000 intervals (1 week); Page 9 of 12

12 Meter Specifications Customer Response the recorded average 10 minute CP95 weekly values shall not exceed the prescribed emission limits, and the recorded average 10 minute CP99 weekly values shall not exceed 1.3 x the prescribed emission limits, and no recorded average 10 minute value shall exceed 1.5 x the prescribed emission limits. To determine compliance with unbalance requirements, measurements must be obtained using a power quality meter with a minimum of Class S performance as defined by CAN/CSA (Class A requires both zero and negative-sequence unbalance to be calculated, while Class S has Class A measurement specifications, but only measures negativesequence unbalance.) Some of the key required metering specifications are: average 12-cycle r.m.s. voltage measurements are required over 10 minute intervals; The effect of harmonics is minimized by the use of a filter or by using a DFT algorithm to extract the fundamental component for evaluation, and negative-sequence unbalance values are recorded and aggregated over 10-minute intervals. When a customer s operation violates prescribed emission limits on the BC Hydro system, mitigation must occur within 60 days of notification. Where mitigation requires a complex engineered solution, an extension of time may be granted, at BC Hydro s discretion, up to 90 days to allow for design, procurement, and commissioning of new equipment and/or process. BC Hydro Power Quality Engineers are available to assist with standards interpretation, data collection and analysis and compliance evaluation: They may, where applicable, offer general advice relating to potential mitigation options. They will not make recommendations of a specific nature for any customer facility. Page 10 of 12

13 BC Hydro Response Customers should act solely on advice from qualified professional electrical engineers, who are hired by the customer, when mitigation is required at their facility. When a customer s operation violates prescribed emission limits, BC Hydro will notify the customer of their violation. Written notification may be communicated via electronic or regular mail, and the notice will be clear that the customer s use of electricity is causing a disturbance to the BC Hydro electrical system. Where previous communication has been established electronically between BC Hydro and the customer, notification shall be considered suitable written notification. The written notice to the customer should outline that if a customer fails to meet emission limit requirements within the approved timeframe, then BC Hydro may disconnect the customer from the public network to prevent further disturbing operation. Where upgrades can be installed on the BC Hydro network to mitigate the disturbance, BC Hydro can offer to install such upgrades at the customer s cost. The customer s written confirmation for assuming responsibility of these costs should be obtained before installing upgrades for the benefit of the customer. References ES55 Q1-04 Customer Apparent Power Assessment ES55 Q2-07 Network Planning Limits Voltage Unbalance Sources Neilson, J.B., Buchholz, Vern L. BC Hydro Power Quality Guide. (2005). CAN/CSA-C Electromagnetic compatibility (EMC) - Part 2-2: Environment Compatibility levels for low frequency conducted disturbances and signalling in public low-voltage power supply systems. CAN/CSA-C Electromagnetic compatibility (EMC) - Part 3-13: Limits Assessment of emission limits for the connection of unbalanced installations to MV, HV and EHV power systems. CAN/CSA-C Electromagnetic compatibility (EMC) - Part 4-30: Testing and measurement techniques - Power quality measurement methods. ANSI/NEMA MG Motors and Generators. Page 11 of 12

14 Bolen, M.H.J. IEEE Power Engineering Review (November 2002). Definitions of Voltage Unbalance. US Department of Energy. Energy Tips: Motor Systems. Retrieved Dec. 15, 2013 from EC&M (1999). The Basics of Voltage Imbalance. Retrieved Dec. 15, 2013 from Page 12 of 12